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Dispersal, survival, and population regulation of the vole Microtus townsendii Beacham, Terry D. 1979

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DISPERSAL, SURVIVAL, AND  POPULATION  REGULATION  OF THE  MIGBOTUS TOWNSENDII by TERRY DALE BEACHAM B. S c . ( H o n s . ) , U n i v e r s i t y o f M a n i t o b a ,  A THESIS SUBMITTED  I N PARTIAL FULFILMENT OF  THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF  PHILOSOPHY in  THE FACULTY OF GRADUATE STUDIES (Department o f Z o o l o g y )  He a c c e p t t h i s  thesis  to the required  as c o n f o r m i n g standard  THE UNIVERSITY OF BRITISH COLUMBIA March c  Terry  1974  1979  D a l e Beacham,  1979  VOLE  In p r e s e n t i n g t h i s  thesis in partial  an a d v a n c e d d e g r e e a t the L i b r a r y I  further  for  shall  the U n i v e r s i t y  make i t  agree that  freely  of  extensive  s c h o l a r l y p u r p o s e s may be g r a n t e d  this  thesis for  written  It  f i n a n c i a l gain shall  Department of ,  Zoology  The U n i v e r s i t y o f B r i t i s h C o l u m b i a 2075 W e s b r o o k P l a c e V a n c o u v e r , Canada V6T 1W5  Date  2  April  1979  I agree  r e f e r e n c e and copying of  this  that  not  copying or  for  that  study. thesis  by t h e Head o f my D e p a r t m e n t  i s understood  permission.  the requirements  B r i t i s h Columbia,  available for  permission for  by h i s r e p r e s e n t a t i v e s . of  f u l f i l m e n t of  or  publication  be a l l o w e d w i t h o u t  my  ii  Abstract A number o f f i e l d dispersal  i s an  investigate two  important  i f voles  vole-proof  voles could  studies  ou  from  enclosures, These  two  control  June  n e a r V a n c o u v e r , Canada.  All the  populations  four Microtus  1976  summer,  i n c r e a s e d d u r i n g the to  delayed  survival  capture  four  breeding  in  began  females  in  t o 0.76  spring  two  a t 0.90.  spring  than  during  sharply  later.  populations reproductive that  attaining prompted All  four  1977,  c o n d i t i o n than behavioural  sexual the  the  sampled  were 2-15% decline  two  populations  owing minimum  weeks.  months  after  concurrent  dispersed  of  survival survival peak  1978.  reached  than  1977  Mounding  a  peak  six  common i n t h e  peak  i n d i v i d u a l s more o f t e n  larger  smaller  live-traps  minimum  v o l e s was  until  during  female per  i n t e r a c t i o n s between and  two  season,  were r e s i d e n t s u b a d u l t s .  maturity  and  male minimum  spring  these  the  h e a v i e r i n the  began and  with  which  low  d e c l i n e s being  d e c l i n e , and  I made  1976  by  and  v o l e s . More m a l e s  d i s p e r s a l o f the vole  Hay  were  were 0.92  the  D i s p e r s a l of s u b a d u l t  aggressive  from  weeks, whereas f e m a l e Voles  into  declined f o r three  when b r e e d i n g  o f summer  area  populations  Male  that  r e g u l a t i o n . To  non-breeding  winter  1977,  this  high  weeks  winter  1976  smaller  per  suggested  populations,  populations  live-traps.  spring  remained  increased  and  of the  during  fenced  population  vole populations  dispersal  dropped  fall  r a t e s d u r i n g the  all  with  the  an  were t r a p p e d  townsendii  and  declining  each h a v i n g  unfenced 1978  have  mechanism o f p o p u l a t i o n  disperse  disperse.  voles  I  smaller  sexually-mature  in  suggest animals adults  individuals.  declined for  a  second  time  in  October the  1977  for  e i g h t months, w i t h  n o n - b r e e d i n g and  subsequent  losses  occurred  i n the  neither  wounded  nor  especially survival  poorly  survival  was  breeding  began  populations  both  0.81  The  in  two  female  interference limited  1978,  The  supply  tendency  may  a t t h e same a g e ,  minimum o f this  vole  of  Spring  for  survived  female  minimum  four  was  0.65.  weeks  the  wounding  and  p e r two  weeks,  I  suggest  small  accounted  when  that  voles  for  some  no  for  a  of  the  on  the  by  grids  (lifetime)  and  avian  l o s s from  declined  avain denser  and  during small  litters  heritable  summer-born d i s p e r s e r s  leave  mature. accounted  for  tagged vole p o p u l a t i o n s , the  winter  individuals,  predation vole  indicates a  predators  the  among  was  of  1977.  and  and The  mortality  density-dependent  populations  a  suffered  in  the  higher  rates.  Juvenile in  were  Minimum  minimal  become s e x u a l l y  1977,  of the  populations;  lowest  voles  weight.  d r o p p e d t o 0.47  have  s e l e c t e d males and  predation  with  time s p e n t  when t h e y  fall 15%  accounted  within  peak p o p u l a t i o n s , w h i c h  percentage  predators  the  males  were non-randomly d i s t r i b u t e d  f o r these' t r a i t s .  In the  of  season l o s s e s .  i n i n c r e a s i n g and basis  Most  body  between l a r g e and  known l e n g t h o f  dispersal  lost  and  survival  competition  non-breeding  smaller  weeks, w h i l e  disappeared,  minimum  food  season.  both  rate of d e c l i n e a c c e l e r a t e d r a p i d l y  d i s p e r s a l . M a l e minimum s u r v i v a l while  The  sexes  per  spring  virtually  d e c l i n e spanning  n o n - b r e e d i n g s e a s o n , when t h e  and  0.82.  breeding  dispersing.  o f m a l e s was  the  s u r v i v a l was  highest  i n increasing populations  d e c l i n i n g ones, whereas n e s t l i n g  survival  was  and  lowest  iv  in  peak  populations.  nothing  to  dependent  Nestling  population  live-traps category  failed of  parameters owing  declines.  upon t h e t r a p p i n g  The c o n c u r r e n t  enter  This  adult survival  showed  to  size  enumerate Moreover,  solely  on  live-traps.  study  increasing  and  completely the  any  estimates  live-trap  Live-traps  According  reguired  beginning  peak  populations  vole  data  although may  for  these  of  quantity  also  were i n v o l v e d  particular  instance.  the  view t h a t  i n population  be  inaccurate before dominant  dispersal  from  stop  f rota  declines:  d i f f e r e n c e s between  i n the properties  that  demographic  the  only  the  c h a r a c t e r i s t i c s o f t h e 1978 d e c l i n e p a r t i a l l y and  was  o r sex  individuals  them  the  subseguent  one  explanation  voles  d e c l i n e s have been s u b j e c t e d and  or  subordinates.  to the Chitty hypothesis  pressures,  hypothesis  may  i t can not account f o r  account  to changes  the  that  o f t h e two  selection due  to  pitfalls  of  sample  indicates  increasing indefinitely, decline.  little  technique.  individuals preferentially,  are  Apparent  t o d i s p e r s a l o f young, s e x u a l l y - m a t u r e  they  is  contributed  use of l i v e - t r a p s and p i t f a l l s  voles. based  mortality  to  at  the  different  t h e two  declines  individuals.  The  support  the C h i t t y  changes i n food  q u a l i t y or  regulation at least  in this  V  TABLE OF CONTENTS  Abstract  i i  T a b l e of c o n t e n t s ......................................... List  of tables  List  of f i g u r e s  v ix xiv  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . x v i i General i n t r o d u c t i o n Literature Section vole  1.  .................  cited  1 4  D i s p e r s a l during  population  f l u c t u a t i o n s o f the  Mierotus townsendii  6  Introduction  6  Methods ..  7  H €S UX tS * • • • • * • • •  * * • • »*•••**.**•••*•'*<••;*• * * * * * * * * * * * . ^ ^  Trappability  ••.........  Population  density  Population  density  S u r v i v a l and  12 15  and d i s p e r s a l . . . . . . . . . . . . . . . . . . . .  24  d i s p e r s a l ..............................  32  D i s p e r s a l and t i m e o f b i r t h  36  Sex r a t i o s o f d i s p e r s i n g v o l e s  ......................  39  and d i s p e r s a l . . . . . . . . . . . . . . . . . . . . . . .  42  Body w e i g h t and d i s p e r s a l . . . . . . . . . . . . . . . . . . . . . . . . . . .  47  Sexual  maturity  Discussion Literature Section  2.  53 cited  .......................................  Dispersal  littermates during Mierotus townsendii  tendency  population  and  duration  f l u c t u a t i o n s of  of l i f e the  61  of  vole  ....................................  66  Intiroductxon * • • • « * • . • « • - • * • • • . * * • * • • - * • . • • • • • - * • • • • • * • • • . * • * • * ' • •  66  vi  Methods ..........  ..  .  67  Results  70  Population  density  and  Littermate  lifespan  Littermate  dispersal  dispersal  ....................  70 71  tendency  76  Discussion  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79  Literature  cited  Section  3.  .......................................  Selectivity  Populations  of the vole  Introduction  of  Avian  85  Predation  i n Declining  Microtias townsendii  ............  87  ..................................... -  87  Hethods  ..................  ..... . . . . . . . . . . . . . . . .  87  Results  . . . . . . . . . . . . . . . . . . . . . . ^ . . . . . . . . ... . . . . . . . . . . . ...  89  a b u n d a n c e ................. « r .  .-.  89  ..............................  91  .....................................  92  Vole  Incidence  of predators  Predator  impact  Sex r a t i o o f v o l e s e a t e n by p r e d a t o r s Body w e i g h t d i s t r i b u t i o n s Probability  ,  95  ...........................  96  of predation  D iscu ss ion  .......................  Literature  cited  Section  in fluctuating  townsendii  Introduction  100  populations  of the  104  vole  ....................................  106  ...........................................  106  M 6 tllO&S • ••••••• R6SllX t S  ....................  .•«•• . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  4. / S u r v i v a l  Microtus  98  •  •••*• •• ••m  • • • * « • • • • * • • • * • • • • «.« * ••<• • • • m * • » m  •••*••  • 106  ••  • • • • -108  A  Trappability  108  Population  109  Juvenile  density  survival  ..................................,  111  vii  Preweanling adult  survival  and s u b a d u l t  . . . . . . . . . . . . . . . . . . . . . . . . . «^  survival  •  114  .........................  124  Discussion  128  Literature Section  5.  cited  136  Demography o f d e c l i n i n g  Mierotus townsendii Introduction  populations  o f the  vole  ....................................  139  ...........................................  139  Methods  139  Results  ...  Population  density  140  .................................  140  Body w e i g h t s  150  Hates of d e c l i n e  .....................155  Size-selectivity G l T O Wt- tl  IT t i t Q S  Wounding  m m • *  <D * * * • m  Literature  cited  6.  m m m m • m m m. m m • • m m m m m  m »m'^\ m • m • • ^ • • •  :  • • • mm*  •  9  versus  populations  Introduction  • m • • • • m m m m m • • •  live-trap  of the v o l e  enumeration  Mierotus townsendii  ..................  Body  177 178  179 density  capture weight  live-traps Dispersal  177  179  TrappaMlity  First  162  of  • • • • • • « . • • • • • * • • • ••.•-••••••••••.*.•*.•••.*••••••••••.••  Population  161  ,164  ...........................................  M©tliocls  159  173  Pitfall  fluctuating  Results  .......,.................  r a t e s •••••••••••••••••••••••••>•••••••••••»  DXSCUSSXOn  Section  of s u r v i v a l  ..................................  of voles  in live-traps  differences and p i t f a l l s  at  and p i t f a l l s  first  capture  ...  180 189  in  ..........................  193 199  viii  Juvenile  survival  200  A comparison o f a d u l t  males caught i n l i v e - t r a p s  and  pitfalls Why  202  a r e some m a l e s n o t c a u g h t i n l i v e - t r a p s ?  Growth  rates  live-traps  of  voles  caught  in  and o f t h o s e c a u g h t o n l y  ....... 206  pitfalls i n pitfalls  and ..210  Discussion.  • •••••*•.••••••.•••••*••••••*•••••*•••••**•••**••  211  Literature  cited  220  G e n e r a l summary  ...........................................  223  ix  L I S T OF  Table  1.1.  Trappability  Longworth  Table  1.2.  live-traps  trappability, 1.3.  All Table from  1.4.  Table  D.  T  is  grids  rate  B and C.  of  of  1.5.  T  is  increase,  Mierotus  Mean  weight  given  (•  1  and  of  dispersing  before  of  the  1977  the s t a r t  caught  from  the  next  of  an i n d i v i d u a l d i s p e r s i n g  i t s month o f f i r s t  capture  1.8.  Sex r a t i o s  i n 1976.  of  experimental  Both  Mierotus grids.  35  37  during sexes  ..........................  ( p r o p o r t i o n o f males)  populations  parentheses.  during  and  .........................................  Probability  and  caught.  successful  an i n d i v i d u a l f i r s t  B and C a r e c o m b i n e d .  control  both  ..........................................  grids  dispersing  grids  on  dispersers  of  30  disappearing  populations  SB)  to control  Probability  to winter  1.7.  and p i t f a l l  14  townsendii.  townsendii  g r i d s and number o f t a g g e d  decline.  1.6.  Mierotus  12  townsendii i n  ..........................  density,  Number o f t a g g e d  breeding season.  and  f o r Mierotus  N i s sample s i z e .  combined l i v e - t r a p  spring  1977  and  d a t a a r e mean v a l u e s f o r t h e p e r i o d . . . . . . . . . . . . . . . . . .  spring  Table  A  ..........................  on e x p e r i m e n t a l  unsuccessful recruits  Table  grids  rates f o r populations  experimental  Table.  size.  estimates  Population  dispersal  f o r Microtas townsendii i n  control  N i s sample  Trappability  Longworth  Table  estimates  l i v e - t r a p s on  trappability,  TABLES  38  i n r e s i d e n t and  townsendii Sample  size  on  both is  in  . . . . . . . . . . .... ... . . . . . .. ...... .... ..... . .... . .  40  X  Table  1.9.  P r o p o r t i o n o f males  dispersing  and  townsendii. Table  1.10-  size Table  resident  Sample  in  of  age  populations  of  subadult  group  Microtus  in  populations.  Median w e i g h t a t s e x u a l m a t u r i t y f o r g r i d  B  f e m a l e r e s i d e n t s and d i s p e r s e r s . 9555 c o n f i d e n c e  cLITQ  XQpclXr  ©lit }XQ S6S«  1.12.  Median  and  or  1.13.  Table  2.1.  in dispersing  seasons. Litter  2.2.  defined  Sample sizes  size  f r o m t h e c o n t r o l and  2.3.  as the time  resultant  litters  are thus a subset 2.4.  within 1976  and 2.5.  between  first  Analysis of variance within  male  limits  litters of s i z e o f those  to  ........  with  dispersers  1 were  elimimated.  i n Table  2.2  was  c a p t u r e s . ..... test  removed. A l l These  data  ..................  of v a r i a n c e t a b l e s f o r d i s p e r s a l tendency  litters  born  1977.  ...........  Analysis  of  experimental  table  75  populations during  ............................  variance  74  non-random  Analyses  in  72  non-random  litters..Lifetime  to  50  experimental  test  and l a s t  tables  46  during  ............................ tables  45  that are j u v e n i l e  i s i n parentheses.  among H i c r o t u s t o w n s e n d i i  lifetimes  Table  C  and r e s i d e n t p o p u l a t i o n s  A n a l y s i s of v a r i a n c e  lifetimes  Table  f o r grid  ......................................  p o p u l a t i o n s i n 1976 and 1977.  Table  limits  • • • • •. • • • • • • • • •  P r o p o r t i o n o f m a l e s and f e m a l e s  subadult  43  male  f e m a l e r e s i d e n t s and d i s p e r s e r s . 95% c o n f i d e n c e  different  Table  •• ••  weight at s e x u a l m a t u r i t y  are i n parentheses. Table  Sample  ..................................  •. * •  41  breeding  and  Table  in  ...............  voles  r e s i d e n t and d i s p e r s i n g  i s i n parentheses.  1.11.  to  s i z e i s i n parentheses.  Percentage  condition  according  to  test  whether  77  xi  dispersing  M.  townsendii  s i b l i n g s have t h e same  lifespan.  ..................... ^.......^..... Table  3.1.  Number  Mierotus from  Table  townsendii  avian  pooled  of  tagged  79  v o l e s d i s a p p e a r i n g from  p o p u l a t i o n s and  predator  pellets.  percentage  R e s u l t s from  a l l grids  3.2. P e r c e n t a g e  of  predators  vole  and  mortality  average  M.  accounted  townsendii  1977-1978. . . . . . .  3.5.  P e r i o d i s from  i n eaten  and  Probability  killed  first  in  and  Sample  sizes  resident  populations  o f an i n d i v i d u a l  1977.  in  are i n  A l l grids  and  juvenile  survival  Mierotus  rates  townsendii  fall ...  townsendii  to both  week  of  sexes are  14-day  period  for  combined l i v e - t r a p and  p o p u l a t i o n . ............................. .........  4.2. Minimum  Mierotus  survival  townsendii  populations  rates  caught  were p o o l e d  per  14-day  by L o n g w o r t h  f o r each  period  4.3.  Mierotus  113  for  live-traps. A l l  interval.  Sample  sizes  are i n parentheses. Table  98  99  per in  96  Mierotus  from  Mierotus  relation  94  resident  ............................  4.1. Minimum  pitfall  by  s p r i n g 1977 t o s p r i n g 1978. ..  by a v i a n p r e d a t o r s  capture  pooled.  Table  o f eaten  populations.  93  density f o r  t o s p r i n g 1978. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  being  Table  for  3.4. Number o f l a r g e and s m a l l male and f e m a l e  townsendii  Table  (males:females)  townsendii  parentheses.  1977  were  .............  3.3. Sex r a t i o s  Mierotus  Table  recovered  i n each p e r i o d . ...................................  avian  Table  four  125  Minimum  survival  townsendii  caught  rate by  per  pitfalls.  14-day  period f o r  A l l populations  xii  are  pooled  parentheses. Table  %  weights  population)  5.2.  o f heavy-weight  females,  breeding  decline  and l a t e r  per  week,  of  body  size  i s in  over  townsendii  in pitfalls,  first  and t h o s e  the entire  163 caught i n  caught  study. first  and those  only i n  . . . . . . . . . 191 caught  caught  in  only i n  .  populations.  192 capture  i n l i v e - t r a p s and  Data f o r e a c h g r i d  (• 1 SE) o f  pitfalls  a r e pooled  for  over  four  the e n t i r e  Sample s i z e s a r e i n p a r e n t h e s e s .  6.4.  Numbers  of Microtus townsendii  class  at f i r s t  capture  6.5. P e r c e n t a g e  between  body  f o r m a l e s and  Sample  6.3. Mean w e i g h t i n grams a t f i r s t  study.  of  and v a r i a n c e  Hicrotus townsendii  6.2. Numbers o f H i c r o t u s  Hicrotus townsendii  first  live-traps Table  variance  of  declines.  Data a r e pooled  live-traps.  Table  a t t h e s t a r t and e n d ,  d e c l i n e s o f 1977 and 1978. ... 158  and l a t e r i n l i v e - t r a p s ,  live-traps  sex  of  127  number  ......... .....................  Numbers  pitfalls.  in  d e c l i n e s o f 1977 and 1978. ... 157 rates  during  6.1.  are  week,  5.3. A v e r a g e number o f wounds p e r a n i m a l  pitfalls  Table  sizes  per  and  Instantaneous  parentheses.  Table  males  males,  during the breeding  females  Table  of  during the breeding  weights  Sample  r a t e s of d e c l i n e  o f heavy-weight  proportion  Table  interval.  .............................................  of  proportion  Table  each  5.1. I n s t a n t a n e o u s  (and  Table  for  in  by s i z e c l a s s  pitfalls  of Microtus townsendii.  6.6. P e r c e n t a g e  i n e a c h s i z e and  i n l i v e - t r a p s and p i t f a l l s .  distribution  capture  194  distribution  of  and f i r s t  ... 195  interval  capture i n  .. . . .... , . . . . . . . . . . . . . .  by s i z e c l a s s  of  interval  197  between  first  townsendii combined. Table  6.7.  and  last  pitfall  caught only i n  captures  pitfalls.  1976  of and  Microtus 1977  are  ....................... .........................  Number o f d i s p e r s i n g  live-traps  and  pitfalls.  Microtus  townsendii  Data a r e pooled  caught i n  over the e n t i r e  study.  200  T a b l e 6.8.  Juvenile  juvenile  townsendii. 6.9.  minimum  survival  p o p u l a t i o n s and  Table  of  for  6. 10.  live-traps. 6.11.,  townsendii caught Table  and DeLong  live-trap  townsendii  1976  and  index  of  (1965) f o r t o t a l  populations  in pitfalls  Demographic caught  during  of  Microtus  Microtus  and l i v e - t r a p s .  attributes summer  of  1977  male  in  Demographic first  caught i n  attributes pitfalls  of  ....  in  1976  and o f t h o s e m a l e s t h a t  Demographic  attributes  f i r s t caught i n  pitfalls  i n l i v e - t r a p s and o f t h o s e  of  and  in  1977  206  Hicrotus that  were  were n o t . ... male  204  Microtus  pitfalls  male  202  townsendii  ..............................................  in live-traps  6.12.  caught  Krebs  rates  D e m o g r a p h i c a t t r i b u t e s o f male  townsendii  Table  survival  ............................... ......... ......  c a u g h t d u r i n g summer Table  198  208  Microtus that  were  m a l e s t h a t were n o t . ...  209  xiv  L I S T OF FIGURES  Figure and  1.1. E x p e r i m e n t a l D a r e unfenced  enclosed Figure  Figure  Figure  1.4.  fenced  grid  Figure  grid  fenced  Figure  those  Population  are 7 on  Microtus  townsendii  d e n s i t y of Microtus townsendii  grid  1.8.  Mean  20  Microtus  townsendii  on  that dispersed  and  dispersing  Microtus  weights  of  27  resident  male  M.  ( C R ) , and  (BD, C D ) .  48  weight d i s t r i b u t i o n s  of resident  townsendii  (N=1255)  from  fenced  B d u r i n g summer 1977. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.  Population  density  determined  by l i v e - t r a p s  78.  sexes  Both  25  townsendii  p o p u l a t i o n s B (BB) and C  (N=92) male M.  22  townsendii  C resident population  i n fenced  1.9. Body  Microtus  B r e s i d e n t p o p u l a t i o n . ................  body  18  on  C and number o f d i s p e r s e r s . . . . . . . . . . . . . . . . .  grid  16  on  B and number o f d i s p e r s e r s .  Figure  Figure  C  D and number o f d i s p e r s e r s . . . . . . . . . . . . . . . .  fenced  townsendii  and  A and number o f d i s p e r s e r s . . . . . . . . . . . . . . . .  1.7. S i z e c l a s s e s o f d i s p e r s i n g  from  B  density of Microtus townsendii  1.6. S i z e c l a s s e s o f d i s p e r s i n g  from  grid  grids  Grids A  ..........................  1-5- P o p u l a t i o n d e n s i t y o f  fenced  Figure  grid  i n t h i s study.  and  1.3- P o p u l a t i o n d e n s i t y o f  control  Figure  Population  grid  used  controls  experimentals.  1.2.  control  design  of  on c o n t r o l  Microtus grid  townsendii  D during  a r e combined. Non-breeding  1976-  periods are  shaded. F i g u r e 4.1. P o p u l a t i o n d e n s i t y d e t e r m i n e d  52  90 from  live-traps  XV  of Both  B i c r o t u s townsendii sexes  are  on f e n c e d  combined.  grid  C during  Non-breeding  1976-78.  periods  shaded. ..................... Figure  4.2.  Survivorship  townsendii  on  recruitment. from  4.3.  Figure  on  4.4.  on  4.5.  y e a r was  females;  B  lactating  fenced  the  for  and  estimated number  young  between  i n each  of  Mierotus  birth  and  y e a r was e s t i m a t e d  females;  5.1.  increase,  grid  C  lactating  the  number  of  for  young  between  i n each  on  control  Mierotus  birth  and  y e a r was e s t i m a t e d  females;  curves  grid  The number b o r n lactating  Population and  minimum  minimum  D  for  the  number  young  between  i n each  of  ' birth  the  instantaneous  two-week  survival  grid  density, two-week  Mierotus and  y e a r was e s t i m a t e d  females;  density,  on c o n t r o l  Population and  118  120  Survivorship  Mierotus townsendii 5.2.  curves  The number b o r n  t h e number o f  increase,  birth  was known.  recruitment.  Figure  grid  Survivorship  townsendii  Figure  i n each  curves  The number b o r n  t h e number o f  recruits  from  between  Mierotus  was known. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  recruitment.  Figure  ft  young  116  fenced  t h e number o f  townsendii  from  lactating  Survivorship  recruitment.  recruits  grid  for  was known.  townsendii  from  ... 110  curves  The number born  t h e number o f  recruits Figure  control  are  & during  of  rate  of  rate  for  1976-1978.  instantaneous survival  number  rate rate  of for  142  xvi  Microtus  townsendii  Non-breeding Figure  5.3.  on c o n t r o l  D during  1976-1978.  p e r i o d s a r e shaded  Population  increase,  grid  and  density,  minimum  Microtus townsendii  144 instantaneous  two-week  on f e n c e d  grid  rate  survival B  during  rate  of for  1976-1978.  Non-breeding p e r i o d s a r e shaded. ....................... Figure  5.4.  Population  increase, Microtus  and  density,  minimum  townsendii  two-week  on f e n c e d  Non-breeding p e r i o d s a r e shaded. Figure  5.5.  Histogram  Microtus townsendii Figure  5.6.  Histogram  Microtus townsendii Figure  6.1.  control Figure  6.2.  fenced Figure  6.3.  fenced Figure  on f e n c e d o f body  6.4.  C during  for  1976-1978.  .......................149  grid  C.  grid  B.  of  male  .................. of  .......  Microtus  B.  151  male  townsendii  A.  153 on ...181  d e n s i t y of M i c r o t u s townsendii  on  .........................................  Population density of  grid  grid  of  rate  weight d i s t r i b u t i o n s  on f e n c e d  Population  grid  survival  rate  o f body w e i g h t d i s t r i b u t i o n s  Population density of grid  instantaneous  146  Microtus  townsendii  on  C. Population  183  185 density of Microtus townsendii  on  ACKNOWLEDGMENTS  I  wish  Charles  t o express  J.  Krebs,  especially  greatly  this  those  study.  Nordstrom  and  C.  the data  presented  access  to  His  criticisms,  use  part  Dennis  those  along  with  Chitty,  computer  The C a n a d i a n of their  Dr.  programs Wildlife  property  both  from  postgraduate s c h o l a r s h i p s .  whose h e l p I  o f Ms. B. W i t h l e r , h a v e made  who a s s i s t e d  i n this thesis.  area. I received support NBC  Dr.  and c o n c i s e . , Thanks  Fleming  several  demographic d a t a . to  of  a p p r e c i a t e d , and a l s o  t h i s t h e s i s more r e a d a b l e  and  Dr.  o f t h e o t h e r members o f my c o m m i t t e e . D r . J . Myers, D r . L.  Gass,  me  t o ray s u p e r v i s o r ,  f o r h i s c o n t i n u i n g a d v i c e , e n c o u r a g e m e n t and  a s s i s t a n c e throughout those  my s i n c e r e t h a n k s  are  due  to  me i n g a t h e r i n g some o f Charles for  Krebs the  Service  gave  me  analysis  of  kindly  allowed  on S e i f e l I s l a n d as a  OBC  L.  teaching  study  assistantships  1  GENERAL INTRODUCTION  Many causes  microtine  of  Dispersal  these is  populations  fluctuations  an  important  f l u c t u a t e i n numbers, are  Dispersal Krebs, to  i s  of most  the genetic  occur  population  (Krebs  et  on t h e r e m o v a l g r i d  experimental  regulation behavioural  al.  common i n i n c r e a s i n g p o p u l a t i o n s  i n d e c l i n e s . These  trapped-out area  understood.  and  1971; K r e b s e t a l . , 1976), b u t h a s n o t b e e n  based  al.  the  poorly  mechanism o f p o p u l a t i o n  ( K r e b s e t a l . , 1969), and may a l t e r characteristics  still  results technique,  are defined  design  originate  as  whereby  dispersers  and t h e  ,  1973).  (Myers and  demonstrated  from  a  design  voles colonizing a and  removed.  r e s t s on many t e n u o u s a s s u m p t i o n s  This  (Krebs e t  , 1976). I chose  enclosing  to  study  dispersal  natural  field  populations  providing  a  vole-free  dispersers t o enter. design, voles  This  in  Mierotus  i n a vole-proof  unfavourable technique,  habitat  unlike  has the advantage of enumerating  must e n t e r  an u n f a v o u r a b l e  disperse.  Therefore,  separating  dispersers  dispersers  by  the  fence  for  The  and I g i v e  a heritable basis i s populations.  Hilborn  because  from r e s i d e n t s ,  I was a b l e  during  to  catch a l l  t h e c o u r s e o f a two-year decline  1.  o f whether o r n o t c e r t a i n c h a r a c t e r s often  difficult  (1975) r e p o r t e d  to  an u n f a v o u r a b l e h a b i t a t f o r  the r e s u l t s i n Section  determination  grid  area)  c y c l e . I examined whether o r n o t d i s p e r s a l o c c u r s d u r i n g periods,  and by  removal  (vole-free  by  potential  a l l dispersers  habitat  providing  f r o m two p o p u l a t i o n s  townsendii  to  that  ascertain  lifetime  in  (time  have field  between  2  first  and  litters thus  last  only  was  of  was  M.  investigate  inherited.  townsendii  heritability  within  litters,  among  and d i s p e r s a l  townsendii  populations  Predation  may  be  the  predisposed Few  tendency  in  i n Section  2.  have life  to  predation  histories  predators  were p r e s e n t  time  declining  l o s s accounted predators  segment  in  and  dispersal  vole  the  fall  peak  M.  populations by  predators  population  reasons  followed  tendency  and  and  (Errington,  be  1956).  a marked p o p u l a t i o n o f  o f t h e a n i m a l s and  related  1977). I n my s t u d y , and w i n t e r  them avian  o f 1977 a t  a  numbers. I examined t h e p r o p o r t i o n o f  f o r by p r e d a t i o n  were s e l e c t i v e  to  the h e r i t a b i l i t y of  increasing  of  late  order  c h a r a c t e r s . I was a b l e t o  (Boonstra,  during  and  and c o m p l e t e  f o r declines i n vole  intensively  voles to obtain subsequent  traps  1974). However, i n d i v i d u a l s e a t e n  vulnerable  among  populations,  partial  and I e v a l u a t e  to mortality f o r social  studies  of  pitfall  of l i f e  i s not necessary  ( K r e b s a n d Myers,  trapped  of d i f f e r e n t  i n duration  and  I  in  compare v a r i a b i l i t y  lifetime  non-randomly d i s t r i b u t e d  i n unfenced, i n c r e a s i n g Microtus  lifetime  litters  captures)  and d e t e r m i n e d  f o r e i t h e r the size  whether  or  not  or sex of t h e prey  (Section 3). Survival fluctuation,  of j u v e n i l e voles i s v a r i a b l e during with  (Krebs and Myers, nestlings  trapping  must  poor  survival  be  a r e i n the nest used  t o estimate  r a t e s o f a group o f v o l e s technique.  In  my  study  may  population  i n the d e c l i n e  1 9 7 4 ) . L i t t l e i s known a b o u t t h e  because they  techniques survival  particularly  a  survival  and u n t r a p p a b l e ; their be  survival.  dependent  of  indirect Apparent upon  the  I measured j u v e n i l e s u r v i v a l  3  directly and  by e n u m e r a t i o n  juvenile  populations. in  s u r v i v a l r a t e s o f e a c h a g e and s e x c l a s s  and p i t f a l l s  the is  major  the  of  and t h e f o o d  total  two  live-traps  is  some v o l e s both  hypothesis  i n four  (Pitelka,  that  However,  Do l i v e - t r a p s c a t c h  Spacing  in  and  live-traps  and  pitfalls  These q u e s t i o n s  Because t h e study related  these  sample  in  (1978)  in several  of  the  studies this  adult  use  assumption  whereas  pitfalls,  the  a r e answered  c a n be d i v i d e d  but  decline  only is  most o f t h e l a r g e r a n i m a l s ?  in pitfalls,  between  townsendii  used  most  and  or  1958).  most  a r e caught only live-traps  a  a detailed  M.  o f the  enumeration technique  captured.  differences  are  1974).  t h e K r e b s and B o o n s t r a  one, assumes  demographic  population?  a l l declines to  5 I provide  declines  t o enumerate t h e p o p u l a t i o n  tested.  population  c o n d i t i o n f o r d e c l i n e s by  the c h a r a c t e r i s t i c s  population  including this  population  that  during  Myers,  In S e c t i o n  to the C h i t t y hypothesis,  The  caught  as a n e c e s s a r y  and e v a l u a t e  hypothesis,  and  microtine  (1978), because i t can cause e i t h e r d e a t h  analysis  populations  depended  concerning  mortality  (Krebs  from t h e p o p u l a t i o n .  demographic  If  of  condition  K r e b s and B o o n s t r a  rarely  questions  cause  behaviour i s postulated  studies,  survival  T h e r e h a s been a t e n d e n c y t o r e l a t e  necessary  relation  t o see i f estimated  (Section 4).  of  dispersal  praweanling  I compared  fluctuations  single  and compared  r a t e s i n i n c r e a s i n g , p e a k , and d e c l i n i n g  upon t r a p t y p e  declines.  pitfalls,  survival  live-traps  One  with  self-contained,  are  t h e n a r e t h e r e any  groups same  of  voles?  segment  i n Section  into  others  six  the  6.  major  I decided  of  Do  aspects  t o w r i t e the  4  sections slight  as independent papers s u i t a b l e revision.  sections  with  literature  Therefore,  regard  there  design,  after  among t h e  trappability,  and  cited.  R.  1977. P r e d a t i o n  Errington,  P.  L . . 1956.  populations. R.  Science  1975.  siblings  cited  on H i c r o t u s t o w n s e n d i i  Impact and v u l n e r a b i l i t y .  Hilborn,  publication  i s some r e p e t i t i o n  to experimental  Literature  Boonstra,  for  populations:  C a n . J . Z o o l . 55: 1631-1643.  Factors  limiting  higher  vertebrate  124: 304-307.  Similarities  in  dispersal  i n four species of voles  tendency  among  (Microtus). Ecology,  56:  1221-1225. K r e b s , C. J . , B. L. K e l l e r , and R. H. population  biology:  populations  o f M.  southern  Indiana.  Demographic  ochrogaster Ecology,  K r e b s , C. J . , M. S. G a i n e s , Tamarin. 179: Krebs,  Tamarin.1969. changes  and  M.  fluctuating  £§flfisylyaniens  J . H. Myers, and R. H.  cycles i n small rodents.  C. J .  and J . H. M y e r s . 1974. P o p u l a t i o n  cycles i n small  Res. 8: 267-399.  K r e b s , C. J . , I . W i n g a t e , J . LeDuc, J . A. . R e d f i e l d , R.  Dispersal J.  Science,  35-41.  mammals, adv. E c o l .  and  in  5 0 : 587-607.  B. L. K e l l e r ,  1973. P o p u l a t i o n  in  Microtus  Zool.  Hilborn.  1976.  i n fluctuating  1  Hicrotus  populations  M.  population  Taitt, biology:  o f M. t o w n s e n d i i .  Can.  54: 79-95.  K r e b s , C. J . and S. B o o n s t r a .  1978.  Demography  of  the  spring  5  decline  i n populations o f the vole  Anim. E c o l .  Mierotus  townsendii. J .  47: 1007-1015.  Myers, J . H., and C . J . K r e b s reproductive  attributes  pennsylvanicus  1971.  Genetic,  behavioral,  of d i s p e r s i n g f i e l d  and M i e r o t u s  ochrpqaster.  voles  Ecol.  and  Mierotus  Monogr.  44:  53-78. Pitelka,  F . A. 1958. Some a s p e c t s  of population s t r u c t u r e i n the  short-term  cycle  Cold  H a r b . Symp. Quant. B i o l .  Spring  o f t h e brown  lemming i n n o r t h e r n 22: 237-251.  Alaska.  6  SECTION  1.  DISPERSAL DURING POPULATION FLUCTUATIONS OF THE  VOLE  MICEOTUS TOWNSENDII  Introduction Many size,  raicrotine  and  dispersal  population 1975).  rodent populations f l u c t u a t e may  be  regulation  Dispersal  food  supply  are  not  in  voles  characteristics is  populations been  invoked  and  reported  to  demonstrated (Chitty  as  and  employ  the  on  and K r e b s  1971b, H i l b o r n  Fairbairn  of  ,. L i d i c k e r  the l e v e l  s e t by t h e  random s u b s a m p l e to  alter  common  sufficient  and  Krebs  However,  Hilborn  dispersing  assumptions  dispersers that area.  this  of  I t may  increasing  1976),  Myers  and  but  and  Krebs  o f some  designating  1976,  has  Krebs (1976)  declines.  individuals  Krebs  et  and set  (Myers  al.  does not biweekly  be n e c e s s a r y t o enumerate  and  individuals  experimental design  live-traps  1976).  voles i s d i f f i c u l t ,  ( K r e b s e t a l . , 1976)  do n o t e n t e r  population  in  1966,  a trapped-out area as d i s p e r s i n g  1978).  o f the  t o account f o r d e c l i n i n g  explanation  and K r e b s  voles  1971b, K r e b s e t a l .  most  method  normal  1973  1971b, K r e b s e t a l .  (1963)  identification  captured  removal  be  d i s p e r s a l as a p l a u s i b l e  studies  several  Krebs  of  because d i s p e r s i n g  potential  P h i p p s 1966,  However, P e a r s o n  The most  (Myers  (Myers and K r e b s  populations 1971b).  below  or g e n e t i c a l l y the  component  (Krebs e t a l .  ( K r e b s e t a l . , 1 9 6 9 ) , and  p o p u l a t i o n , d i s p e r s a l has  not  necessary  regulates densities  a behaviourally  Dispersal  a  periodically in  1976,  involves enumerate in  the  a l l dispersers  7  from a p o p u l a t i o n i n order t o d e t e c t d i s p e r s a l declines.  In t h i s  disperse  into  study,  from  I  fenced  provided  for  i s a necessary  area  voles  or s u f f i c i e n t  and  population  f o r voles  populations of Mierotus  hoped t o e n u m e r a t e a l l d i s p e r s i n g dispersal  an  during  to  townsendii;  to  I  determine i f  c o n d i t i o n ( C h i t t y , 1960)  population declines i n voles.  Methods The Biver was  study  delta  near  on  although  species, also  ftgrostis  present.  waterfowl  was  short  Eeifel  Vancouver, B r i t i s h  p a r t o f a T i m o t h y hay f i e l d  Service.  by  was c o n d u c t e d  Timothy  during  owned by t h e  was c u t i n t h e f a l l  scarce  the  Fraser  The s t u d y  Canadian  area  Wildlife  was t h e d o m i n a n t  perenne, and H o l e a s l a n a t u s  the winter  and cover  in  Columbia.  (Phleura p r a t e n s e )  a l b a , Lplium The f i e l d  Island  were  of 1975 and g r a z e d  o f 1975-1976, s o t h a t  when t r a p p i n g s t a r t e d  the  grass  i n the spring  o f 1976. In  May 1976 I s e t up two e n c l o s u r e s  unfenced  grids  townsendii females  population  were  live-trap an  area  season and  the  added  to  a  and  until each  D) June  i n s i d e each e n c l o s u r e  trapped  1978.  enclosure  every  Three after  five  f t ) wide and 53.3 m  c o n t r o l g r i d s I mowed an a r e a 1977 a n d 1978 I r e d u c e d  Mierotus  males and t h r e e the  first  two  I mowed  two weeks d u r i n g t h e g r o w i n g  which d i s p e r s i n g v o l e s had t o p a s s .  (60  each  individuals.  i n order t o create a h a b i t a t unfavourable  m  during  and  s e s s i o n s produced l e s s than  across  18.3  (grids  ( g r i d s B and C) and two  (175 f t ) l o n g  f o r the voles, This (Fig.  of equal s i z e during  i t s width  t o 9.1  m  area  was  1 . 1 ) . On 1976,  (30  but  f t ) to  8  F i g u r e 1.1. E x p e r i m e n t a l d e s i g n used i n t h i s s t u d y . G r i d s A and D are unfenced controls and grids B and C are enclosed ex p e r i m e n t a l s . The s t u d y a r e a was s i t u a t e d i n a T i m o t h y (Phle_M! pratense) hayfield and bounded on one s i d e by a w a t e r - f i l l e d ditch.  Ditch  < >  c  B  <  i >  Disperser Traps  Scale  100 ft 30 m  c*  Fence  Mowed Area  D  10  attempt  to  grids.  catch  more  I a l s o mowed an  fence  perimeter  to  dispersing area  0.6  voles  from  m wide i n s i d e  discourage  voles  from  these  and  unfenced  outside  the  burrowing near  the  fence. I constructed the enclosures hardware c l o t h e x t e n d i n g  0.5  where  bordering  areas,  ground  level  where b o r d e r i n g  grids  were  bounded  by  mowed a r e a ,  the  were b o r d e r e d the  mowed  on  (Fig.  one  The  before  on  had  commenced,  Each t r a p - s i t e  trap s i m i l a r  (1978).  In October  with  had  to the 1976,  at each t r a p - s i t e .  supplied  other  two  high In  cotton  and one  type an  and  locked  winter  of  The  open on  above ground  level  below and  The  the fence in.  experimental  and  The  above  on  one  side  control  ditch,  on  grids  another  by t h e r e m a i n d e r of t h e  Pitfall  1976,  through  They were n o t  used  traps  by hay  i n the  (25 f t ) a p a r t , i n a 7 x 7  were  Longworth  were  b a i t e d with  s e t every and  i n s t e a d o f two  1977,  and  and and  one Krebs  to  second  During  and  week  on and  summer,  nights  only.  c h e c k s were made on  f r o m May April  was  oats  afternoon,  morning.  trapping  used  Boonstra  as  Longworth l i v e - t r a p  morning  Heknesday  weeks  replenished  live-trap  d e s c r i b e d by  live-traps,  were  September  m  oats  restricted  1977-78, one  Tuesdays. April  mesh  areas.  additional  bedding,  daytime temperatures  the  in)  m  (1/4  were p r e b a i t e d f o r f o u r  Monday a f t e r n o o n , c h e c k e d Tuesday checked  mm  0.6  a water-filled  Longworth l i v e - t r a p s  pitfall  placed  both  s i d e s by  49 t r a p s i t e s 7.6  trapping  necessary.  and  a l s o fenced  by  the  below and  unmowed  three  side  u s i n g 6.3  1.1) .  Each g r i d pattern.  m both  which was  mowed a r e a , and  field  on  by  through  through  October  June  i n t e r v e n i n g p e r i o d s because of  1978. winter  11  flooding. checked  They  were  afternoon.  week, t h e p i t f a l l s been l o c k e d  When  both  leaving  traps  a  were s u b j e c t t o c a p t u r e at  8.9-m  1.1). One-gallon  were two l i v e - t r a p s  pitfalls  cotton,  prevented  capture  remained  relative  released classified  as  after  follows: About  from  along the  between  pitfalls.  i n the enclosures,  totalling  half-way  on  the  controls.  These  s e t , except  pitfalls on e a c h  on f i r s t  from  that  rain  November  crossing  traps  c a p t u r e , and i t s  were r e c o r d e d .  being  adult  weight,  the  A l l animals  processed.,  >42  g  ;  Voles  subadult  were r e c o r d e d  inside  grids.  were were  30 - 42 g; on  nearly  study. entire  width  an e n c l o s u r e  were u n t a g g e d d i s p e r s e r s . the  to  visit.  o f t h e mowed a r e a  a s d i s p e r s e r s . , I assumed t h a t u n t a g g e d v o l e s c a u g h t  disperser and  five-gallon  Longworth  30,000 c a p t u r e s  5,000 i n d i v i d u a l s i n t h i s  defined  18... 3-m  b a i t e d w i t h o a t s and s u p p l i e d  large  s i z e o f hip glands  <30 g.  Voles  had  s e x , r e p r o d u c t i v e c o n d i t i o n , wounding on rump,  immediately  juvenile  these  v o l e was e a r - t a g g e d  location,  fitted  continually  A l l t r a p s were c h e c k e d  Each  on  with  site  and l i v e - t r a p s ,  t h e use o f  February.  per  in  (29 f t ) i n t e r v a l s  cans  per s i t e  12 p e r e n c l o s u r e , a n d one  with  live-traps  to cross the entire  t u n n e l s were s e t a l o n g t h e same f e n c e  disperser  morning,  m o r n i n g , and c l o s e d  t h e Longworth  trapping area  installed  (see F i g .  There  Wednesday  t y p e s o f t r a p s were s e t i n one  were s e t a f t e r  o f t h e mowed a r e a  pitfall  and  on  open.  Voles  fence  week  Wednesday a f t e r n o o n a n d T h u r s d a y  Thursday  width  s e t every  Untagged  had been b o r n  Dispersing v o l e s caught  voles  were in  on t h e g r i d  were  removed  i n d i s p e r s e r t r a p s on  12  control place  grids was  were n o t c l a s s i f i e d  unknown.  also c l a s s i f i e d during  1976,  The  escaping  dispersers;  12 d u r i n g  total  determining  as  Animals  1977,  as d i s p e r s e r s a s  and  there  the  were  16  2 during  enumeration technique  population size.  from  birth  enclosures known  were  escapes  1978.  of Krebs  A l l of the  their  (1966) was  used  in  demographic a n a l y s i s  to  follow  assumes t h a t most o f t h e i n d i v i d u a l s i n a p o p u l a t i o n  caught  during  trappability  each  used  sampling  period.  =  estimate  of  was: Number o f a c t u a l  Trappability  The  are  c a p t u r e s f o r an  Number o f p o s s i b l e c a p t u r e s  animal  f o r that  animal  N where  N  is  trappability times  of  possible  the  number  of  voles  caught  i s summed o v e r a l l N a n i m a l s .  capture  are excluded  c a p t u r e s , b e c a u s e an  from  animal  the  more t h a n The  first  twice and  and last  summation o f a c t u a l  must  be  caught  at  and  these  times.  Results  Trappability  The both  trappability  control  trappability 1.2. equal  grids on  Trappability  is  both was  f o r m a l e s and  of  t h e v o l e s i n Longworth l i v e - t r a p s presented  experimental  in  A 60%  1.1,  and  g r i d s i s presented  g e n e r a l l y above  females.  Table  60%,  and  trappability  in  on the  Table  approximately level  combined  13  T a b l e 1.1. T r a p p a b i l i t y e s t i m a t e s for Longworth live-traps on control t r a p p a b i l i t y , N i s sample s i z e . Grid Males  Hicrotus grids A  A  townsendii and D. T  Grid Males  Females  in is  D Females  T  N  T  N  T  N  T  N  Hay - O c t . 1976 Nov. -Feb. 1977 Mar.-Oct. Nov.-Feb. 1978 Mar. - J u n .  0.36 0.63 0.64 0.59 1.00  14 88 169 97 14  0.53 0.60 0.66 0.58 0.73  27 89 192 129 36  0.66 0.69 0.67 0.64 1.00  32 116 202 111 8  0.64 0.65 0.66 0.65 0.81  39 112 190 96 25  Total  0.65  382  0.63  473  0.67  469  0.66  462  Period  14  T a b l e 1.2. T r a p p a b i l i t y e s t i m a t e s for Longworth live-traps on experimental t r a p p a b i l i t y , N i s sample s i z e .  Grid Males  Mierotus townsendii. i n grids B and C. T is  a  Grid Females  Males  D Females N  T  N  T  N  T  N  May - O c t . 1976 Nov. - F e b . 1977 Mar.-Oct. Nov.-Feb. 1978 Mar. - J u n .  0.60 0.59 0.64 0.64 0.91  40 152 272 130 42  0.65 0.63 0.63 0.56 0.74  70 147 235 88 37  0.61 0.67 0.69 0.64 0.95  28 116 220 96 32  0.61 54 0. 57 107 0.62 212 0.64 118 57 0.91  Total  0. 64 636  0.68  577  0.69  492  0.64  Period  T  548  15  with  good  survival  estimate  is  indicates  that  80%. of  the  spring  Population  four  relatively  The  began i n May  townsendii trappable  live-traps,  a period  p o p u l a t i o n s were s i m i l a r  on a l l  until  They i n c r e a s e d  the  greater  populations  onset to  losses  approximately  the than  that occurred  A f t e r October  season  decline, females,  in the  October  the  population  before  the start  1977, t h e p o p u l a t i o n s and w i n t e r .  At  the  the rate of d e c l i n e increased  50% p e r week f o r males and  and t h e p o p u l a t i o n s  season i n  5% p e r week u n t i l  a t 5% p e r week t h r o u g h o u t t h e f a l l  o f t h e 1978 b r e e d i n g  females, the  season.  9%  capture i n  1977 b r e e d i n g  i n numbers i n c r e a s e d  s i z e s t o near o r above t h e l e v e l s breeding  a t approximately  Following  increased at approximately  T h i s summer r e c o v e r y  declined  the  (Figs.  on a l l f o u r g r i d s c o v e r i n g t h e  which  sustained  t h e summer  owing t o d e l a y e d  of  1977.  males  rapidly  and w i n t e r , start  were low when t r a p p i n g  low t h r o u g h o u t  A spring d e c l i n e occurred  the  over  with  12-week p e r i o d t o mid-June  of  except  was u s u a l l y  was c o n c u r r e n t  populations  1976 and r e m a i n e d  week d u r i n g t h e f a l l  1977.  (Hilborn  constant  of 1978, when t r a p p a b i l i t y  1.3, 1.4, 1 . 5 ) .  March.  size  alive  Density  grids.  1.2,  number  density.  C h a n g e s i n M.  per  was  This increase i n trappability  low v o l e  minimum  q u i t e c l o s e to t h e a c t u a l p o p u l a t i o n  e t - a l . - . , • 1976) . . T r a p p a b i l i t y during  the  18%  d e c l i n e d t o very  s p r i n g a n d e a r l y summer o f 1978.  per  week  low l e v e l s  for  during  16  F i g u r e 1.2. P o p u l a t i o n d e n s i t y o f M i e r o t u s t o w n s e n d i i on c o n t r o l grid A and number of dispersers. Non-breeding periods are shaded. Both sexes a r e combined i n the d i s p e r s a l plot. Dispersers were defined as voles crossing an unfavourable h a b i t a t and were removed a s t h e y were c a p t u r e d .  Grid A Residents  18  F i g u r e 1.3. P o p u l a t i o n d e n s i t y o f M i e r o t u s t o w n s e n d i i on c o n t r o l grid D and number of dispersers. Non-breeding periods are s h a d e d . B o t h s e x e s a r e combined i n t h e d i s p e r s a l p l o t .  200  Grid D Residents  1976  1977  1978  20  F i g u r e 1.4. P o p u l a t i o n d e n s i t y o f H i c r o t u s t o w n s e n d i i on fenced grid B and number o f d i s p e r s e r s . Non-breeding periods are s h a d e d . V e r t i c a l l i n e s mark t h e l i m i t s of the seasons. Both s e x e s a r e combined i n t h e d i s p e r s a l p l o t .  Grid B Residents  11  I  1  1~  i  i  I •  i . *  «*  _i  4  Grid B Dispersers  M  J  S  1976  N  J  M  M  J  1977  S  N  J  M  M  1978  J  22  Figure 1.5. P o p u l a t i o n d e n s i t y o f H i c r o ' t u s t o w n s e n d i i on f e n c e d grid C and number of dispersers. Non-breeding periods are shaded. Vertical lines mark t h e l i m i t s of the s e a s o n s . Both s e x e s a r e combined i n t h e d i s p e r s a l p l o t .  23  Grid C Residents  Grid C Dispersers  1976  1977  1978  24  Population  The  And D i s p e r s a l  dispersal  presented those  Density  in  patterns  Figures  1.2 a n d 1.3.  o f t h e two f e n c e d  that  the  number  controls,  of  populations  dispersers  probably  because  o n l y a c r o s s t h e mowed a r e a Voles via  dispersing  from  t h e mowed a r e a .  could  avoid  the  f o r the  control The t r e n d s  (Figs.  are  were s i m i l a r t o  1.4 and 1.5), e x c e p t  recovered  was  lower  on  the  d i s p e r s i n g animals could leave not  b u t a l s o from t h e s i d e s o f t h e  the experimental  Dispersing voles mowed  populations  area,  grids could leave  from  and o n l y  the:control  grid. only grids  an unknown f r a c t i o n o f  d i s p e r s e r s was e n u m e r a t e d . The were the  numbers o f d i s p e r s e r s r e c o v e r e d  -higher  with  On  voles  grid were  grid  the  randomly  first  C,  half  the  of  Potential located  first  capture  study adults  the  grid  grids  from  were  locations  (X =0.34, 2  the h a l f  of  adjacent.  P>0.50), the  grid  However, on  more v o l e s o r i g i n a t i n g t h e mowed a r e a of  from  (X =12.25,  grid  2  C  may  the  P<.001). not  have  a r e a and t h u s r e m a i n e d i n t h e p o p u l a t i o n .  four periods of increased dispersal  The f i r s t  and s u b a d u l t s  of  l o c a t i o n s o f d i s p e r s i n g v o l e s were  d i s p e r s e r s a t t h e f a r end  period.  1.7).  with  bordering  the dispersal  There  fenced  capture  distributed  t o t h e mowed a r e a a s f r o m t h e h a l f  non-randomly d i s t r i b u t e d ,  and  B  a s many d i s p e r s e r s o r i g i n a t i n g  distal  the  b e c a u s e d i s p e r s i n g v o l e s had no o p t i o n b u t t o c r o s s  mowed a r e a .  dispersing  on  occurred  i n the  fall  of  dispersed during t h i s i n t e r v a l  D i s p e r s a l was low d u r i n g  t h e t h e 1976-77  during the  1976.  Both  (Figs.  1.6  winter  but  25  Figure fenced  1.6. grid  S i z e c l a s s e s of d i s p e r s i n g B resident population.  Mierotus townsendii  from  27  Figure fenced  1.7. S i z e c l a s s e s o f d i s p e r s i n g grid C resident population.  Mierotus towjisendii  from  29  increased Adult  dramatically  after  d i s p e r s a l increased  which  i t  dispersal  was  low  sharply  until  increased  the  1977 b r e e d i n g  from  October  mid-March  1977.  October.  r a p i d l y i n June a n d J u l y ,  1977-1978 1978  rapidly  decline.  i n the f a l l during  was h i g h  during  was  two-week dividing  no d i s p e r s a l To  i n t h i s study  counting  interval  the  number  s e a s o n by t h e  average  density  populations. areas  dispersal  rate  for  results  enough  possibility  to provide  dispersal  of c o r r e l a t i o n  season.  statistically  1.3  experimental  on t h e  control  between  these  two m e a s u r e s  resident  populations,  I constructed  t o examine s e a s o n a l significant  More v o l e s  a variable  o f s p r i n g = 1 , summer=2, f a . l l = 3 , dispersal rates.  correlation  of the resident populations  (r=.52, n=16, P<.05).  by  ( r ) , o r t h e s e a s o n was i n v e s t i g a t e d by  s e a s o n and g a v e i t a v a l u e  winter=4 i n order  was  Table  f o r the  of the  i n t o eight periods.  the data  The  a meaningful a n a l y s i s .  grouping  density  subadult  number o f d i s p e r s e r s p e r  that  change  only  of d i s p e r s a l  o f d i s p e r s e r s p e r two weeks i n a  rate of population  and  precipitous  s e a s o n s , and t h e s e c o n d  o f d i s p e r s a l r a t e and t h e d e n s i t y  called  period  The number o f d i s p e r s e r s c o l l e c t e d  was n o t h i g h The  the  was measured i n two ways.  average  f o rdifferent  average  the  the winter of  o f 1976 and 1977, a d u l t d i s p e r s a l i n c r e a s e d  the  presents  a  September  t h e 1977 summer. rate  by  after  through  i n late  during  summarize,  May,  declined  d i s p e r s a l during  t h e 1977 s p r i n g d e c l i n e , and  Dispersal first  was l i t t l e  and e s s e n t i a l l y  spring  occurred  There  to  began.  S u b a d u l t and j u v e n i l e  August and e a r l y September, and t h e n i n c r e a s e d and  season  was  between  The the  and t h e number o f d i s p e r s e r s  dispersed  from  high  density  30  T a M e 1.3. P o p u l a t i o n d e n s i t y , r a t e o f i n c r e a s e , and d i s p e r s a l rates f o r p o p u l a t i o n s o f M i c r o t u s t o w n s e n d i i . . . a l l d a t a a r e mean values f o r the period.  Period  Season  Average Density  Rate o f Populn. Growth*  Av. No. Disper.  2  Dispersers Density 3  Grid B Summer 1976 F a l l 1976 W i n t e r 1976- 77 S p r i n g 1977 Summer 1977 F a l l 1977 W i n t e r 1977- 78 S p r i n g 1978  2 3 4 1 2 3 4 1  9.7 71.8 248. 1 176.6 208. 1 215.3 143.9 53. 5  +0.1413 +0.1590 +0.0390 -0.0601 +0.0417 - 0 . 1170 -0.0443 -0.1720  0.7 10.3 1.9 30.8 25.7 22. 0 2.3 1.8  0.072 0.143 0.008 0. 174 0. 124 0. 102 0.016 0.034  Grid C Summer 1976 F a l l 1976 W i n t e r 1976- 77 S p r i n g 1977 Summer 1977 F a l l 197 7 W i n t e r 1976-•77 S p r i r i g 1978  2 3 4 1 2 3 4 1  11.5 60.8 164.7 168.0 179. 8 201.5 153. 1 49.7  +0.1129 +0.1438 +0.0318 -0.0224 +0.0264 -0.0121 -0.0379 -0.2762  0.6 4.0 0.5 19.5 12.0 10.5 1.8 0.5  0.052 0.066 0.003 0. 116 0.067 0.052 0.012 0.010  1 i n s t a n t a n e o u s r a t e p e r week 2 p e r two weeks 3 d i s p e r s e r s p e r two weeks p e r r e s i d e n t  31  populations population number  from  increase  was  of  weeks  rate  per  The  1978  correlation  (r=-.75,  n= 14,  investigated  was  by  1.3-  density 200  and  a  and  strong  winter having density  on  with  100  the  dispersal  two  weeks p e r  In  summary,  the  season  Dispersal  rate  population  had  (P<.08).  there  is  a  (r=-.64,  per  resident from  spring  dispersal  rate  May and  variance  on  greater  assigned  a value  t h a n 200  the  was  (P>.08),  no  with  equal d i s p e r s a l  data of  between  100  all  effect  three  rates  and  There  (P<-01),  strong  in  1 when  animals.  dispersal rate  There  were  with of  density  (dispersers  resident). the  and  the  number o f d i s p e r s i n g v o l e s density  increased a  two  i n the  2 when i t was  e f f e c t on  statistically  not  the  analysis,  on  was  animals,  rate  did  winter.  index  lowest r a t e .  two  declining  significance  highest  effects  3 when i t was seasonal  c l a s s e s having per  density  or  and  dispersers  with  per  r e s u l t indicates that  and  density  l e s s than  animals,  This  fall  dispersers  number o f d i s p e r s e r s  a two-way a n a l y s i s o f  The  was  the  d i s p e r s a l was  l o w e s t i n the  Seasonal  Table  1978,  statistical  number o f  P<.005).  to February  summer and  and  P>.80) o r  season  of  average  More d i s p e r s e r s  between  and  rate  the  than a slow-growing  from  season  season  of  P>.30).  excluded  between  (r=.06,  number  (r=.23,  The  with e i t h e r  weeks  correlation  P<.02) and  1976  two  population  is  populations.  correlated  measures a p p r o a c h e d  spring  n=14,  not  measured a s t h e  a fast-growing  dispersal  density  per  resident  population.  If  low  dispersers  disperssal  leave  than  moderate  of  the  r a p i d l y i n the decline,  was  resident spring  decreased  correlated populations.  i f the  resident  throughout  the  32  summer low  and  during  population the  fall the  when t h e  winter.  resident  D i s p e r s a l can  i s increasing  rate  population  or  occur  d e c l i n i n g and  increased, when  population  increase.  the  resident  population  s u f f e r s a severe spring  and  Boonstra,  Survival  And  To from  grids  what can  listed  accounted  population  voles  in disperser  are  dependent  D i s p e r s a l does not  proportion  be  v a r i e s with those  captured  resident  occur  decline  on  when  (Krebs  Dispersal  a population  examined  was  1978) .  determine  proportion  the  i t i s not  of  and  tagged traps.  of  the  f o r by density  on  the  Only the  l o s s of i n d i v i d u a l s  d i s p e r s a l and c h a n g e s and  trapping results  this  season,  g r i d s and for  a s I assumed a l l d i s p e r s e r s  how  I  later  the  fenced  from t h e s e  grids  were e n u m e r a t e d . Host tagged d i s p e r s e r s t h a t in  Longworth  after last  leaving capture  type of and  the  2.8  previously  trapping  weeks  t r a p was for  6 weeks  for  entered  dispersers that  traps  p r i o r to  B and  7.9  dispersers  a  The  were  the  grid  and  weeks f o r  majority  been c a p t u r e d  166  8.0  204  of  at  shortly  C.  The  C.  either  The  grid  interval  dispersers  weeks f o r 348  or greater  in  a n i m a l s from  that  average  l e a s t once  a n i m a l s from g r i d  10-week  l e a s t once  caught  capture  Longworth l i v e - t r a p s . had  at  a v e r a g e i n t e r v a l s between  animals from g r i d  d i s p e r s a l was  weeks f o r had  101  the  3.2  been c a p t u r e d  dispersal  grid.  i n l i v e - t r a p s on  disperser  l e s s than  for  l i v e - t r a p s before  had  in  l a g between l a s t  was had  interval pitfall  a n i m a l s from About 25%  B  of  grid these  pitfall  33  capture The  on t h e g r i d  higher  mean  inoperable voles  lag  during  dispersed  and s u b s e q u e n t c a p t u r e i n  the f a l l  before  Dispersing  time  and w i n t e r  the p i t f a l l s  animals  could  l i v e - t r a p s , p i t f a l l s , both losses  of  voles  populations, loss  T. 4  by  that  accounted  f o r over  population  when  accounted  for  a higher  i n either increase  to the rate  (r=.05,  n=16,  populations excluded,  of  P>.80),  losses  loss  from  of the  population  of t h e l o s s d u r i n g summer a n d f a l l , Survival  size,  rates  experimental grids  of  the  always  grid  1977  B than decline  by  the  d i s p e r s a l was n o t  resident  density  of  populations the resident  However, i f s p r i n g from spring  that there  1978  is  t o winter  i s no s e v e r e  spring  d i s p e r s a l c a n a c c o u n t f o r o v e r 50%  the spring,  and s t i l l  t h e 1978  or after i t .  the percentage of l o s s decreased  in  tagged  Dispersal  in  explained  Provided  juvenile  total  o f t h e l o s s from  before  to  and  decline dispersal the  10%.  were h i g h e r  nor  pitfall  excluded, but during than  increase  and  the  The r e s u l t s i n T a b l e  the  loss  gives  and t h e p e r c e n t a g e  spring  were  trapped i n  1.4  live-trap  Juvenile  (r=.12, n=16, P>.60).  (r=-.78, n=14, P<. 0 0 1 ) . decline  Table  1977  period  and  were o p e r a b l e i n t h e s p r i n g .  dispersers,  proportion  of  season,  the  Dispersal rates  percentage  some  combined  for less  than  related  C.  of  juveniles  grid  rainy  from t h e a n a l y s i s .  50%  from  The  the  during  i t accounted  were  , or neither.  dispersal.  are excluded  indicate  spring,  from  traps.  because t h e p i t f a l l s  h a v e been p r e v i o u s l y  t h e number o f t a g g e d  explained  dispersers  occurred  disperser  less  of  the  loss  during  the  l e s s o f any w i n t e r l o s s . male  Mierotus  townsendii  were c l o s e l y r e l a t e d t o t h e p o p u l a t i o n  on  the  growth  34  T a b l e 1.4. Number o f t a g g e d Mierotus townsendii disappearing from combined live-trap and p i t f a l l p o p u l a t i o n s on b o t h experimental grids and number o f tagged dispersers caught. J u v e n i l e l o s s e s and d i s p e r s e r s a r e e x c l u d e d .  Period  No. tagged v o l e s l o s t from r e s i d e n t population Males  Females  Grid B Summer 1976 F a l l 1976 W i n t e r 1976-77 S p r i n g 1977 Summer 1977 F a l l 1977 W i n t e r 1977-78 S p r i n g 1978  12 39 74 130 153 58 83 62  7 41 47 89 166 36 61 40  Total  611  487  Grid C Summer 1976 F a l l 1976 W i n t e r 1976-77 S p r i n g 1977 Summer 1977 F a l l 1977 W i n t e r 1977-78 S p r i n g 1978  19 29 53 86 137 54 65 42  11 46 28 53 142 36 73 53  Total  485  442  Number o f tagged dispersers Males  Females  % loss e x p l a i n e d by dispersal Males  Females  6 12 15 60 67 11 1 1  42 46 30 56 32 38 5 8  86 29 32 67 40 31 2 3  173  33  36  6 1 4 46 34 10 1 0  2 4 2 31 23 7 1 3  32 3 8 52 25 19 2 0  18 9 7 58 16 19 1 6  102  73  21  17  5 18 22 74 49 22 4 5 199  35  rates  of  the  survival rates  resident  rates  males o r  of  The  (r=.61, n=16,  r e l a t e d t o the  There  l o s s explained  with  Since  moderately P<.07).  females.  correlated  the  sere  (r=.43, n=16,  percentage  populations  by  was  no  d i s p e r s a l accounted during  the  f o r the  1977  a n i m a l s c a u g h t on  the  I  then  determined  proportion  the  what  population  heavier  than  unsuccessful  females  were  more than  and  Becruitment  rates  because the  population  Time Of  The  of  and  season  1.7  month  disperse  first before  animals born Thus,  of  show the of  was  of  mainly  that  the  not on  were  the  being  the  dispersers  1.5  was  recruits  were  indicates  that  recruited  into  the  animals generally  were  weight  difference  statistically  grid  dispersers.  A t h a n on  significant.  grid  grid  between  D,  A at  probably  was  l o w e r on  this  time.  of  young v o l e s i s d e p e n d e n t  Birth  (Kalela  capture. the  d e c l i n e , I assumed  Table  heavier  1957,  p r o b a b i l i t i e s of  i n the  fall-born  was  dispersal  birth  from  successful  in  that  density  loss  of  recruits.  were h i g h e r  D i s p e r s a l And  their  if  although  non-recruits  time  and  m a l e s , and  of  control grids  successful  more s u c c e s s f u l r e c r u i t s , recruits  between  survival rates  majority  spring  adult  populations  growth  of d i s p e r s a l l o s s  new  to  population  season.  population  recruited  Female  correlation  d i s p e r s a l and  percentage  PC.02) .  start late  Anderson  d i s p e r s a l as  V o l e s born of  the  next  i n the  breeding  summer o r e a r l y f a l l  dispersers  1970).  tend  t o be  Tables  1.6  a function  of  spring  summer  and  season, disperse  older  upon  than  the  whereas after i t .  spring-born  36  Table 1.5. Mean weight {+ 1 SE) o f s u c c e s s f u l and u n s u c c e s s f u l r e c r u i t s t o c o n t r o l g r i d s d u r i n g t h e 1977 s p r i n g d e c l i n e . Grid  Females  Males No. l i v i n g l e s s t h a n 2 weeks Weight a t f i r s t capture  (2. 4)  52.0 (2.0)  59.3  32  No. living 2 or more weeks Weight a t f i r s t capture  26  15  62.7 (2. 0)  55.6  ^successful  70  88  recruits  Males  20  2  11 57.3  Grid  &  (2.3)  64.0 61  D Females  7 (2. 3) 58.4  (4.  7)  30 (1. 6) 55. 1 (1.?) 81  37  T a b l e 1. 6. P r o b a b i l i t y o f an i n d i v i d u a l f i r s t c a u g h t f r o m s p r i n g next breeding start of the to w i n t e r d i s p e r s i n g b e f o r e the s e a s o n . G r i d s B a n c C, 1976 and 1977, and b o t h s e x e s a r e p o o l e d . Month April May June July Aug. Sept. , Oct. NovDec.  New  animals  81 284 632 447 359 286 201 72 66  Number 30 94 111 80 44 18 15 4 0  dispersing  Probability 0.37 0.33 0. 18 0. 18 0. 12 0.06 0.07 0.06 0.00  38  Table 1.7. Probability g i v e n i t s month o f f i r s t and C a r e c o m b i n e d .  Month May June July Aug. Sept. Oct. Nov. Dec.  New in  animals 1976  11 59 32 250 265 182 70 55  o f an i n d i v i d u a l d i s p e r s i n g d u r i n g 1977 c a p t u r e i n 1976. B o t h s e x e s and g r i d s B  Number d i s p e r s i n g i n 1977 0 2 1 20 41 41 17 14  Probability dispersal 0.00 0.03 0.03 0.08 0. 15 0.23 0.24 0.25  of  39  dispersers.  Sex  Ratios  Of  Dispersing  I compared t h e by  tallying  summing  the  for  dispersing  is  (Myers and  a  The  by  sex  tallying for  but  an  t o those on  grid  during I  the  on  T h e r e was (X =3.98, 2  in  Dispersing  a l l  were c a l c u l a t e d  the  if  age  there  the  were  summing t h e female  on sex  Dispersing  r a t i o s than  by  data  juvenile  1.9),  similar  both g r i d s .  age.  were compared  (Table  sex  used  of  i n d i c a t i o n of  s u b a d u l t s had  higher  the  excess  classes  e x c e s s of  P<.05)  an  ratios  significantly  of  Although  sex  s u b a d u l t s on  that  The  a l l grids during  c a u g h t and  juvenile  population.  summer.  The  an  four  dispersing  categories  i t was  sex  in  inquired  age  the  population  the  study.  on  and  resident  a crude  three  alive  gives  than  were t h e r e f o r e  difference  had  higher  decreased  voles  than females.  excess  classes.  of r e s i d e n t C  ratio  animal each time  2  i s an  a l l age  (X =1.85, P>.15).  1.8  population  m a l e s i n the  in  of  Table  same g r i d .  and  from g r i d B  significant  the  i n age,  whole  dispersers  on  of  recovered  w e i g h t and  known t o be  populations  usually  vary  ratios  the  sex  resident  pitfall-enumerated  1971a), t h e r e  occurred  defined  was  same s e a s o n  The  declines  resident  and  more m a l e s d i s p e r s e d  deficiency  Dispersers males  that  ratio  Krebs  population. spring  sex  f o r the  usually  dispersing  resident  l i v e - t r a p - and  These d a t a i n d i c a t e  residents  and  r a t i o s f o r the  combined  dispersers'  r a t i o s of  data over seasonal periods.  Sex  the  sex  e a c h a n i m a l e v e r y t i m e i t was  r a t i o s f o r the grids.  Voles  but grid  no C  ratios adults resident  40  T a b l e 1.8. Sex r a t i o s ( p r o p o r t i o n o f males) i n r e s i d e n t and dispersing populations o f M i e r o t u s t o w n s e n d i i on b o t h c o n t r o l and e x p e r i m e n t a l g r i d s . Sample s i z e i s i n p a r e n t h e s e s . Grid D  C  B  A  Season  Residents 0.31 0. 40 (416) 0.42 0. 44 (1482) 0.49 0. 51 , (3341) 0.46 0. 40 (2174) 0.46 0. 55 (5658) 0.47 0.59 (1111) 0.44 0. 58 (1364) 0.32 0. 51 (379)  (453) (1282) (2583) (1985) (5021) (1052) (1218) (324)  0. 37 0. 45 0. 52 0. 40 0. 49 0. 53 0. 55 0. 25  Summer 1976 F a l l 1976 w i n t e r 1976 S p r i n g 1977 Summer 1977 F a l l 197 7 W i n t e r 1977 S p r i n g 1978  0. 48 0. 43 0. 47 0. 40 0. 43 0. 44 0. 43 0. 24  (298) (606) (1598) (1400) (3943) (907) (1192) (263)  Total  0. 43  (10207) 0. 51  (15925) 0.45  (13918) 0. 48  Summer 1976 F a l l 1976 W i n t e r 1976 S p r i n g 1977 Summer 1977 F a l l 1977 W i n t e r 1977 S p r i n g 1978  (0) "50 (2) (0) "80 (5) ~0~. 0. 50 (8) 0. 50 (8) 0. 33 (3) 1. 00 (2)  0. 83 0. 50 0. 63 0. 50 0. 49 0. 50 0.,76 0. 73  Dispersers 0.80 (6) 0.47 (38) (30) 0.67 0.54 (173) 0.56 (187) 0.62 (88) 0.64 (21) 0.00 (11)  (5) (15) (6) (116) (84) (42) (14) (3)  0. 80 0. 33 0. 00 0. 57 0. 75 1. 00  Total  0. 53  0. 52  (554)  0.56  (285)  0. 66  oZ  (28)  —  (645) (867) (2009) (1871) (4244) (961) (1189) (182) (11968)  (10) (3) (D (7) (8) (3) (0) (0) (32)  41  T a b l e 1.9. dispersing Sample s i z e 1978.  Proportion of males according to age group i n and resident populations of Mierotus townsendii. i s i n p a r e n t h e s e s . D a t a a r e summed f r o m 1976 until  Population  Adult  Resident Grid B Grid C  0.55 0.50  Dispersers Grid B Grid C  0.61 0.71  Subadult  Juvenile  (6480) (4696)  0.46 0.37  (1621) (1884)  0.51 0.44  (112) (1088)  (290) (136)  0.44 0.41  (231) (117)  0.33 0.56  (33) (32)  42  adults  (X =24.03,  p<.0001),-as d i d d i s p e r s i n g  2  (X =3.94, 2  P<.05).  The e x c e s s  grid  B  was c o n f i n e d  to  was a s s e s s e d  by  adults  o f male d i s p e r s e r s  on  adults.  Sexual Maturity  The  and  Dispersal  reproductive  external  characteristics.  presence  of s c r o t a l  perforate  vagina,  or  condition  a litter  Maturity  enlarged  and  condition  more  dispersers  during  frequently adult  and  the  grid  in  the  with  fall  that  of  {Table  resident  1.10).  grids and  than  (P<.001). resident  during  each  subadults  1977  maturity  that  was c a p t u r e d .  a This  o f 1976  females  f o r both  statistically sufficient difference  was  P<.001). dispersing analysis. subadult  experimental subadult  significantly  population between  f o r both  dispersing  significant  number was  were 2  of dispersing  mature  adult  (X =8. 18,  of d i s p e r s i n g  The d i f f e r e n c e i n m a t u r i t y  season  than  2  i n the resident  was  same  dispersers  (X =9.88,  subadults  were s e x u a l l y  subadults  the  t o make a m e a n i n g f u l  The p r o p o r t i o n  the proportion  in  female  B i n the f a l l of  these periods  m a l e s and f e m a l e s which greater  the  symphyses,  condition  I d i d not c a p t u r e enough a d u l t  C during  populations  by  same s e a s o n , a s were  reproductive  I compared the r e p r o d u c t i v e voles  were  However, a d u l t  f e m a l e s on g r i d  in  Unfortunately, from  judged  mammary g l a n d s , o p e n p u b i c  male r e s i d e n t s and d i s p e r s e r s .  P<.005)  i n m a l e s was  t e s t e s , and i n f e m a l e s by t h e p r e s e n c e o f a  residents  reproductive  resident  the voles  i n a trap.  Juvenile  were  of  of  (P<.01)  dispersing  most marked  during  43  Table 1.10. P e r c e n t a g e o f s u b a d u l t v o l e s i n b r e e d i n g c o n d i t i o n i n resident and d i s p e r s i n g populations. Sample size i s in parentheses. Grid  B  Resident Male Summer 1976 F a l l 1976 W i n t e r 1976 Spring  1977  Summer 1977 Fall  1977  W i n t e r 1977 S p r i n g 1978 T o t a l minus winters 1  Female  48 33 (24) (48) 26 12 (100) (112) 4 0 (80) (93) 44 38 (45) (53) 4 5 (467) (431) 0 0 (38) (15) 0 0 (27) (62) 50 55 (22) (4) 9 (663)  15 (696)  Grid C Dispersing  Resident  Male F e m a l e Male 100 100 (2) (D 80 33 (10) (6) 0 0 (8) (3) 57 94 (23) (32) 42 35 (55) (71) 35 9 (11) (19) 0 (3) " ( 0 ) 100 "0) (D 40 (97)  58 (134)  Female  56 32 (52) (19) 11 23 (94) (72) 5 0 (108) (185) 45 20 (41) (47) 7 8 (381) (448) 3 0 (34) (94) 0 0 (40) (175) 0 0 (4| (90) 9 (551)  13 (825)  Dispersing Male  Female  100  (D  TO)  (0.) 53 (17) 36 (25) 20 (5)  l0) 78 (27) 32  100 (1)  ~0)  57 (7)  (22)  50 (12) 0 (D IMII  „  t  ~0)  (0)  43 (49)  53 (68)  44  t h e summer and and  1.5).  fall  L e s s than  m a t u r e whereas mature. 30% in  10% o f t h e r e s i d e n t  about  30%  of  s u b a d u l t males and  the d i s p e r s i n g  the  dispersing  at  were  s e a s o n s i s summed,  about  determine  mature sexual  sexual  maturity  a t t h e same w e i g h t maturity  for  the technique of L e s l i e  data  3-month p e r i o d s t o d e t e r m i n e  classes,  subadults  weight a t  by  for  sexually  f e m a l e s were i n b r e e d i n g c o n d i t i o n  calculated  weight  s u b a d u l t s were  1.4  populations.  voles  weight  (Figs.  p o p u l a t i o n s than i n the r e s i d e n t  calculated  dispersing The  when d e n s i t i e s were h i g h  I f the data f o r the non-winter  more  I  o f 1977  and  then f i t t e d  to  as r e s i d e n t  live-trapped  et a l .  i f  voles.  voles  (1945).  I  was  grouped  p e r c e n t a g e mature i n  a probit  line  t o the  4-g  percentage  mature d a t a . The  weight  at sexual maturity f o r resident  v o l e s from  grid  B i s shown i n T a b l e 1.11.  cycle  the  median w e i g h t  in  p o p u l a t i o n f o r both or  none  at  dispersing resident  females.  females in  matured  a l l  sample o f d i s p e r s e r s .  significantly was  some  grid  C  lower  dispersers  of these  maturing  And  a t lower  seasons The  for  dispersing  annual resident  heavy  voles  the winters. weights which  Both  than  in  t h e r e was  males matured  the c o n f i d e n c e i n t e r v a l s . trends,  a t lower  1-12) .  Body H e i g h t  an  an at  weights i n a l l seasons, but i n females t h e r e  overlap  confirmed  was  O n l y a few  a l l were s e x u a l l y m a t u r e d u r i n g  males and  dispersing  at sexual maturity i n the  males and  populations  adequate  There  and  Dispersal  with  weights  both  male  The  data  and  than the r e s i d e n t s  from  female (Table  T a b l e 1.11. Median female residents parentheses.  Residents  Period  June-Aug. Sept..—Nov. March-May June-Aug. Sept.-Nov. March-May  w e i g h t a t s e x u a l m a t u r i t y f o r g r i d B male and and d i s p e r s e r s . 95% c o n f i d e n c e l i m i t s a r e i n  1976 1976 1977 1977 1977 1978  J u n e - A u g . 1976 Sept.-Nov 1976 March-May 1977 J u n e - A u g 1977 S e p t . - N o v . 1977 March-May 1978  Males 42.0 64.1 48.6 56.5 >70 56. 1  Dispersers  1  (40.8, (59.4, (46.7, (53.8,  43. 3) 69. 2) 50. 6) 59. 5)  49.8 40.7 38.7 66.6  (47.3, (38. 2, (37.5, (53. 8,  52.3)* 43.4) * 40.0) * 82.5)  38.4 30.0 38.4 59.2  (12.5, (21.5, (29. 8, (31.6,  118.2) 42.0) 49.6) * 111.2)  (53.9, 58. 5)  Females 33.7 (29.4, 49.9 (47.7, 44. 1 (40. 1, 54.0 (52.5, >70 42.4 (33.0,  38. 7) 52. 3) 48. 5) 55. 5) 54. 8)  1 Median w e i g h t a t s e x u a l m a t u r i t y was above 70 g f o r r e s i d e n t m a l e s and f e m a l e s f r o m D e c . - F e b . 1977 and S e p t . - F e b . 1978. * P<.05  46  T a b l e 1. 12. Median female residents parentheses.  w e i g h t a t s e x u a l m a t u r i t y f o r g r i d C male and a n d d i s p e r s e r s . 95% c o n f i d e n c e l i m i t s a r e i n  Residents  Period  Dispersers  1  June-Aug. Sept.-Nov. March-May June-Aug. March-May  1976 1976 1977 1977 1978  M a l es 46.0 60-6 48. 1 51.2 52.3  (43.7, (56.9, (46.8, (49.5, (50.8,  48.3) 64.4) 49.4) 52.9) 55.0)  June-Aug. Sept.-Nov. March-May June-Aug. March-May  1976 1976 1977 1977 1978  Females 34.3 (30.4, 50.4 (46.7, 39.6 (35.5, 47.2 (45.7, 36.4 (29.5,  38.6) 54.4) 44.2) 48.8) 44.9)  44. 1 37.8  (39.5, 49.2) (36.5, 39. 1) *  29.3 21.2  (17.8, 48.2) (0.0, >200)  1 Median w e i g h t a t s e x u a l m a t u r i t y was above 70 g f o r r e s i d e n t males a n d f e m a l e s f r o m Dec.1976-Feb. 1977 and S e p t . 1977 - F e b . 1978. * P<.05  47  Mean dispersing grids  body  and  weights  resident  were compared and  Fig.  The d a t a  summing  Thus,  s e v e r a l times  a  single  individual  1.13).  dispersers in  for  indicated  grid  female  and  than  1977  1.7 i l l u s t r a t e s  o f both  summer  1977  P<.001,  grid  females  (grid  C  for  summer  with t h e  f o r both  and s u b a d u l t males than P<.07).  B X =70.12, P<. 0001,  with  the  P<. 02)  sharp  ( X = 17.28, 2  significant. increase  and f a l l  males c o n s t i t u t e d (grid  The same r e s u l t grid  B  grids  a higher X =1G.Q5, 2  occurred i n  C X = 2 7 . 6 1 , P<.0001). 2  in  o f 1977.  experimental  expected  and  significant.  1977  2  X =3.54, 2  female  (X =6.30, P<.02) were a l s o  o f these data  2  male and  2  summation  proportion of dispersing  (Table  was s t a t i s t i c a l l y  this result  juvenile  females  (X =5.57,  The  that  than  F o r example, t h e d i f f e r e n c e  and j u v e n i l e d i s p e r s a l i n t h e summer  indicate  r e s i d e n t males,  compared  subadult total  measurements  p r o p o r t i o n o f d i s p e r s e r s was  1.9).  comparisions  fall  resident  weights  P<.0001)  2  B  P<.0001) Figure  (X =19.66,  captured  males were l i g h t e r  weight category  (Table  each  However, d u r i n g t h e same p e r i o d ,  and f a l l  B males d u r i n g both  1977  weight  by  o f t h e mean.  that a higher  population  grid  fall The  1.8).  t h e j u v e n i l e and s u b a d u l t  resident  might have been  and a l l i t s  were l i g h t e r  The 1977 summer  throughout  m a l e s were h e a v i e r t h a n  (Fig.  d i s p e r s i n g females  of  B and C a r e p l o t t e d i n  of a l l individuals  1977 and 1978, d i s p e r s i n g  males  experimental  The mean w e i g h t s  weights  1976, d i s p e r s i n g  resident  of  f o r r e s i d e n t p o p u l a t i o n s were c a l c u l a t e d  during a season,  whereas d u r i n g  t h e two  seasons.  are i n c l u d e d i n the determination In  distributions  on  r e s i d e n t males f o r g r i d s  the capture  season.  weight  populations  for different  dispersing 1.8.  and  &  48  F i g u r e 1.8. Hean body w e i g h t s o f r e s i d e n t male ti. townsendii i n f e n c e d p o p u l a t i o n s B (BE) and C ( C E ) , and t h o s e that dispersed (BD, CD). Data a r e grouped o v e r t h e s e a s o n s i n d i c a t e d i n F i g s . 1.4 and 1.5. V e r t i c a l b a r s g i v e 95% c o n f i d e n c e l i m i t s .  65r  E  60  O  55  ro i_  I U)  O CO  2  BR  CR BD  45  CD  BR CR-L CDi  40 35  I I  BRj  BD  50  c  CD <U  I  CD  I  I  CR  I  BR  BD CD  CR  B  I BRj  BD BR  I D  i.  BR  BR  CR  I  CR  I  CR CD  IT  1  BD  CD  CR  BD  BD CD  30 SUMMER  !976  FALL  WINTER  SPRING  SUMMER  1977  FALL  WINTER  SPRING  1978  50  T a b l e 1.13. P r o p o r t i o n o f males a n d f e m a l e s t h a t a r e j u v e n i l e o r s u b a d u l t i n d i s p e r s i n g and r e s i d e n t p o p u l a t i o n s d u r i n g d i f f e r e n t s e a s o n s . Sample s i z e i s i n p a r e n t h e s e s . Grid Resident Male Summer 1976 0.54 (110) 0.58 F a l l 1976 (395) W i n t e r 1976 0.08 (915) S p r i n g 1977 0.27 (658) Summer 1977 0.57 (1255) 0.04 F a l l 1977 (336) W i n t e r 1977 0.06 (474) S p r i n g 1978 0.06 (167) Total  Grid  B Dispersing  Resident  Female  Male Female M a l e  0.55 (171) 0.45 (558) 0. 16 (898) 0.17 (950) 0.56 (1149) 0.20 (190) 0. 18 (282) 0. 18 (146)  0.40 (5) 0.32 (19) 0.16 (19) 0.30 (87) 0.70 (92) 0.20  W)  0.17 (16) 0.00 (8)  1.00 (D 0.68 (1:9) 0. 64 (11) 0.46 (81) 0-78 (95) 0.38 (42) 0.00 (5) 1. 00 (D  C  0.37 (94) 0.69 (277) 0. 18 (645) 0.18 (732) 0.61 (986) 0. 10 (266) 0. 12 (345) 0.05 (98)  Dispersing  Female  Male  Female  0.4 8 (179) 0.44 (425) 0.36 (592) 0.18 (841) 0.57 (1330) 0.38 (248) 0.59 (424) 0.46 (196)  0.25 («»)0. 14 (7) 0.00 (4) 0.38 (63) 0.69 (48) 0.22 (27) 0.00 (9)  1.00 (4) 0.88 C8) 0.50 (2) 0.62 (53) 0.78 (37) 0.69 (13) 1.00 (D 0.00 (1)  •  (0)  •  0.67 0.40 0.43 0.33 0.57 0.39 0. 33 0. 30 (4310) (4344) (290) (255) (3443) (4235) (162) (119)  51  higher than  proportion  did  a b o u t 5%  1.9  shows t h a t  o f the  resident  indicated  and  male  and  T h e r e was  no  on  subadult  grid  male d i s p e r s e r s  females  the  the  B during  the  were o v e r 54  population  that during  disperse,  the  juvenile  dispersed  males.  Figure  the  of  weighed  summer l a r g e  same r e s u l t was  g,  summer o f  but  more  t h a n 54  male v o l e s  obtained  started  i n the  fall  of  1555  f o r the  of  g.  tend  evidence of d i s p e r s a l o f heavy-weight  decline that  over  1977  This  not  fall  to data.  males  during  1977.  Discussion I  undertook  dispersal  can  this  study  between  study, voles  were d e f i n e d  unfavourable  habitat.  Dispersal by  weeks  1978). removal  in  on  resident  a removal g r i d  the  removal g r i d  (Myers and  This  design  grid,  assumes  Longworth l i v e - t r a p s s e t  The  basic  are  resident  and  that  animals.  they  populations  has  s i t u a t e d near  that long  enough t o be  biweekly. discussed  Further  growing  this The  habitat  days  voles  Fairbairn find  the enter  assumptions of  this  al.  constituted  be  grid, every  (1976).  i n t h i s study i s that  to  an  mainly  a control  b a r r i e r separated  b a r r i e r seemed  this  t r a p p e d , and  i n Krebs et  season  In  been  1976,  i f  examine  crossed  f o r two  dispersing  assumption i n v o l v e d  mowed b i w e e k l y i n t h e habitat,  if  occurring  to  voles.  K r e b s 1971b, K r e b s e t a l .  remain there  experimental design  determine  and  dispersing  dispersers  mammal  to  declines  and  as  small  means o f  with t r a p p i n g two  dispersal  account f o r population  differences  studied  of  areas  unfavourable dispersing  effective  and  because  52  F i g u r e 1.9. Body w e i g h t d i s t r i b u t i o n s o f r e s i d e n t dispersing (N=92) m a l e M. t o w n s e n d i i from fenced summer 1977.  (H=1255) and grid B during  Grid B Residents  Summer 1977  Body Weight, G r a m s  54  the  number  of  tagged  p o p u l a t i o n s was experimental  about  15% o f t h e  populations,  populations.  This  alternative,  voles  and  the  habitat  were n o t  on  the  similar  foray  i n the  sought  voles  attempting  the  given  a r e a s and  a brief  control  from  when  probable that  were  merely  were  that  mowed  I t appears  from  originating  densities  indicates  avoided  unfavourable  population  number  and  result  avenues o f d i s p e r s a l . this  dispersers originating  to  an other  crossing leave  into  the  adjoining  areas. The  idea  the time 1953,  that  of p u b e r t y  Smith  d i s p e r s a l i n s m a l l mammals i s u n d e r t a k e n i s generally  accepted  (Howard  1968,  Christian  1970).  the suggestion that  dispersal  occurs mainly  summer, 1957,  owing  that  The  1970).  socially  results  increased  Hurray  (1967)  of  the  b o t h young  during  periods suggests that  with  they  may  y o u n g Peromyscus m a l e s a r e d r i v e n  population  by  with  may adult Voles  be  forced  females born  male  aggression.  to d i s p e r s e through  and  time  (Kalela  Christian  (1970)  of  young  be  driven  more a g g r e s s i v e  (1967) t h a t  sexes  spring  there i s  young f e m a l e s  the s u g g e s t i o n of S a d l e i r  dominant  idea i s  disperse.  increased dispersal  p o p u l a t i o n s by l a r g e r , agrees  and  males and  f a l l . . The  result  the  Blair  this  present study i n d i c a t e that  t h e summer and  resident  in  subordinate individuals  d i s p e r s a l by  these  Stemming f r o m  to the attainment of puberty at t h i s  Anderson  suggest  1949,  at  during animals  from  the  animals.  This  (1965) and  Healey  from  the  resident  Young v o l e s o f  aggressive  both  interactions  too., near  the  have a h i g h e r p r o b a b i l i t y  start  of the s p r i n g  of d i s p e r s i n g  breeding  before the s t a r t  season of  next  55  year's  breeding  breeding that  season than  season.  spring-born  year  of  higher  their  This  result  birth.  breeding  Late  average  fall-born  voles  more f a 1 1 - b o r n This result  occurs  during  dispersal  of  and  dispersal  removal g r i d dispersal  to  grid  population  disperse  within  the  f a l l - b o r n v o l e s have a start  of  the  under  six  months,  of d i s p e r s i n g  comparisons  voles are  moderate  next  counterparts.  probability  alive  spring  season.  spring  is  higher  simply at  this  decline  Little  decline  or  (Krebs  no and  during  Krebs  a  d i d not  (r)  and  removal  expel the  correlation  on  presumed  because the  grids,  not  no but  a  that dead the  presumed d i s p e r s e r s . beteen  the r a t e  of  number o f d i s p e r s e r s l e a v i n g a grid  evaluation  of  dispersal  p o p u l a t i o n s are growing  g r e a t e s t number o f d i s p e r s e r s . i s found  growing  D i s p e r s a l has  (1976)  detect these  the  faster  d e c l i n e p e r i o d s with  lead to a positive c o r r e l a t i o n  Thus, the  they  with  decline  v o l e s were f o u n d  design  increasing populations  1976)  during  i n d i c a t e s t h a t , when c o n t r o l  no  in  more d i s p e r s a l .  H i l b o r n and  tagged  growth  population.  rapidly,  idea  is  breeding  increase  occurred  These r e s u l t s  study,  a  severe  relatively  design.  radioactively  rates  (1970)  spring-born  spring-born  K r e b s 1971b, K r e b s e t a l .  demonstrated  removal  Anderson's  spring-born  following a  the  the  voles  applies only i f  the  p o p u l a t i o n s having been  their  end  1978).  Bate (Hyers  of  of  to  their  have a h i g h e r  than  accompanies  Boonstra,  do  lifetime  the f o l l o w i n g s p r i n g than  time.  supports  of d i s p e r s i n g a f t e r  should  near t h e  summer- and  season than  Since the  because  v o l e s born  v o l e s a r e more l i k e l y  probability  year's  do  between t h e  rate of  In  most this  population  56  growth  and  the  correlation periods.  was  not  I f the  (Krebs e t highest  al.  1976) , t h e  week  growth  dispersal  rates  was  t o the  The  results  moderate  High  removal  possible Male  this as  grid  or  the  per  2  per  rate  of  had  the  populations  week), but  per  population  correlation  design,  a  resident. growth,  between  high  was  but  population  because,  Why,  experimental design  detected  by  the  rates, lack  live-traps  unlike  dispersal rates  declining population.  mortality  to  pitfalls  by  growth  weeks  of  periods  dispersers  (15%  two  rate  d e c l i n e s , does the  on  were  during in  this  removal-grid  of a t t r a c t i o n t o  i t ,  or  than l i v e - t r a p s i n c a t c h i n g  greater dispersers  explanations.  voles  usually  1969,  decline,  as  t h i s r e s u l t may  male be  show  K r e b s and  an  lower observed much  positive  disperser  (Krebs e t a l . spring  the  fall  study  per  d i s p e r s a l t h a t i s not  e f f e c t i v e n e s s of are  i n the  removal g r i d  spring  design?  (0.12  time  year.  with a moderately  study r e v e a l  the  1976  the  winter  characterized  study  positive  grouping of  d i s p e r s a l rate i n t h i s study  with the  concurrent  the  a  summer was  dispersers  a  the  of  i n t o summer and  The  r e l a t e d to  of  lack  an a v e r a g e p o p u l a t i o n 1.3).  season of  lack  in  recorded  0.10  not  growth r a t e and  the  rates  (Table  rate of  Dispersal rather  1977  This  upon  grouped  r e s i d e n t ) , but  per  highest  dependent  data are  dispersal  weeks p e r -0.4%  number o f d i s p e r s e r s .  increase survival.  periods Myers  fewer female d i s p e r s e r s  1974).  low  spring survival  During  i n male d i s p e r s a l can  a  the  accounted  as compared  1977 f o r by  with  spring there  males.  moderate  account  Female s u r v i v a l d i d n o t  survival during partially  of  for  decrease  decline, having  and been  57  Lidicker exhibit  (1962,  presaturation  before  the  shows t h a t  not  fall  vole  maximum  the  animals.  These  Figure  fall  a v e r a g e male r e s i d e n t  may  emigrate  density.  summer and  when d e n s i t i e s were low  density.  populations  individuals  necessarily subordinate  l a r g e r t h a n the  animals dispersed  which  reaches  male d i s p e r s e r s d u r i n g  t o be  1976-77  suggests that  dispersal in  population  i n d i v i d u a l s are  tended  1975)  1.8  of  1976  vole.  as compared  These  with  F e m a l e d i s p e r s e r s , however, were  the  smaller  than female r e s i d e n t s during  the  same t i m e  interval.  Although  the  was  s m a l l , the  number o f  dispersers  number  corrected summer  of  dispersers  for density  and  fall  periods  d i s p e r s e r s tended that  to  be  density  by  increases. proportion  more v o l e s indicates  do  1977.  population  increase  one  would  emigrating  there  from i s no  (dispersers density.  seasonal  effects  dispersal  rate.  disperse  an  suggested  disperse  that  emigrating  from  An  Krebs  After  that  fall  mammal  a  there  not  weeks  was  The  suggests  correlation  density.  per  no  Although  this  study  proportion  resident)  and  indicated that effect of  as  of  voles  proportion  of  with  density does  on not  dispersers  density-dependent.,  males t h a n f e m a l e s d i s p e r s e d , Krebs  1976,  emigrating  populations,  proportion  a denser p o p u l a t i o n .  of  the  populations  positive  population  dense  in  which  proportion  expect  two  Thus a h i g h e r  and  the  small  i n the  and  that  animals.  a n a l y s i s of variance  removed,  (1971b)  with  c o r r e l a t i o n between t h e  per  seasonally-dependent, More  and  of  comparable  l i g h t e r than r e s i d e n t s ,  (1962) has  regulated  between  was  be  was  t h e s e d i s p e r s e r s were s u b o r d i n a t e Lidicker  may  i n 1976  et  al.  i n agreement with (1976).  This  Hyers larger  58  proportion  of  male  ratio,  especially  about  35-40%  constituted  females  apparently  that  pennsylvanicus  The  an  there  condition.  Sexual  of  Krebs  sexually show  clearly  a  of the  al.  (1976)  same  used a r e m o v a l p l o t whether  some  disperse.  However, n o t  factor  to  linked  with  a l l seasons i n  all  probably  were  lowering  the  the  the  unable  males.  they  My  dispersing  during this  to  could mature area  results before  subadults  1977  increased  may  have  proportion  of  However,  the removal  maturation.  and  gives  (1974),  sexually  d e n s i t y on  of  of  i n reproductive  become r e p r o d u c t i v e l y mature  maturity,  mature d i s p e r s i n g s u b a d u l t s .  be  design,  became  High d e n s i t i e s  median w e i g h t a t s e x u a l  but  subadult  reproductive  the  increased proportion  experimental  o r w h e t h e r t h e low  subadults  was  females  for  in  (1971b), Myers  an  some , s u b a d u l t s  s e x u a l l y mature.  contributing  effect  Microtus  d o m i n a n t v o l e s and  Krebs  found  but  proportion  subadults with  spring  summer.  During  voles  they  Fairbairn  dispersers  higher  subadult  and  that i n  appears t o  voles.  Myers and  stimulated rapid  that  male  maturity  interactions  guickly  were  of  dispersed,  dispersal  they  excess  maturation  However,  dispersers.  (1977a) f o u n d  subadult  before  show  an  resident  the  determine  decline.,  v o l e s were f e m a l e s ,  mature d i s p e r s i n g s u b a d u l t  because they not  in  subadults. et  Tamarin  of sexual  aggressive  expulsion  r e s i d e n t s ' sex  of female d i s p e r s e r s i n the  subadults than  and  was  excess  dispersing  to  spring  dispersing  died.  tendency  some  rise  1977  the  male P e r o m y s c u s d i s p e r s e d i n t h e  attainment  dispersal which  the  the  depressed  a s i z e a b l e p r o p o r t i o n of the  found  and  during  of  (1977)  winter  dispersers  been  the a  of sexually  59  Tamarin between  (1977) s u g g e s t s t h a t  density  saturation random  dispersal.  to  population.  and  differ  at  more  The  r e s u l t s of  this  of  dispersers;  which must by  the  of  results does  not  Thus t h e r e act.  for  In  Clearly,  a  no  a  sex  a l l  seasons  r a t i o s and  age  random sample o f  the  dispersers  than  are  saturation  are  residents. regarded  dispersal occurring  at  not  a  as peak  dispersal,  d i s p e r s e r s are  suggests severe  mortality this  view  of  dispersing predation  1978  accelerated neither  (see loss  necessary  vole  cycles  d e c l i n e s , and  for the  two  that  dispersers. reasons.  severe  1978  can  random  light  predation during nor  during  section) the  1978  predation,  conditions  for  are  This  study  There  was  decline.  predators  the and  the  predation  spring  a n i m a l s upon which was  dispersal,  that  among t h e  d i s p e r s a l during  addition,  i n t e r a c t i o n s are declines.  also  high  were no  spring the  (1978)  support  essentially  dispersing  condition  because t h e  d i s p e r s a l during in  indicate that  because  be  are  resident  not  definition  a  the  e x a m p l e , the  therefore  as  be  population.  Tamarin result  from  d i s p e r s e r s i n t h i s s t u d y c a n n o t a l l be  a s e l e c t i v e process,  sample o f  i t i s to  dispersers  voles during  seasons,  reproductive  correlation  dispersal  presaturation  study  are  f o r most  the  more l i k e l y  quality  For  dispersers  in  presaturation densities,  and  in  population  the  the  i n q u a l i t y from r e s i d e n t  often  Clearly,  process,  densities.  higher  views s a t u r a t i o n  different  differing  resident  and  Tamarin  be  distributions  be  dispersal rate,  non-selective  presumed  voles  and  the  could  late  winter  cannot  account  breeding nor severe  any  season. of  their  population  60  Hilborn  and  townsendii.  Krebs  found  presumed  that  Dispersal  was  no  (1976),  dispersal  dispersal measured  by  the  voles.  fenced  populations.  The  study  and  K r e b s and and  Boonstra  Krebs  Microtus I  demonstrated  for  necessary in  important  be  aggressive  and  for  a  severe  spring  than  to  resident  animals.  The  for  may  subsequent and  and  decline.  disappearance the  opposite  of  in  the  r e s u l t s of  this  to  explain.  that  of  Hilborn  the  results  e f f e c t not  generally  dispersal  is  valid  a  for  sufficient  decline i n voles However,  period  and  the  population.  voles  the  may  , but  vole  and  is  constitute  an  Iverson  (Chitty  November 1977 by  no  no  dispersal.  populations  by  less  Chitty  less  animals  1970).  wounding and  no  declines  between  to  aggressive  breeding  T h e r e was  tend  are  aggressive  1967,  until  interactions  postulated  of  large  voles  presumably  d i s p e r s a l of  is  dispersal  T u r n e r and  Dispersing  proportion  processes  aggressive as  males.  severe breeding season  almost  declines  stop  decline  Krebs i s d i f f i c u l t  fall  were c h a r a c t e r i z e d  increased  these  Microtus  severe  between the  consistent  increase  d e c l i n e s from  1978  wounding  a or  decline.  dominant over s m a l l  should  the  moderate  exactly  moderate p o p u l a t i o n  which a f f e c t p o p u l a t i o n  March  of  that i n Microtus pennsylyanicus populations.  smaller  The  that  s e l e c t i v e f o r c e on  are  animals  a  the  (1973) f o u n d males  area  study  townsendii. have  common  and  (1978) s u g g e s t an  a  presence  discrepancy  be  a  during  I found  Hilborn  (1976) may  condition not  of  in  occurred  radioactively-tagged  those  in  began  in  dispersal, had  t h u s no  little evidence  individuals  (1967, 1 9 7 0 ) .  from i n c r e a s i n g i n d e f i n i t e l y ,  in  Dispersal but  it  61  cannot  account  This  f o r the subsequent  study  further,  suggests  because  the  examined.  Dispersal  aggression  and  alteration  of  and  a  moderate  rates.  Winter  be i n d u c e d  areas  aggression  spring  r a t e s should  Beeman, E .  A.  behavior Blair,  W.  Symp.  1947.  Boonstra,  R.  Microtus Chitty,  D.  J.  Genet. Krebs.  townsendii. 1960.  J.  19 67.  behaviour 2:  and i f 1947),  influence dispersal dominant  should  rates  individuals  decrease  increase i f  dispersal  breeding  can  s t r u c t u r e and gene f l o w i n  Soc.  Lond.  Zool.  i n animal  aggressive  20: 373-405.  natural  and o t h e r  5: 1-41. 1978.  Pitfall  Can.  of  i n t h e v o l e and t h e i r  J.  selection  populations.  trapping  59: 136-148.  Population processes  The  26: 299-325.  o f male hormone on  Mammal.  relevance t o general theory. ,  (Beeman,  P o p u l a t i o n dynamics o f rodents  Adv.  And C.  Zool.  Physiol.  1953.  s m a l l mammals.  linked  linked,  be  cited  Ecological  The e f f e c t  of mice. F.  be  should  time.  1970.  s m a l l mammals.  are  decline  Literature  K.  may  l e v e l s should  dispersal  P.  dispersal  Removal o f l a r g e ,  during that  Anderson,  t h a t s h o u l d be e x p l o r e d  inducing  levels  hormone  (Krebs e t a l . , 1977). during  some  factors  hormone  decline.  Proc.  Zool. of  38: 99-113.  self-regulatory  E c o l . . Soc.  Aust.  51-78. ,  1970.  Variation  and p o p u l a t i o n d e n s i t y .  Symp.  Zool.  62  SocChitty,  Lond.  D.  in  And  E.  mixed  Ecol.  26:  Fairbairn,  J. and  1970.  J.  D.  ,  1977. Can.  1978.  M.  size  and  Can. A.  reliability voles.  Can.  W.  Howard,  E.  longevity George Lab. Kalela,  0.  8 4 - 9 2 .  deer  causes  and  effects  of  and  1976.  R e d f i e l d , and  Peroayscus on  fitness.  self-regulation 48:  Zool.  J.  1949.  J.  population  43:  Regulation  Acad.  Sci.  J.  1966. of  the  Mierotus  recapture  On census  the of  amount of  of p r a i r i e Univ.  inbreeding, d e e r m i c e on Mich.  and the  Contrib.  1-52. of reproductive  Ser.  Demographic  Mierotus  1976.  Krebs.,  rate i n  vole Clethrionomys  Fenn.  of  1019-1024.  Dispersal,  Biol.  disappearing  1507-1518.  C.  54:  of  populations 54:  of  377-392.  Fates  e n u m e r a t i o n f o r mark and  Ann.  populations  death  ,  mice,  Ecology,  Zool.  J.  of  C.  of  fluctuating  populations  Krebs,  55:  Reserve, Southern Michigan.  1957.  1550-1560.  Zool.  Krebs.  in a local  Vert.  population  146:  Science,  Aggression  J.  C.  townsendii. J.  subordination,  J.  i n deermice.  in  R.,  Anim.  s p r i n g d e c l i n e i n deer mice:  1967.  individuals  Hilborn  J.  171-193.  C.  population R.  survival  The  Proximal  32:  Oecologia,  Hilborn,  changes i n  species of vole.  Social  Dispersal  manicnlatus.  Healey,  two  mammalian e v o l u t i o n .  or d i s p e r s a l ? •  of  Seasonal  313r331.  J.  density,  1966.  Phipps.  populations  35:  Christian,  327-333.  A 4:  rufocanus  (Sund.).  1-60.  changes  californiens.  subarctic  in  Ecol.  fluctuating Monogr.  36:  63  239-273. K r e b s , C.  J.,  B.  Micrptus  L.  population  fluctuating  C.  and  B.  H.  rodents. Krebs,  C.  S.  Science  K r e b s , C. and  J . , I. B.  1973.  H.  Adv.  Wingate, J .  Can.  Zool. J.,  Keller,  54:  Z.  1974.  Hes.  T.  H a l p i n , and J .  populations  of  Mierotus  J.  spring  P.  and  the  R.  vole  in  J. J.  determination r a t s reach  cycles  Myers , in  small  M.  population of  N.  the  Boonstra.  decline  H.,  H.  M.  Taitt, biology:  townsendii.  M.  spring  Smith.  1977.  decline  townsendii.  in  Can.  J.  of  the  430-437.  townsendii. Leslie,  587-607.  79-95.  and  K r e b s , C.  J.  Bedfield,  populations  testosterone,  55:  50:  M.  267-399.  Mierotus  Aggression,  Zool.  and  in  Population cycles i n  8:  LeDuc, J .  1976.  i n fluctuating  C.  L.  Myers.  Dispersal  Krebs,  Ecology  1969.  changes  ochrogaster  Population  Ecol.  Hilborn.  J.  Tamarin  Demographic  M.  B.  fl.  179: 35-41.  and J .  s m a l l mammals.  B.  Indiana.  Gaines,  Tamarin.  J.  of  i n southern  J . , H-  and  biology:  populations  pennsylvanicus Krebs,  Keller,  Anim.  1978.  populations Ecol.  47:  S.  P e r r y , and J .  of  the  maturity.  Demography  of  the  vole  Mierotus  1007-1015. S.  Watson.  1945.  The  median  b o d y - w e i g h t a t which f e m a l e  Proc.  Zool.  Soc.  Lond.  115:  473-488. Lidicker,  W.  Z.  permitting carrying  the  1962.  Emigration  regulation  capacity.  Amer.  of  a s a p o s s i b l e mechanism  population  N a t : 29-33.  density  below  64  ,  1975.  The  role  of  d i s p e r s a l i n t h e demography  s m a l l mammals.  Pp.  Petrusewicz,  and  L.  productivity  and  p o p u l a t i o n dynamics.  Vol. Murray,  5.  103-134. Byszkowski  Cambridge Univ.  B.  G.  In  1967.  F.  (eds.)  B.  Golley,  Small  mammals:  Int.  Biol.  P r e s s , Great  of K.  their  Progr.,  Britain.  Dispersal in vertebrates.  Ecology  48:  975-978. Myers, J .  H.  house  mouse  Ecology Myers, J .  55:  H.  and  B.  and  C.  vole 105:  and  J.  C.  J.  53-78.  P.  1963. Ecology  M.  agonistic  F.  behaviour  and  Smith,  Sex  ratios  feral  in  open  implications.  of  dispersing  H i s t o r y o f two 44:  Genetic,  Microtus  behavioral field  voles  pchroqaster.  local  outbreaks  Ecol.  of  feral  540-549.  S.  1965. and  The  relationship  population  mouse, P e r o m y s c u s m a n i c u l a t u s 14:  of  Island, California.  1971b.  attributes  44:  house m i c e .  1971a.  Krebs.  Monogr.  R.  Grizzly  p o p u l a t i o n s : Demographic  pennsylvanicus  Sadleir,  on  structure  325-344.  reproductive  0.  social  Krebs.  Microtus  Pearson,  and  747-759.  Nat.  Myers, J .  Genetic  populations  enclosed  Amer.  and  1974.  changes  (Wagner).  J.  between  i n the  deer  Anim.  Ecol.  old-field  mouse,  331-352. M.  H.  1968.  Dispersal  Peromyscus p o l i o n o t u s .  Bull.  of  the  Georgia  Acad.  Sci.  26:  45-51. Tamarin*  R.  Ecology,  H.  1977.  Dispersal in island  58:  1044-1054.  and  mainland  voles.  65  ,  1978.  K-selection Turner,  B.  Dispersal, in field  N a n d  aggression relation  S. in  mice.  L. male  population Amer.  Iverson.  Nat. 1773.  Microtus  to population parameters.  regulation, 112:  545-555.  The a n n u a l  pennsylyanicus. Ecology,  and  54:  cycle of and  its  967-981,  66  SECTION 2.  DISPERSAL TENDENCY AND  DURATION OF  LITTERMATES DURING POPULATION FLUCTUATIONS OF  THE  LIFE  OF  VOLE HICROTUS  TOWNSENDII  Introduction M i c r o t i n e rodent Krebs  et a l -  populations  (1973) have s u g g e s t e d  for  normal p o p u l a t i o n  not  a random s u b s a m p l e o f  Krebs et the  genetic  a  genetic  necessary  Dispersing  (Myers and  voles  Krebs,  s e l e c t i o n t h r o u g h d i s p e r s a l may  composition,  age  a population.  I f tendency  control,  and  sex  Howard  traits  ratios,  and  are 1971;  alter  reproductive  (1960) s u g g e s t e d  than  within l i t t e r s .  species  disperse  littermates  and  that  lifespan  should  be  t h a n i n d i v i d u a l s from d i f f e r e n t  variability  dispersal  to  then  the  study  population  and  there  polymorphic g e n e t i c b a s i s f o r d i s p e r s a l tendency w i t h i n a  population.  these  the  periodically,  that d i s p e r s a l i s  regulation i n voles.  a l . , 1 9 7 6 ) , and  a t t r i b u t e s of is  fluctuate  was  during  i n these  traits  In  fact,  non-random periods  of  populations  variability  f o r each  by  lifespan  among  (1975)  reports  for  trait.  Methods  four  in so,  litters that  Microtus  The  present  tendency t o  disperse  in increasing comparing  If  greater  increase. and  under  more s i m i l a r  litters.  litters  population  are h e r i t a b l e c h a r a c t e r i s t i c s  be  Hilborn  within  e x a m i n e s whether o r n o t  townsendii  should  are  among  and and  peak  Microtus  within-litter  67  This study River was  delta  near  o u t on R e i f e l  Vancouver,  British  p a r t o f a T i m o t h y hay f i e l d  Service.  The  waterfowl  field  was  cut  in  Columbia.  June  1978.  grids  two weeks d u r i n g t h e g r o w i n g s e a s o n unfavourable  An a r e a i n s i d e  f o rvoles.  wide and 53.3 m (175 f t )  and g r a z e d by cover  i n order t o  and  was  adjacent  A t each t r a p - s i t e  were two L o n g w o r t h  one p i t f a l l t r a p s i m i l a r  t o the type  and  Krebs  l i v e - t r a p s and p i t f a l l s  (1978).  oats,  Longworth  and  provided  with  pitfalls  from  May t h r o u g h  to  a  each  through  periods,  June  the  1976, A p r i l  1978.  pitfalls  During could  week.  through  the not  be  live-traps  by  were b a i t e d  two  Pitfalls September  intervals used  Boonstra  A l l grids  f o r two days e v e r y  f o r two d a y s e v e r y  October  described  with c o t t o n bedding.  with Longworth l i v e - t r a p s  and  April  create  was 18.3 m (60  and  trapped  I set  h a d 49 t r a p - s i t e s s e t 7.6 m (25 f t ) a p a r t , i n a 7  pattern.  with  and M.  grid.  Each g r i d 7  Wildlife  e a c h e n c l o s u r e was mowed  The mowed a r e a  long  area  i n May 1976 and t r a p p e d  every  x  Canadian  so t h a t both  until  trapping  Fraser  The s t u d y  t h e 1975 f a l l  them  ft)  the  were s c a r c e when t r a p p i n g began i n May 1976.  two e n c l o s u r e s and two u n f e n c e d  habitat  Island i n  owned by t h e  during the f o l l o w i n g winter  townsendii up  was c a r r i e d  were  weeks,  were  used  1977,  between  and  these  because o f winter  flooding. E a c h v o l e was e a r - t a g g e d capture and  On  weight,  l o c a t i o n , s e x , r e p r o d u c t i v e c o n d i t i o n , wounding on rump,  relative  released  on f i r s t c a p t u r e , and i t s  size  of h i p glands  immediately each g r i d  after  being  were r e c o r d e d .  A l l animals  were  processed.  I placed traps t o catch voles that crossed the  68  18.3  m width  installed the  o f t h e mowed a r e a .  a t 8.9  mowed  m  (29ft) i n t e r v a l s along  area.  were s e t a l o n g  Five-gallon  One-gallon  were two l i v e - t r a p s p e r s i t e  half-way  live-traps,  baited  remained c o n t i n u a l l y pitfalls  from  each  visit.  and  entering  between p i t f a l l s .  oats  s e t , except  were removed f r o m  to February.  families  caught  in  littermates. of  weeks  killed  Pearson discrete  most  common  pitfall  The l i f e t i m e  examined  variable  and  of  A l l t r a p s were c h e c k e d  on  width  of the  mowed  area  v o l e s u n d e r 40 g i n  and i n t h e same  unit.  at  o f each  its first  of 4 g from  (1960) s u g g e s t e d Microtus  same  the  t h a t runway  population  Thus new  the  weight  small  time  units, Microtus  should  be  v o l e i s d e f i n e d a s t h e number  and  last  was e l i m i n a t e d f r o m  capture.  Any  vole  the a n a l y s i s along  with  littermates. I  of  cotton,  t h e use  a maximum w e i g h t r a n g e  by  same  between  accidentally its  the  the  prevented  on t h e t r a p p i n g g r i d s  to the l i g h t e s t .  s y s t e m s were o c c u p i e d with  pitfalls  the populations.  c h e c k and h a v i n g  heaviest  The l a r g e  12 p e r  t r a p s were c o n s i d e r e d t o be d i s p e r s e r s and  i n t h e same p i t f a l l  trapping  There  s u p p l i e d with  when r a i n  L i t t e r m a t e s were d e f i n e d a s new caught  and  Voles c r o s s i n g the e n t i r e these  bordering  i n the enclosures, t o t a l l i n g  with  November  fence  with Longworth t u n n e l s  e n c l o s u r e , and one p e r s i t e on t h e c o n t r o l s . and  t r a p s were  the  cans f i t t e d  t h e same f e n c e  pitfall  tendency  f o r each i n d i v i d u a l  two i f t h e a n i m a l the  remained.  to  start  of  d i s p e r s e by c o n s t r u c t i n g a dummy  in a litter  and g i v i n g  d i s p e r s e d between t h e t i m e next  I considered  year's  of f i r s t  breeding season  only l i t t e r s  with  i t  a  value capture  and one i f i t  two o r more  dispersing  69  siblings time. from  t o d e t e r m i n e whether l i t t e r m a t e s Non-dispersing  the a n a l y s i s .  week, l i t t e r relative  number w i t h i n and  population  according  of  1976  spring  to fall  was  t o summer  was  caught during  same  were e x c l u d e d  recorded  analyzed  the  capture  separately  u s e d by K r e b s  et  grid, weight,  1978 a d e c l i n e the decline  because,  al.  1977 was an i n c r e a s e p e r i o d  o f 1977 a peak p e r i o d  1977  at the  dispersal.  phase  t o spring  I  week, l i f e t i m e , f i r s t  t o the c l a s s i f i c a t i o n  spring  these l i t t e r s  For each i n d i v i d u a l ,  growth r a t e ,  Each  members  dispersed  (1973),  (r>0.03/wk),  (-0.02<r<0. 03) ,  period  (r<-0.02).  s o no r e s u l t s  and  O n l y one  fall litter  f o r the decline  are  presented. I  u s e d an a n a l y s i s  lifetime  due  among-litter analysis  area  variance  living  longer  than  o f c a p t u r e , from  I  used  a  to  litters  begins.  population  may  live  declining  population.  longer  fourth  source  differences.  on  than  from  two-way  combination  resulting  differences i n an  born  in  of grid  in  peak  or  variability week.  from  the  increasing a  and  the p a r t i t i o n i n g  The  among-litter of  from d i f f e r e n t  from  of  l i t t e r s on o n e g r i d  t e r m removed  resulted  resulting  source  The weeks s o u r c e o f  from  voles  variation  variation  variation  The g r i d  another.  resulting  The d e s i g n a l l o w e d  squares i n t o and  of  resulting  The i n t e r a c t i o n  t o any p a r t i c u l a r  w i t h i n - and  partially-nested  Thus, v o l e s born e a r l y  peculiar  in  examine f o u r s o u r c e s o f v a r i a t i o n i n  removed v a r i a b i l i t y  breeding  variation  week  removed v a r i a b i l i t y  variation  weeks  separate  to the within-litter v a r i a b i l i t y .  variation  of  to  and  variability.  of  addition  time  to  of v a r i a n c e  a grid-week  weeks  sum  first-capture interaction.  70  However, t h e d e s i g n litters grids. of  were  such  cases  a totally  disadvantage,  variation If  into the  mean  within-litter indicates  average  mean  of  litters  should A  i n average  litter  Thus,  litter  variation, tests  mean  mean  square  of litter square  square.  In  o f v a r i a n c e , which interaction  tests  i s t h e same, t h e n the  litter  life.  'error' mean  Grids,  divided  by e r r o r  interaction  the s i g n i f i c a n c e  mean s q u a r e i s  o f weeks and g r i d s  or  square  weeks, and  i n the s t a t i s t i c a l  variation,  and t h e i n t e r a c t i o n  of significance  mean  equal  larger  used.  over  degrees  variation.  square.  litter  which  pair of  the  incorporating  a r e c o n s i d e r e d random e f f e c t s  significance  in  t o be i n s u f f i c i e n t t o  nested a n a l y s i s  litters  tests  weeks  caused  lifespan of a l l l i t t e r s  for  differences  condition  interaction  however,  t h e weeks  square  the  interaction  estimate of the  I used  as  c a p t u r e d were n o t t h e same on e a c h  f o r the grid-week  a reliable  has t h e  the  first  unbalanced,  Sometimes t h i s u n b a l a n c e d  freedom  give  was  models  mean mean  square square  of interaction  the  divisor  for  variation.  Results  Population  The  D e n s i t y And  Dispersal  M i e r o t u s t o w n s e n d i i p o p u l a t i o n s were l o w when  began i n May  1976 a n d r e m a i n e d  low t h r o u g h o u t  a t about  the  1.4).  They  increased  winter,  owing  to delayed recruitment t o l i v e - t r a p s  trapping  summer ( F i g .  9% p e r week d u r i n g t h e f a l l  and  (see t r a p p i n g  71  section). of  A spring decline started  breeding,  sustaining  i n March  and o c c u r r e d i n a l l f o u r greater  losses  than  populations,  females.  numbers i n c r e a s e d t h e p o p u l a t i o n s i z e s that  occurred  declined  i n March.  started  decline  increased  about  week f o r f e m a l e s ,  both  during  to and  the  in  March  populations  t h e s p r i n g and e a r l y  occurred  dispersing  i n t h e 1976 f a l l  (Fig.  1.4).  males  1977 t o l e v e l s  males  and  1978,  females  the f a l l the  rate  declined  to  1977 b r e e d i n g s e a s o n  June late  and  July  September  low  during the study.  The  summer o f 1978.  with both s u b a d u l t s  Dispersal  was  increased  began.  low  Subadult  1977, d e c l i n e d t h r o u g h and  October  i n September  of  very  and a d u l t s  during  the  w i n t e r b u t i n c r e a s e d d r a m a t i c a l l y owing t o a d u l t d i s p e r s a l the  and  50% p e r week f o r m a l e s and 18% p e r  T h e r e were f o u r p e r i o d s o f d i s p e r s a l first  with  a t a r a t e o f a b o u t 5% p e r week t h r o u g h o u t After breeding  the onset  A summer r e c o v e r y i n  by O c t o b e r  A f t e r October,  winter.  levels  1977 w i t h  1977,  dispersal increased i n  adult  dispersal  T h e r e was l i t t l e  during  t h e 1977 w i n t e r and e s s e n t i a l l y  severe  1978 s p r i n g  after  A u g u s t , and i n c r e a s e d i n  and  and O c t o b e r .  1976  no d i s p e r s a l  also  dispersal  during  the  decline..  Littermate Lifespan  The  observed  experimental The  average  and  Krebs,  litter  sizes  i n each year  and c o n t r o l p o p u l a t i o n s a r e l i s t e d litter  size i s five  litters  in  Table  i n Microtus townsendii  1978), s o t h a t most l i t t e r s  a subsample o f t h e  f o r v o l e s i n the  born.  2.1.  (Boonstra  o f s i z e two o r t h r e e a r e  The  analyses  of  variance  72  T a b l e 2.1. L i t t e r s i z e s from the c o n t r o l and experimental populations i n 1976 and 1977. L i t t e r m a t e s were d e f i n e d as new v o l e s u n d e r 40 g i n w e i g h t c a u g h t i n t h e same pitfall i n the same trapping check and h a v i n g a maximum w e i g h t r a n g e o f 4 g from t h e h e a v i e s t t o t h e l i g h t e s t . r  —  — T  | Litter  |  I  |  size  !— Experimental 1976  I  grids  T  I  T  1976  1977  .,  , I  1977  |  77 32 10 4 1 1  | | | I | I  Control grids r  t  I i | I | |  2 3 4 5 6 7  i | | i | |  72 20 10 3 1 1  | |  135 41 13 7 1 1  |Hean+1SE| 2.54 + 0. 091 2.49+0.08 i.. i i  | | j | |  28 7 1 1 0 0  I 2. 32+0.11 — i —  | | | ) |  | 2.58+0. 111 —L- _  , i  73  indicate in  that  f o r both  both y e a r s ,  than expected for  litters  living  the  littermates by  chance  were n o t  longer,  tended t o  (Table  lifetime.  both  the  control  and  1976.  In  not  in  weeks,  whereas  interaction grid  was  source  indicated both  the  grids.  Hilborn  (1975)  to  litters  this  h y p o t h e s i s on  not  the  analyses  non-random grids  that  within  (Table  2.3)-  in of  the  was  for  significant in  1977  l i f e t i m e was  l i f e s p a n of  The  year.  insignificant, litters  significant  significant  was  the  only  By  removing  which  in  lifespans  control  litters,  was  entirely  able  and  to  there  test  was  a  I could  test  grids  only  experimental  for  indicate both  Some l i t t e r s  factor  I was  were c a u g h t .  I  This  was  clearly  grids.  variance  litters  one,  on  the  dispersers  among s i b l i n g s .  born i n the  a l l dispersers  of  then  were  known  The  same  F-ratios  results  21  grid-week  g r i d i n any  always  at  variation  weeks.  non-dispersing  even when d i s p e r s a l l o s s e s not  11  s i z e was  litters  case f o r the  The  my  resultant  assortment  assuming  was  to d i s p e r s a l .  whose  by  was  capture  r e l a t i o n between w e i g h t  weeks s o u r c e o f  attributed  i f  hypothesis that,  random  no  s i g n i f i c a n t f o r any  average  F-ratios  t e s t s t o non-random d i s p e r s a l among l i t t e r s .  examine  attributable  significant  average l i t t e r m a t e  i t  variation  p a i r s of  able  The  the  1977  not  of  that  among-litter  the  in  The  experimental g r i d s 1976  populations  l a r g e r animals at f i r s t  b e c a u s e t h e r e was  c a p t u r e and  experimental  have l i f e s p a n s more s i m i l a r  2.2).  a r e s u l t of  first  but  c o n t r o l and  1976  that and  survived  were e l i m i n a t e d .  acting  to  1977  better  lifespan on  the  than  was fenced others  Thus, d i s p e r s a l  produce d i f f e r e n t i a l  was  lifespans  74  T a b l e 2.2. Analysis of variance tables to test non-random lifetimes among Microtus townsendii litters. Lifetime was d e f i n e d as t h e t i m e between f i r s t and l a s t c a p t u r e s .  Source  df  Grids Weeks GXW Litters Error  1 12 3 20 49  Total  85  Sum o f Sguares  Mean Square  F  C o n t r o l g r i d s 1976 0. 10 45.3 45.3 0.86 374.8 4498. 1 1311.8 437.3 0.92 2.71 9543.4 477.2 176.3 8638.8  P 0.7587 0.6364 0.4527 0.0024**  24037.4  Grids Weeks GXW Litters Error  1 18 14 91 198  C o n t r o l g r i d s 1977 0.60 136.9 136.9 2. 11 484.1 8713.2 1.04 3198.4 228.5 220.6 1.46 20077.7 151.5 30005.4  Total  322  62131.6  Grids Weeks GXW Litters Error  1 15 11 79 164  Total  271  102370.9  Grids Weeks GXW Litters Error  1 17 15 164 295  E x p e r i m e n t a l g r i d s 1977 0.6314 0.25 43.1 43. 1 0.0022** 4.74 14084.8 828.5 0.7978 0.68 174.8 2622.8 0.0000*** 255.4 1.83 41887.5 139.2 41071.3  Total  492  99709.5  * p<.05 ** p<.01 *** p<.001  0.4573 0.0396* 0.4 266 0.0154*  E x p e r i m e n t a l g r i d s 1976 0.00 0.9 165 0.6 0.6 2. 55 0.0619 717.3 10759.5 0.8679 0.54 281.9 3100.3 0.0009*** 1.79 4 0946.9 518; 3 4 7563.6 288.2  75  T a b l e 2.3. A n a l y s i s of variance tables to test non-random lifetimes w i t h i n l i t t e r s w i t h d i s p e r s e r s removed. A l l r e s u l t a n t l i t t e r s o f s i z e 1 .were . e l i m i n a t e d . These d a t a a r e t h u s a subset of t h o s e i n T a b l e 2. 2 Sum o f Squares  Mean Square  F  P  Source  df  Grids weeks GXW Litters Error  1 14 11 68 143  Total  237  Grids Weeks GXW Litters Error  1 15 13 113 199  91046.0 E x p e r i m e n t a l g r i d s 1977 0.2694 1. 33 323.3 323.3 760.0 3. 12 0.0 231* 11400.2 0.4672 0. 99 243.2 3161.2 0.0336* 1. 35 27792.8 246.0 182.4 36289.9  Total  341  78967.4  * P<.05 ** P<.01  E x p e r i m e n t a l g r i d s 1976 0.6381 0. 24 69.6 69.6 0.2010 483.2 1.66 6764.4 0.8672 0. 54 290.9 3200.0 0.0036** 535.6 1. 71 364 20.3 311.8 44591.7  76  among  litters.  significant  Littermate  I  1976 both  dispersed  not  was  New  from  grid  with t h i s  The  error to  suggest  distributed  among  Given litters,  that did  i n 1977,  in  the  i n the l a t e  but  not  significant spring  and  breeding  summer and  fall  source o f v a r i a t i o n  indicates  C  that  that  (see d i s p e r s a l  more  was  voles  section).  One  variable  was  dispersal  as assumed by an a n a l y s i s o f v a r i a n c e .  was  only the  still  1.68  probability  levels.  times l a r g e r than  the  the r o b u s t n e s s o f a n a l y s i s o f v a r i a n c e  t h e u n u s u a l l y low that  tendency  probability  t o d i s p e r s e was  levels,  the  non-randomly  litters. tendency  siblings  siblings.  among-litter  which  would b i a s  answer t h i s q u e s t i o n was dispersing  caught  i n the  indicated  among weeks was  grids  B than g r i d  Given  and  again  born  d i s p e r s e b e f o r e t h e next  The  year,  square  mean s q u a r e .  data  variance  a n a l y s i s i s that the  mean  non-normality  1977  first  animals caught  However, n o n - n o r m a l i t y litter  variation  to  normally d i s t r i b u t e d  The  of  for l i t t e r s  non-random among l i t t e r s  section). each  was  1976.  analyses  prone  in  variation  dispersal  animals  more  dispersal  problem  The  were new  significant  not i n  of  Tendency  (Table 2.4).  than  source  non-random  tendency  were  season (see  Dispersal  years.  summer  weeks  but  populations.  dispersal  in  i n 1977  examined  fenced  in  The  to  disperse t o examine  I f they  variability  disperse  in  was  non-random  a t t h e same t i m e ? the  duration  do d i s p e r s e a t t h e  One of  among way  life  same t i m e ,  d u r a t i o n o f l i f e s h o u l d be  to of  then  larger  77  Table 2.4. analyses of variance t a b l e s f o r d i s p e r s a l tendency w i t h i n l i t t e r s b o r n i n e x p e r i m e n t a l p o p u l a t i o n s d u r i n g 1976 and 1977. D i s p e r s a l t e n d e n c y i s measured by a v o l e t s w i l l i n g n e s s t o cross unfavourable habitat.  Sum o f Squares  Mean Square  F  P  Source  df  Grids Weeks GXW Litters Error  1 15 11 79 165  Total  271  Grids Weeks GXW Litters Error  1 17 15 164 295  21.055 E x p e r i m e n t a l g r i d s 1977 0.0097** 8. 68 1.541 1.541 0.0019** 4. 88 0.866 14.715 0.5587 0. 91 0. 178 2. 663 0.0001*** 0. 196 1. 68 32.123 34.414 0.117  Total  492  85.456  * P<.05 ** P<.01 *** P<.001  E x p e r i m e n t a l g r i d s 1976 7. 72 0.0173* 0. 197 0. 197 0.0010** 7. 64 0. 195 2.931 0.976 3 0. 33 0.003 0.281 0.2838 1. 11 0.078 6. 129 0.070 11.517  78  than w i t h i n - l i t t e r v a r i a b i l i t y . of  variance  degrees for or  to investigate  o f freedom  I used a t r i p l y - n e s t e d  this possibility  were i n s u f f i c i e n t  t h e i n t e r a c t i o n mean s q u a r e . more a n i m a l s d i s p e r s i n g  season.  I  for  that  in  Table  was  year. 2.5.  dispersed,  insignificant at  about  the  same  to  age  and  at  last  indicate that  leave  at  the  that  independent  before  observed  t h e same a g e .  captures  animals dispersed,  March  for  are  when  same  of  their  1978  littermates time.  2-5 the  dispersed  week o f f i r s t  born  in  weeks.  both  left  greater  The i n s i g n i f i c a n t g r i d s *  1977  and  I f fall-^born  following  l i f e t i m e was t h e r e f o r e  a v e r a g e age o f d i s p e r s e r s  The  The a v e r a g e i n t e r v a l between  was in  analysis presented  littermates  littermates  they d i d so  summer-born v o l e s . the  few had two  Thus s p r i n g - b o r n and summer-born d i s p e r s e r s  dispersing  their  results  tended  population  first  1976 l i t t e r s  u n a b l e t o make a m e a n i n g f u l  weeks F - r a t i o i n d i c a t e s  the  capture.  Among  a r e l i a b l e estimate  t h e r e s u l t s f o r 1977 l i t t e r s  These  they  t o give  interaction  b e f o r e t h e s t a r t o f t h e 1977 b r e e d i n g  therefore Only  because  analysis  spring,  and  t h a n s p r i n g - and  F - r a t i o means  was a b o u t t h e same on e a c h  that  grid.  Discussion From not  these  disperse,  both  the  tendency phase  littermates  increasing  to disperse  and  littermates The  a n a l y s e s , we s e e t h a t ,  likely  t o have  similar  lifespans  and peak p h a s e s o f p o p u l a t i o n was non-random w i t h i n  litters  non-random i n t h e i n c r e a s i n g  dispersed  average  tended  whether t h e y d i d o r d i d  a t about  lifetime  growth; in  the  in that peak  phase; and t h a t  t h e same a g e .  (interval  between  first  and  last  79  T a b l e 2.5. A n a l y s i s o f v a r i a n c e t a b l e t o t e s t whether M. t o w n s e n d i i s i b l i n g s have t h e same l i f e s p a n . Mean Square  So ur ce  df  Sum o f Squares  Grids Weeks Litters Error  1 14 11 41  Experimental 6.9 6.9 12.5 174.6 23.7 260.4 73.3 1.8  Total  67  515.2  ***  P<.001  F  P  g r i d s 1977 0. 4764 0. 55 0. 8709 0. 53 13. 23 0. 0000***  dispersing  80  captures) There  of  was  lifetime  no of  breeding have  a  season.  this  the  born  Litters  average  1977  peak  decreased  significant litters  longer  during  litter  peak  phase  s e a s o n had  litters  born  later  born  litters  the d e c l i n e a f t e r that  followed  spring of  1977,  began  time of  increased  born  heterogeneity  Hilborn  (21  interval  weeks)  season  than  (11  a significant  can  be  of the  The  litters  to born  During  beginning (14  than  timing  The  of of  decline  begin u n t i l  d e c l i n e f o l l o w i n g the  in the  lifetime  the  season.  Thus l i t t e r s  of  weeks)  shortened  s e a s o n d i d not  same f a l l .  tended  weekly v a r i a t i o n  a c c o u n t e d f o r by breeding  average  weeks).  born near the  breeding  the  1977  the  breeding  born c l o s e t o a  m o r t a l i t y have s h o r t e r l i f e s p a n s than  o n l y one  in  average  litters  (1975) r e p o r t e d  unfenced  litters  Longworth Microtus (Boonstra  lifespan  still that  was  had  lifetime  townsendii and  However,  avoid capture  K r e b s 1978,  have a n a l y z e d but  among  litters,  differential was  random  among  partial by  see  pitfall  is  in live-traps  until  T h i s problem  t r a p p i n g I was  able  in  t h a t many adulthood  so t h a t was  litter  trapped  evidence  trapping section)  litters.  litters  H i l b o r n ' s method o f  there  as  lifetimes.  d e p e n d e n t upon s m a l l v o l e s b e i n g  live-traps.  study,  of s e v e r a l f a c t o r s i n f l u e n c i n g the  increasing populations.  classification  this  1976  this  average l i f e t i m e  was  non-dispersing  may  end  1976  the  earlier. Dispersal  in  the  in  i n c r e a s i n g phase  (7 w e e k s ) .  whereas t h e  season  the  Litters  i n 1977  the  during  was  n e a r t h e end  cycle.  in  phase b r e e d i n g  a longer  population  variation  lifetimes  average l i t t e r l i f e t i m e .  the  weekly  born d u r i n g  there  breeding  during  Hilborn  present  in  t o t r a p more  81  nearly-complete were u s e d .  litters  a t a younger age than  Misclassification  variability  within  o f l i t t e r m a t e s would i n c r e a s e  litters  (error  decrease t h e chance o f d e t e c t i n g Tendency but  to disperse  n o t i n 1976.  i f only l i v e - t r a p s  mean  square),  d i f f e r e n c e s among  d e n s i t i e s during  t h e s p r i n g and e a r l y summer  If  voles  from  same g r i d would  and r e m a i n e d  there,  they  pattern  also lacked  to  detect  random d i s t r i b u t i o n The was  analyses  non-random  to leave litter  early  within  litters  basis f o r dispersal.  et  (1976)  idea  but  direct  tendency The leave that  that  resident  there  fact  at  that  the  subadult  of  This  i n 1976. tendency  in  (1960) h y p o t h e s i s  of  result  (1971) and K r e b s in allelic  i s consistent  basis f o r dispersal  variability  in  litter  with  tendency, dispersal  test.  and  summer-born  same a g e , i n d e p e n d e n t  would mean t h a t  f o r the  Differentiation  dispersing voles d i f f e r  voles.  a stronger spring  time.  dispersing  litters  o f week of b i r t h ,  suggests  d i s p e r s a l age may be g e n e t i c a l l y d e t e r m i n e d .  weeks e f f e c t  in  dispersal  H y e r s and K r e b s  i s a genetic  measurement  provides  The 1976  and d i s p e r s i n g l i t t e r m a t e s t e n d e d  s u p p o r t s Howard's  that  but  i n 1977 ( s e e d i s p e r s a l s e c t i o n ) .  a t t h e same  report  1.4).  dispersed  increase  i n d i c a t e that  a genetic  from  have  d i s p e r s e r s i n 1976 may a c c o u n t  of variance  d i s p e r s a l tendency  frequencies  (Figure  by  t o v a c a n t a r e a s on t h e  would  t h e summer  1977,  explained  o f d i s p e r s a l t e n d e n c y among l i t t e r s  the population  a l .  in  a s s u c h b y my methods.  j u v e n i l e d i s p e r s a l observed  Failure  the  birthsite  n o t h a v e been r e c o g n i z e d  dispersal and  their  would  litters.  may be p a r t i a l l y  low  dispersed  which  was non-random among l i t t e r s  The 1976 r e s u l t  the  dispersing l i t t e r s  A significant from  different  82  weeks d i s p e r s e cues, For  as  at  the  in  the  aggressive  behaviour  the  weeks  to f a l l age  littermates age  were  growth  means t h a t  could  account  indicates average  for  that  in  greater  the  fall  observed  suggested  at the  was  that  so  2.5  environmental  environmental  throughout sexual  mature  voles  same age.  the  cue  entire  maturity  at  interactions  may  Anderson  result (1975)  in  found as  that synchronous l i t t e r m a t e maturation at  and  the  spring-born  Howard voles  age.  the  those  study  have  dispersed this  Anderson  at  an  individuals  n e x t s p r i n g must  (1949) and  the r e s u l t s o f  This  dispersers  therefore  dispersing  lifetimes.  same  summer-born  weeks and  and  autumn-born v o l e s , and  same age  an  The  h i g h l y c o r r e l a t e d among l i t t e r m a t e s ,  dispersal  of  any  dispersers  season.  t h i s period, aggressive newly  spring-  lifetime  r u l e out  season.  dispersers i f  breeding  I f young v o l e s a t t a i n  these  rates,  the  simply  dispersing  breeding  spring-born  does n o t  throughout  at puberty  the  summer-born  result  period.  between a d u l t s and  born  than  d i s p e r s i n g animals at the  same  that  longer  that  declines during  f o r d i s p e r s a l , but  spring  changes during  expected  population  nonsignificant  a c t s on  ages, thus i m p l i c a t i n g environmental  environment  e x a m p l e , i t m i g h t be  remain  cue  different  have  (1970)  younger a g e s  study  than  support  this  view. Hilborn litters  (1975)  during  periods  peak and  decline  measure  of  non-random study  found  that  lifetime  of population  periods.  By  indicates  litters  during  among-litter  non-random  i n c r e a s e , but  assuming  d i s p e r s a l t e n d e n c y , he within  was  that  concluded  increase  lifetime  that  periods.  variability  for  random  within during is  dispersal The  a was  present  both d i s p e r s a l  83  tendency  and  lifetime  lifetime  and  tendency  dispersal  lifetime  ones.  should  study  lifetime  be  of  the in  enclosed  peak  in  The  an  should al. ,  lifetime heritable,  so  1969),  was  indicative  study  heritability  complete  and  that  densities,  i f  and f o u n d  analyses  T h i s study  is with  or  a  effect.  Hilborn's  species. of variance high  i f lifetime  to test  mortality,  were s t r o n g l y among  litters  indicates that lifetime i s  be  i n declining  dispersal  a fence  townsendii  t h a t even  may  a  then  (1977) e n c l o s e d  i s that i n periods of  litters  provide  i n numbers compared  f o r Microtus  using  short  of lifetime  of  If dispersal i s  peak  even i f d i s p e r s e r s a r e e l i m i n a t e d .  nearly  suggest  should  Boonstra  townsendii  with  heritability.  in  randomness o f  t h e r e m i g h t n o t be enough d i f f e r e n c e s  detect  heritable  be  because  i n c r e a s e i n d e n s i t y f a r above  a r e u n i q u e t o M.  of l i f e t i m e  may  at  population.  of Microtus  difficulty  heritability  this  of  heritable  of d i s p e r s a l .  regulation  are not generally valid  One  of  no change i s e x p e c t e d  results  heritability  that  a peak p o p u l a t i o n  et  unfenced  dispersal  b u t n o t i n peak  also  populations  linked  component o f p o p u l a t i o n s r e g u l a t i o n i n  Fencing  (Krebs  i s  suggested  results  populations  then  these  results  Hilborn  population  peak p o p u l a t i o n Either  that  to increasing populations  regulatory aspects  levels  unimportant, those  Hilborn  and suggested  lifetime  i s an important  important  of  that  voles.  peak p o p u l a t i o n s .  to  confined  dispersal.  natural  tendency  among l i t t e r s i n peak  dispersal  test  populations.  I t i s d i f f i c u l t t o s e e why  shows  non-dispersing  little  peak  was h e r i t a b l e i n i n c r e a s i n g p o p u l a t i o n s  or d e c l i n i n g  this  in  Very l a r g e  necessary populations.  to  samples detect  8U  Chitty threat they  (1967)  behaviour  suggested  that  d o m i n a t e peak and  animals  declining  have been s e l e c t e d f o r s u r v i v a l  interference,  and  because  under c o n d i t i o n s of  mutual  change  i n d i v i d u a l s a l s o reduces t h e i r  fitness  the d r i v i n g this  Chitty  mechanism b e h i n d  change  hypothesized phase o f  has  a  that  assumes  peak  frequency  and  support  differentiated tendency  and  with  respect  was  no  aggressive  increases,  populations  The  behaviour  the high of  Litters  in  were  tendency  to  disperse.  behaviour  are  both h e r i t a b l e  provides  animals in  associated  was  a driving  the  Krebs  found  with  increase. of  voles i n  (1970)  were l e s s  also  aggressive  that  in  behaviour  selection  mechanism o f  If  mechanism  should  t h i s study  (1975)  change not  a  ones,  t h e g e n e t i c i n f l u e n c e upon a g g r e s s i v e genetic  in  results  hypothesis.  more a g g r e s s i v e .  Anderson  that  modified  c h a r a c t e r i z e d by  dispersal  indication  (1973)  population This  voles.  that  increasing  density  voles i n declining populations  i n peak p o p u l a t i o n s .  that  to  al.  increasing  p r o p o r t i o n of aggressive  populations being  townsendii,  in  change i s and  the  animals.  of t h i s  aggressive  declining  so  et  during  as  likely  i n v e r s e l y r e l a t e d , then  that  such  some a s p e c t s  there  low,  of  non-dispering  However,  than  dispersal  p a s s i v e v o l e s d i s p e r s e so  t o d i s p e r s e and  which the  found  Krebs  the  environmental  fluctuations,  e a r l y d e c l i n e phase a r e  peak I p o p u l a t i o n s ,  by  endure  basis.  that  of aggressive,  study  are  p r o p e r t i e s of  that t h i s behavioural  genetic  frequency  density-intolerant,  and  to  population  selective  the  hypothesis  this  i n the  g r o w t h b r i n g s a b o u t a change w i t h i n t h e  increases  late  suggested  their  populations  this  conditions.  that  selected for  M. was upon  population  85  regulation. The p r e s e n t explored  further.  evaluate high  study  suggests  Fencing  the importance  density.  peak  for  establish  and  L.  1975.  variability  of  the  Phenotypic  Hicrotus Zoology,  should  be  populations  will  utero  marking  and  enable  would direct  t e n d e n c y and l i f e t i m e .  cited  c o r r e l a t i o n s among  i n reproductive  vole  Department o f  in  of dispersal  Literature  that  i n population r e g u l a t i o n at  littermates  measurements o f h e r i t a b i l i t y  Anderson, J .  areas  microtine  of dispersal  Techniques  conclusively  some  performance  townsendii.  University  in  populations  Ph.  of  relatives  D.  British  Thesis Columbia,  V a n c o u v e r , BC. Anderson,  P.  K.  1970.  s m a l l mammals. Boonstra,  R.  Symp.  1977.  population  A  of  E c o l o g i c a l s t r u c t u r e and gene f l o w i n Zool.  fencing  Hicrotus  Soc.  Lond.  experiment  26: 299-325.  on  townsendii.  a  Can.  high-density J.  Zool.  55:  1166-1175. Boonstra,  R.  and C.  J.  Hicrotus townsendii. Chitty,  D.  1967. .  behaviour 2: Hilborn,  The  i n animal  Krebs. J.  1978.  Mammal.  natural  Pitfall  of  59: 136-148.  selection  populations.  trapping  Proc.  of s e l f - r e g u l a t o r y Ecol.  Soc.  Aust.  51-78. R.  siblings  1975.  Similarities  in  i n four species of voles  1221-1225.  dispersal  tendency  (Hicrotus).  among  Ecology,  56:  86  Howard,  W.  E.  longevity George Lab. ,  1949-  in a local  Reserve, Vert.  1960.  Innate  C.  J.  C.  and  B.  H.  rodents. Krebs,  C.  Taitt,  Science,  and  B.  H.,  and  C.  J.  pennsylvanicus  Pearson, by  and  H.  Behavioral  cycle  Ecology, Keller,  Population  M.  Tamarin.  1969.  changes  ochrogaster  L.  in  51:34-52.  Demographic  B.  J.  Hilborn.  Dispersal  Contrib.  and  50: J.  in M.  587-607. H.  cycles  Myers,  in  small  35-41.  , I. „ wingate,  Can.  B.  Indiana.  1973.  179:  the  individual  biology:  Ecology,  M.  Gaines,  J.  LeDuc, J .  1976.  in  Zool.  54:  1971.  Redfield,  M.  population  populations  of  79-95.  Genetic,  of d i s p e r s i n g f i e l d  Mierotus  A.  Mierotus  fluctuating  Krebs  reproductive attributes  44:  of  J.  townsendii.  Myers, J .  and  i n southern S.  on  152-161.  population  biology:  Tamarin.  biology: M.  the  Keller,  M.  63:  pennsylvanicus.  populations  J.,  Mich.  d i s p e r s a l of  population  with  population  pennsylvanicus C.  M.  L.  fluctuating  Nat.  Mierotus  and  J . , B.  Mierotus  Krebs,  environmental  associated  ochrogaster Krebs,  and  Oniv.  and  1-52.  Midi.  1970.  changes  Michigan.  43:  Am.  amount o f i n b r e e d i n g ,  p o p u l a t i o n of p r a i r i e deermice  Southern  Biol.  vertebrates. Krebs,  Dispersal,  ochrogaster.  b e h a v i o r a l , and voles  Mierotus  Ecol.  Monogr.  californicus  revealed  53-78. O.  P.  automatic  231-249.  1960.  Habits of Mierotus  photographic  recorders.  Ecol.  Monogr.  30:  87  SECTION 3-  S E L E C T I V I T Y OF  POPULATIONS OF  THE  AVIAN PREDATION IN  DECLINING  VOLE MICROTUS TOWNSENDII  Introduction The area  cause of  of  continuing  concerning and  the  Myers,  predation duration  cyclical  indicated  Pearson  during  declines  that  (1964,  Several  1977).  (Maher Other  on  study  was  whose  population  renders of  position  the  them  in  the  were  selective  for  population  declines i n voles.  the  was size  amplitude  to and  have  microtine  al. avian  and  1974, predators The  impact o f  avian  population.  social to  that  Maher 1970).  predators  the  study  predators  1955,  (Krebs  studies  et  townsendii  vulnerable  present  declines  i n f l u e n c e some  to a s s e s s  (1956) s u g g e s t e d t h a t  individuals  both  an  consensus  suggested  MacLean  (Thompson  a declining Hicrotus  Errington  objective  this  1971)  s t u d i e s suggest that  a c c o u n t f o r some d e c l i n e s objective of  no  additional  may  1967,  abundance i s  is  population  1966,  mammalian p r e d a t i o n  Fitzgerald  predators  there  influences  fluctuation.  declines  first  and  mechanisms u n d e r l y i n g  population  may  research,  1974).  of the  f l u c t u a t i o n s i n vole  are  selective  structure  in  for the  predation. ,  A  second  determine  if  avian  sex  of  prey  during  Methods The Canadian  study  area  Wildlife  was  p a r t o f a T i m o t h y hay  S e r v i c e on  field  R e i f e l I s l a n d i n the  owned by Fraser  the  River  88  delta  near  Vancouver,  British  Columbia.  townsend i i  populations  were  live-trapped  live-traps  and  traps  from  pitfall  T h e r e were two u n f e n c e d experimental of  grids  biweekly  the  (B and C ) .  sampling  weekly s a m p l i n g winter  control grids  from  of  with  the  population with  with  pitfall  25 trap  ear-tagged  with  reproductive All  were  located  late  fall  because winter area,  predators  February and e a r l y  little  regurgitate  1978 t o J u n e  between p e l l e t s  and  stream tags  poured  saved.  were  were  recorded.  processed.  indigestible  were c o l l e c t e d  food  once per  P e l l e t s deposited  searched  included  to the study  i n the intact,  predatory  for  over a fine-mesh  study  and n e a r b y  to distinguish  s p e c i e s because t h e r e  o v e r l a p among them.  hydroxide  the  area,  I made no a t t e m p t  o f water t o remove d i g e s t e d h a i r  were  Voles  of the p e l l e t s occurs during the  adjacent  barns.  size  s o a k e d i n 10% p o t a s s i u m were  1978.  Areas  of d i f f e r e n t  considerable  remains  with  and one  w i n t e r were assumed t o h a v e r e m a i n e d  1977).  trees  being  some  and t h e s e p e l l e t s  ditches, the f i e l d s  often  station.  released after  decomposition  (Boonstra,  coniferous  each  pattern  live-traps  c o n d i t i o n , and w e i g h t o f e a c h a n i m a l  components a s p e l l e t s , month from  at  During  aluminum e a r t a g s , a n d t h e t r a p l o c a t i o n , s e x ,  v o l e s were i m m e d i a t e l y Avian  Longworth  l i v e - t r a p s and  because of f l o o d i n g .  stations  Two  consisted  traps.  had 49 t r a p p i n g s t a t i o n s s e t i n a 7 x 7  pitfall  J u n e 1978.  (A and D) and two e n c l o s e d  c o u l d n o t be used  apart.  Longworth  1976 u n t i l  Each g r i d  feet  Mierotus  The t r a p p i n g t e c h n i q u e  spring t o f a l l  the p i t f a l l s  May  Four  24  to  The p e l l e t s 72  hours.  s c r e e n a n d washed and f e a t h e r s .  is were The  with a  A l l ear  89  Results  Vole  Abundance  The  v o l e d e n s i t i e s were low when t r a p p i n g began d u r i n g t h e  summer o f 1976 and r e m a i n e d low u n t i l populations  i n c r e a s e d i n numbers d u r i n g  owing t o d e l a y e d peak  of  capture  abundance  fall  equal  the  the  size  October  1977  occurred  declines  lasted  in  u n t i l June  of interest  d e c l i n e spanning  in  a  relative  A moderate s p r i n g  to l e v e l s  spring.  1977, b u t  approximately  Declines  starting  populations,  and  1978, when t h e s t u d y study  o f 1976-1977,  i n the spring of  a l l four  this  3 . 1 ) . The  and were a t  had r e c o v e r e d  of t h e  (Fig.  the winter  s p r i n g o f 1977.  occurred  populations  t o o r above those  period  in live-traps,  in  decline i n population by  the f a l l  was  this  terminated. last  an e i g h t - m o n t h p e r i o d f r o m O c t o b e r  in  these The  population 1977 t o J u n e  1978.  Incidence  The  Of  Predators  major  blue  heron  (Ardea  The  highest  avian  predators  herodius)  density  and  s e e n on t h e a r e a marsh  hawk  o f herons occurred  s e v e n h e r o n s were s i g h t e d on t h e t r a p p i n g predators  included  the  barn  (ftsip f l a m m e u s ) , g r e a t - h o r n e d (Nyctea  scandiaca),  and  were t h e g r e a t  (Circus  cyaneus).  i n December 1977, when grids.  Other  avian  o w l ( T y t o a l b a ) , s h o r t - e a r e d owl o w l (Bubo v i r g i n i a n u s ) ,  red-tailed  hawk  (Buteo  snowy  owl  lagogus).  90  F i g u r e 3.1. P o p u l a t i o n d e n s i t y o f M i e r o t u s t o w n s e n d i i d e t e r m i n e d by l i v e - t r a p s on c o n t r o l g r i d D d u r i n g 1976-78. B o t h sexes are combined. Non-breeding p e r i o d s a r e shaded.  Minimum Number Alive CA)  O O T  v  1—I  I I I II  O O T  c_  (D  CD  9  CD  2  0 8  2  1  92  Predators winter  were i n h i g h e s t  of  spring of other  1977 1978  and  one  heron  was  p r o p o r t i o n of the  predation believe  is that  predation  on  the  of  the  were  overlooked  absolute  pellets  known  may  and  in  By  the  the g r i d s  and  have  However,  amount i s predators  due  I  had  the  to predation  between  to  although  15%,  some  because  not  pellets  may  in  that  the  Table  of  the  have  been  I  may  area.  I searched,  pellets  that  a l l  far  relative 3.1  and  from  the  amount  even  if  of the  (Table 3.1).  Wore o f t h e  and  loss i n  i n females f o r each p e r i o d .  for  by  at least  predation 25%  of the  tended loss in a  The  to  be  single  predation.  impact of the  I missed c o l l e c t i n g  g r e a t e s t impact i n e a r l y winter  than  accounted  to  of  deposited  i n spring  of l o s s  due  to  the  areas  assume  proportion 10  s e v e r a l reasons  underestimated.  l e s s important  m a l e s was  to  intensity  some  the  i s accurately estimated  Avian  density  early  due  d i t c h e s near the study  pellets  predators  predation  The  1978.  v o l e s w h i c h was  There are  tagged p o p u l a t i o n s .  in water-filled  area.  tagged  underestimate  values  deposited  transient  l o s s of  these  areas  week was  and  sighted.  3.1.  roosting  Avian  o c c a s i o n a l l y s e e n on  shown i n T a b l e  unknown p r o p o r t i o n  were  fall  Impact  The  study  late  d e c l i n e d i n abundance t h r o u g h  d i u r n a l r a p t o r s were p e r i o d i c a l l y  Predator  have  abundance i n t h e  of the  vole  predators  had  avian  predators  populations the  (r=.99,  was  c o r r e l a t e d with  P<.02)  g r e a t e s t i m p a c t on  the  (Table highest  the  3.2). density  93  T a b l e 3- 1. Number o f t a g g e d v o l e s d i s a p p e a r i n g f r o m f o u r Mierotus townsendii populations and p e r c e n t a g e recovered from avian p r e d a t o r p e l l e t s . R e s u l t s from a l l grids were pooled i n each period. Males Number disappearing  Time Oct.-Nov. Dec.-Jan. Jan.-Feb. Mar.-Jun. Total  1977 1978 1978 1978  Females  %  recovery  Number disappearing  %  recovery  147 169 154 172  9.5 15.4 13.0 4.7  108 170 147 164  2.8 10.6 11.6 4. 3  642  10.6  589  7.6  94  Table 3.2. P e r c e n t a g e o f v o l e m o r t a l i t y a c c o u n t e d f o r by a v i a n p r e d a t o r s and a v e r a g e M. t o w n s e n d i i d e n s i t y f o r 1977-1978. Data f o r p e r i o d s i n T a b l e 3-1 a r e p o o l e d f o r e a c h grid. Both sexes a r e combined.  Grid D A C B  % mortality due t o p r e d a t i o n 6. 9 7.6 9.2 13. 3  Average v o l e density 109 111 121 134  95  vole  Sex  p o p u l a t i o n and  R a t i o Of  To from eaten  V o l e s E a t e n By  determine  the  predators  populations. determined  end  The  by  p r e d a t o r s was been  The  eaten  the  each  sample  (Table 3.3).  sex r a t i o  of  the  the r e s i d e n t  by  the s p r i n g  of the  of  sex  sample  resident  populations  was  s e x known t o  be  1977  until  the  o f t h e sample o f v o l e s e a t e n  including  a l l those animals  t h e same p e r i o d . was  t h e sex r a t i o by  ratio  one  The  total  known  by  (X =10.28,  P<.001).  to  162. (males/females)  was  higher  in  p r e d a t o r s t h a n i n the r e s i d e n t p o p u l a t i o n  difference  voles eaten  by  number o f  When t h e d a t a f o r a l l g r i d s were p o o l e d , t h e r e  a significant  2  of  from  been e a t e n  eaten  ratio  removed  minimum number o f e a c h  during  grid,  t h e sex  sex  ratio  determined  v o l e s known t o have For  the  trapping period  of the study.  have  compared  with sex  predation.  Predators  I  summing t h e  i n each  density-dependent  i f predators p r e f e r e n t i a l l y  populations,  by  alive  exhibited  between t h e s e x  p r e d a t o r s and  those  Hales  ratios  of the  were  o f t h e sample  resident  selectively  was of  populations  removed  by  avian  predators.  Body Weight  To I  Distributions  see i f p r e d a t o r s s e l e c t e d  grouped  the  animals  Males g r e a t e r than large;  into  57 g and  o t h e r s were s m a l l .  v o l e s a c c o r d i n g t o body  'small*  females I used  and  'large'  greater  these weights  than  size,  f o r each 45  so that  g when  sex. were the  96  Table 3.3. Sex ratios (males:females) of e a t e n and r e s i d e n t Mierotus townsendii populations. Sample sizes are in p a r e n t h e s e s . P e r i o d i s f r o m s p r i n g 1977 t o s p r i n g 1978.  Eaten Grid  k  by  predators  Males:Females  Besident Males:Females  B C D  1.33 2.40 1.33 1.00  (35) (51) (42) (34)  0.72 1. 10 0.83 0.91  (7857) (10962) (9862) (8644)  Total  1.49  (162)  0.90  (37325)  97  sexes the  were  • s m a l l " and  predators, size  I  were a p p r o x i m a t e l y  »large' c a t e g o r i e s . used  their  last  3.4  frequently resident the  indicates  in  the.  that  samples  the  voles  known w e i g h t t o a s s i g n  populations  numbers i n eaten  by  them t o  life,  measured  higher  i n females eaten  smaller  and  Probability  a  I vole  This  October  would be  august.  Table  by  weeks) t h a n i n  is  increased there  a  few  will  predators  of being  was  assumed  smaller  indicates that  was  from  However, b e c a u s e t h e  by  predator  week o f f i r s t  For example, those  y o u n g e r and  3.5  the  capture  animals.  eaten  last  males  s e l e c t i o n of  males.  i t s probability  that  1977  (21.2  c o n s i s t e n t with  suggests  summer o f  whereas  was  analysis.  only  predators,  capture,  age  be  to  and  Seek o f f i r s t  may  males tended  t i m e between f i r s t  predators.  predators  the  of  whether o r n o t  related to  in this  in  duration  predators  The  than  Predation  examined was  the  avian  more  average  presumably younger  Of  Smaller  likely.  occurred  predators  s e l e c t e d by  least  weeks).  by  2  f e m a l e s were t h e  (18.0  animals  (X =4.45, P<-05).  a n i m a l s most l i k e l y  as  smaller eaten  larger  by  For  equal  category. Table  be  combined, t h e r e  capture  s e l e c t e d by t o be  first  probability  spring  to  fall.  s e l e c t i o n f o r y o u n g e r and lifespan  of  months, v o l e s b o r n  n e c e s s a r i l y have a because  many  an  i n the lower  were  caught  of in  caught  of being  a  avian  a measure  voles f i r s t  than those  the  of  in  eaten  This  result  thus  smaller  individual  vole  s p r i n g and  early  chance  already  of  being  dead when  the  98  Table 3.4. Number o f l a r g e and s m a l l male and f e m a l e M i e r o t u s t o w n s e n d i i i n e a t e n and r e s i d e n t p o p u l a t i o n s f r o m f a l l 1977 to spring 1978. Eaten  by p r e d a t o r s  >58g  <46g  Grid  <58g  a B c D  5 23 13 8  7 5 4 4  6 7 8 3  Total  49  20  24  Females  Males  Females  Males  Besident  <58g  >58g  <46g  >46g  7 4 4 7  364 608 546 460  283 364 153 306  271 264 531 213  560 360 330 478  22  1978  1106  1279  1728  >46g  99  Table 3-5. Probability o f an i n d i v i d u a l Mierotus b e i n g k i l l e d by a v i a n p r e d a t o r s i n r e l a t i o n t o week c a p t u r e i n 1977. A l l g r i d s and b o t h s e x e s a r e p o o l e d .  Month  New animals  May June July Aug. Sept. Oct.  624 1022 639 189 75 46  Number eaten 19 58 34 14 7 7  Probability of p r e d a t i o n 0.030 0.057 0.053 0.074 0.093 0. 152  townsendii of first  100  density  of predators  increased  i n the  fall.  Discussion Selective of  the  predation  vole  population,  season.,  This  predation  of Microtus  similar avian  for  frequently  a l s o may avian  by  sample Thus  have been s e l e c t i o n o f Thompson  preyed  upon  M.  male brown  barn  owls  was  but  (1968) r e p o r t e d  no  f o r Apodemus o r C l e t h r i o n o m y s .  study  o f Pomarine J a e g e r  brown  lemmings  resident  in  population  killed  both  sexes  P<.25),  but they  Unfortunately, populations,  equally  preferred  and  thus  did the  (1977) a r e  the  males  by  i n 1956 males  (Lemmus  sibirieus)  (cited  in  Boonstra  californicus  Southern  and Lowe  either  sex  (1970) r e p o r t e d predation  not  showed  1960  census  actual  that  (Maher's T a b l e in  sex  the  the  snowy o w l s  owls.  Maher  there  that  Microtus  Maher  more  resident  Assuming a 50:50 s e x r a t i o  o f lemmings,  Maher  in  ( S t e r e g r a r i u s pomarinus)  Alaska.  sex.  males o c c u r  reported  Stendell  that  either  selection for  owls  In a  i n Boonstra's study  not by s h o r t - e a r e d was  predators.  then  than  s e l e c t i o n f o r male  that there  preferential  for  lemmings  e a r l y , b u t n o t l a t e summer. found t h e r e  was  townsendii  (1955)  nonbreeding  4 of Boonstra  considered,  avian  structure  (1977) r e p o r t e d  preference  P<.01).  2  Boonstra  sex the  there  i n Table  periods  (X =7.32,  selectively during  no  presented  the three  predators.  that  townsendii,  i n the tagged  population  during  t o w n d e n d i i m a l e s by a v i a n  exhibited  i f t h e data  t h e age and  especially  indicates  on M.  predators  pooled  1977)  study  study  However,  would a l t e r  7)  by on a on  i n the  predators (X =2.30, 2  (X =5.28,  P<.05).  resident  lemming  2  ratio  i s  unknown.  101  Generally  there  microtine  tends  populations  t o be  (Myers and  a p p a r e n t s e l e c t i o n (50% it  is  difficult  movement by  of  populations This  been that were tie,  M.  avian  resultant  may  also often  was  mean w e i g h t s o f  c o n s i s t e n t l y lower This  no  and  However,  5 below).  M.  predators, without  be  due  greater  exposure  to  may  to  predators  account  was  for  resident  Boonstra  d i f f e r e n c e i n body  those  tagged  data  the  tagged sample  than  those  of  that  P=0.03).  (1955)  owls took a h i g h e r  known  i n Boonstra's Table  test,  5 indicate  by  was  predators  found  result  of  selection  in  the  be  larger  animals,  s e l e c t i n g smaller animals,  s a m p l e d by there  was  the no  only  traps. size  ftpodemus s y l v a t i c u s  valid  by  of  traps  predators  i f b o t h c l a s s e s of  S o u t h e r n and selection  populations  on by  Lowe  for  did  his  selecting or  both.  versus  those  v o l e s have  been  reported  that  (1968)  Clethrlgngmy.s o w l s a t any  (1  summer,  young than  However, t h i s c o u l d  are  predators  (one-tailed sign  that/  proportion  S i z e c o m p a r i s o n s between v o l e s k i l l e d  have  the c o n t r o l p o p u l a t i o n there  by  a  eaten  of  between to  traps.  killed  vole  (1977)  weights  voles  the  predators  the  s e l e c t i v e predation  townsendii.  a n i m a l s i n t h e sample e a t e n  not  no  1971).  indicates  Lockie  by  characterizes  light  short-eared  that despite  certainty  partially  i n d i c a t e s that there  there  with  predators  the  Krebs,  townsendii  eaten. the  conclusions  presumably younger  that  1971), so  population.  which  (Myers and  and  resident  It  males  study  reported  and  1955).  deficiency  prey  males by  males  (Thompson,  smaller  of  Krebs  males) i n a sample e a t e n  t o draw any  more k n o w l e d g e o f t h e Selection  a d e f i c i e n c y of males i n r e s i d e n t  glareolus  time of the  or  year.  102  Errington prey  (1956) s u g g e s t e d  predators  upon i n d i v i d u a l s f r o m a p o p u l a t i o n  individuals  whose  to predation. subordinate smaller  In  vole populations,  (Turner  and  vulnerable  and  hawk  whereas a c a t removed  mammalian  hispidus) predators  their  social  cats  s e l e c t e d tagged  i n s t e a d remove them  to  Wolfe  removed  of  the  more  (1974) r e p o r t e d  that  subordinate  Pearson  be  supports  i n d i v i d u a l s are  population.  position.  vulnerable  individuals  individuals  able  more  those  S e l e c t i v e removal  dominant  are  randomly  1973).  and  selectively  not  a n i m a l s tend  that subordinate fioberts  do  smaller  subordinate  predation.  red-tailed  (Sigmodon  Iverson,  presumably  to  but  s o c i a l p o s i t i o n renders  Errington's hypothesis  a  that  in  a  (1966)  animals,  cotton  rat  suggested  that  t o remove i n d i v i d u a l s r e g a r d l e s s  Christian  Microtus  (1975)  found  pennsylvanicus  that  of  domestic  a t random from  the  population. Mammalian and  duration  1971,  predators o f low  MacLean  predators presumed  et  lack to  even  vole d e n s i t i e s . inverse  1974,  mobility heavy  and  v o l e numbers, positive  when  avian  Mammalian  thereby  and  avian  Fitzgerald of  avian  avian  no  predators  1977).  have l e f t  extended  predators impact  on  and  owing t o  low  cause  declining  vole  periods study  of  low  showed  declining  n a t u r a l mammalian p r e d a t o r s appeared to  thus  to  in this the  are  microtine  presumed in  1966,  Mammalian  declining  are  mortality  fluctuation  (Pearson  predators  on  predators  account f o r  whereas t h e  There are  microtine cycle  predators  density-dependent  populations.  r e s p o n s i b l e f o r the  predation  density-dependent  populations,  Island,  the  be  numbers i n a al.  exert  populations  may  on  a  vole Seifel  have g r e a t e s t e f f e c t  at  103  high  vole  accounted density to  population f o r by  low. of  The  areas  declined  m o b i l i t y of  when t h e  prey  these  population  significant  predation  (Boonstra,  i s not  simply  Rather, i t  one  microtine The  of  i n the  1977),  be  a  sufficient  them  became absence  and  thus  microtine  condition,  several contributing factors, resulting  major  difficulty  of  voles  unknown u n l e s s predators  100%  are  this  study,  the  absolute scats  and  in  hypothesis  that  population  pellets  total and  rate.  o f an  predation  microtine  or  i n some  Unless  nearly  descriptive  experimental  d e c l i n e s , one  a predator-proof  from  design  could  a l s o t e s t the  that  mammalian  abundance a f t e r  reaching  hypothesis  a decline.  for  not of  100%  o f the  or  of  intensifies extended  to  To  in periods  in  be  test  the  sufficient  to  a  microtine area  prevent  densities.  Pearson  the  pellets  should  approach.  high  by  is  determining  studies  in order  on  satisfied  w i t h i n w h i c h an  abnormally  predation accounts  produced  could enclose  fence,  study  of predation  method  i s e i t h e r necessary  populations  and  impact  scats  to d i s p e r s e i n t o i s provided  populations,  descriptive  T h i s c o n d i t i o n was  recovered,  favour  with  the  a  i s no i n d e p e n d e n t  predation be  doing  that  of the  there  can  for  is  in  recovered.  abandoned  voles  allowed  densities  occur  loss  population  component o f d e c l i n e s i n  may  of  declines.  predation  account  a necessary  vole  predators  d e c l i n e can  any  proportion  as  population  populations.  the  predation  High r a t e s of  predation  and  avian  declined.  move t o new  d e n s i t i e s , because the  (1966, declining of  low  for the This 1971) vole vole  104  Literature  Boonstra,  R.  1977.  populations: 55:  cited  Predation  impact  on  Mierotus  and  vulnerability.  1975.  Vulnerability  townsendii  Can.  J.  Zool.  1631-1643.  Christian,  D.  Mierotus Amer.  P.  pennsylvanicus,  Mid.  Errington,  P.  Hat. L.  populations., Fitzgerald,  B.  California. K r e b s , C.  J.  small HacLean,  Science,  of  the  mammals.  1974.  H.  domestic  Population  M.  on  (Mierotus  cats.  vertebrate  fies.  cyclic  montanus) i n  Population  in  cycles  in  8: 267-399.  F i t z g e r a l d , a n d F.  predation  a  46: 367-397. 1974.  Ecol.  cycles  and  higher  predation  vole  Myers.  Adv.  limiting  Weasel  montane  F . , J r . , B.  reproduction  by  voles,  124: 304-307.  Anim., E c o l .  and J .  S.  Factors  1977.  J.  predation  meadow  93: 498-502.  1956.  M.  population  to  of  Arctic  by w e a s e l s .  A-  Pitelka.  lemmings: Arct.  winter  A l p . Res.  6: 1-12. Maher,  W.  J.  1967.  Predation  by  weasels  population  of  lemmings.  Banks  Territories.  Can.  Field-Nat.  8 1 : 248-250.  ..,  1970.  in  northern  Myers, J .  H.  enclosed Nat. P e a r s o n , 0.  The p o m a r i n e j a e g e r Alaska.  and C.  Wilson  J.  Krebs.  vole populations:  Island,  a s a brown  Bull.  on  lemming  a  winter Northwest  predator  82: 130-157.  1971.  Sex r a t i o s i n open and  demographic i m p l i c a t i o n s .  Amer.  105: 325-344. P.  1964.  Carnivore-mouse p r e d a t i o n :  an example o f  105  its  intensity 1966.  and b i o e n e r g e t i c s . The  mouse a b u n d a n c e . 1971.  prey J.  of  Anim.  Additional  Roberts,  W.  and J .  susceptibility  c a r n i v o r e s d u r i n g one c y c l e o f Ecol.  35: 217-233.  voles  of  the  impact  of  (Mierotus c a l i f o r n i c u s ) .  J.  L.  to  Wolfe.  1974.  predation  S o c i a l i n f l u e n c e s on  i n cotton rats.  J.  Mammal.  869-872.  Southern,  H.  N.  and 7.  P.  W.  Lose.  distribution  of  prey  and  territories.  J.  Anim.  Ecol.  Thompson, North Turner,  4 5 : 177-188.  52: 41-49.  M.  55:  Mammal.  measurements  c a r n i v o r e s on C a l i f o r n i a Mammal.  J.  B.  D.  Q.  Am., N.  aggression relation  1955.  Ecology  predation  a n d S.  Iverson.  male  in  o f t h e lemmings.  Proj.  in  The p a t t e r n o f  Mierotus  tawny  owl  3 7 : 75-97.  F i n a l Rep. L.  1968.  ONR-133. 1973.  Arct. , Inst.  63p. The a n n u a l  pennsylvanicus,  t o p o p u l a t i o n parameters.  Ecology,  cycle of and  i t s  54: 967-981.  106  SECTION  4.  SURVIVAL IN  FLUCTUATING POPULATIONS OF  THE  VOLE  MICROTUS TOWNSENDII  Introduction Microtine  rodent  populations  f l u c t u a t e i n abundance,  changes i n s u r v i v a l r a t e s i n f l u e n c e p o p u l a t i o n (Krebs in  and  Myers,  1974).  declining populations  juvenile  population  are  difficult be  change  to  trappability, juveniles  and  (Boonstra objective  survival  of  population  ( K r e b s and  this  Krebs et of the  Myers,  may  vary  and  Krebs,  of  is  and  voles  al.  low  1969),  and  subsequent  rate  Juvenile  voles  1974). an  is  index  of  survival  ( K r e b s and  affected  by  DeLong, juvenile  owing t o c h a n g e s i n t r a p p a b i l i t y o f 1978).  t h i s s t u d y was  juvenile, cycle,  index  subadult, to  to and  determine  technique i n f l u e n c e s observed of  1955;  in  juvenile voles  determine j u v e n i l e s u r v i v a l  However,  The  (Godfrey  t o t r a p i n l i v e - t r a p s , but  used  1965).  s u r v i v a l of  s u r v i v a l i s a major d e t e r m i n a n t  of  can  The  growth  and  measure d i r e c t l y adult  voles  whether  during  o r not  minimum s u r v i v a l o f  a  minimum a  trapping  size  class  voles.  Methods The Canadian delta  study  Wildlife  near  pratense  area  was  Service  Vancouver, was  the  part  o f a T i m o t h y hay on  Reifel  British  dominant  Island  Columbia.  species,  field i n the  owned by Fraser  Although  Agrostis  alba,  the  River Phleum Lolium  107  perenne,  and H o l c u s  Four Mierotus May a  1976 u n t i l  l a n a t u s were a l s o townsendii  June  vole-proof  1978.  fence,  controls.  Each g r i d  apart  a  in  Longworth  live  described with on  7  x  present.  p o p u l a t i o n s were l i v e - t r a p p e d f r o m  Populations  and  B and C were e n c l o s e d  populations  A  and  D were  had 49 t r a p s t a t i o n s s e t 7.6 7  pattern.  Each  t r a p s and one p i t f a l l  by B o o n s t r a  and K r e b s  m  trap similar  (1978).  checked  and l o c k e d open on Wednesday  Pitfall September  t r a p s were used f r o m  1977,  between t h e s e winter  Tuesday  and  April  to  flooding. Wednesday  Thursday  afternoon.  week, t h e p i t f a l l s  afternoon  to  June  types  were s e t a f t e r  the  type  baited  second  week  and a f t e r n o o n ,  to October 1978.  1976, A p r i l  and  to  In the i n t e r v a l s  c o u l d n o t be used  owing  week on Wednesday  and T h u r s d a y  When b o t h  feet)  morning.  T h e y were s e t e v e r y  checked  (25  The l i v e - s t r a p s ,  morning  May  p e r i o d s , the p i t f a l l s  unfenced  s t a t i o n c o n s i s t e d o f two  o a t s and s u p p l i e d w i t h c o t t o n , were s e t e v e r y Monday a f t e r n o o n , c h e c k e d  by  to  morning,  m o r n i n g , and c l o s e d on  o f t r a p s were  the l i v e - t r a p s  set  in  had been  one  locked  open. All  v o l e s were e a r - t a g g e d ,  location, recorded  sex,  on e a c h c a p t u r e .  processing  and  the  total  Voles  capture  wounding  were  were r e l e a s e d i m m e d i a t e l y  after  as f o l l o w s : adult>42g;  enumeration technique  used  and  weight,  subadult  <30g.  determination  trappability  condition,  were c l a s s i f i e d  30-42g; j u v e n i l e The  reproductive  and t a g number,  of was:  demographic  o f Krebs data.  (1966) was The  used i n  estimate  of  108  number o f c a p t u r e s f o r an a n i m a l Trappability  =  number o f p o s s i b l e c a p t u r e s f o r t h a t  animal  N where  N  i s  trappability times  the  number  i s summed  of  v o l e s caught  over a l l N animals.  o f capture are excluded  must be c a u g h t  a t these  more t h a n t w i c e , and The f i r s t  and  i n t h e summation, b e c a u s e  last  animals  times.  Results  /  / Trappability  The  trappability  calculated  estimates of live-traps  independently  for  v o l e s known t o be p r e s e n t f r o m in  live-traps,  those  each type of t r a p .  by  in  live-traps.  females  in live-traps  summer  1976 u n t i l  spring  1978.  trappability  and r e l a t i v e l y  of  I n t h e 1976 summer  animals.  Low  male t r a p p a b i l i t y to  densities  i n the spring  t o 95% f o r 96 a n i m a l s  of  m a l e s and  constant  from  trappability  was 6 1 % f o r 324 a n i m a l s , and i n t h e 1977 summer i t was 1692  but not  estimates  Trappabilities  were s i m i l a r  were  F o r example,  capture i n p i t f a l l traps,  were n o t i n c l u d e d  captured  and p i t f a l l s  65% f o r  o f 1978 i n c r e a s e d  and f e m a l e  trappability  8 1 % f o r 155 a n i m a l s . Hale  and  Trappability animals  and  Trappability  female  trappabilities  in pitfalls in  the  in pitfalls  d u r i n g summer  1977  summer  c o u l d n o t be c a l c u l a t e d  was  1976  was  28%  f o r spring  were  similar.  35%  for  727  f o r 1726 a n i m a l s . 1978 b e c a u s e  no  109  animal  was  one-half lower in  captured  more t h a n  as e f f e c t i v e  as l i v e - t r a p s  trappability  The 1976,  were o n l y  i n recapturing  i n d i c a t e s that voles avoid  about  voles.  This  repeated  capture  Density  vole populations  and  the  during  the  capture  in live-traps,  1977.  vole than  fall  populations those peak  October  had  of the s p r i n g .  particularly opportunity  severe  1976-1977,  owing t o d e l a y e d  increased  i n the spring  of  attributable  to  largely  section),  but  to l e v e l s equal  by  fall  June  1978,  characterized  when  t h e 1978 b r e e d i n g  (see d e c l i n e s e c t i o n )  t o compare j u v e n i l e s u r v i v a l  and p r o v i d e s with  the  to or greater  Declines i n a l l four populations  The r a t e o f d e c l i n e d u r i n g  May  sizes  The summer o f 1977 was  until  in  Population  declines,  recovered  began  by l i v e - t r a p s r e m a i n e d low  and d e n s i t i e s were h i g h  1977 and p e r s i s t e d  peak  of  (see d i s p e r s a l  densities.  ended.  4.1).  winter  spring  occurred  sampled  (Fig.  and  Moderate  dispersal,  were low when t r a p p i n g  population  t h r o u g h o u t t h e summer  and  Pitfalls  pitfalls.  Population  by  twice.  began i n  the  study  season an  was  ideal  that i n increasing  populations.  Juvenile Survival  The  estimate  of juvenile survival  numbers o f j u v e n i l e s were e n u m e r a t e d pitfalls.  Survival  rates  in  i s direct either  because l a r g e live-traps  a r e b a s e d on t h e combined  or  live-trap  110  Figure 4.1. Population density determined from l i v e - t r a p s o f M i e r o t u s t o w n s e n d i i on f e n c e d g r i d C d u r i n g 1976-78. B o t h sexes are combined. Non-breeding p e r i o d s a r e shaded.  112  and  pitfall  decreased density,  during  than  (X =6.87,  average,  (X =10.20,  juveniles  than  Juveniles  may h a v e s u r v i v e d  in  the g r i d  effect  of  1978;  adults  fiedfield  peak  total  size adult  The a v e r a g e a d u l t  density  (X =2.81, upon  was o b t a i n e d  class  during  density  for  males  The  a  (1976 r=-.34,  year, populations not  period  was  of adults.  of the year.  and  juvenile  populations  with higher  within  density  r=-.37, 1977 r=-.22, 1978  survival  did  lower  Bowever,  whereas j u v e n i l e s u r v i v a l and a d u l t  (r=.48,  one  year  1978 r=-.04) o r  r=-.41).  females  There  and j u v e n i l e  within  d e n s i t i e s of e i t h e r adult  significantly  and  obtained  (r=.43, n=12, P>.10) o r f e m a l e s  1977 r=-.49,  have  t h e number o f  by t h e p r o p o r t i o n  no r e l a t i o n s h i p between mean a d u l t  females  of  c a p t u r e s f o r each sex.  the breeding portion  (1976  was  proportion  breeding  revealed  males  juveniles  and j u v e n i l e s u r v i v a l  The c o m p a r i s o n  in  and  survival  f o r each sex i n a p e r i o d  of a l l four  B  c o r r e l a t i o n s have been  P>.10).  survival  declines  populations  by summing  was no c o r r e l a t i o n between a d u l t d e n s i t y  better,  1978 s p r i n g  1978).  c a p t u r e s by t o t a l  D a t a were summed d u r i n g  years  summer o f 1977  juvenile  inverse  a l .  on  P<. 10) .  2  m u l t i p l y i n g the average d e n s i t y  among  better,  summer s u r v i v e d  female density  et  the population each  i n t h e low  survived  i n the enclosed  because s i g n i f i c a n t  captures o f  by  1976  survival  b e t t e r i n t h e summer o f 1976 t h a n  between mean a d u l t  in  dividing  the  h and D p o p u l a t i o n s  investigated  adults  in  juvenile  Juveniles  juveniles i n the severe  C  (Boonstra  cycle.  phase summer o f  P<.01).  2  reported  population  that  P<.01), and j u v e n i l e s i n t h i s  2  The  T a b l e 4.1 shows  a  increasing  average,  on  captures.  one  males o r  juvenile survival. density  were  not  113  T a b l e 4.1. Minimum s u r v i v a l r a t e s p e r 14-day p e r i o d f o r j u v e n i l e Microtus townsendii in combined live-trap and pitfall p o p u l a t i o n . A l l p o p u l a t i o n s and both sexes a r e pooled f o r each i n t e r v a l . Sample s i z e s a r e i n p a r e n t h e s e s .  A  Grid Summer  1976  Summer 1977 Spring  1978  0.58 (113) 0.58 (398) 0.25 (4)  B 0.70 (380) 0.56 (668) 0.25 (6)  C 0.66 (319) 0.63 (695) 0. 00 (2)  D 0.44 (142) 0.50 (425) 0.25 (8)  Total 0.63 (954) 0.58 (2186) 0.20 (20)  114  related,  there  was  t h e p o p u l a t i o n and n=12,  establish  been  lower  difficult  untrappable,  periods  apart. 1978),  than  was  and  pitfall  (r=-.93,  adult but  P>.10).  males  this  had  does  not  because preweanlings  in  g.  rate  animal  study,  the  weaning  has  was  estimated.  in  virtually  live-traps.  caught  total  capture  five of  two  estimate  The  breeding  be  lactating  weeks o r  be  born  of both  males and  converted  1969).  Krebs, each  calculated.  Krebs,  of  B during  The  females  t o age  Preweanling  F o r e x a m p l e , on g r i d  more  in  by  less  the  use  1978), and  the  5 weeks o l d a t f i r s t weeks  minimum  ( B o o n s t r a and  can  t o be  to  to  young  ( B o o n s t r a and  of l a r g e  three  season  to occur a t three Conway,  the  lactating  number  breeding  able  assumed  i s approximately  estimated  and  are  manner.  was  T h i s w e i g h t was  assumed  was  following  female  eguations  was  I  A  weight a t f i r s t 24  caught  d u r i n g each of  the  each  are  females  size  and  t r a p s permitted the capture  in  i f she  thus  Richmond be  males (r=-.46,  of  survival,  juveniles  known.  Litter  Weaning was  then  estimate few  litters  growth  1941;  juvenile  survival  4 0 g was  average  proportions  juveniles i n this  population average  to  lactating  different  females  r a t e o f v o l e s between b i r t h  and  preweanling number o f  for  adult  relationship.  because  numbers o f  for  Survival  survival  However,  not  higher  a causal  Preweanling  The  but  with  significantly  between p r o p o r t i o n o f a d u l t s i n  juvenile survival  P<.0001),  Populations  of  a relation  age  capture. (Hamilton,  survival  could  summer  1976,  115  juvenile 343  minimum  survival  post-weanlings.  was 0.70 p e r two weeks, and I c a p t u r e d  The  maximum  number  of  weaning c a n be e s t i m a t e d a s 343/0.70 = 490. young p r o d u c e d Preweanling  by 141 l a c t a t i n g  survival  Preweanling subject on  a  grid  reached  were  40 g.  summer,  so  lactating of  rates  to error f o rseveral  reported  reasons.  females born  a l l 24  g  voles  was  two-week  2  in  this  3  study  or p i t f a l l s  were  underestimated.  five  survival  the  4.1.  may be  in  increasing In  I  assumed  that  for  each  specific  c o m b i n a t i o n , t h e minimum  Figures  4.2,  curves f o r preweanlings  survival  survival  rate of  4.3, 4.4, a n d 4.5 show t h e and p o s t - w e a n l i n g s  t h r e e breeding seasons.  Preweanling  was  i na l l  Several trends  highest  in  the  1976  p o p u l a t i o n s and l o w e s t i n t h e 1977 peak p o p u l a t i o n s .  1976, i t was  also  unenclosed  populations.  the  decline  1978  I  of t r a p p a b l e j u v e n i l e s  post-weanlings.  are apparent.  the  error,  all  over  number  although  o f t h e t r a p p a b l e j u v e n i l e s was t h e minimum s u r v i v a l  populations  the  weeks o l d . , Some  rate  four  born  before they  throughout  However,  survival rates  and b r e e d i n g s e a s o n  survivorship  were  estimates are accurate..  minimum  a r e shown i n T a b l e  a s 705.  (490/705) / .  may have a v o i d e d c a p t u r e , s o t h a t  assume t h a t t h e r e l a t i v e  population  number o f  Hot a l l o f t h e v o l e s  i n live-traps  absolute estimates of preweanling  The  The t o t a l  Growth r a t e s o f young v o l e s v a r y not  young  caught  a l i v e at  f e m a l e s c a n be e s t i m a t e d  p e r two weeks was t h e n  survival  voles  than  higher  the  Preweanling in  calculated  f o r t h e 1978 g r i d  survival  was  zero.  in  the  enclosed  survival  1977  peak,  survival  i n the  was g r e a t e r d u r i n g b u t c o u l d n o t be  C p o p u l a t i o n because  Preweanling  than  was  post-weanling higher  than  116  F i g u r e 4.2. S u r v i v o r s h i p c u r v e s f o r young M i e r o t u s t o w n s e n d i i on c o n t r o l g r i d A between b i r t h and r e c r u i t m e n t . The number born i n each year was e s t i m a t e d f r o m t h e number o f l a c t a t i n g f e m a l e s ; t h e number o f r e c r u i t s was known.  117  3000 r  0.57  1000h 0.58  500  1977  (D > 0.58  CD £  1976 100  50  \0.25 \  \  \  \ \ D  Weaning  10 0 1  i  Age, Weeks  1978  ^ A v e r a g e Age at R e c r u i t m e n t  118  F i g u r e 4.3. S u r v i v o r s h i p c u r v e s f o r young M i e r o t u s t o w n s e n d i i on f e n c e d g r i d B between b i r t h and r e c r u i t m e n t . The number born in each year was e s t i m a t e d from t h e number o f l a c t a t i n g f e m a l e s ; t h e number o f r e c r u i t s was known.  119  3000 — 0 . 6 8  <D £ Z  100 -  at Recruitment  A g e ; Weeks  120  F i g u r e 4.4. S u r v i v o r s h i p c u r v e s f o r young M i e r o t u s t o w n s e n d i i o n f e n c e d g r i d C between b i r t h and r e c r u i t m e n t . The number born in each, year was e s t i m a t e d from t h e number o f l a c t a t i n g f e m a l e s ; t h e number o f r e c r u i t s was known. No recruits were c a u g h t in 1978.  30001 0.63  1000  -  0.63  1977  0.92  500^  0.66  1976  100  50  A v e r a g e Age at Recruitment  Weaning  10' 0  J  1  L  Age, Weeks  J  i  122  F i g u r e 4.5. S u r v i v o r s h i p c u r v e s f o r young M i e r o t u s t o w n s e n d i i on c o n t r o l g r i d D between b i r t h and r e c r u i t m e n t . The number b o r n i n each year was e s t i m a t e d from t h e number o f l a c t a t i n g f e m a l e s ; t h e number o f r e c r u i t s was known.  123  3000  1000  500 r  t> 1977  > < 1976  <D _Q  E  13  100 h 0.68  50  \  \  0.25 \  \  Weaning  10. 0  2  3  Age, Weeks  4  \  \  1978 -'-•Average A g e 5 at Recruitment  124  post-weanling also  less  survival  variable.  Survival  weaning, presumably  Adult  age  group  growth  can  be  an  ( K r e b s and M y e r s ,  Values  the t r a p p a b i l i t y will  o f t h e young v o l e s  important 1974).  decreased  was after  determinant  L o s s from both  o f animals.  of  a specific  f o r death  f o r s u r v i v a l rates are also  and  i n f l u e n c e d by  An age g r o u p w i t h low t r a p p a b i l i t y  t e n d t o have a p p a r e n t l y low s u r v i v a l . , Minimum s u r v i v a l r a t e s o f d i f f e r e n t  Longworth the  interval  end  of t h e s p r i n g  and  the non-breeding  season.  between t h e s t a r t decline,  Survival  declined  drastically  equally  with  better  subadults  season,  in the  than  Adults  and  the onset  larger females  2  survived  as  defined  between  the  survived  summer,  breeding  spring  non-breeding  in  and 1978.  (X =15.68, P<.001) 2  Juvenile  juveniles  (X =59.76,  animals  1977  of  2  females  winter  peak  (X =15.68, P<.001) and i n f e m a l e s b e t t e r than  S p r i n g was  by  o f b r e e d i n g i n March and t h e  males as a d u l t s  2  caught  were h i g h i n i n c r e a s i n g p o p u l a t i o n s ,  (X =6.67, P<-01). well.  survived  groups  summer a s t h e i n t e r v a l  values  moderately  survived  age  l i v e - t r a p s a r e shown i n T a b l e 4.2.  as  in  and  the nest.  o f a n i m a l s i n c l u d e s a component  emigration.  seasons  Survival  survival  population  breeding  at the time they l e f t  And S u b a d u l t  Adult  i n a l l three  males and f e m a l e s  decreased Females and  as  survived  b e t t e r than s u b a d u l t s i n males (X =35.96, P<. 0 0 1 ) . 2  i n males  P<.001).  (X =22.50, 2  These  survived  better  than  better  than males.  results  smaller  Subadults  P<-001)  and  indicate  that  ones  Subadult s u r v i v a l ,  and  that  compared  125  T a b l e 4.2. Minimum s u r v i v a l r a t e s p e r 14-day p e r i o d f o r M i c r o t u s townsendii c a u g h t by L o n g w o r t h l i v e - t r a p s . A l l p o p u l a t i o n s were p o o l e d f o r e a c h i n t e r v a l . Sample s i z e s a r e i n p a r e n t h e s e s . Females  Males Adults  Subadults  Juveniles  Adults  Subadults  Summer 1976 Winter 1976 Spring 1977 Summer 1977 Winter 1977 Spring 1978  0.70 (228) 0.92 (1727) 0.78 ( 1502) 0.82 (1129) 0.82 (2551) 0.48 (141)  0.56 (48) 0.93 (201) 0.42 (19) 0.76 (409) 0.70 (142) 0.25 (4)  0.47 (34) 0.83 (12) 0.08 (25) 0.73 (108)  (3)  0.86 (468) 0.93 (1590) 0.91 (1638) 0.84 (1637) 0.82 (1868) 0.66 (339)  0. 67 (115) 0. 93 (446) 0. 85 (157) 0.75 (426) 0.81 : (663) 0.60 (109)  Total  0.82 (7278)  0.76 (823)  0.59 (182)  0.86 (7540)  0.81 (1916)  loT 0.33  Juveniles 0.47 (43) 0. 76 (25) 0.22 (18) 0.63 (142) 1.00 (1) 0.60 (5) 0.59 (234)  126  with t h a t o f a d u l t s , dropped diminished  survival  dispersal  of  in  smaller  s h a r p l y i n t h e s p r i n g o f 1977.  the  subadult  animals  at  group  this  was  time  The  a result  (see  of  dispersal  section) . Hinimum than  s u r v i v a l o f m a l e s was  during  subadults  1977 w i n t e r  (X =43.84, 2  survival 1977  the  was a l s o  winter  (X =30.54,  i n adults  P<.0001)  in  adults  Male and female  same d u r i n g t h e w i n t e r s , b u t f e m a l e spring  breeding  (X =108.60, 2  pitfalls caught  season  The  survived  better  subadults  (X =4.03,  survived  equally  survived than  than  juvenile P<.01). better  whereas However,  survived  These r e s u l t s  better than  males, larger  again  caught  by  caught  males a s a d u l t s  but  i n 1977  subadults  indicate  subadults,  the  R a t e s f o r a d u l t s and  also  2  during  groups  data  (X =34.96,  were t h e  P<.001).  pitfall  well.  females  than  2  than  e q u a l l y poorly, while  subadults  better  (X =14.18,  lower  P<-05),  subadults  male s u r v i v a l  much  2  minimum  during the  and  were  than  than  minimum s u r v i v a l  higher than  i n T a b l e 4.3.  pitfalls  live-traps.  Female  minimum s u r v i v a l  s u r v i v a l r a t e s o f t h e t h r e e age  a r e shown in  was  P<.001) and i n 1978  Minimum  4.2).  P<.0001)  2  winter  2  (Table  (X =92.50,  t h e 1976  (X =85.79, -P<..0.001) and  h i g h e r d u r i n g t h e 1976 w i n t e r  P<.0001).  2  higher during  females  2  juveniles adult  P<.001).  and  males Adult  better  better  that  in  (X =88.41, P<.001) and a s  juveniles  not  f o r those  than  indicate  than  of  either  subadult  but that s m a l l e r animals  better survived  juveniles,  subadults that  males  survived females  females  sex  and  (X =24.92, 2  survived  tended  to survive  types  of  animals.  Because o f d i f f e r e n t  trends i n  the  two  traps.  127  T a b l e 4.3. Minimum s u r v i v a l r a t e p e r 1,4-day p e r i o d f o r M i e r o t u s townsendii caught by p i t f a l l s . A l l p o p u l a t i o n s a r e p o o l e d f o r e a c h i n t e r v a l . Sample s i z e s a r e i n p a r e n t h e s e s . Males  Females  Adults  Subadults  Juveniles  Adults  Subadults  Juveni  Summer 1976 Spring 1977 Summer 1977 Spring 1978  0.38 (549) 0.39 (596) 0.20 (693) 0.00 (5)  0.36 (274) 0*16' (156) 0.31 (928) 0.00 (4)  0.41 (402) 0.42 (332) 0.46 (633) 0.02 (7)  0.56 (744) 0.62 (961) 0.26 (642) 0.09 (37)  0.52 (332) 0. 16 (112) 0.31 (975) 0.06 (4)  0.57 (410) 0.36 (329) 0. 44 (678) 0.04 (5)  Total  0.30 (1843)  0.30 (1362)  0.43 (1374)  0.48 (2384)  0.34 (1423)  0.45 (1422)  128  minimum  survival rates  l i v e - t r a p s and survival and  rates  pitfall  P<.15), as P<.15). of  were n o t  good.  correlated  within  a given  As  (r=.88, n=5,  not  adults  be  from  correlated  of t r a p ,  respective  in adults  the  minimum  not  n=6, in  subadults  a n i m a l s enumerated the  age  by  and  and  (r=.98, (r=.91,  rates  were  in  juveniles  (r=.69).  Minimum  l i v e - t r a p s and  group being  s i m i l a r t r a p p a b i l i t y i n both t y p e s of  n=4,  age  male  juveniles  survival  P<.03)  n=4,  the c o r r e l a t i o n  were c o r r e l a t e d P<.07), and  live-trap  (r=.88,  captures,  <r=.87»  comparable unless  within  in  Minimum  (r=.88,  pitfall  (r=.93,  P<.05), but  r a t e s of  correlated.  determined  type  determined from  live-traps,  in  c l a s s e s caught  f e m a l e minimum s u r v i v a l r a t e s  subadults  From  sex  strongly  m a l e s , as  minimum s u r v i v a l r a t e s  survival  and  f e m a l e minimum s u r v i v a l r a t e s  was  P<.10).  age  were m o d e r a t e l y  were a d u l t  P<.02),  may  various  subadult  However,  groups  n=4,  of  captures,  male and  female  pitfalls  of  pitfalls  considered  has  traps.  Discussion Survival effect  on  the  variability elucidation  i n d i f f e r e n t age rate in  of  rates  o f c h a n g e s i n the the  in  a proposed  evaluating Results  of  v a r i a b l e than  did  not  groups can  growth, is  but  still  mechanism o f  post-weanling  peak p h a s e l e v e l s .  The  the  of  strong  basis  different  c y c l e may  population  of The age  be b e n e f i c i a l regulation.  preweanling s u r v i v a l i s  survival.  worse i n t h e  have a  controversial.  survival rates  study i n d i c a t e that  become m a r k e d l y over  sex  course of a population  this  less  improved  population  survival  groups d u r i n g  and  Preweanling s u r v i v a l  decline  p h a s e , but  depression  of  instead  preweanling  129  survival  i n peak p o p u l a t i o n s may  be  due  i n part  to d i s p e r s a l  young a n i m a l s  b e f o r e t h e y have been enumerated by  30%  v o l e s t h a t d i s p e r s e d from  of  the  were u n t a g g e d voles  was  highest  section).  individuals  by  i n t h e peak  decreases  the  and  thereby  reduces  alive  at  thus reduced. comparing  Hoffmann  the  number o f p l a c e n t a l Microtus  two  weeks  assuming Krebs  (1978)  estimated  a  increasing  M.  in declining  1976,  preweanling per  nestling a  and  73%  of  age.  survival  The  of  populations.  preweanling  and  78%  t h e r e was  Two  open p o p u l a t i o n s o r t h a t  in  in  decline. of  77%  montanus.  a  two  M.  (1964)  weeks i n a  of t h i s  p e r r two i n peak  and  peak  Krebs  results  study  weeks  in  populations,  mortality  i s not  vole populations.  post-weanling  possible  increased dispersal  survival  Boonstra  per  Preweanling  i n the e n c l o s e d p o p u l a t i o n s than possible.  the  rate  weeks.  a t 63%  in a decline. survival  survival with  i n M.  survival  of  survival  population  survival  two  component o f d e c l i n i n g both  number  preweanling  t o w n s e n d i i p o p u l a t i o n s , 63%  i m m i g r a t i o n was either  during  californicus  preweanling  a significant  higher  28%  calculated  that  preweanling  lemmimg p r e w e a n l i n g  indicate  In  average  p o p u l a t i o n t o be 86%  peak p o p u l a t i o n and  70%  increased  estimated  post-weanlings  mammae o f f e m a l e s  weaning o c c u r s a t t h r e e weeks  townsendii  and  found  small  dispersal  (1958) e s t i m a t e d p r e w e a n l i n g  He  M.  of  Calculated  scars.  in  of  p o p u l a t i o n s (see  the  of a c t i v e  r e p o r t e d an  dispersal  number  number  montanus  Hoffmann per  weaning.  Over  the e n c l o s e d p o p u l a t i o n s and  1977  This  enumerated  is  (see t r a p p i n g s e c t i o n ) ,  traps.  of  survival  were  i n p o p u l a t i o n s i n which explanations  of post-weanlings  immigrants reduced  both  are  that  from  the  preweanling  and  130  post-weanling  survival.  post-weanling dispersal  dispersal  section).  deleterious of  1976  not  fluctuation survival  1974).  Boonstra adult As  their  Myers,  an  survival  increased, et  originate reduced  recruited  i n population  adult  interpreted  causation  of  that  in  summer  population Poor  declining  vole  K r e b s and  Myers  restricted  t h a t 90%  of  month  determinant  of  to  voles  of  life.  population  inverse correlation  survival (1978)  found  lactating Boonstra  of  that  in  of males, the f e m a l e was  toward  young a  inversely that  young may These  types  the  M.  number  of  key  was  experiments guestion  the hypothesis proportions  be  results  p r o p o r t i o n of males  I f these  higher  townsendii.  introduced  Microtus.  to  between  (1978) s u g g e s t e d  females  relation  with  of  depleted  m i c r o t i n e d e c l i n e s , then populations  i n the  had  decline.  not  first  i n which the  f a r below n a t u r a l l e v e l s .  a r e t o be  must be  per  dynamics  from p o p u l a t i o n s  have  o f young M i c r o t u s  al.  female d e n s i t y . of  (see  1974).  voles  important  is  in  reported  responses  1969;  died  of  behavioural  of  (1960) e s t i m a t e d  population a r t i f i c i a l l y  with  the  Getz  be a s i g n i f i c a n t  (1978)  1976  the  in  characteristic  townsendii  correlated  low  survival  Redfield  young  during  juvenile  and  density  decreased.  may  Krebs et a l .  f e m a l e d e n s i t y and  female  immigrants  1955;  s u r v i v a l can (Krebs  in  increased  year.  is  population  of  populations  decreased  populations.  growth r a t e  evidence  of small animals  particularly  (Godfrey  b o r n i n a low Juvenile  other  was  However, low  declining  no  open  survival  juveniles  populations  in  survival and  in  on  i n any  Juvenile  was  I t appears t h a t  effects  but  There  of  generated or  higher  131  densities in  of  females should  any c y c l i c  survival  phase.  that  proportion  o r between j u v e n i l e  were a d u l t s .  o f males t h a t  suggests that  in  adult  and H e a l e y  findings  for  Mierotus  Juvenile  s u r v i v a l may be r e d u c e d  evidence Why decline and  radically  is  died  and  follow  reduced  leads  juvenile  reported  phase  with  to  Either  glareolus.  i s no  populations. rates  was  juvenile  Adult in  adult  death  have been  Peromyscus  (Sadleir,  juvenile  apparent  study,  during  the  presumably  behaviour  the disappearance o f j u v e n i l e s  These  survival  In t h e present  s e c t i o n ) , and j u v e n i l e s aggressive  or  males  c y l e and  not  in  Chitty  (Flowerdew, 1974).  are breeding.  adult  the sex  but there  survival  either  survival  (see d i s p e r s a l  similar  o r peak p e r i o d s ?  rates.  by j u v e n i l e s  males  a g g r e s s i v e b e h a v i o u r by  an a n n u a l p o p u l a t i o n  dispersal  lower  and i n Apodemus  when t h e a d u l t s  Sadleir  i n p o p u l a t i o n s i n which  that  which  Peromyscus  Clethriongmys  much  survival  1967)  on t h e g r i d s .  connected  and  i n either increase  low  to reduce  decline  (1966)  in  exhibit  e s p e c i a l l y males,  increased  survival  of  between  survival,  t o favour females,  (1966) s u g g e s t e d  adult  survival,  do j u v e n i l e s  Healey,  species  was a r e l a t i o n  t h e same e f f e c t i n n a t u r a l  emigration  1965;  Phipps  agrestis  altered  p e r i o d s than  reported  and  showed  proportions  to illustrate  Phipps  voles,  Chitty  study  t h e presence o f a d u l t  juvenile  and  juvenile  s u r v i v a l and  juvenile  that  maniculatus,  i s  This  and j u v e n i l e  males i n f l u e n c e d  survival  that  s u r v i v a l and  were a d u l t  the breeding season reduced  ratio  density.  However; t h e r e  (1967) f o u n d  juvenile  (1969) f o u n d  was no r e l a t i o n between j u v e n i l e  female density  (1965)  Krebs e t a l .  was n o t d e p e n d e n t on f e m a l e  that . there  females  experience loser  i n this  was  not  decline.  132  or the  kind  of  behavioural  j u v e n i l e s changed so Survival and  pitfalls  survival  survival  were n o t was  will  be  rates  sp.)  (Boonstra,  increased  and  1977a).  than  survival,  survival  was  post-peak  phase  increasing  population  Voles  the  time,  addition, selected  of other since  for  than  size,  the  mutual  shorter supply abundant,  can  than  be  during  and  two  are  there  is  summer.  accounted  in during  than during  the  a  direct  quality, the  the  1976  winter,  and  botflies  1977  this  (Chitty, proportion  t h a t food winter,  winter.  The  is  and  and hay  with  of  survival  change  in  fitness  to  1965). of  In  animals  that  cover  will the  were i n  predators was  a  possible.  dispersion  view  was  a time  are  l o s s e s through  and  vigil)  winter,  reduces t h e i r  high  by  period.  1977  alternative  1977 1976  this  explanations  also  Lower for  parasitic  the  conditions  The  subadult  h a v e been s e l e c t e d f o r  now  more  that i s consistent  interference,  environmental  minimum  f l y (Wohlfahrtia  during  individuals  ' t h r e a t ' , the  changed  because  a d u l t and  animals during  winter  more p r o n o u n c e d .  habitat  i n winter  result  lower  post-peak  under c o n d i t i o n s o f properties  enumerations  flesh  a  d i s p e r s a l of smaller  in  However,  i n f e s t a t i o n s of  grey  live-traps  data.  adults  the  classes in  of  and  dispersing.  Subadult s u r v i v a l i n the s p r i n g of  adult  Adult  withstand  higher  of  without  adults  than i n p i t f a l l s ,  discussion  live-trap  was  between  correlated.  live-trap  the  b a s e d on  of adults  age  in live-traps  o f s m a l l a d u l t s and  (Cuterebra  be  different  strongly  higher  survival  dispersal  also  the  Accordingly,  Survival  lower  of  r a t e s c a l c u l a t e d from  reliable.  summer  t h a t young v o l e s d i e d  rates  trappability  interactions  sparser  more and  133  shorter this  during  the  physical  1977  summer t h a n d u r i n g  deterioration  account f o r the lowered explanations  may  i n the  survival  have  1976  q u a l i t y of  during  applied  the  the  t o the  summer,  the  1977  and  habitat  winter.  may Both  populations  during  this  subadults  captured  period. minimum s u r v i v a l r a t e s o f a d u l t s and pitfalls  were  less  animals captured possible. their  by  Older  observed  trappability  voles  in  pitfalls.  the  and  several  avoid rates  caught  simply  explanation and  catch  live-traps,  thus  pitfall-caught the  in  pitfalls  lowering  voles.  and  may  may  Survival groups. that  during  What  be  types;  In  between  the  types  of  low  1  (docile)  lower  different voles  before  some  entering  survival  of  the m a j o r i t y  voles  caught  to  the of in  different  traps. 1978  was  for  in  all  (1967) a  'threat'  age  implied  population  i n t e r a c t i o n s are  and  i n d e c l i n e season d e c l i n e s the  poor  Chitty  necessary  peak s e a s o n d e c l i n e s t h e 'avoidance  was  r a t e s , because  between  survival?  i n t e r a c t i o n s are  lower  subordinate  attributable  s p r i n g of  caused t h i s  behavioural  decline.  two  a  a c c c o u n t f o r some o f  apparent  is  and  plausible.  catch  dispersed  survival  pitfalls  t r a p p a b i l i t i e s i n the  the  most  However, I s u g g e s t t h a t  d i f f e r e n c e i n apparent  live-traps  reflect  a c l a s s of  are  in pitfalls,  seems  pitfalls  This explanation  only  comparable  explanations  capture  d i f f e r e n c e i n o b s e r v e d minimum s u r v i v a l  voles  to  this  pitfalls  poorly.  and  of a p p a r e n t l y  T r a p p a b i l i t y in p i t f a l l s  live-traps  c l a s s e s , and survive  rates  could  minimum s u r v i v a l  Alternatively,  that  the  live-traps,  than i n l i v e - t r a p s ,  social  than  by  supposed  (aggressive)  i n t e r a c t i o n s are  supposed  134  to  be  mostly  hypothesized  between that  threat  during  late  types. declines  numbers, v o l e s become s o a g g r e s s i v e as successfully,  or  if  successful.  mating  is  concluded rate  t h a t t h e r e was  varied  Hoffmann in  in relation  (1958) and  declining  behavioural and  the  However, no  I found  populations  then  f o r t h e low  spring  1978.  lower  in  periods unable  1978  must  will  destroy  young  and  Myers  to indicate  that  (1974)  pregnancy  the  population  cycle.  that  preweanling  survival increased  compared  with  animals  adverse  survival  Moreover,  peak  ones.  increases  during  in  If  declines  as  Chitty  behavioural interactions the  breeding decline  result  low mate  severe  and  of  wounding r a t e s were  t h a n i n 1977  death  were  decline  (see  these hypothesized behavioural i n t e r a c t i o n s ,  present,  of to  they  However, b e c a u s e d i s p e r s a l  the  section),  (1977b)  i n t e r a c t i o n s become more i n t e n s e i n s e v e r e  suggested,  necessary  be  Krebs  evidence  to  and  to  mate, t h a t  proportion of aggressive  (1967)  are  i f t h e y do  Boonstra  without heavy  decline i f  they  wounding  or  dispersal. Boonstra which females males  (1977b) p r o p o s e d compete f o r s p a c e  compete f o r f e m a l e s  of i n c r e a s e d a g g r e s s i v e explain in  periods  dispersal,  non-breeding non-breeding breeding and  little  a  i t  seasons,  and  interactions survival  for  behaviour  to  for rapid  young,  While  between  account in  changes  this  model  declines  survival  i n which t h e r e  may  by an i n c r e a s e  for  in  in  whereas  individuals  accompanied  differences  begins during a decline wounding.  vole  i n order to r a i s e  inadequate  seasons,  of  w i t h w h i c h t o mate.  o f reduced is  model  between  survival i s no  in  when  dispersal  135  fire  individuals  aggressive  of  (1970) f o u n d aggressive in  behaviour  in  directly is it  measured by  highest seems  behaviour the  i n the  to  breeding  The  present  adult  Chitty  populations, could  (1967)?  Krebs  by  l e v e l s of  population related  Iverson  Males  season  than  evidence  that  more  (1973)  aggression  density.  that  intermediate  was  and  most  did  were  in  the  not more  winter  aggressiveness,  i n the l a b o r a t o r y ,  microtine time.  cycle;  Thus  f o r low  in  aggressive  survival  f a c t o r s (food, predators, with  fact  during  etc.) the  are a  onset  of  study The  suggests  several areas  that should  be  c a u s e o f low 1978  juvenile survival  i s of prime i n t e r e s t .  during  the  Removal  of  an a n t i c i p a t e d s e v e r e  decline  j u v e n i l e s u r v i v a l i f b e h a v i o u r a l i n t e r a c t i o n s are  of  low  (1967) and  suggesting  Turner  poor e x p l a n a t i o n  s p r i n g d e c l i n e of  cause  most  season.  further.  improve  the  peak  observations  male o r a d u l t f e m a l e s b e f o r e  should  one  lessened  and  f o r r a p i d changes i n s u r v i v a l  1978  severe  the  behavioural  d e c l i n e ; but e x t r i n s i c  explored  phase.  T h e r e i s no  seems t o be a  Chitty  i n t h i s case  d e c l i n e phase o f the  have  poor e x p l a n a t i o n the  behaviour  breeding  non-breeding season.  by  increase  population  the  populations  i n peak d e n s i t i e s were t h e  pennsylyanicus,  c o n s i s t e n t l y with  aggressive  suggested  between  to c y c l i c a l  i n M.  vole  whereas d e c l i n e males were  Thus a g g r e s s i v e  that  vary  phase, as  individuals,  to d e n s i t y than found  declining  t h a t male M i e r o t u s  aggressive  males.  any  in  that and  juvenile  Krebs  and  spacing  survival Boonstra  behaviour  presumably can  administer  i n severe (1978)  for  If  correct  in  are  i s more s e v e r e  account  declines.  the  drugs t o reduce a g g r e s s i o n  in  low  declining survival,  (Vessey,  19 67)  136  or  increase  aggression to  (Krebs  population  about  undergo  demographic  consequences.  et  a severe  Literature  Boonstra,  R.  on  1977a.  Microtus  Effect  al. •,  in  d e c l i n e and  a  vole  observe i t s  cited  of the  townsendii  1977)  parasite  Wohlfahrtia  populations.  Can.  conspecifics  on  J.  vigil  Zool.  55:  1057-1060. —  , 1977b.  Effect  population E c o l . . 46: ,  Boonstra,  H.  Microtus Chitty,  declines  D.  in  Microtus  of  adult  survival  townsendii.  during  J.  Anim.  835-851.  1978.  townsendii  of  Effect  Townsend  ) on  survival  o f young.  and  C.  Krebs.  J.  townsendii.  1965.  J.  populations.  Proc.  Ecology,  1978.  Mammal.  Predicting  voles 59:  Pitfall  59:  136-148.  qualitative  changes  12th  Int.  Congr.  (Microtus 242-248. trapping  in  of  insect  Entomol.  Lond.  384-386. ,  1967.  The  natural  selection  behaviour i n animal populations. 2: Chitty, in 35:  Proc.  self-regulatory Ecol.  Soc.  Aust.  51-78. D.  and  mixed  E.  Phipps.  populations  1966.  o f two  Seasonal changes i n  vole species.  J.  survival  Anim.  Ecol.  313-331.  Flowerdew, the  of  J.  social  mouse  R.  1974.  b e h a v i o u r and  (Apodemus  Field  and  laboratory experiments  population  §xlvaticus).  dynamics J.  Anim.  of  on  the  wood  Ecol.  43:  137  499-511. Getz,  L.  L.  1960.  A population study  pennsylvanicus. Godfrey,  G-  K.  Amer. 1955.  decline  in  Mammal.  36: 209-214.  Hamilton*  H.  Exper. Healey,  C.  H o f f m a n n , R.  size  S.  Monogr.  on  the  Mierotus  nature  o f the  populations.  Reproduction  (Ord) .  vole,  64: 392-05.  Mierotus  J.  of the f i e l d  Cornell  Univ.  vole  Agric.  The r o l e  and  self-regulation  of  E c o l o g y , 4 8 : 377-392. o f r e p r o d u c t i o n and  fluctuations  of  voles  mortality  (Mierotus).  Ecol.  28: 79-109.  J.  Tech.  Aggression  i n deermice.  1958.  1964.  Territories,  Krebs,  two  1941.  1967.  population  C.  of  the  S t a . , Memoir, 237: 1-23.  population  Krebs,  numbers  pennsylvanicus  M.  in  Nat.  Observations  J., Jr.  Mierotus  Midi.  of  Pap.  C.  The lemming c y c l e  during No.  J.  1959-62.  15.  1966.  a t Baker Lake,  Arctic  Inst.  Northwest N.  Amer.  pp 1-104. Demographic  populations o f Mierotus  changes  californicus.  Ecol.  in  fluctuating  Monogr.  36:  239-273. ,  1970.  Mierotus  associated M. Krebs,  with the p o p u l a t i o n c y c l e  pennsylvanicus. C.  J.  and K.  T.  with supplemental Krebs,  C.  p o p u l a t i o n b i o l o g y : b e h a v i o r a l changes  J . , B.  Mierotus fluctuating  o c h r o g a s t e r and  E c o l o g y , 5 1 : 34-52. DeLong.  food.  L.  i n M.  J.  Keller,  population populations  1965. Mammal. and  biology: of  M.  A Mierotus population 46: 566-573.  R.  H.  Tamarin  demographic ochrogaster  1969.  changes i n and  M.  138  pennsylvanicus Krebs,  C.  J.  i n southern  and J .  s m a l l mammals. Krebs,  C.  J . , Z.  Adv. T.  Aggression,  H.  Krebs,  55:  C.  and  B. ,  decline  J.  in  J.  1974.  Hes.  8:  and  50: 587-60 7.  Population cycles i n 267-399.  N.  M.  the  spring  Microtus  Experimental  Boonstra.  Smith.  1977.  decline  townsendii.  1978.  populations  Anim.  A.,  Hicrotus  vole  Ecology,  Can.  in J.  430-437.  townsendii. Redfield,  Ecol.  testosterone,  J.  spring  Myers.  H a l p i n , and J .  p o p u l a t i o n s o f the Zool.  Indiana.  M.  Ecol.  J.  47:  Taitt,  alteration  townsendii.  of  o f sex a  field  Demography  the  vole  o f the  Hicrotus  1007-1015. and C.  J.  Krebs.  1978.  ratios  in  populations  vole.  Can.  J. Zool.  of 56:  ;  17-27. and c.  Richmond, H.  reproductive in  H.  Conway.  B.  H.  agonostic  F.  Lab.  S.  behaviour  Turner,  and  Care,  The  qchroqaster,  19: 80-87.  relationship  population  changes  (Wagner).  and  J.  between  i n t h e deer  Anim.  Ecol.  331-352. B.  N.  and S.  L.  in  male  aggression relation Vessey,  Anim.  1965.  mouse, P e r o m y s c u s m a n i c u l a t u s 14:  breeding  pervormance o f t h e v o l e H i c r o t u s  a laboratory colony.  Sadleir,  Management,  S.  1973.  Hicrotus  t o population parameters.  1967.  laboratory 367-376.  Iverson.  pennsylvanicus, Ecology,  E f f e c t s of chlorpromazine populations  of  The a n n u a l  on  w i l d house m i c e .  54:  cycle of and  i t s  967-981.  aggression  in  Ecology,  48:  139  SECTION 5.  DEMOGRAPHY OF  DECLINING POPULATIONS OF  THE  VOLE  MICROTUS TOWNSENDII  Introduction Microtine abundance, Myers, the  the  1974).  start  populations  annual  1972;  Boonstra  result  of  either  death  provide  breeding  Fairbairn  spacing  1977),  duration  (1978) s u g g e s t  (Krebs  behaviour  by the  hypothesis  to the C h i t t y  (1978) h y p o t h e s i s , and 1964,  (Krebs  and  spring  at  and  Phipps  be  variable  Boonstra,  1978).  animals,  d e m o g r a p h i c a n a l y s i s o f two  relation  Schultz  and  can  population.  townsendii,  1958,  and  breeding  populations of Mierotus  Boonstra  (Chitty  the  in  1966; in  Krebs  t h a t s p r i n g d e c l i n e s i n voles are  o r d i s p e r s a l from  a detailed  season  changes  controversial  Sudden d e c l i n e s u s u a l l y o c c u r i n  m a g n i t u d e , r a t e , and and  periodic  c a u s e o f which i s s t i l l  of the  Flowerdew  undergo  and  the  food  causes  In t h i s s t u d y , declines in  interpret  (Chitty,  which  a  four  the r e s u l t s  1967), t h e K r e b s hypothesis  I  in and  (Pitelka  1969).  Methods The Canadian delta  study  area  Wildlife  was  part of a  S e r v i c e on  near Vancouver, B r i t i s h  dominant s p e c i e s : A g r o s t i s lanatus  were a l s o  Four Mierotus  T i m o t h y hay  field  owned by  R e i f e l I s l a n d i n the F r a s e r Columbia.  alba,  Lolium  P a l e am- p r a t e n s e perenne.  and  the  River  was  the  Holeus  present. townsendii  p o p u l a t i o n s were l i v e - t r a p p e d f r o m  140  May a  1976 u n t i l  vole-proof  Each  grid  fence,  and  described  baited  second  with  apart  by B o o n s t r a  in a 7 one  and Krebs  o a t s and s u p p l i e d  and c h e c k e d  from  May  with  checked  and l o c k e d  through  t h r o u g h September 1977, and A p r i l n o t be u s e d a t o t h e r  afternoon, When b o t h  open on  t r a p types  times  had been  on a c c o u n t o f w i n t e r  locked  location^  wounding  recorded.  sex,  1976,  1978, but flooding. Wednesday afternoon.  were s e t i n o n e week, t h e p i t f a l l s  weight, capture  a f t e r being  June,  m o r n i n g , and c l o s e d on T h u r s d a y  v o l e s were e a r - t a g g e d ,  were  through  October  week o n Wednesday m o r n i n g , c h e c k e d  Thursday  the l i v e - t r a p s All  controls.  week on Monday a f t e r n o o n ,  afternoon,  t r a p s were used  They were s e t e v e r y  after  m (25 f e e t )  by  morning.  Pitfall  could  live-traps,  morning  Wednesday  A and D were u n f e n c e d  t o the type  were s e t e v e r y  Tuesday  B and C were e n c l o s e d  Each s t a t i o n h a d two L o n g w o r t h l i v e - t r a p s and  The  cotton,  Populations  populations  trap similar  (1978).  April  1978.  had 49 t r a p s t a t i o n s s e t 7.6  x 7 pattern. pitfall  June  were s e t  open.  and on e a c h c a p t u r e , reproductive  t a g number,  condition  A l l v o l e s were r e l e a s e d  ,  and  immediately  processed.  Besults  Population  The but  not  Density  population identical  changes i n M i c r o t u s on a l l f o u r g r i d s ;  townsendii hence I w i l l  were  similar  discuss  each  141  separately relevant  by u s i n g  one a s  differences  in  c h a n g e s on c o n t r o l g r i d the  live-trap  the  summer,  winter,  owing  declined This  decline  declined The  average r a t e  males density 62%.  almost  entirely  breeding  o f t h e 1978  (Fig. 5.1).  5.2 shows d e n s i t y  i n males b u t in  both  not  24  times  (Fig.  5.2).  1977,  Initial  the  the f a l l  population  i n March 1977.  by t h e  males,  a peak i n O c t o b e r  breeding  which  of  of decline  an  accelerated  season  and  at  1977, and  for  I n 1978, t h e i n i t i a l  rate  and  2% p e r week f o r f e m a l e s .  both  breeding  females of both  c h a n g e s on c o n t r o l g r i d  1977 t h e p o p u l a t i o n in  females.  six  times  faster  1978 b r e e d i n g  was males  D.  After  underwent a  spring  The  rate  and  populations  of  decline  season  female  began, decline  declined  rate  of  both  decline sexes  a s t h o s e o f 1977.  c h a n g e s on e x p e r i m e n t a l g r i d  The p o p u l a t i o n with  during  f a s t e r t h a n t h o s e o f t h e 1977 b r e e d i n g  were a b o u t 25% as l a r g e Density  low  The  s e x e s a f t e r t h e 1978 b r e e d i n g  w i t h male d e c l i n e r a t e s rates  began,  t i m e s f a s t e r i n 1978 t h a n i n 1977.  s t a r t e d i n March  increased  5.3.  with  o f m a l e s was 46% o f t h a t i n 1977,  Figure  trapping  The r a t e o f d e c l i n e  However, t h e b r e e d i n g s e a s o n  decline  when  density  t h e summer and e a r l y f a l l  a t 355 p e r week.  f e m a l e s was f i v e  5.1 shows  a t 8% p e r week d u r i n g  during  describing  was low and r e m a i n e d  o f ,3% p e r week, r e a c h e d  females  Figure  by  onset of the b r e e d i n g season  a f t e r the s t a r t and  then  capture i n l i v e - t r a p s .  increased  declined  rapidly  and  the  and  1976,  8% p e r week compared  population  then  increased  was s u s t a i n e d  at  I n May  density  to delayed  with  model,  the others.  A.  population and  a  after  about f o u r  B a r e shown i n F i g u r e  breeding  began  in  March  t i m e s f a s t e r i n males t h a n i n  142  F i g u r e 5.1. P o p u l a t i o n d e n s i t y , i n s t a n t a n e o u s r a t e o f increase, and minimum two-week s u r v i v a l r a t e f o r M i e r o t u s t o w n s e n d i i on control grid A during 1976-1978. Non-breeding periods are s h a d e d . I n s t a n t a n e o u s r a t e o f i n c r e a s e c u r v e smoothed by 3 - p o i n t running average. The h o r i z o n t a l l i n e on t h e s u r v i v a l p l o t i s a survival rate of 0.707, below which half the population d i s a p p e a r s e v e r y f o u r weeks.  Density  1976  1977  1978  144  Figure 5.2. P o p u l a t i o n d e n s i t y , i n s t a n t a n e o u s r a t e o f i n c r e a s e , and minimum two-week s u r v i v a l r a t e f o r M i c r o t u s townsendii on control grid D during 1976-1978. N o n - b r e e d i n g periods are shaded.  145  146  F i g u r e 5.3. P o p u l a t i o n d e n s i t y , i n s t a n t a n e o u s r a t e o f i n c r e a s e , and minimum two-week s u r v i v a l r a t e f o r M i c r o t u s t o w n s e n d i i on f e n c e d g r i d B d u r i n g 1976-1978. N o n - b r e e d i n g p e r i o d s a r e s h a d e d .  Density  m  Rate of  1976  Increase  1977  1978  148  females. decline and  a f t e r breeding accelerated  five  times  decline.  The  The  population  30%,  The  of d e c l i n e  with 20  male  times  males  declined  l o s s per at  28%  rates  10  1977  week.  the  1978  5-5  seasonal grids  and  were  density.  males  was  1977  38%,  spring  and  the  1977  1978 and  a t an  grid  C.  breeding females.  breeding  began,  female r a t e s  spring  breeding  the  populations.  of  1977  breeding d e n s i t i e s i n both  63.5  and  larger  grid  over  decline.  average r a t e females  The of  88%  declined  d e n s i t i e s were  one-half  sexes.  g i n March  in  1977  g  and  females  larger,  C  v a r i a t i o n i n body  heaviest  same p a t t e r n  grid  was  15%  the  and  grid  of  g i n March  g respectively.  on  the  males  other 8%  shown  T h e r e was Voles  1977,  C a t the  larger in spring  B  5.6.  are  weights.  spring  54.6  44.1  found  grid  males  B males i n F i g u r e  a v e r a g e male w e i g h t on  and  2%  of  yearly  w e i g h t s were 50.9  were  for  times f a s t e r than  e x t i n c t i o n i n May  Initial  the  Weights  Figure  The  onset  a f t e r the  Body w e i g h t d i s t r i b u t i o n s o f  was  i n the  1977  times greater  than those of to  than  week a f t e r b r e e d i n g s t a r t e d , whereas per  initial  Body  decline  1978,  c h a n g e s on e x p e r i m e n t a l  about t h r e e  accelerated  of  times g r e a t e r  equivalent  a f t e r the  declined  greater  spring  females  density  declined  males  the  breeding population  of the  shows  season; rate  for  male i n i t i a l  5.4  in  to r a t e s about t h r e e  greater  female population Figure  began  in much  on  all  a time o f  peak  start 1978,  of  breeding  and  female  Males averaged  1977  than s p r i n g  grids.  l a r g e r , and  Grid grid  A  16% 1978.  males  D males  8%  149  F i g u r e 5.4. P o p u l a t i o n d e n s i t y , i n s t a n t a n e o u s r a t e o f i n c r e a s e , and minimum two-week s u r v i v a l r a t e f o r H i c r o t u s t o w n s e n d i i on f e n c e d g r i d C d u r i n g 1976-1978. N o n - b r e e d i n g p e r i o d s a r e s h a d e d .  Density  R a t e of  Increase  Survival  M  J  S  1976  N  J  M  M  J  S  1977  N  J  M  M  1978  J  151  F i g u r e 5.5. H i s t o g r a m o f body weight M i e r o t u s t o w n s e n d i i on f e n c e d g r i d C.  distributions  of  male  153  Figure 5.6. Histogram o f body H i e r o t u s t o w n s e n d i i on f e n c e d g r i d  weight B.  distributions  o f male  155  larger  i n spring  1977 compared  were 3% l a r g e r , g r i d larger i n spring Figures spring  present  than  during  this  on  C.  R a t e s Of  The  weight o f  males  late  winter  this interval.  interval  there  2%  on  grid  B.  Smaller  until  the  w i n t e r and  C  was  less  m a l e s were  start  of  B, s m a l l e r  The f i g u r e s  also  the males  show  that  was an a b s e n c e o f l a r g e r - s i z e d males  Decline  average r a t e  of. individuals  of decline  p e r week was used a s a  was  related  to  The  breeding  declines  the  same y e a r .  the  rate  spring  Thus, t h e magnitude o f t h e  of  1978  was  of d e c l i n e  (r=-92,  l a s t e d about as l o n g  t o t h e r a t e , not t h e d u r a t i o n ,  The  with  population,  the  spring  of  I  examined  in  relation  breeding  began.  I  breeding  as t h e average o f t h e f o u r  population  variance t o the rate  calculated  by h i g h  was  body  rate  voles  weights  of  preceding  r a t e s of  of decline  of d e c l i n e before  the  within  decline  heavy-weight  of  P<.001) .  of d e c l i n e .  characterized  proportion  n=8,  loss  both i n  on a l l g r i d s  s o t o d e t e r m i n e whether o r n o t t h e r a t e  correlated  measure  The o v e r a l l p e r c e n t a g e  males (r=.94, n=8, P<-001) and i n f e m a l e s  decline,  D females  the l a t e  1978, whereas on g r i d  i n t e n s i t y o f the s p r i n g d e c l i n e .  related  Grid A females  i n s p r i n g 1978.  during  o f t h e m a l e s on g r i d  C from  during  with those  5.6 show t h a t  s e a s o n i n March  disappeared  grid  that  on g r i d  breeding  of  and  o f 1978 t h e body  variable  1978.  B f e m a l e s 5% l a r g e r , and g r i d  1977 compared  5.5  with s p r i n g  in a  within  a  and a f t e r  decline  sampling  was  before periods.  156  covering of  an  body  eight-week p e r i o d p r i o r t o breeding.  weights  was  a weighted  previous four periods. with  rates  n=4,  of  P<. 10)  (Table variance slower  Variance  decline  ( T a b l e 5.1)  5.2).  This  rate  and  the  rate  after  in  females  the  before  breeding  of decline a f t e r  began  was  lower  interval  d e c l i n e r a t e once b r e e d i n g I examined the  determine  if  proportion eight-week females,  of  the  the  I  because  there  few  rankings  o f t h e g r i d s d i d not  In  1977,  animals Also, higher  lowered  populations faster  weight  which accounted  populations.  In  the  the  rate of  related  of  tended  to  which  decline t o have a  i n 1977  and  with  g was  males,  calculated and  calculated  cut-off  to  the p r o p o r t i o n In  1977  above 58  i n 1978  for  an  1978.  In  f o r 1977  weight  g , and  the  in  the  and 1978  relative  change.  with  higher  (males  variance  in  g was  females  populations with higher  section), these  were  declined  body  began  p r o p o r t i o n above 58  g f o r 1978.  1978,  P<.05),  populations.  breeding  above 50  the  began.  p o p u l a t i o n a b o v e 70  period after the  n-U,  d e c l i n e r a t e s were c o r r e l a t e d in  P<. 13)  The  inversely  breeding  r a t e of d e c l i n e both  of heavy-weight animals  in  a higher r a t e  before  (r=-.92,  t h a t the h i g h e r  began  (r=-.95,  the  correlated  n=4,  b r e e d i n g began.  began i n 1978  breeding  eight-week  was  (r=-.88,  indicates  i n d i c a t e s that grids that experienced in  variances of  i n body w e i g h t s  correlation  d e c l i n e before breeding  of the  variance  a f t e r b r e e d i n g began i n m a l e s  i n body w e i g h t s the  average  The  proportions  r=.90,  r=.92, f e m a l e s  p r o p o r t i o n s of h e a v y had  f o r the  more  higher  contrast,  populations  heavy P<.10).  animals  dispersal rates  of  of  and  (dispersal decline  i n 1978  with  in a  157  T a b l e 5.1. I n s t a n t a n e o u s r a t e s o f d e c l i n e p e r week, number (and % of population) o f males a t the s t a r t and e n d , p r o p o r t i o n o f heavy-weight males, and variance o f body weights during the breeding declines of 1977 and 1978. Body weight v a r i a n c e i s c a l c u l a t e d f o r an e i g h t - w e e k p e r i o d b e f o r e b r e e d i n g b e g i n s .  Rate o f Decline  Number (%) at s t a r t  Number a t end  A B C D  0.0754 0.1038 0.0586 0.0809  84 132 105 103  (51) (50) (51) (52)  1977 b r e e d i n g d e c l i n e s 35 (35) 58.3 0.232 38 (29) 71.2 0.371 39 (33) 62.1 0.208 39 (31) 62.1 0.231  84.3 82.2 99.4 86.5  0.3806 0.2792 0.8815 0.4580  42 56 34 39  (46) (59) (37) (51)  1978 b r e e d i n g d e c l i n e s 2 (11) 95.2 0.235 6 (35) 89.3 0.241 : 0 (0) 100.0 0.000 1 (8) 97.4 0. 108  71.0 73.5 36.0 50.8  A B C D  (%) % loss  Prop.> 70g.  Body Ht. Variance  Grid  158  T a b l e 5.2. I n s t a n t a n e o u s r a t e s o f d e c l i n e p e r week, proportion o f h e a v y - w e i g h t f e m a l e s , and v a r i a n c e o f body w e i g h t s d u r i n g t h e breeding declines of 1977 and 1978. Body w e i g h t v a r i a n c e i s c a l c u l a t e d f o r an e i g h t - w e e k p e r i o d b e f o r e b r e e d i n g b e g i n s .  Grid  k B C D  k B C D  Prop. > 58 g.  Body Wtw Variance  breeding d e c l i n e s 65 20.7 94 28.2 81 20.5 88 6.4  0.089 0.20 6 0.146 0.068  52.7 54.0 50.8 46.9  1978 b r e e d i n g d e c l i n e s 16 67.3 49 11 71.8 39 6 89.7 58 12 67.6 37  0.534 0.481 0.086 0.543  29.8 33.0 22.9 41.0  Bate of Decline  Number at s t a r t  0.0206 0.0304 0.0203 0.0055  1977 82 131 102 94  0.1399 0.1582 0.2835 0.1308  Number a t end  1  %  loss  1 1978 p r o p o r t i o n > 50 g .  159  higher  proportion  of  heavy a n i m a l s  (males r = - . 9 3 , f e m a l e s r=-.99, The the  f a s t e r the  dispersal  r=-.90, P<.10)« in  the  modest  of  decline  breeding season, the  ( m a l e s r=-.82, n=4,  Thus d i s p e r s a l can  1977  slower r a t e s  P<.10).  d e c l i n e i n the  o f e i t h e r sex  had  decline than  less  P<.20;  a c c o u n t f o r more o f  in  the  severe  females the  1978  was  loss  breeding  decline. In of  summary, t h e  m a l e s and  population with  f e m a l e s was  decline  periods  of  were s l i g h t l y could  be  voles of  the  there  decline.  1977  females  of  T h e r e was of  These  Of  and  also a  data  rate of  and  declines when  animals, voles,  males not  that  i s the  with  the  faster  the  between  (r=.83# 1978  the  n=8,  relevant  density  P<.01)  females  the  rate  and  (r=-.79,  proportion  of  variable  to  decline.  Survival  to determine g  decline  However,  correlation  spring  of  c o r r e l a t i o n between  suggest  density,  rate  of  concurrent  dispersal  e x a m i n e d minimum s u r v i v a l r a t e s d u r i n g  59  population  were  season.  heavy-weight  P<.10), b u t  a n i m a l s , not  Size-selectivity  one  heavy-weight  heavy-weight  measure t o p r e d i c t t h e  above  breeding  When d e c l i n e s  a stronger  of  (r=.91, n=4,  P<.25).  decline  within  proportion  proportion  I  the  dispersal, density  was  proportion  heavy-weight  of  c o r r e l a t e d with subsequent r a t e s  spring.  density-dependent  and  n=4,  i n the  density  c o r r e l a t e d , which s u g g e s t s t h a t  d e c l i n e and  spring  not  increased  dispersed,  higher  initial  if  females  survival above 49  was  the  1977  breeding  size-selective.  g were ' l a r g e ' ; o t h e r s  Males were  160  •small .  The  1  was  greater  and on  survival than  the  when i t was the  minimum  of large animals  a  lower,  sign  test  each  This  analysis  larger  indicated (23+,  9-,  P<.01).  Dispersal  decline  (dispersal  lighter I  m a l e s and  calculating  that  P<.05);  of lighter  females  minimum  and  may  and  or  the  rates  for  20% p r o p o r t i o n s of the  period  over  then  (8  until  t h e end  of  the  weight  I  group  of  v o l e s was  individuals Grid  in  at  relatively  insufficient 5.5).  A  indicated better  sign that  than  the the  d u r i n g the  1978  8-, this  rates  survival  heavier  percentage  rather  their  this  body  80%  light-weight  for  analysis and  the  males males  advantage  and  decline.  and male  a  fixed  survival  of a  lightest  because  week. they  therefore provided  to  act  upon  (Fig.  remaining three survived  (16+,  over  than  the  by  sampling  at a particular  size  of  was  calculated  being  for selection  heavy-weight  breeding  (24+,  breeding season  the  population  analysis  m a l e s were a t a s e l e c t i v e  male  I  upon  uniform  test  than  dispersal.  see whether or not  the  p h e n o t y p i c range  better  survival  to  1978  decline.  during  decline.  C m a l e s were e l i m i n a t e d from  were  and  to  dependent  present  lower  and  weeks) b e f o r e b r e e d i n g s t a r t e d  rates of a f i x e d wanted  the  male p o p u l a t i o n f o r e a c h  minimum s u r v i v a l because  of l a r g e  common  not  during  survival  four periods  the  i t  conducted  with females  be a t t r i b u t e d  lighter  from  was  during  was  when  d u r i n g a week,  males s u r v i v e d  animals  whether  before  grid  similarly  section),  investigated  size-selective  analysis  ••'  s u r v i v a l r a t e s estimated biweekly  a n i m a l s o f b o t h s e x e s on  males  scored  s u r v i v a l o f small animals  small  smaller  was  5-,  significantly  P<-04).  lighter  grids  males  Heavier before  161  Female  survival  the b r e e d i n g analysis  showed  l a r g e r and  Growth  season  in a  no  similar  significant  s m a l l e r females  in  voles  59  rate  but  the  during  sign  differences i n survival  from  g r o w t h r a t e s were examined f o r  mid-December  until  g.  I d i d a four-way  grid,  y e a r , and  indicated  that  test  between  significant grids,  March i n 50-59  w e i g h t as  time  in  growth  o r between y e a r s ; but  there  of  year,  was  a  between  year  size  c l a s s was  f o l l o w i n g comparisons  significant of  winter  loss  of  0.20%  0.27%  p e r day  1977,  but  weight  i n both  per i n 1978.  lost  0.03%  i n 1978.  Small  in  1977,  larger  same w i n t e r .  They l o s t  voles lost  0.10%  i n 1978. per day  weight  voles gained  there  the  effects  V o l e s 40-49 g g a i n e d p e r day  The  greater growth sex,  analysis were  no among  between  interaction  (F=9. 14,  weight over  y e a r s , a b o u t 0.09%  day  but  day.  g lost  1978.  P<.001).  growth r a t e s a r e  mean g r o w t h r a t e s c o r r e c t e d f o r s i z e V o l e s u n d e r 40  and  difference  (F=5.4Q, df=3,4602, P<-01), and  analysis.  and  r a t e s between s e x e s ,  classes  The  g,  classes  a c o v a r i a t e , and  weight  and  1977  as the f o u r i n d i c e s .  this  differences  size  a n a l y s i s of covariance with  weight c l a s s during  late  40-49 g,  as t h e dependent v a r i a b l e ,  upon  and  1978.  S i z e c l a s s e s were: u n d e r 40 g, than  manner,  before  Bates  Instantaneous of  r a t e s were a l s o compared  by  based  covariance  w i n t e r , an per  day  0.12%  Voles  i n 1977 per  over  i n 1977,  more r a p i d l y  average  49  and  day g  in  gained  0.22%  i n 1978  w e i g h t more r a p i d l y  and  during  per than the  162  Hounding  Rates  Table during  5.3  the  Wounding  shows t h a t wounding  1977  However, and  s p r i n g d e c l i n e than  increases after  weeks t o r e a c h the  i n 1978  was  breeding  maximum i n t e n s i t y  i n the begins  v o l e s may  spring  and  had  than  the  number o f wounds p e r i n d i v i d u a l f o r t h e  periods  wounding was comparison  (6 weeks) o f t h e two  higher i n a given was  scored analysis  in  breeding  1977  decline  in  i n males  1978  decline  not  in  was  average males  with  in  animals  compared  those  and  When  in  1978,  a c c o m p a n i e d by  the  scored higher  1-,  2  ties,  P>.10).  The  unusually  of  those  i n females  also  wounding  to  have  high  more s e v e r e  lost.  T h e r e was  breeding  (r=.68, n=8,  lower  and  breeding  season P<.10).  tended  survival  to of  significant  condition i n voles  during  i n s u r v i v o r s of the no  animals  declines.  that disappeared  was  heavy  p r o p o r t i o n s o f heavy a n i m a l s  during the  the  s u r v i v e d and  Wounding  declines.  5-,  between p r o p o r t i o n  P<-05) and  higher  wounding  light-weight  that  was  a relation  (r=.73, n=8,  more  I  four  (13+,  number o f wounds i n c u r r e d d u r i n g t h e  Populations have  I compared first  i t was  (11*,  to  levels.  There and  lower  females  significantly  wounding  than  1977,  time  showed t h a t wounding l e v e l s were  P<.01), b u t n o t breeding  breeding  week i n 1977  ; when i t was  1  This sign test the  possibility,  1978). in  enough  wounds.  sampling  investigate this  decline.  Boonstra,  s h o r t e r i n 1978  not have  sexes  r e q u i r e s about s i x  accumulate average  To  simply  1978 but  (Krebs  b r e e d i n g d e c l i n e was  the  more s e v e r e i n b o t h  each  1978  difference  decline.  d e c l i n e than in  wounding  163  T a b l e 5.3. A v e r a g e number o f wounds per animal f o r males and f e m a l e s d u r i n g b r e e d i n g d e c l i n e s . Sample s i z e i s i n p a r e n t h e s e s . D  C  B  A  Grid Male  Female  Male  Female  Male  Female  Male  Female  1977  1. 10 (4 61)  0.06 (619)  1. 15 (299)  0.04 (424)  1.05 (503)  0.07 (547)  1.05 (425)  0.09 (536)  1978  0.40 (59)  0.04 (139)  0.85 (157)  0.02 (133)  0.28 (95)  0.03 (187)  0.62 (42)  0.02 (94)  164  between lasted  survivors longer  declines,  than  non-survivors  the  1978  surviving voles  than  those t h a t  for  by  decline.  the  longer  1978  1977  to  voles  During  both  reflects  be  accounted  were l e s s  often  the  1977  fact  that  voles  the  1978  condition  during  breeding  reproductive  soon a f t e r b r e e d i n g  which  i n breeding  voles that  weeks o f t h e  decline,  decline.  were l a r g e r  become  1977  r e s u l t can  light-weight  four  period  disappeared  The  difference  first  the  were more o f t e n  mature t h a n  The  surviving  of  of  breeding  disappeared.  dispersal  reproductively  a  and  decline  than  had  those  that  started.  Discussion Population Chitty on  (1955) r e c o g n i z e d  the  of the  rate  in  spring  maximum during  decrease during  which  in  the  1966,  Myers and  mammal  i n one  both i n r a t e  Krebs  breeding  and  year.  one  the  are  declines  duration  very  rapid  t o a low  during  Type  declines  y e a r and  G  have no  or  recovery  more y e a r s  and  based timing  population  the  Type H d e c l i n e s i n v o l v e  o v e r one  mammals.  winter are  or  of  in  a  numbers  a  gradual  w i t h some  recovery  season.  declines  small  survival  declines  small  microtine  d e c r e a s e and  season.  numbers  many  the  M  variable in  types of  numbers f a l l  of  breeding  highly  three  a f t e r a peak  breeding  Spring  after  Type  duration the  are  of p o p u l a t i o n  decline.  decreases early  declines  i n population populations 1971). season  s i z e are  ( C h i t t y and  The begins  or both sexes o c c u r . i n duration  a characteristic  rate  of  and These  ( K r e b s and  P h i p p s 1966, decline  marked  Boonstra,  Krebs  increases  decreases  declines  of  can  1978).  in vary  165  Variability on  the  time to  properties of  the  i n the  rate  of  i n d i v i d u a l s i n the  the  decline.  of  some s p r i n g  K r e b s and  over  70  g i n March and  that  there  was  heavy-weight following  a  increasing  or  and  Peak  Therefore,  smaller the  of  the the  rate  populations  breeding the  and  males  decline,  showed  decline  of  i n the  the  the  rate  body  size  and  introduced.  rate  Thus  P o p u l a t i o n s with sharply  in  decline  Correlations may  that higher  when  decline.  They  of  populations slower  a  are  decline  in  phase  i f  and  Boonstra  males t h u s a r e  following  spring'*  time  lag  suggestion  that  programmed t o simply  is  drop  shows t h a t  a  peak.  biological  rates  large-sized  at  faster decline low  and  had (see  rates.  during  proportions  decline  body s i z e this  within study  the  of  than populations  faster dispersal In  with  rates  phase  breeding season,  w i t h few  of  section),  decline  large-sized  a  shows  a peak, p o p u l a t i o n s  voles  more d i s p e r s a l  was  and  relevance,  common, a s  a l s o had  of  decline Krebs  dispersal  rates  phase  one-year  the  with higher  peak  a  which a c c o u n t s f o r the populations,  than i n  of  d i s p e r s a l was  proportions  the  a c o r r e l a t i o n between  between d e c l i n e  have g r e a t e r  in  be  large-sized  phase f o l l o w s  than  must n e c e s s a r i l y  the  numbers  the  of d e c l i n e  peak p h a s e b r e e d i n g s e a s o n i s l e s s t h a n i n t h e season, there  in  decline  increasing  peak p h a s e , and  of  voles  higher i n d e c l i n i n g populations in  failing  of  larger  rates  the  proportion  spring  contain  ones,  at  proportion  of  depend  (1978), a f t e r  between  because males  than those  breeding  year  the  declining  breeding season are ones.  rate  correlation  males  year.  the  may  population  Boonstra  d e m o n s t r a t e a c o r r e l a t i o n between  declines  voles large  and had  voles.  166  K r e b s and  Boonstra  (1978) s u g g e s t  populations  with higher  have  more  intensive  this  study  spring  d u r i n g the  in  vole  extrinsic  (behaviour, this  study  size  1977  t h a t was  one  from  October  mechanisms  support  until  t h e a b s e n c e o f wounding and during the breeding  wounding,  but  not  hypothesis  (Chitty,  d e c l i n e s have been  Two  1978  dispersal  declines;  from  the  Chitty  declining  by  a  dispersal,  marginal to  individuals assumption expense the  in  show  increasing  i s that selection of  increase.  the t r a i t s  more or  peak  and  the  due  The  to two  to  prediction  than  in do  fundamental  f o r a b e h a v i o u r a l change i s a t  peak and  in  property  individuals  behaviour  ones.  and  selection  main  the  average,  threat  of  d e c l i n e s are  i s behaviour,  to  Chitty  required  different  two  that are s e l e c t i v e l y  Voles i n l a t e  the  v o l e s a t the b e g i n n i n g  between t h e  in  season  increase  explanation i s  to  of  c h a r a c t e r i z e d by  According  h y p o t h e s i s i s t h a t , on  populations  March  i n the n o n - b r e e d i n g  subjected  assumed t o c h a n g e i n f r e q u e n c y  results  declines  one  t h a t was  with  intrinsic  The  w i t h wounding and June  differences  or  c h a n g e s i n t h e p r o p e r t i e s o f t h e i n d i v i d u a l s . , The  from  of  heavy-weight  explained  etc.)  i n t h i s study;  1967), o n l y one  breeding  been  decline.  dispersal.  for  and  have  of  season,  account  pressures,  these  in  However,  whatever the cause  both; v i e w p o i n t s .  concurrent 1977  g i n March  year.  decline,  predators,  were r e c o r d e d  June  townsendii,  to environmental c o n d i t i o n s .  (food,  partially  and  breeding  populations  genetics)  population  and,  d e c l i n e s the next  1978  v o l e s seemed b e t t e r s u i t e d Declines  Hicrotus  p r o p o r t i o n s o f males above 70  showed t h e o p p o s i t e r e s u l t ,  disappearance  either  that i n  advantageous  the  during  d e c l i n i n g p o p u l a t i o n s have  167  been  selected  for  interference,  and  this  also reduces t h e i r (Chitty,  survival  under  change i n p r o p e r t i e s  f i t n e s s to  endure  and  Boonstra  (1978)  behaviour causes v a r i a b i l i t y  during  breeding of  present  seasons.  more v i g o r o u s in  that  decline. the  started  onset  dispersal  with of  the  highest  in  those  in  body  heavy-weight v o l e s , observations  conditions  voles  smaller,  and  The  in  the  i n the  r a t e of  0.93  section).  breeding,  wounding i n c r e a s e d ,  that the  per  to  Boonstra  argued t h a t  of  accounted  f o r by  what n e e d s t o be  of  These  interactions the  (1978)  Chitty  extrinsic  explained  population  minimum s u r v i v a l but  well  1976  winter  did and  below (see  not  discuss  Phipps  (1966)  relatively  factors.  are  non-breeding  the  i n the  when n o n - b r e e d i n g l o s s e s a r e  be  and  highest  proportion  a  period,  suggested may  losses,  the  decline  levels.  both  70%  weeks o b s e r v e d  K r e b s and  of  and  individuals.  weeks i n t h i s  two  decline  led to dispersal of  season  they  had  aggressive  non-breeding that  rate  wounding  s t u d y had 60  The  highest  a  hypothesis  decline.  subordinate,  two  are  individuals  Boonstra  The  in  decline  declines  the  n o n - b r e e d i n g component; w i t h  per  of  breeding  condition  and  rate  by  K r e b s and  highest  in this  variability  breeding  voles increased.  breeding  that  the  hypothesis that  component,  above 0.80  survival  of  the  second d e c l i n e  disappeared  The  weight,  presumably  breeding  in  populations  and  support  between  the  environmental  behaviour  1977  smaller-sized  variability  rates  individuals  Bore s e v e r e  spacing  adequately explains  and  mutual  the  proposed  spacing  was  of  of  1965).  Krebs  result  conditions  slight,  Instead,  drastic  changes  they in  168  survival, and  f o r example, those  breeding  Chitty  and  any  behaviour  different,  decline. more  during  i .  e.  the  was  non-breeding  breeding  (1978) would  the b r e e d i n g  that i t  between t h e  within  (1967) and K r e b s and B o o n s t r a  spacing was  seasons,  that occur  predict  p o r t i o n s o f both  more  intense  in  the  1978 b r e e d i n g  that  declines  in  The l o g i c a l a s s u m p t i o n i s t h a t , on a v e r a g e ,  aggressive  season.  the  1978  voles  d e c l i n e than  were  during the  1977 d e c l i n e . If  severity  vigorous that,  o f s p r i n g d e c l i n e s i s simply  spacing  during  light-weight should  behaviour,  the  then  severe  voles should  wounding  i t seems r e a s o n a b l e  1978  breeding  have been  levels.  Also,  this  and  one  must  than  without  reported  results  either  death  reject  hypothesis. in  rate data Chitty  the  wounding  of  with  may  made  o r no  wounding  in  Barnett  wild rats  levels  or  some (1958)  (Battus  i n weight  no  predictions  of  loss  wounding  phases of the m i c r o t i n e c y c l e ,  cyclical  in  wounding.  suggested  phase,  and  i f  to  should  be  be  but i f  the  most  the frequency  i s a f u n c t i o n o f t h e number o f a g g r e s s i v e  presumably  rates  n o t be r e l e v a n t t o t h e C h i t t y  populations are  any  higher  observed  death  dispersal..  little  o r may  (1967)  different  voles i n declining aggressive  or  as  t h i s hypothesis,  i n t e r a c t i o n s between  of subordinates  1977,  with  were  i n the l a b o r a t o r y sometimes r e s u l t e d  Wounding  levels  wounding  that aggressive  norvegicus) and  heavy  expect  s h o u l d h a v e had f a s t e r  assume t h a t t h e b e h a v i o u r a l i n t e r a c t i o n s c a u s e d manner  to  in  populations  P r e c i s e l y the opposite  study,  o f more  d e c l i n e , d i s p e r s a l of  higher  p r o p o r t i o n s o f heavy-weight animals of d e c l i n e .  the r e s u l t  highest  animals, in  of  then  declining  169  populations. distinction  However, t o both  have h i g h death the  Chitty  hypothesis  index  in  declines.  If  fighting  in  declining  wounding.  of  low  is  applies  populations  can  suggests,  i f  wounding, w h i c h  correct,  t h a t wounding may  of behaviour  that  without  may  n o t be  should  show  aggression,  marked  a  be i n v o l v e d  f o r a p o p u l a t i o n d e c l i n e , then  populations  and  animals  increases  in  I n M i c r o t u s t o w n s e n d i i . wounding d u r i n g t h e w i n t e r i s ( K r e b s and in  declines  in  the  non-breeding  and  Boonstra,  season 1978),  breeding  in  1971,  winter  a  and  70-80% There  (1976) f o u n d  populations of  pennsylvanicus  i n peak and  M.  62%,  an  early  loss,  and  a l a t e decline of  had  no n e t l o s s .  is  his  Clearly  conclusion  of  Severe  townsendii  during  wounding  (Krebs  of the no  population evidence  a  population  these more  d e f i n i t i o n s are severe  season high  males  had  In a study  population  of  phase  decreased  7 months which had  months i n which  may that  was  (1978) d e f i n e d a peak  as c o m p r i s i n g 12  t h a t wounding  decline periods.  t o 4-month p e r i o d i n which  in  1973).  any  is  M.  1973), and  o c h r o g a s t e r , and  p o p u l a t i o n s . Rose  decline  Iverson  i n M.  onset  to that i n  (Lidicker,  absence o f  over  way  i n v o l v e d i n these non-breeding  Gaines  wounding  and  occur  period. is  i n c r e a s e s with the  similar  Turner  may  i n the  this  Rose and  and  californicus  behaviour  declines.  comparable  spring  i n M.  during  aggressive  1978)  population size  the  disappear  Boonstra,  (Christian  winter breeding  a s a 3-  little  i s necessary  breeding  H*  hypothesis  Females i n d e c l i n i n g  of the s e v e r i t y  pennsylvanicus  in  Chitty  wounding i s a measure o f some f o r m s o f  if  very  sexes.  r a t e s , but  relevant these  the  by  a  42% size  questionable,  as  wounding i n a l a t e d e c l i n e  170  population. necessary  His  data  results  dispersal  of  section  Dispersal  was  Krebs  Krebs et  which  that  and  al.  1973).  the  population  i n the  aggressive.  During  survival  population,  rates  of  were s i g n i f i c a n t l y winter, had  which  may  because  had  selected  been  interference. interact  already  less  the  than during  indicate that  voles  in late  for  left  when  behavioural  peak p o p u l a t i o n s ,  results  (Myers  a  If  those  with  this  in  death.  there  prone  to  e a r l y d e c l i n e have  peak p h a s e ,  mean t h a t t h e y  remains good. 1977  are  the  to Chitty peak and under  These  more  The  minimum winter  increasing  f i t n e s s of  the  may  prone  have  populations  conditions begins,  phase  population  (1965),  declining  so  factors  p o s t - p e a k phase 1976  i n c r e a s e and  few  and  mechanism  population.  b e c a u s e most i n d i v i d u a l s i n the  be  and  peak and  breeeding  to remain behind.  problem  behaviour  hypothesis.  and  the  survival  the  increase  which may  survival  unknown mechanism t h a t l e a v e s One  Chitty  d i s p e r s e , then  late  voles during  a  in  non-breeding season e x t r i n s i c  but  However,  more, but  a n i m a l s tend  the  upon t h e  been r e d u c e d , w h i c h , a c c o r d i n g  occurred  have  the  not  reported  D i s p e r s a l can  i n the  been s e l e c t e d f o r s t a y i n g p u t ,  the  those  support  natural selection acts  leave  is  a non-random s a m p l e o f t h e p o p u l a t i o n  i n d i v i d u a l s present  deplete  wounding  decline.  study  a polymorphism f o r tendency to  disperse  severe  common i n t h e i n c r e a s i n g and  are  1971,  this  partially  dispersers  is  that  condition for a population  The  by  show  of  mutual  the  animals  to  disperse  peak p h a s e , t h e  remaining  individuals  die  by  an  wounds.  view It  is  explaining  seems  i n t e r a c t i o n s become s o s e v e r e  as  how  spacing  implausible t o cause death  that with  171  no  dispersal.  varied  least  only  I n d e e d , on g r i d  i n weight, the males  s i x females  behavioural  because  behavioural  as  declined  It  seems  causes  extinction  extinction  approaches  (see p r e d a t i o n  Avian  predators  zero,  have  been  presumably food  limiting.  1976- 77.  These  and cover  initial  breeding  1977  breeding  season.  during  the  1977  shorter  (0.5 t o 1 m ) . than  present  during  supply. lost  densities  summer,  in  Survival  of  so  weights season  had  faster  with  pre-breeding  in  m a l e s were l e f t would  limiting.  body w e i g h t s . a t t h e end o f  the  growth  was s p a r s e r and  and l e s s dead g r a s s  was  food  the  was  in  shorter  was r e d u c e d , s m a l l  season  variability declines.  populations  Figure  o f the  of  higher  d e c l i n e s were f a s t e r i n t h o s e  variability  animals  animals  excellent,  retarded  in  to  winter  presumably  smaller  w e i g h t , and p o p u l a t i o n s  lower  winter,  the animals  a t the start  grass  m,  the winter of  was  grass  the  was  previous  time,  dead  that  Survival  the  this  so  survival  were h i g h  of  indicated  o f 1.5 t o 2  were p l e n t i f u l d u r i n g  A cover  i n the  F o r example, d u r i n g the  s u r v i v e w e l l and be a b u n d a n t ; b e c a u s e  1977- 78  the r a t e o f  evidence  good c o n d i t i o n s may h a v e h e l p e d  and  the  a c c o u n t e d f o r some o f t h e l o s s  summer o f 1976 t h e T i m o t h y g r a s s grew t o h e i g h t s so  of  a  decrease.  s e c t i o n ) , and c i r c u m s t a n t i a l  may  and  t o accept  f a c t o r s were i n v o l v e d t o v a r y i n g d e g r e e s  decline.  food  to  virtual  density  m a l e s and f e m a l e s  difficult  i n t e r a c t i o n s must n e c e s s a r i l y  Extrinsic  that  survived.  mechanism t h a t  population,  second  C, where both  in  have been t h e b e t t e r c o m p e t i t o r s  t h a t had  Presumably i f resources  The r a t e s o f d e c l i n e i n c r e a s e d s h a r p l y a f t e r  body  Breeding  5.6 shows t h a t o n l y  decline.  voles  lower large these were  breeding  172  began,  which s u p p o r t s  the h y p o t h e s i s  of an i n c r e a s e d demand f o r  resources i n already short supply. If  food  application  abundance of  not  i t may  food-enhanced supplied and  food  that  remained about supplied  and  and  DeLong  (1965)  delayed  slower  the  females  increased  ochrogaster  h i g h e r a d u l t s u r v i v a l , and  declining both  supplementary  densities  seems onset  higher  found  higher  that occurred  in late  accelerated  rapidly  M.  winter on  mean  control  Batzli that  breeding  growth r a t e s than  f e e d i n g d i d not  and  higher  food-enhanced  However, C o l e a n d  showed  (1978)  reported  Apodemus on  males  prevent summer  the  (1978)  although rates, control  or d e l a y and  fall  the in  populations. Decline  1978,  grids than  p o p u l a t i o n and  population  Taitt  and  (1971) b o t h  p o p u l a t i o n s of  control grids.  food-enhanced  population,  Hansson  density,  maniculatus  that population densities  (1971)  maniculatus  population.  spring breeding  the  in  in  Fordham  Peromyscus  initial  a Mierotus  and  i n the c o n t r o l .  Flowerdew  fed  Mierotus  been  grids.  on  a  was  were h i g h e r on t h e f o o d - e n h a n c e d  than  fed  the d e c l i n e  than  (1972) and  the  that although  t o p o p u l a t i o n s o f P.  found  then  found  weights  grids  declines,  p o p u l a t i o n s h o u l d h a v e some  same as t h e c o n t r o l  food  in  to a p o p u l a t i o n of  while  the  excess  townsendii  have  population  excess  found  Krebs  p o p u l a t i o n and  prevented,  involved  food to a declining  measurable e f f e c t . californicus  is  and  the  rates major  unable of  precisely  to  explain  breeding. this  problem w i t h the f o o d the  However,  the  began i n  hypothesis i s that  sudden d r o p  I t seems u n l i k e l y  time.  when b r e e d i n g  in survival  t h a t food role  of  was  with  exhausted  food  can  i t the at be  173  investigated enclosure, the  by  i n c l u d i n g an  amount o f  density  that  and  not  (1978)  f o r the  is  breeding  be  due  voles to  dependent  individual  have  densities,  d e c l i n e with  no  and  Chitty  behavioural breeding  drugs,  in  administer  drugs to  aggression  (Krebs  undergo  a  such as  cause  of  a  survival,  season  decline  c o n t r o l population  Krebs can  to  and be  Boonstra tested  If  account  (Vessey^  for  1967)  and  observe  manipulations disappearance  its  are of  severe  one  or  vole population  by  aggressive  i s no d i s p e r s a l ,  in a  decline  Experimental  the  there  e t a l . , 1977)  presumably  tranquilizers.  reduce a g g r e s s i o n  severe  consequences. determine  the  hypothesis  which  breeding  winter  a breeding  i n t e r a c t i o n s are necessary  declines  then  If  dispersal.  p r e d i c t i o n s from  behaviour-modifying  vole-proof  initial  whereas t h e  behavioural the  upon  higher  and  in a  disperse into.  properties,  to d i s p e r s a l ,  would have a s e v e r e The  to a population  population should  initial might  area  dispersal  food-enhanced higher  supplying food  could  increase about  to  demographic  essential  voles  to  in  severe  •social  stress'  declines.  Literature Barnett, in  S.  A.  wild rats-I.  Res.  3:  Boonstra,  B.  Hicrotus Chitty, the  1958.  D.  cited  Physiological effects The  adrenal  cortex.  of J.  Psychosomatic  1-11. and  C.  J.  townsendii.  1955. viability  Krebs. J.  Mammal.  Adverse e f f e c t s of l a t e r  1978.  of  59:  Pitfall  of  136-148.  population  generations.  trapping  In  The  density Numbers o f  upon Man  r  174  and  Animals.  57-67. .  (eds. J .  Oliver  B.  W.  Pirie)  changes  in  pp.  and Boyd, L o n d o n .  1965.  Predicting  populations.  C r a g g and N.  qualitative  Proc.  12th  Int.  The  natural  Congr.  insect  Entomol.  Lond.  384-386. 1967. behaviour 2:  i n animal  selection  populations.  of self-regulatory  Proc.  Ecol.  Soc.  Aust.  51-78.  Chitty,  D.  in  a n d E.  Phipps.  mixed p o p u l a t i o n s  35:  1966.  Seasonal  changes i n s u r v i v a l  o f two v o l e s p e c i e s -  J.  Anim.  Ecol.  313-331.  Christian,  J.  J.  density  in  1971.  Fighting,  Microtus  maturity,  pennsylvanicus.  and  J.  population  Mammal.  52:  556-567. Cole,  F.  R.  and  supplemental 59:  G.  feeding  D.  J.  1977.  or d i s p e r s a l ? Flowerdew, J .  Ecol.  a  vole  Influence  population.  J.  of  Mammal.  J.  1972. of  Zool.  The e f f e c t  wood mice  55: 84-92. o f supplementary food  (Apodemus  sylvaticus).  J.  on  a  Anim.  4 1 : 553-566.  Fordham, B.  H.  1971.  supplemental food. L.  dynamics.  with  on  1978.  The s p r i n g d e c l i n e i n d e e r m i c e : Death  Can.  B.  population  K r e b s , C.  Batzli.  809-819.  Fairbairn,  Hansson,  0.  J.  1971. Oikos, and K.  Field  populations  Ecology, Small  of  deerraice  with  52: 138-146.  rodent  food,  f e e d i n g and p o p u l a t i o n  22: 183-198. T.  DeLong.  supplemental food.  J.  1965. Mammal.  A Microtus  population  46: 566-573.  175  K r e b s , C.  J . , M.  and  R.  H.  rodents. K r e b s , C.  S.  Science,  J.  C.  B.  Tamarin.  and J .  s m a l l mammals. Krebs,  Gaines,  1973.  Keller,  Population  Myers.  Adv. T.  1974.  Ecol.  Res.  H a l p i n , and J . and  populations  of  Mierotus  J.  spring  and  the  E.  vole  W.  Z.  dynamics. H.  in  J.  of  Ecol.  cycles  in  M.  Smith.  1977.  decline  townsendii.  in  Can. J .  Ecol.  Regulation  Monogr.  and C.  J.  Monogr.  41: 53-78.  F.  A. . 19 58.  short-term  Alaska.  Cold  of  and  of  Spring  vole  Mierotus  in  an  island  v o l e , a p r o b l e m i n community  of  Genetic,  dispersing  Mierotus  Some a s p e c t s  cycle  the  numbers  1971.  atrributes  pennsylvanicus  of  of the  4 3 : 271-302.  Krebs.  Mierotus  Demography  4 7 : 1007-1015.  the C a l i f o r n i a  reproductive  the  cycles i n small  spring  1978.  populations  Anim.  1973.  population  Pitelka,  N.  the  Boonstra.  decline  townsendii.  and  ,  55: 430-437.  K r e b s , C.  Myers, J .  Myers  8: 267-399.  testosterone,  Lidicker,  fi.  Population  Aggression,  Zool.  J.  179: 3 5 - 4 1 . H.  J . , Z.  L.  the  field  voles  ochrogaster.  of population  brown  Harb.  behavioral,  lemming  Symp.  Ecol.  structure i n in  Quant.  northern  Biol.  22:  237-251. Rose, R.  K.  1978.  Mierotus Rose,  R. in  K.  Levels of  pennsylvanicus. and M.  fluctuating  ochrogaster,  S.  wounding J.  in  Mammal.  Gaines.  i n eastern Kansas.  vole,  Levels of aggression  prairie  J.  meadow  In press.  1976.  populations of the  the  Mammal.  vole,  Mierotus  57: 43-57.  176  Schultz,  A.  H.  Arctic  1964.  microtine  relation  to  terrestrial Pp. ,  cycles.  Arctic  and  1969.  II.  microtine  recovery  hypothesis  for  Ecosystem  variables  in  Grazing  in  cycles.  marine e n v i r o n m e n t s  A study in  management York.  The (G.  o f an  (D.  In J.  Crisp,  ed.),  ecosystem: the a r c t i c tundra.  ecosystem H.  Van  concept  Dyne,  ed.).  in  natural  Academic  Pp.  resource  Press,  New  383p.  M.  J.  1978.  Population  maniculatus  austerus  supplementary  food.  University Vessey,  nutrient  57-58, B l a c k w e l l , O x f o r d .  77-93,  Taitt,  The  S.  367-376.  D.  Microtus  Effects  populations of  of  Peromyscus  townsendii  t h e s i s . Department o f  of B r i t i s h Columbia, Vancouver,  1967.  laboratory  Ph.  and  dynamics  of chlorpromazine wild  house  mice.  with Zoology,  BC. on  aggression i n Ecology,  48:  177  SECTION 6.  PITFALL  VERSUS LIVE-TRAP ENUMERATION  POPULATIONS OF  THE  OF  FLUCTUATING  VOLE MICROTUS TOWNSENDII  Introduction Two  main t e c h n i q u e s a r e used  mammal  populations.  assumes  that  capture,  an a s s u m p t i o n  populations  a l l  The  stochastic  individuals  caught  Longworth  study  was  to  individuals and  low  Krebs,  1966;  (1965)  probability  1966).  second  a l l of  et a l . ,  i s untestable  1976). because this  a l l  the  present i n the p o p u l a t i o n a t both  high  live-traps  first  the  of  i f  The  of  rodent  The  virtually  Hilborn  small  purpose  enumerate  densities.  segments Pitfalls  l i v e - t r a p s and  of  a microtine  individuals.  enter  live-traps,  recruited  pitfall  population  tend t o c a t c h younger  older  whereas  of  this  study  traps  ( B o o n s t r a and  c a u g h t by  o t h e r s may  different  Krebs,  1978).  pitfalls  catch  fail  t a k e s e v e r a l weeks t o  ( B o o n s t r a and K r e b s , was  sample  a n i m a l s , whereas l i v e - t r a p s  Many i n d i v i d u a l s  to l i v e - t r a p s  a n i m a l s caught some  t o be  Jolly  equal  t h i s assumption  determine  of  f o r many m i c r o t i n e  l i v e - t r a p s a r e used.  known  Longworth  purpose  an  assumes t h a t  (Krebs,  However, i n most s t u d i e s , only  have  ( L e s l i e e t a l . , 1953;  are  model  that i s i n v a l i d  method, o f d i r e c t e n u m e r a t i o n , individuals  to estimate the s i z e of  1978).  The  fail  d e t e r m i n e i f t h e two  to  t o examine t h e c h a r a c t e r i s t i c s  be  recruited  to live-traps  t r a p t y p e s sample d i f f e r e n t  social  be  second  by b o t h t y p e s o f t r a p s i n o r d e r t o d e t e r m i n e  individuals  to  and  of why to  segments  178  of the  adult  population.  Methods The  study  Canadian delta  area  Wildlife  was  S e r v i c e on  lanatus  species; were a l s o  Aqrostis  1976  until  1978.  fence,  controls.  Each g r i d  apart  a  in  7  l i v e - t r a p s and Boonstra  and  supplied  with  afternoon, and  7  traps  The  were used  September  every  closed  the  Holeus  locked  open.  from  1977,  May and  D were  7.6  m  had  baited  second  two  by  unfenced  (25  feet)  Longworth  described with  week  afternoon,  through April  oats  by and  on  Monday  and  checked  1976, 1978.  pitfalls  could  f l o o d i n g . , They were s e t  pitfalls  sex,  October  through June  periods, the  v o l e s were e a r - t a g g e d , and location,  and  t o the t y p e  Wednesday a f t e r n o o n ,  on T h u r s d a y a f t e r n o o n .  had  C were e n c l o s e d  morning.  of w i n t e r  week, t h e  weight, capture  A  m o r n i n g and  between t h e s e  because  B and  live-traps,  set  Wednesday  i n one  All  trap similar  Tuesday  were s e t been  was  were l i v e - t r a p p e d f r o m  Each s t a t i o n  Wednesday m o r n i n g , c h e c k e d  m o r n i n g , and  the  River  p e r e n n e , and  t r a p s t a t i o n s set  were  intervals  used  Lolium  populations  pattern.  cotton,  the  Fraser  Phleum p r a t e n s e  populations  49  (1978).  During  week on  had  Krebs  l o c k e d o p e n on  be  and  pitfall  through  Columbia.  Populations  one  April  not  x  checked  Pitfall  owned by  I s l a n d i n the  alba,  townsendii  June  vole-proof  field  present.  Four Microtus  a  Reifel  near Vancouver, B r i t i s h  dominant  May  p a r t o f a T i m o t h y hay  When b o t h  were s e t a f t e r  on  the  each c a p t u r e ,  reproductive  every  Thursday  trap  types  live-traps  tag  condition  number, ,  and  179  wounding after 42g;  were  being  recorded.  processed.  subadult  a l l  Voles  v o l e s were r e l e a s e d  were c l a s s i f i e d  30-42g, j u v e n i l e  inside  each  assumed  t h a t untagged v o l e s caught  enclosure  h a d been b o r n  dispersers. The the  were  as f o l l o w s : a d u l t >  <30g.  V o l e s c r o s s i n g a s t r i p 60 f t wide enclosure  defined  (18.3 m) o f as  trappability  used  of  grass I t was  i n disperser traps inside  on t h e g r i d  demographic  an  a n d were t h e r e f o r e u n t a g g e d  enumeration technique  determination  mowed  dispersers.  D i s p e r s i n g v o l e s were removed f r o m  total  immediately  o f Krebs data.  the grids.  (1966) was used i n The  estimate  of  was: number o f c a p t u r e s f o r an a n i m a l  Trappability  =  number o f p o s s i b l e c a p t u r e s f o r t h a t  animal  N where  N  i s  trappability  the  number  of  v o l e s caught  i s summed o v e r a l l N a n i m a l s .  times  of  capture  are  animal  must be c a u g h t  excluded  at these  in  more t h a n The f i r s t  t w i c e , and and  last  t h e summation, b e c a u s e an  times.  Results  Trappability  The  trappability  calculated  e s t i m a t e s of  independently  for  live-traps  each  i n d i v i d u a l s known t o be p r e s e n t f r o m but  not  in  live-traps  were  not  trap  and p i t f a l l s  type.  capture included  in  were  F o r example, pitfall  in  traps  trappability  180  e s t i m a t e s o f those males  and  constant  females  from  trappability was  65%  captured i n in  summer  1976 u n t i l  male t r a p p a b i l i t y  were s i m i l a r  spring  summer  Low d e n s i t i e s t o 95% f o r  had s i m i l a r  calculated twice.  Pitfalls  live-traps  The  in  During  this  May  or  be a l i v e  female  and  could  not  be  was c a p t u r e d more effective  Many v o l e s  as  avoided  traps.  1976  and  remained  live-trap  counts,  when  throughout  which  trapping  t h e summer.  were s i m i l a r t o indicates  that  were c a p t u r i n g p r a c t i c a l l y a l l o f t h e v o l e s known (Figs.  combined  low  population counts  and p i t f a l l  6.1, 6.2, 6.3, 6 . 4 ) .  The  live-trap  o n e - h a l f t h e magnitude of e i t h e r  counts.  throughout  Live-trap the  1976  p o p u l a t i o n s were n o t b r e e d i n g . lag  and  and i n t h e 1977  one-half as  captures of voles.  in pitfall  approximately  increased  o f 1978  in pitfalls,  Trappability  were o n l y a b o u t  period the p i t f a l l  pitfalls  were  i t  Density  t h e combined  to  animals  d e n s i t y o f t h e v o l e p o p u l a t i o n s was low  in  the  96  1978 b e c a u s e no a n i m a l  repeated  capture  began  summer  i n the s p r i n g  1976 i t was 3 5 % f o r 727 a n i m a l s ,  f o r spring  Population  and r e l a t i v e l y  I n t h e 1976 summer  trappability  summer i t was 2 8 % f o r 1726 a n i m a l s .  repeated  1978.  of  t o 8 1 % f o r 155 a n i m a l s .  M a l e s and f e m a l e s  than  Trappabilities  was 6 1 % f o r 324 a n i m a l s a n d i n t h e 1977  trappability  during  live-traps  f o r 1692 a n i m a l s .  increased  live-traps.  population winter  Thus t h e r e  at  the p i t f a l l  counts a  was  time a  i n r e c r u i t m e n t o f young v o l e s t o t h e l i v e - t r a p s  counts  steadily when  the  considerable during  this  181  F i g u r e 6.1. P o p u l a t i o n d e n s i t y o f M i c r o t u s t o w n s e n d i i on c o n t r o l g r i d A. Minimum number a l i v e (MNA) i s g i v e n f o r t h e p o p u l a t i o n known t o have entered live-traps, f o r those known t o have e n t e r e d p i t f a l l s , and f o r t h o s e known t o have entered one o r both t y p e s of t r a p s . Non-breeding p e r i o d s a r e shaded.  400 r  1976  1977  1978  183  Figure 6.2. P o p u l a t i o n d e n s i t y o f M i c r o t u s t o w n s e n d i i on f e n c e d g r i d B. Minimum number a l i v e (MN&) i s g i v e n f o r t h e p o p u l a t i o n known t o have entered live-traps, f o r those known t o have e n t e r e d p i t f a l l s , and f o r t h o s e known t o have entered one o r both t y p e s o f t r a p s . Non-breeding p e r i o d s a r e shaded.  Minimum  78 T  Number  Alive  185  Figure 6.3. P o p u l a t i o n d e n s i t y o f M i c r o t u s t o w n s e n d i i on f e n c e d g r i d C. Minimum number a l i v e (MNA) i s g i v e n f o r t h e p o p u l a t i o n known t o have entered live-traps, f o r those known t o h a v e e n t e r e d p i t f a l l s , and f o r t h o s e known t o have entered one o r both t y p e s o f t r a p s . Non-breeding p e r i o d s a r e shaded.  Minimum Number Alive  98T  187  F i g u r e 6.4. P o p u l a t i o n d e n s i t y o f M i e r o t u s t o w n s e n d i i on c o n t r o l grid D. Minimum number a l i v e (NN&f i s g i v e n f o r t h e p o p u l a t i o n known t o h a v e entered live-traps, f o r those known to have entered pitfalls, a n d f o r t h o s e known t o have e n t e r e d one o r both t y p e s o f t r a p s . Non-breeding p e r i o d s a r e shaded.  Minimum  88T  Number  Alive  189  time.  Figures  populations live-trap  6.1,  declined  population  live-trap  6.2, in  and  6.4  indicate  t h e s p r i n g o f 1977.  estimates  and p i t f a l l  1977 l i v e - t r a p s  6.3,  were  estimates,  were e n u m e r a t i n g  that a l l  By t h i s  similar ; to  time the  the  combined  w h i c h i n d i c a t e s t h a t by s p r i n g most o f t h e v o l e s known  to  be  alive. The early a  pitfall  summer  4%  per  pattern The  population  1977 a t a r a t e o f a b o u t week  increase  o f 1976 r e p e a t e d  pitfall  combined  alive.  reproduction,  the  There  again  live-trap early  were  of  population  b u t by s p r i n g  summer  voles  The  of  similar  whereas  the  with  1977. to the  live-traps known t o be  the tapering  o f f of  d e c l i n e d a t about  20% per  population  considerable  during  populations.  again  summer o f 1977 w i t h  live-trap  rapidly  15% p e r week compared  estimates,  one-half  pitfall  a  increased  i n the  pitfall  the l a t e  week w h i l e  live-traps,  and  the  the  estimates  approximately  During  was  in  itself  population  live-trap  enumerated  estimates  was  time  relatively  lag  1978 t h e l i v e - t r a p s  stable.  i n recruitment were  to  enumerating  most o f t h e a n i m a l s known t o be a l i v e . The it  was  caught  vole  populations  declined  p o s s i b l e t o t r a p with in pitfalls  pitfalls.  a t t h i s time.  were e n u m e r a t i n g more v o l e s t h a n  First  C a p t u r e Of V o l e s  I  compared  the  sharply  During  i n spring  before  T h u s , few a n i m a l s this  the p i t f a l l s  I n L i v e - t r a p s And  1978  period  were  live-traps  were.  Pitfalls  d i s t r i b u t i o n s of i n i t i a l vole captures i n  l i v e - t r a p s and p i t f a l l s .  This comparison  included  only  the time  190  when b o t h t r a p in  live-traps  were u s e d c o n c u r r e n t l y .  during  were e x c l u d e d pitfalls  types  the w i n t e r  from t h i s  could  Initial  when p i t f a l l s  analysis.  were not o p e r a t i n g  An i n d i v i d u a l  be r e c r u i t e d t o l i v e - t r a p s  captures  f i r s t caught i n  a t any t i m e d u r i n g t h e  study. Approximately  3700 v o l e s were f i r s t c a u g h t  a b o u t 450 were f i r s t c a u g h t i n l i v e - t r a p s . 45%  of  the  voles  live-traps. immigration pitfalls  were  not size  unfenced the  46% o f t h e a d u l t  6.2  live-traps.  in  populations  the  of  adult  the  fenced females  are pooled  over  o f animals i n the  live-traps  than  in  (Xz=12.?8, P<.01).  spring to f a l l  pitfalls.  than  and  live-traps  of  f o r 79%.  juvenile  class,  captured  trappable  43% In  When t h e d a t a  failed  enter  i n t o which  m a l e s and 26% o f t h e a d u l t  However,  period  the  have  a  in in  were  higher  of capturing a vole f o r the f i r s t  first-caught  pitfalls adult  accounted  females,  The r e s u l t s a r e e v e n more s t r i k i n g  for  they the  which 94% o f 2213 f i r s t - c a u g h t i n d i v i d u a l s  pitfalls.  population,  live-traps.  289  i n each y e a r  pitfalls  Of t h e 435 f i r s t - c a u g h t a d u l t m a l e s , 80%,  not  that  shows t h a t 40% o f t h e 447 a n i m a l s f i r s t c a u g h t i n  during  trapped  did  whereas  to enter  accounted  than  in  i n live-traps.  experimental  shows  and  males f i r s t c a u g h t i n  c o n t r o l populations  probability  were  caught  6.1  c o n t r o l populations,  proportion  live-traps  for  pitfalls  and s e x c l a s s e s , a g r e a t e r  fenced  time.  not  caught  Table  not  unfenced  in  was p o s s i b l e , 50% o f t h e a d u l t  populations  all  the  caught  were n o t c a u g h t i n l i v e - t r a p s ,  females  were  In  first  Table  in pitfalls  pitfalls  After  voles  are able  have  to detect  entered them  the  earlier  191  Table 6.1. Numbers o f M i c r o t u s t o w n s e a d i i f i r s t c a u g h t i n p i t f a l l s and l a t e r i n l i v e - t r a p s , a n d ~ t h o s e c a u g h t o n l y i n p i t f a l l s . D a t a a r e pooled over the e n t i r e study. Females  Males Group  Juvenile  Subadult Adult  Juvenile  Control F i r s t caught in p i t f a l l s L a t e r caught in live-traps Caught o n l y in p i t f a l l s  Subadult Adult  Total  populations  387  233  224  374  231  147  1596  188  114  111  200  132  84  829  199  119  113  174  99  63  767  Experimental populations F i r s t caught in p i t f a l l s L a t e r caught in l i v e - t r a p s Caught o n l y in p i t f a l l s  638  305  122  675  260  81  2081  371  168  66  388  15 1  60  1204  267  137  56  287  109  21  877  All F i r s t caught 1025 in p i t f a l l s L a t e r caught 559 in l i v e - t r a p s Caught o n l y 466 in p i t f a l l s  populations  538  346  1049  491  228  3677  282  177  588  283  144  2033  256  169  461  208  84  1644  192  T a b l e 6.2. Numbers o f M i e r o t u s t o w n s e n d i i f i r s t c a u g h t i n l i v e - t r a p s and l a t e r i n p i t f a l l s , and t h o s e c a u g h t o n l y in live-traps. First captures o f v o l e s i n l i v e - t r a p s when p i t f a l l s were n o t c o n c u r r e n t l y t r a p p e d were e x c l u d e d . Females  Males Group  Juvenile  Subadult  Adult Juvenile Control  F i r s t caught in live-traps L a t e r caught in p i t f a l l s Caught o n l y in live-traps  Total  33  40  66  33  38  50  260  20  24  33  23  21  33  154  13  16  33  10  17  17  10 6  populations  29  39  23  44  41  11  187  23  21  13  30  20  7  114  6  18  10  14  21  4  73  All F i r s t caught in live-traps L a t e r caught in p i t f a l l s Caught o n l y in l i v e - t r a p s  Adult  populations  Experimental F i r s t caught in live-traps L a t e r caught in p i t f a l l s Caught o n l y in live-traps  Subadult  populations  62  79  89  77  79  61  447  43  45  46  53  41  40  268  19  34  43  24  38  21  179  193  Body H e i g h t  Differences  At  First  Capture  In  Live-traps  And  Pitfalls  Table  6.3  shows  Microtus townsendii  t h e mean body w e i g h t s  at f i r s t  T h e s e means i n c l u d e w e i g h t s in  one  type  already  trap,  been c a u g h t  individuals entered weight  of  live-traps at f i r s t  The  of a l l i n d i v i d u a l s  regardless  trapped  were  in  body  was  weight  about  15  of  than  M a l e s and  in pitfalls.  D were s i g n i f i c a n t l y  compared {Table  with  m a l e s and  6.3).  account  Mean w e i g h t pitfalls.  of  size classes. voles, higher  and  had  F o r example, a l l and  which  a l l  of  later o f mean  capture  {Table  in  the  6.3).  populations  capture  in  enclosed  pitfalls  populations  was  unfenced  higher  difference?  captures  in  than  Table  live-traps  likely  entered  to  capture  the c o n t r o l  p o p u l a t i o n s (X =60.46, p<.0001)  and  juvenile  to capture adults.  live-traps  6.4 and  untagged a c c o r d i n g t o sex  were more l i k e l y  d i d males i n both  in  live-traps  i n the unfenced  in live-traps  or  were more  2  capture  difference.  first  v o l e s tagged  Pitfalls  first  o f heavier animals i n t o the  p r o p o r t i o n of females  fenced  in  capture  whereas l i v e - t r a p s  s u b a d u l t s than  at  What i s t h e b a s i s o f t h i s  shows t h e d i s t r i b u t i o n pitfalls  they  the c a l c u l a t i o n  heavier at f i r s t  for this  at f i r s t  first  females  females  Immigration  p o p u l a t i o n s may  in  capture  or not  g more-than i n p i t f a l l s  12 g h e a v i e r a t  and  of t r a p .  males  F e m a l e s were a b o u t  A  o f whether  in  pitfalls.  at f i r s t  pitfalls  included  and  female  capture i n l i v e - t r a p s .  mean  live-traps  capture i n l i v e - t r a p s  i n the other type  initially  of male and  as j u v e n i l e s  A or  (X =45.57, P<.0001) 2  (Table  6.4).  Young  194  T a b l e 6.3. Mean w e i g h t i n grams a t f i r s t c a p t u r e (+ 1 SE) o f Mierotus townsendii in live-traps and p i t f a l l s f o r four p o p u l a t i o n s . Data f o r e a c h grid a r e pooled over the entire s t u d y . Sample s i z e s a r e i n p a r e n t h e s e s . Pitfalls  Live-traps Grid  Males  Females  Males  Females  A ;  4 8.8+0.5 (378) 46.7+0.4 (480) 4 5.2+0.5 (391) 49.0+0.4 (459)  42.6+0.4 (386) 42.8+0.3 (414) 39.8+0.3 (402) 44.9+0.4 (396)  35. 1+0.8 (426) 29. 1+0.3 (661) 29.1+0.5 (530) 36. 1 + 0.5 (568)  32.4+0.7 (424) 29.1+0.4 (593) 25.8+0.4 (545) 36.9 + 0.7 (516)  47.4+0.5 (1708)  42-5+0.3 (1598)  32. 1+0.6 (2185)  30.8 + 0. 5 (2078)  B C D Total  195  Table 6.4. Numbers o f M i c r o t u s t o w n s e n d i i c l a s s at f i r s t capture i n l i v e - t r a p s and pooled over the e n t i r e study. Live-traps  Total  Control populations 80 147 403 293 488 264 409 984 327  395 256 289  798 520 616  Experimental 137 236 336 577 343 874  702 292 144  1376 625 328  Females T o t a l  Juvenile Subadult ad u l t  67 195 575  Juvenile Subadult adult  99 241 531  class  Pitfalls Females  Males  age  i n e a c h s i z e and sex pitfalls. Data are  Males  populations 674 333 184  196  males a r e l e s s These first is  likely  t o e n t e r l i v e - t r a p s than young f e m a l e s .  results  suggest  capture i n p i t f a l l s  this  time  lag  caught  in  adult  males  took  2  weeks  juvenile  caught  in pitfalls  time  (Table 6.5).  by t h a t pitfalls as  were more l i k e l y  subadults.  pitfalls, The  The  maximum  captures  7  in  pitfalls  were  weeks, whereas  adult  for 5 females  weeks.  Juvenile  4 weeks l o n g e r t h a n juvenile  males  d i d e n t e r l i v e - t r a p s were  adults  Juvenile  those  females  first  to enter l i v e - t r a p s  heavier  t h e sooner  caught  Over 50% o f  which  How  t h e age a n d s e x c l a s s e s ?  f o r about  adults to enter live-traps. first  over  i n live-traps and  i s a t i m e l a g between  capture i n l i v e - t r a p s .  males f i r s t  live-traps  f e m a l e s were n o t c a u g h t  there  and f i r s t  distributed  T a b l e 6.5 shows t h a t not  that  an i n d i v i d u a l  caught  f o r the f i r s t at f i r s t  in time  capture i n  i t tends t o enter the l i v e - t r a p p o p u l a t i o n .  interval  in pitfalls  recorded  in  and l i v e - t r a p s  this  study  between  first  was 54 weeks f o r a s u b a d u l t  female. T a b l e 6.1 pitfalls. long  in  t i m e between f i r s t  of  the  38  life  pitfalls  voles  were  caught  only  in  population  These about  o v e r 50% o f t h e i n d i v i d u a l s  were t r a p p e d o n l y o n c e .  and l a s t  2 weeks.  enumeration of  1644  T a b l e 6.6 i n d i c a t e s t h a t  only  l e s s than  that  Are these i n d i v i d u a l s present i n t h e p o p u l a t i o n f o ra  time?  caught  shows  captures of  these : i n d i v i d u a l s  animals comprise which  had been by l i v e - t r a p s  a substantial  nothing only.  would The  r e c o r d e d f o r an i n d i v i d u a l caught  weeks f o r a j u v e n i l e  female.  The a v e r a g e  be  maximum  was  portion  known  i f  duration  only i n p i t f a l l s  was  197  Table 6.5. Percentage distribution by size class of i n t e r v a l between f i r s t c a p t u r e i n p i t f a l l s and f i r s t c a p t u r e in live-traps of Mierotus townsendii. Classification used i s size at f i r s t capture i n p i t f a l l s . A l l g r i d s and y e a r s a r e c o m b i n e d .  Females  Males  Duration (weeks)  Juvenile  Subadult  Adult  Juvenile  Subadult  Adui'  1-5 6-10 11-15 16-20 >20  43.3 22.7 15.0 9.1 9.9  47.5 20.9 16.3 6.0 9.3  54.8 25.4 10.2 3.4 6.2  44. 5 24. 6  48.1 23.7 13.4 5.3 9.5  71. 3 18. 2 7.0 2. 1 1. 4  Average interval+2SE  9.2+1.3  Sample  559  282  15.0 31.3 53.7  28.1  size  Centering live-traps as: Juveniles Subadults Adults  8.6+1. 1  71.9  7. 1 + 1.2 177  100.0  11.6  7.0 12.3  9.3+1.7  8 .8+3.7  588  283  17.6 43. 4 39.0  39.6 60.4  5 . 1+1. 144  100. 0  198  T a b l e 6.6. Percentage distribution by size class of interval between first and last pitfall captures o f Mierotus townsendii c a u g h t o n l y i n p i t f a l l s . 1976 and 1977 a r e c o m b i n e d .  (weeks)  Juvenile  Subadult  0 1-5 6-10 >10  62.4 29.6 7.5 0.5  Control 67.2 31.1 1.7 0.0  Sample  size  0 1-5 6-10 >10 Sample  size  Females  Males  Duration  adult  Juvenile  populations 54. 0 69.0 27.4 35. 1 9.8 2.7 1. 1 0.9  47.6 43. 1 7.8 1.5  174 113 119 Experimental populations 56. 1 62.4 55.5 30.4 33. 8 34.3 8.4 5.4 7.3 1.7 1.8 2.9  267  137  199  56  287  Subadult  AduH  66.7 25. 3 8.0 0. 0  66. 7 30. 1 1.6 1.6  99  63  68.8 23.9 7. 3 0.0  71.4 23.8 0.0 4.8  109  21  199  Dispersal  The  experimental  dispersing caught  voles.  A dispersing  shows t h a t  dispersers  90%  used  in live-traps, pitfalls,  T a b l e 6.7  only  design  and  part of of  the  the  y e a r , caught  in  capturing  study  b o t h t y p e s of enumerated  60%  of the  Pitfalls dispersers  identified  previously  of tagged d i s p e r s e r s .  tagged d i s p e r s e r s .  live-traps  this  vole could  live-traps  60%  in  traps, about  have  or  total  neither.  30%  Pitfalls,  been  of  all  operating  dispersers  and  were more e f f e c t i v e t h a n before  they  left  the  population. T a b l e 6.1 and  130  females  pitfalls.  127  a  maximum  only  in  subadult  and  pitfalls  only.  of  130  the  populations  Juvenile  an  adult  were c a u g h t  indicates  were  either  62%  that  enumerated  of  the  dispersed.  by  failed  only  adult  before were  live-traps. or  in  Of  adults.  subadult  and  adult  males  There  were  122  known  had  These d i s p e r s e r s accounted f o r a  that  males  there  subadults  female d i s p e r s e r s that  and  193  these i n d i v i d u a l s emigrated  m a l e s not  pitfalls  juveniles,  populations  T a b l e 6.7  of  subadult  females  been t r a p p e d maximum o f  i n the  in 94%  experimental  to enter l i v e - t r a p s .  Survival  Juvenile of  of  i n d i v i d u a l s , 120  Therefore, caught  tagged  excluding  fenced  live-traps?  dispersing  these  i n the  What p r o p o r t i o n  they e n t e r e d 127  indicates that,  index  survival i s usually ( K r e b s and  DeLdng,  measured i n d i r e c t l y  1965), b e c a u s e d i r e c t  by  means  estimation  200  T a b l e 6.7. Number o f d i s p e r s i n g M i c r o t u s t o w n s e n d i i c a u g h t in l i v e - t r a p s and p i t f a l l s . D a t a a r e p o o l e d o v e r t h e e n t i r e s t u d y . Grid B Grid C Males Tagged d i s p e r s e r s previously  84  61  40  83  92  44  35  179  169  96  70  24  7  9  5  203  176  105  75  59  48  58  53  pitfalls  88  96  91  93  dispersers  85  91  58  47  288  267  163  122  42  31  27  33  62  63  59  57  Tagged d i s p e r s e r s previously  dispersers  dispersers  by l i v e - t r a p s  tagged  caught  dispersers  by  Untagged Total  not  pitfalls  T o t a l tagged  %  caught  by p i t f a l l s  Tagged d i s p e r s e r s  caught  not  live-traps  % tagged  dispersers  % total  Females  120  c a u g h t by  by  Males  caught  by l i v e - t r a p s  Tagged d i s p e r s e r s  caught  Females  dispersers  c a u g h t by  live-traps  % t o t a l dispersers c a u g h t by p i t f a l l s  201  of  survival  is  juveniles.  dependent  This  condition  comparison  between o b s e r v e d  Krebs  and  DeLong  rates  per  two  populations Krebs  weeks  DeLong  pitfall-caught juvenile data  satisfied  made.  highest  i n the  index  for  1978  solely 1977  large  (Table  numbers  i n t h i s study, the  townsendii populations,  the  a of  minimum s u r v i v a l  in  1976  the  increasing  6.8),  and  i s not  (r=.22).  In  K r e b s and  the and  both measures  (r=.69, P<.02).  and  was  live-trap  However,  f r o m l i v e - t r a p p i n g show t h a t  minimum s u r v i v a l r a t e s  and  Juvenile  combined  peak p o p u l a t i o n s ,  of  index  d e c l i n i n g o n e s , as  the  s u r v i v a l were c o r r e l a t e d  lowest i n the  Jl-  be  population  derived  observed  was  were  lowest  capturing  j u v e n i l e s u r v i v a l and  (1965) can  and  and  upon  the  these high  DeLong  the  index  correlated  index  of  is  with  density is  too  low.  A C o m p a r i s o n Of  It capture  has  adult  already  juvenile  Hales Caught In  been  voles,  demonstrated whereas  However,  adult  there  demographic d i f f e r e n c e s  any  pitfalls  v o l e s can  and  those  subordinate v o l e s h a v e pitfalls In  sampled  addition to  examined  their  i n both  segments  maturity  the  reproductive  and  tend  capture  adults.  voles  of  caught  Assuming  to  Are in that  i f l i v e - t r a p s and the  wounding  infection  Pitfalls  pitfalls  live-traps?  more wounds, I e n g u i r e d  parasitic  and  t y p e s of t r a p s .  between a d u l t  in  different social  of  that  live-traps  caught  caught  F e m a l e s were e x c l u d e d f r o m wounded and  be  reproductive  degree  Live-traps  population.  levels,  in adult  male  I  also  voles.  a n a l y s i s b e c a u s e t h e y were s e l d o m maturity  was  difficult  to  assess.  202  T a b l e 6.8- J u v e n i l e minimum s u r v i v a l r a t e s and i n d e x o f j u v e n i l e survival o f Krebs and DeLong (1965) f o r t o t a l p o p u l a t i o n s and for live-trap populations of Microtus townsendii. Data are pooled over the entire breeding season i n e a c n y e a r . 1978 i n c l u d e s March t o June only. The t o t a l population includes combined l i v e - t r a p a n d p i t f a l l p o p u l a t i o n s . Index Minimum s u r v i v a l t o t a l population p e r 2 weeks  Index live-traps  Year  Grid  1976  D A B C  0.44 0.58 0.70 0.66  1.06 1.16 2. 54 2.55  0.38 0.46 0.82 0.77  1977  D A B C  0.50 0.58 0.56 0.63  0.83 1.07 1.27 0.60  0.13 0.21 0.16 0.13  1978  D A B C  0.25 0.25 0.25 0.00  0.55 0.40 0.60 0.50  0.50 0.40 0.43 0.00  20 3  because  females  externally  in  the  and  10-g  live-traps  state  or  externally recorded  each  concurrently.  Male  testes size  for  are  and  Data  on  i n which  botfly  (Cuterebra  sp.|,  captured maturity  These  t y p e , and  calculated  population.  for  ft  live-traps,  pitfall  males When  comparison  data  in  i n the  each  year).  the  no  o f 20 c o m p a r i s o n s had  comparison  fewer  in  were A sign  trap-type  more wounds t h a n males was  caught test  each  were made i n e a c h  scored  wounds, t h e c o m p a r i s o n  males 'NC.  class  i n one  analysis  as a • + *, was  male  year. caught  when  scored  as  or both c l a s s e s , was  used  to  the a the  analyze  data. Table  6.9  40-59 g c a u g h t condition  than  indicates in  that  pitfalls were  i n t h e summer o f  were  more  similar-sized  often  1976,  in  t h e s e weight c l a s s e s caught i n maturity  at l i g h t e r  live-traps. more wounds  In  (12+, 3-,  pitfalls  attained  r e p r o d u c t i v e males caught 5NC,  of  males c a u g h t i n l i v e - t r a p s  weights than d i d s i m i l a r - s i z e d  1976,  males  reproductive  (40-49 g X*=3.89, P<.05; 50-59 g X*=15.93, P<-01). , The  had  were  t h e d a t a were  were summed o n l y  t o September  weight  in pitfalls  had  was  each  total  When males c a u g h t  in  was  a v e r a g e number o f wounds p e r male and p e r r e p r o d u c t i v e  were  in  b o t h t r a p t y p e s were o p e r a t e d  parasitism  (from J u l y  Males The  males  position.  not  40-90 g .  reproductive  c a p t u r e i n each t r a p  periods of i n f e s t a t i o n  the  pregnancy  botflies  recorded  pitfalls.  j u d g e d by for  were  summed o v e r t h e i n t e r v a l s  in  of  weight c l a s s e s from  o f wounds, number o f p a r a s i t i c  reproductive  The  stage  d i s t i n g u i s h a b l e from n o n r e p r o d u c t i v e females.  were o r g a n i z e d i n t o f i v e number  early  males i n  reproductive males in  P<.05) a s d i d a l l males  caught  pitfalls (13+,  3-,  204  Table 6.9Demographic a t t r i b u t e s o f male B i c r o t u s t o w n s e n d i i c a u g h t d u r i n g summer 1976 i n p i t f a l l s and l i v e - t r a p s . A l l grids are c o m b i n e d . The t i m e i n t e r v a l i s t h e e n t i r e summer e x c e p t f o r t h e b o t f l y r e s u l t s when males i n t h e b o t f l y s e a s o n (July through September) were s c o r e d . Sample s i z e s a r e i n p a r e n t h e s e s .  Weight  %  scrotal  Wounds/male  W o u n d s / s c r o t a l male  40-49 50-59 60-6 9 70-79 >80  23.5 62-3 76.0 92.8 91.7  (259) (199) (146) (56) (12)  P i t f a l l c a p t u r e s o f males 0.98 (61) 0. 53 (259) 1.34 (124) 1.28 (199) 0.96 (111) 0. 82 (146) 0.65 (52) 0.68 (56) 0.45 (11) 0.50 (12)  Total  53.4  (672)  0. 83 (672)  Bots/male  0.38 0.33 0.68 0.63 0. 18  (213) (153) (120) (49) (11)  1.04 (359)  0. 45  (546)  40-49 50-59 60-69 70-79 >80  13.6 38.7 73.8 93.5 100.0  (88) (111) (107) (46) (6)  L o n g w o r t h l i v e - t r a p c a p t u r e s o f males 0.28 1.08 (12) 0.41 (88) 0.39 0.67 (43) 0.70 (111) 0.44 0.74 (79) 0.61 (107) 0.24 0.56 (43) 0.56 (46) 0.00 0.17 (6) 0.17 (6)  Total  51. 1  (358)  0.58 (358)  0.69 (183)  0.37  (72) (92) (86) (29) (2) (281)  205  4NC,  P<.05).  botfly  Males  caught i n p i t f a l l s  infestation  Table during  6.10  1977  similar  that  males  males c a u g h t i n l i v e - t r a p s males c a u g h t  caught  males  shows  1 t i e , 4NC,  (18+,  botflies In  higher  P<-01).  Hales  (7+,  10-,  3 ties),  1978  males  caught  had  (17+,  had  more wounds  caught  in  in pitfalls, (6+,  botfly  data  are  the b o t f l y  pitfalls  1-,  whether  In  and  tend  lower  were  higher  populations  Why  had  and  maturity  rates  greater  males  sign  ended  caught  levels  reproductive  in live-traps.  t h a n 79 g  lower  1976  in  of  infestation.  in  wounding.  infestation  Not Caught  first  test.  the study  than d i d medium-sized  lower r a t e s o f  botfly  were  were n o t  were t h e same i n t h e  than i n the i n c r e a s i n g  Are Some Males  Some  were more o f t e n  M a l e s 40-49 g and  wounding  V e r y l a r g e males  the  have h i g h e r  o f t h e same s i z e  B a t e s o f male r e p r o d u c t i v e  have  fewer  or not they  because  in pitfalls  in p i t f a l l s  c o n d i t i o n t h a n were males  weight c l a s s e s .  had  fewer.  t o have g r e a t e r r a t e s of b o t f l y  M a l e s 40-49 g c a u g h t  did  began.  summary, m a l e s c a u g h t  wounding  than  12NC), b u t t h e r e  a v a i l a b l e i n 1978  season  this  P<.01) a s d i d r e p r o d u c t i v e  but not s i g n i f i c a n t l y  more wounds  were  At  2  enough c o m p a r i s o n s t o i n d i c a t e s i g n i f i c a n c e by  before  pitfalls  (X =21.80, P<.001),.  3-,  of  P<.05).  40-49 g c a u g h t i n  in p i t f a l l s  in live-traps  2-,  reproductive,  No  levels  were more o f t e n i n r e p r o d u c t i v e c o n d i t i o n t h a n  peak d e n s i t y males  (12+, 3-,  a l s o had  heavier  tended  to  adult  males.  Wounding  rates  t h e peak  1977  populations.  In L i v e - t r a p s ?  caught  in  pitfalls  later  entered  206  Table 6.10. Demographic attributes of tale J i c r o t u s townsendii c a u g h t d u r i n g summer 1977 i n p i t f a l l s and l i v e - t r a p s . A l l grids are combined. The i n t e r v a l i s t h e e n t i r e summer e x c e p t f o r t h e b o t f l y r e s u l t s , when males i n t h e b o t f l y s e a s o n o n l y were scored. Sample s i z e i s i n p a r e n t h e s e s . Hounds/scrota1  male  Bots/male  %  40-4 9 50-59 60-69 70-79 >80  18.8 53.7 93.4 98. 1 99.0  (768) (296) (274) (267) (101)  P i t f a l l c a p t u r e s o f males 0.20 (14 5) 0.04 (768) 1.78 (159) 0.96 (296) 1.85 (256) 1.76 (274) 1.72 (262) 1.70 (267) 1.19 (100) 1.21 (101)  0. 16 0.22 0.34 0.40 0.16  Total (1021)  54.0  (1706)  0.81  1.48 (922)  0.20  (489) (386) (468) (434) (105)  L o n g w o r t h l i v e - t r a p c a p t u r e s o f males 0. 10 0.49 (45) 0.06 (489) 0.28 1.27 (202) 0.70 (386) 0.49 1. 17 (423) 1.09 (468) 0.24 0.93 (415) 1.00 (434) 0.08 1.15 (105) 1. 15 (105)  (1882)  0.72  40-4 9 50-59 60-69 70-79 >80 Total (1049)  scrotal  Hounds/male  Height  9.2 52.3 90.4 95.6 100.0 63.2  (1706)  (1882)  1.08  (1190)  0.26  (681) (197) (82) (42) (19)  (459) (265) (172) (127) (26)  207  live-traps; these was  males?  in  did  not.  Are t h e r e  An a n a l y s i s s i m i l a r  conducted  only are  some  on t h e s e  pitfalls  any d i f f e r e n c e s  between  t o t h e one p r e v i o u s l y o u t l i n e d  two g r o u p s o f i n d i v i d u a l s . , H a l e s  are defined  as PO m a l e s w h i l e  those  caught  males  who  caught  in  e v e n t u a l l y c a u g h t i n l i v e - t r a p s a r e PL m a l e s . Table  pitfalls  6.11  indicates that  only  were  more  i n 1976  often  males 40-59 g  i n reproductive  were m a l e s e v e n t u a l l y c a u g h t i n l i v e - t r a p s , b u t was  not  significant.  PO  males had  the  than  difference  more wounds t h a n t h e i r  counterparts  (10 + , 6-,  6-,  4NC), b u t t h e d i f f e r e n c e s were n o t s i g n i f i c a n t .  2 ties,  males had h i g h e r (10*,  4-, In  often  r a t e s of  6NC), b u t a g a i n 1977,  males  i n reproductive  live-traps  wounds  (3+,  14-, 3NC,  this  reproductive  infestations  d i f f e r e n c e was  40-49 g c a u g h t o n l y  only  Hales  (Table  6.12).  How  ( 3 - , 14+, males  that  P<.05).  Under  eventually  were  had  than  (8+, PO  males  significant. were more  PL  entering  males  P<.05) a s d i d PL r e p r o d u c t i v e  3NC,  PL  had males  PL m a l e s a l s o high  density  recruited those  to  males  in pitfalls. 40-49  l o w e r wounding also  not  PL  in pitfalls  l i v e - t r a p s had more wounds and more b o t f l i e s caught  than  P<.05) t h a n t h e i r PO c o u n t e r p a r t s .  those  males  c o n d i t i o n t h a n were 40-49 g m a l e s  (4+, 13-, 4NC,  more b o t f l i e s  conditions,  botfly  {X*=22.55, P<.001)  more  had  4NC) , as d i d PO  condition  g  and g r e a t e r  t h a n 79 g a g a i n  r a t e s than d i d medium-sized  lower  r a t e s of p a r a s i t i c b o t f l y r a t e s i n t h e peak  t o have  males.  infection.  higher  wounding  PL  males  i n t h e 1976 i n c r e a s i n g p o p u l a t i o n s . , P r e s u m a b l y  is  heightened  to  enter  the  population  PL  had  competition  1977  adult  tended  resident  than  They males did there  live-trap  208  Table 6.11. f i r s t caught and of t h o s e interval is males i n t h e parentheses. Height  Demographic a t t r i b u t e s o f male H i c r o t u s t o w n s e n d i i i n p i t f a l l s i n 1976 t h a t were caught in live-traps m a l e s t h a t were n o t . A l l g r i d s a r e c o m b i n e d . The t i m e t h e e n t i r e summer e x c e p t f o r t h e b o t f l y r e s u l t s , when b o t f l y s e a s o n o n l y were s c o r e d . Sample size is in  % scrotal  Wounds/male  40-49 50-59 60-69 70-79 >80  Males 28.8 67. 1 85.7 75.0 100.0  first (90) (64) (28) (8) (D  caught 0.70 1.30 1.21 1.75 0.00  Total  52.4  (191)  1.01  40-49 50-59 60-6 9 70-79 >80  Hales 20.7 60.0 73.0 95.8 90.9  first (169) (135) (119) (48) (11)  caught 0.44 1.27 0.72 0.50 0.54  Total  53.7  (482)  0.75  H o u n d s / s c r o t a l male  i n p i t f a l l s but 0.92 (90) 1. 12 (64) 0.91 (28) 2-00 (8) 0.00 (D (191)  1.06  i n p i t f a l l s and 0.94 (169) 1. 30 (135) 0.89 (119) 0.52 (48) 0.50 (11) (482)  0.94  Bots/male  not i n l i v e - t r a p s 0.38 (76) (64) 0.50 (48) (43) 1.00 (15) (24) 1.00 (7) (6) —:— •(0) (D (138) later (35) (81)(87) (46) (10) (259)  0.51 in  (146)  live-traps 0.38 (137) 0.25 (105) 0.63 (105) 0.57 (42) 0. 18 (11) 0.43  (400)  20 9  Table 6.12. f i r s t caught and o f t h o s e interval is males i n t h e parentheses.  Demographic a t t r i b u t e s o f male M i c r o t u s t o w n s e n d i i i n p i t f a l l s i n 1977 t h a t were c a u g h t in live-traps males t h a t were n o t . A l l g r i d s a r e c o m b i n e d . The t i m e t h e e n t i r e summer e x c e p t f o r t h e b o t f l y r e s u l t s , when b o t f l y s e a s o n o n l y were s c o r e d . Sample size i s in  scrotal  Wounds/male  Weight  %  40-49 50-59 60-69 70-79 >80  Males 13.8 52.5 93. 1 98.6 99.0  first (550) (236) (262) (284) (95)  caught 0.05 1.02 1.80 1.61 1.20  Total  57. 3  (1427)  0.92  40-4 9 50-59 60-69 70-7 9 >80  Hales 28. 1 58.2 100.0 100.0 100.0  first (229) (67) (15) <*)• (4)  caught 0.08 0.54 0.66 1.00 0. 00  Total  39.5  (319)  W o u n d s / s c r o t a 1 male  Bots/male  in p i t f a l l s b u t not i n l i v e - t r a p s 0.16 0.34 (76) (550) 0.22 1.92 (124) (236) 0. 35 1.89 (244) (262) 0.4 3 1.62 (280) (284) 0.16 (94) 1.21 (95) (1427)  1.58  i n p i t f a l l s and 0. 15 (229) 0.92 (67) 0.66 (15) 1.00 (4) 0.00 <<*>  0.21 :, (319)  0.47  0.20 (803)  (818) later (65) (39) (15) (4) (4) (127)  (518) (152) (74) (40) (19)  in  live-traps 0.18 (163) 0.20 (45) 0.25 (8) 0.00 (2) ____ (0) 0. 19 (218)  .210  population  in  peak p o p u l a t i o n s  indicative  of  males  1977  This to  in  increased  low l e v e l  and g r e a t e r  interactions  wounding  among  l e v e l s are  individuals.  PO  had l o w e r wounding r a t e s t h a n pO males i n 1976.  o f wounding i n 1977 may  the d i s p e r s a l  of  PO  males  be p a r t i a l l y  before  they  attributable  accumulated  many  wounds.  Growth  Rates  Of V o l e s C a u g h t I n P i t f a l l s  and of  and L i v e - t r a p s  Those Caught Only I n P i t f a l l s  The enter  a n a l y s i s so f a r i n d i c a t e s t h a t  live-traps  attained  weights than d i d males growth  rates  in  investigate this (Brody, two  included.  of  question,  voles.  I performed  grid,  The  analysis  of  I examined  Growth a  5-way  at  lighter Were  different?  i n s t a n t a n e o u s growth  rate  captures of  d a t a from  analysis  to  live-traps.  voles  g r o u p , w e i g h t c l a s s , and y e a r a s indicated that  g r o w t h r a t e s between s e x e s  among  maturity  entering  groups  failing  of  To  rates these  both sexes covariance  were with  r a t e a s a d e p e n d e n t v a r i a b l e , w e i g h t a s a c o v a r i a t e , and  sex,  in  two  voles  1945) a s d e t e r m i n e d f r o m p i t f a l l - o n l y  groups  growth  reproductive  eventually  these  male  grids  <F=2.87,  there  (F=132.46,  (F=13.47,  P<.001).  The  that  there  i s additional variability  with  body  growth r a t e s  size that on body  df=1,  between g r o u p s  among w e i g h t c l a s s e s  five  was a s i g n i f i c a n t  <F=47.29,  P<.05) ,  the  P<.0001),  3426,  indices. difference P<.0001),  (F=8.60,  and  P<.01),  between  s i g n i f i c a n c e of the c l a s s index i n growth r a t e s  i s n o t removed by t h e l i n e a r weight..  years shows  associated  regression of  211  The mean  following  growth  analysis. Voles  comparisons  rates  corrected  M a l e s grew a b o u t  voles,  in  for  1977.  difference  mean g r o w t h always  which  growth later  effects  per  day  based  i n summer  groups  by c o v a r i a n c e than  1976 grew  between w e i g h t  w i t h i n each class  upon  females.  8% f a s t e r p e r day t h a n  i n d i c a t e s that there  between  of  than  did  8% f a s t e r  than  c l a s s and group was  about  the  weight c l a s s .  The  the  PO  voles  were  t h a n f o r t h e same w e i g h t c l a s s f o r t h e PL v o l e s .  This analysis c l e a r l y pitfalls  are  PO v o l e s grew 11% f a s t e r  r a t e s f o r each weight  higher  size  g r i d s grew  The i n t e r a c t i o n  was n o t s i g n i f i c a n t , same  grids.  whereas i n d i v i d u a l s  summer  rates  24% f a s t e r  on t h e u n f e n c e d c o n t r o l  v o l e s on t h e e n c l o s e d PL  o f growth  i n d i c a t e s that those voles f i r s t  caught i n  b u t n e v e r c a u g h t i n l i v e - t r a p s had h i g h e r i n s t a n t a n e o u s  r a t e s than d i d those v o l e s f i r s t  caught  in  pitfalls  and  in live-traps.  Discussion S m a l l mammal p o p u l a t i o n statistical et  models  a l . , 1953), d i r e c t  Krebs,  1966)  rodent  populations  statistical equally papers)  that  or  used  direct  live-traps It  that  the  i s  deterministic  models  (Chitty  Phipps,  1966;  ( S m i t h e t a l . , 1975) .  Small  major  a l l marked  (Krebs, 1966).  population  population.  methods  violate  c a n be e s t i m a t e d by s t o c h a s t i c  1965),  enumeration  other  models  catchable  estimating is  (Jolly,  size  enumeration  and  assumption  of  the  a n d unmarked a n i m a l s a r e  T h e r e f o r e , Krebs as  (Leslie  (1966,  an a l t e r n a t i v e  size.  The major a s s u m p t i o n  catch  a l l the  known  that  adult  juvenile  method o f  of this  animals  later  method  in  the  v o l e s do n o t r e a d i l y  212  enter l i v e - t r a p s , survival to  indirectly  K r e b s and  but n o t  (1966) s u g g e s t e d  was  density Krebs  no  Krebs f e l t et  al.  ochrogaster  the  (50%  eventually  live-traps  upon  of  introduced  succeed  townsendii.  from when  parameters?  This  never  in live-traps  had  caught  entering  live-traps.  live-trap  data underestimate  males  caught  Thus,  only  only  faster growth  are  study,  growth  of  574  caught  in that  enumerating use  only  What used  any  age  live-traps, is  based  errors to  rates  are  estimate that voles  than  those  rates derived solely growth  pitfalls  213 were  assumption  demonstrated  the average in  used  violated.  only.  study  that  62%  characteristics  data  M.  that of  were e v e n t u a l l y  voles  of  (Krebs e t a l . ,  (1978) f o u n d  Studies t y p i c a l l y  live-trap  90%  suggested  p o p u l a t i o n s , the  enumerated  enumerated.  In the present  56%  this  pennsylvanicus  pitfalls,  of demographic  only  M.  i n completely  demographic  adult  in  Above  was  to  acre.  avoid snap-traps  enumerate a l l a d u l t s i s c l e a r l y  data  per  enumerated  of  Krebs  caught  townsendii  determination  150  enclosures  and  in pitfalls,  I n M.  c l a s s o f M. the  up t o  most o f t h e i n d i v i d u a l s  first  L i v e - t r a p s do n o t  and  summer)  in live-traps.  a d u l t s f i r s t caught live-traps.  in  Boonstra  caught  live-traps  a b l e t o enumerate 80  assumed t h a t v o l e s d i d n o t  townsendii  of  expected.  of the p o p u l a t i o n  trap-out  trap-out.  a d u l t M.  was  juvenile  of c o n f i r m i n g t h i s e s t i m a t e .  enumerated  I t was  a l l adults, i s  in densities  t o 808  75% A  live-trapping 1969).  60  Failure  (1969) b e l i e v e d t h a t t h e y  and  populations.  in  method  (1965) measured  index.  t h a t he  of M i c r o t u s c a l i f o r n i c u s  There  DeLong  by means o f an  catch a l l juveniles, Krebs  90%  and  are  rate. more  from  Smaller often  in  213  reproductive  condition  live-traps.  I t appears that  earlier  tend  Median  weights at  data  than  to disperse  overestimate  which  are  recruited  males which a t t a i n  without  sexual the  those  being  maturity  caught  derived  actual values  for  in  solely the  live-traps  reduces  reproductive  i n d i v i d u a l s i n the  individuals  also  assumption class  has  by  of the  both  survival  a  dispersing  they  individuals.  time  more  parasitic  that  parasite  present  loads  indicates  to  are  i n the  the  demographic  townsendii  data  estimates  that  relative  populations,  determined.  For  maturity  be  will  example, biased  of  although  i f only  and  sex  a c t u a l minimum estimated early  pitfalls  by  maturing,  during  the  have more wounds derived  The  may  produce  and  study segment biased  peak p o p u l a t i o n s ,  and  However, a l t h o u g h  the  be  i n error, I  parameters median  live-trap  and from  present  e n u m e r a t e any  and  well.  the  The  a c t u a l wounding l e v e l s  o f many p a r a m e t e r s may magnitude  these  Disappearance  data  f o r b o t h i n c r e a s i n g and  absolute  a l l  totally  as  of  representative  The  population.  for declining populations  of  age  that  operating  underestimate  loss  is a  catch  Demographic  before  proportion  class.  in  The  survival rates.  be l o w e r t h a n  t h a t l i v e - t r a p s d i d not  Mierotus  that  live-trap  maturity  particular  Males caught  botflies. may  of  fail  live-traps  live-trapping  the  finite  d i s p e r s a l component.  group w i l l  because  The  enumeration  minimum s u r v i v a l  age  same  likely  minimum  live-trap  d e a t h and  o f an  live-traps  of the  population.  live-traps.  from  apparent  i s made t h a t s u r v i v a l o f a  obtained  estimate  affects  the  maturity  population.  l o s s through d i s p e r s a l of voles a t t a i n i n g sexual entering  sexual  to  data  is  accurately  weight are  suggest  used  at  sexual in  its  214  determination, populations  these  have t h e  peak o n e s the This  data  lowest  Mierotus  could  that there  townsendii  of l i v e - t r a p s other  1977).  used  workers However,  was  greater  and  1978  populations,  among  voles  positive traps  them.  the  relation  that increasing maturity  and  to  d i d not  accurately  One  explanation  t h i s study  trappabilities  there  a v a i l a b l e to  to  and  i n the  Van  capture  of  Vleck  density  a ; constant  65%  low  in  density  competition  (1968)  and  used  Tamarin  about  were i n d i c a t i o n s  live-traps.  density  1969;  were  o v e r 80%  the  catch  densities  Krebs e t a l .  between p o p u l a t i o n  necessary  In  than or equal  peak p o p u l a t i o n s  for  live-traps  1966;  because  increasing spring  few  to enter  (Krebs,  live-traps  populations.  were t o o  individuals likely  by  demonstrate  median w e i g h t a t s e x u a l  demonstrates that  enumerate  all  still  highest.  study  be  will  the  found  number  proportion  of  a of  the  population. Individual voles  behaviour  are  caught  in  live-traps  sample  mainly  Kikkawa  (1964) o b s e r v e d  live-traps  and  away s m a l l e r , also  found  likely Gliwicz in  to  noted  be (1970)  live-traps,  and  in  determining  There are  larger,  which  i n d i c a t i o n s that  dominant  individuals.  encounters of Clethrionomys g l a r e o l u s at that  l a r g e r , dominant  voles.  were  that  suggested  Summerlin  dominant  than  reported  h e i r a r c h y , such as relationship  the  heavier,  trapped  important  live-traps.  subordinate that  is  and  Wolfe  smaller,  subordinate  young v o l e s have low  that  (1973)  Sigmodon h i s p i d u s were more  i n d i v i d u a l s low  j u v e n i l e s , have low  between s o c i a l  i n d i v i d u a l s chased  dominance and  trappability  i n the  trappability.  rats.  social  A positive  trappability  has  been  215  demonstrated  in  Andrzejewski  and  appears from  several Rajska,  t h a t some f o r m  entering This  between  1972;  of  (Davis  first  deters  capture  in< p i t f a l l s  practically  a l l of t h e i n d i v i d u a l s  populations,  low  whereas  and  in  summer  Microtus  live-traps  townsendii  enumerated  population  densities  and  in pitfalls  about a  50%  until alive.  1977  Even  at  of  the  1977  considerable  time  was  animals. lag  to low  to enter pitfalls  known t o be a l i v e ,  of  the  i n the  t o be r e l u c t a n t  summer  When  enumerated  spring  appears  and l i v e - t r a p s  enumerate  pitfalls  took be  in  whereas At  peak  between  first  maintained.  Live-traps  vole populations only i n l a t e  winter  early spring. Small  inhibition pitfalls similar  v o l e s seem i n h i b i t e d accounts and  time  pitfalls  i n entering live-traps,  f o r the time  live-traps.  l a g s f o r M.  and l i v e - t r a p s ,  l a g between f i r s t  Boonstra townsendii  and K r e b s  areas  from  by Watts  adults  captures  (1970)  live-traps  were  present.  at a  younger  age  I n unmanipulated  in  than  in  supports  He f o u n d  which he removed l a r g e male C l e t h r i o n o m y s  j u v e n i l e s entered where  captures  between f i r s t  and t h e s t u d y  and t h i s  (1978) r e p o r t e d  t h e h y p o t h e s i s t h a t young v o l e s a v o i d l i v e - t r a p s . on  voles  populations.  present  e n u m e r a t e d most o f t h e a n i m a l s  adeguately  It  capture  known t o be  I n t h e s p r i n g and e a r l y  live-traps  first  1976  e n u m e r a t e most o f t h e v o l e s known t o  capture  1974).  subordinate  peak, a n d d e c l i n i n g  were  again  1956;  i n d i c a t e s that there i s a considerable time l a g  densities  live-traps.  Emlen,  J o u l e and Cameron,  behaviour  i n increasing,  densities  and  live-traps.  study  live-traps  species  that  gap.peri, on  areas  populations, the  216  young p r e s u m a b l y survival  avoided  proposed  by  live-traps.  Krebs  trappability  of  juveniles.  by  in  juvenile  changes  derived  from  trapping  when Direct  used  estimation  this  of juvenile  caught  live-traps. to  and  possibly  definitely  to  enter  immigrants  born  that low  this  pitfall  more  reliable  fl.  townsendii  t i m e l a g may  be  partly  i n the s o c i a l h e i r a r c h y .  still  up  While  in  this  the  require  and  social  explanation i s  weeks.  It  and  of  weeks  area,  Krebs,  live-traps,  56%  However, o n l y 33% appears  of  an a d u l t * s a b i l i t y  to enter  to  have  than  females.  measure. be  New  immigrants  assumed  bute  1976;  adult  factors  difficult  Some w o r k e r s  (Hilborn  which  that  open p o p u l a t i o n s may  the  a number  entering  due Time  t h e f e n c e d p o p u l a t i o n s , o f 66  influence  rates are  on  as  o f s i x weeks t o e n t e r  w i t h m a l e s b e i n g more a f f e c t e d  i n unfenced  are  density  as by  a  adult  5 weeks t o e n t e r them.  more t h a n 5  the study area.  voles  that  an a v e r a g e  pitfalls  immigration  Immigration  to  In  in  more t h a n  live-traps,  caught  took  immigrants,  live-traps.  than  index  live-traps  such  provide  The  i n t h e f e n c e d p o p u l a t i o n s some a d u l t s ,  not  60 f e m a l e s t o o k  affected  peak  entered  f o r t h e s e a n i m a l s t o move  males f i r s t caught  other  (1978) f o u n d  of immigrants  correct,  required  may  are  1977  juveniles,  gain access t o l i v e - t r a p s .  are  index  by  demography.  They s u g g e s t e d  would be r e q u i r e d  of  juvenile  in live-traps.  first  study,  in pitfalls  the presence  system  voles  of  (1965) i s i n f l u e n c e d  l o w e s t i n the  enumeration  B o o n s t r a and K r e b s first  trappability  few  in  DeLong  index  Changes i n t h i s  l i v e - t r a p p i n g was  populations, juveniles.  and  An  that  Bedfield  that  new et  adult  or new  voles native small  voles  al.,,  are  1978).  217  In  the  fenced  weight c l a s s  at f i r s t  immigrants.  The  populations  capture  et a l .  derived  population  size  trappability  by  in  In  population between  the  t h a t new  i s a b o v e 65%, enumeration  about  10%.  1976,  by  70%;  50  55 t o  actual  in July  was  increasing  voles and  phase  in  the  were e n u m e r a t i n g  known  1977  the  alive  t o 75  while  %  of  population  when p i t f a l l s  e n u m e r a t e d from  63  The  total  results  accurately early  spring  vole of  this  estimate provided  considerable  delays  l i v e - t r a p s suggests the  population  months  population  after  study  t o 72%  the  indicate  that >  there  has  winter  first  t h a t l i v e - t r a p s do  breeding  not  the  density 1976  number  of  between  30  42  1977, to  49%  at that  time.  live-traps  may  the  capture  was  i n July  alive  in  during  low  live-traps  populations  between  actual  result of  total  vole  been no  by  mid-October  occurred  known t o be  the  underestimate  In  and  when  size the  l i v e - t r a p s enumerated  peak e n u m e r a t e d  the  study,  were i n a  summer. ,  The  of  44%.  t o be  69  actual  underestimated  populations 1976  the  population  live-traps.  townsendii  alive  a b o u t 65%,  absence o f a l a r g e c l a s s of v o l e s from Mierotus  number  this  is a direct  pitfalls  60%.  in  This discrepancy  The  and  minimum  summer  live-traps and  were  i n unfenced  underestimated  during of  none  a s i m u l a t i o n model t o show  Therefore,  underestimate  although  adult  wrong.  the  from  October  v o l e s were i n t h e  adult animals  (1976) d e v e l o p e d  live-traps  model p r e d i c t s an 10%.  of  in live-traps,  assumption  when t r a p p a b i l i t y  estimate  52%  are a l l immigrants i s thus  Hilborn that  populations,  late  winter  breeding. in  pitfalls  accurately  and The and  enumerate  the b r e e d i n g  season, nor  f o r about  four  stops.  alternative  methods  of  218  enumeration, one  such  as p i t f a l l  t o e n u m e r a t e more f u l l y  t r a p p i n g , are necessary  the  vole  population  t o enable  during  this  a peak  phase  period. These during  H.  townsendii  t h e 1977 summer.  between  401  and  enumerated f o r Boonstra  a  maximum  Other  maximum  a  H.  per  acre  and M.  voles  sizes  maximum and M.  acre  i n the l a t t e r  recorded  as  substantial  pitfalls  pitfalls pitfall,  Krebs  d e n s i t i e s o f M.  vole  o f 300 v o l e s  per  (1966) r e p o r t e d  a t 40 v o l e s  have  at  per acre. recorded  The a c t u a l  been  a  Krebs e t a l .  pennsylvanicus  (1977).  may  recorded  population.  d e n s i t i e s were e a c h  Tamarin  of  enumerate  c a l i f o r n i c u s per acre.  studies  results  They  to  a density  ochrqqaster  ever  much  M. at  population  higher  of live-traps  60  than  t o enumerate  portions of the populations. and l i v e - t r a p s ,  when o p e r a t e d  segments o f t h e p o p u l a t i o n . have  infestations Although  the  townsendii  live-traps  a r e s u l t o f the f a i l u r e  Pitfalls different  by  varied  b e c a u s e b o t h p i t f a l l s and  population.  pennsylvanicus  per  Only  f o r a M.  (1952) r e c o r d e d  aqrestis  recorded  i n t h e i r study.  only  d e n s i t y o f 324 M.  breweri 50  used  in  a c r e , among t h e h i g h e s t  populations.  concurrently  Chitty  were  densities  (1978) a r e s i m i l a r ,  have  (1969) e s t i m a t e d voles  per  o f 519 v o l e s p e r a c r e  studies  in  voles  microtine  were used  populations. acre  The maximum  530  and K r e b s  live-traps  populations  more  wounds  and  t h a n do s i m i l a r - s i z e d  some  of  the  may be a r e s u l t  wounds  Adult  higher  males  rates  males caught  accumulated  of confinement  I suggest that these  concurrently,  with  of in  sample  caught  in  parasitic live-traps.  by m a l e s c a u g h t i n other  voles i n the  wounds c o n s t i t u t e a minor  portion  219  of  the  total  wounding l o a d .  If  v o l e s h a v e more wounds, t h e n subordinate Rajska  individuals  ( 1 9 7 2 ) , by  l i v e - t r a p s and they  more  to those  to  l i v e - t r a p s than exerted  by  population.  less  of  social  live-traps.  be  greater  the and with  became o l d e r ,  so i n  age,  pitfalls.  had  similar  Thus i t a p p e a r s  heirarchy,  they  More s m a l l e r f e m a l e s  m a l e s , which s u g g e s t s may  sample  concurrently  individuals  irrespective  ascend i n the  adults  subordinate  Andrzejewski  of r e s i d e n t j u v e n i l e s .  enter do  t h a t as  i n l i v e - t r a p s and  animals,  individuals  likely  the  found  were more t r a p p a b l e  as  that  trapping a vole population  Newly i n t r o d u c e d  that  assumes  i t appears that p i t f a l l s  in  pitfalls,  trappabilities  one  that s o c i a l  towards  are enter  aggression  young  males  than  females. An males of  a n a l y s i s of covariance were  faster  than  those  voles caught i n p i t f a l l s  voles  caught  in  both  associated  with  behavioural  i n t e r a c t i o n s with  Summer  of females,  t h a t growth  sexual  other faster  populations  compared  with  those  populations  and  i n the  i n the experimentals.  rates  by  between  individuals in  either  directly  The capture  increasing  or  present a large  frequency high  indirectly study  portion  in  the  density  growth r a t e s  are  which  leads  may  to  voles.  low  density  1976  peak  density  1977  populations  influence  of  behavioural  density  populations  growth  interactions and  thereby  r e t a r d growth.  demonstrates of  the  rates of  r e p r o d u c t i v e l y mature in  of  rates  lower d e n s i t y c o n t r o l  Vole  rates  growth  Higher  maturation,  were  faster  and  than  of t r a p s .  rates  than  growth  growth  were f a s t e r  types  faster  i n d i c a t e d that  the  vole  that  live-traps  populations,  fail  and  to  that  220  trapping  with  pitfalls  obtaining  a comprehensive  suggests  some a r e a s  factors  that prevent  i s a n e c e s s i t y i f one i s i n t e r e s t e d i n demographic a n a l y s i s .  that should  be e x p l o r e d  smaller, subordinate  live-traps  could  be  investigated  aggesssion,  odour, o r o t h e r  and E.  R.  Microtus B r o d y , S.  and  1945.  C o r p . , New Chitty,  D.  1972.  Chitty,  York.  1952.  in 35: Davis,  Soc.  D.  from  entering  the  role of  Trappability  of  Acta T h e r i o l . 1978.  Mammal.  Pitfall  bank  17:41-56. trapping of  59: 136-148.  and  growth.  Reinhold  Publ.  1023p. Mortality  Lond., S e r .  among v o l e  Phipps.,  A.  in  CMicrotus  1936-9.  agrestis) at  Phil.  Trans.  236: 505-552.  1966.  Seasonal  changes i n s u r v i v a l  p o p u l a t i o n s o f two v o l e s p e c i e s .  J.  Anim.  Ecol.  313-331. D.  E. ,  trappability Mgmt. Gliwicz,  J.  Montgomeryshire  a n d E.  mixed  Krebs.  Bioenergetics  L a k e Vyrnwy, Roy.  J.  townsendii.  The s o c i a l  cited  Rajska.  C.  voles examining  v o l e s i n p i t f a l l s and l i v e t r a p s . Boonstra,  further.  also  factors.  Literature  A n d r z e j e w s k i , R.  by  This study  and  J.  of r a t s  T.  Emlen.  1956./  Dif f f e r e n t i a l  a c c o r d i n g t o s i z e and age.  J.  Wildl.  20: 326-327.  J.  1970.  individuals Theriol. H i l b o r n R., J .  R e l a t i o n between  in  a  population  trappability of  the  bank  and  age  vole.  of Acta  15: 15-23. A.  R e d f i e l d , and C.  J.  Krebs.,  1976.  On  the  221  reliability voles. Jolly,  Can.  G.  data  o f e n u m e r a t i o n f o r mark and r e c a p t u r e c e n s u s o f  H.  J.  1965.  with  both  Zool.  54: 1019-1024.  Explicit death  estimates  and  from  capture-recapture  immigration-stochastic  model.  B i o m e t r i k a , 52: 225-247. Joule,  J.  and  G.  demographic population Kikkawa, J . small  parameters:  1964.  J.  estimation of  Sigmodon  hispidus  5 5 : 309-318. and d i s t r i b u t i o n  glareolus  Anim.  Demographic  of Mierotus  Field  of  Mammal.  Clethrionomy.s  1966.  populations  influence  J.  i n woodland.  J.  1974.  Movements, a c t i v i t y ,  rodents  C.  Cameron.  structure.  sylvaticus Krebs,  N.  Ecol.  Apgdemus  3 3 : 259-299.  changes  californicus.  and  o f the  in  Ecol.  fluctuating Monogr.  36:  239-273. Krebs,  C.  J.  and K.  T-  with supplemental Krebs,  C.  J . , B.  Mierotus  DeLong.  food.  L.  pennsylvanicus i n southern Leslie,  P.  H.,  estimation  D.  and  of  and  Ecology  H.  o f p o p u l a t i o n parameters from  practical  1969.  changes and  in M.  50: 587-607.  Chitty.  means o f t h e c a p t u r e - r e c a p t u r e method. the  Tamarin  ochrogaster  Indiana.  Chitty,  H.  Demographic  M.  population  46: 566-573.  8.  biology:  populations  A Mierotus  Mammal.  Keller,  population  fluctuating  J.  1965.  1953.  data  The  obtained  by  I I I . An example o f  a p p l i c a t i o n s o f t h e method.  B i o m e t r i k a , 40:  137-169. Redfield,  J.  A., M.  Experimental Mierotus  J.  Taitt,  alteration  townsendii.  of  a field  and  C.  sex  r a t i o s i n populations of  vole.  J.  Can.  Krebs.  J.  Zool.  1978.  56:  222  17-27. S m i t h , M. and  H., E. H.  H.  mammal  H.  O'Farrell.  Petrusewicz,  S u m m e r l i n , C. trap  Ecology, T a m a r i n , B.  Van  T.  and L.  and J .  response  Kaufman,  estimations of small  25-54,  In  Small  (F.  mammals:  B.  Golley,  R y s z k o w s k i , eds.) Cambridge  L.  of  1977.  Wolfe.  the  1973.  cotton  Univ.  Social  influences  r a t . Sigmodon h i s p i d u s .  Dispersal i n island  Vleck,  D.  B.  relation  t o depopulated  S.  Mammal.  and mainland  1968. , Movements o f H i c r o t u s  1970.  areas.  & field  i n t e r a c t i o n s i n the red-backed J.  W.  54: 1156-1159. H.  H.  D.  451p.  58: 1044-1054.  W a t t s , C.  Gentry,  Density  Pp.  Ecology  in  B.  and d y n a m i c s o f p o p u l a t i o n s  P r e s s , London.  on  1975.  populations.  productivity K.  Gardner, J .  51: 341-347.  J.  pennsylvanicus  Hammal.  experiment  on  voles.  49: 92-103. intraspecific  vole, Clethrionomys  gapperi.  223  GENERAL  I in  designed  population  types  interested  population  declines  to as  spring  breeding  last  and  a spring  compared  recovers  by  the  or  decline.  I suggest  The  of  which  the  same as  i s explained  voles  coming  of  decline  continued  population  started  was  when was  behavioural  in  the  breeding  lost  The  lack  Rather,  the by  1977  fall and  began  during  in  until  after  in  by  in  the  spring  breeding  behaviour  and  population. study  J u n e 1978.  spring.  non-breeding  declines  aggressive  this  rate  the  smaller  this  breeding  the  Here  stopped,  of  decline  Most o f  season,  by  and  the the  n e g l i g i b l e wounding and e s s e n t i a l l y  o f w o u n d i n g and  interactions  decline.  the  winter,  characterized  dispersal.  typified  d e c l i n e from October  through  accelerated  was  wounding  those  into  second  after  population  than females during  from t h e  decline  two  typically  These  individuals disperse  type  is  dispersing  population.  decline  in  from  are  decline  subordinate  the  this  this  after  between  results  first  spacing  eight-month  no  disperse this  there  increased  the  decline  During  months,  that  was  because i t s t a r t s s h o r t l y  to d e n s i t i e s about the  males  This  The  I  dispersed  indicated that  the r e s i d e n t  interactions  condition.  i f voles  preponderance  four  fall  More  smaller,  is a  r o l e of d i s p e r s a l  townsendii.  in voles.  decline  with  spring.  behavioural  study  commences.  there  for three  Mierotus  in determining This  referred  adults  of  populations.  of  increases  t o i n v e s t i g a t e the  regulation  specifically declining  t h i s study  SUMMARY  were n o t those  the  animals  d i s p e r s a l suggested d r i v i n g mechanism that  that  behind  disappeared  and  224  therefore  died  were t h e s m a l l e r i n d i v i d u a l s i n t h e  These i n d i v i d u a l s during had  the  last  positive  indicate that  were c h a r a c t e r i z e d by half  growth  that  food  of t h e  was  I  populations  resource.  from  I suggest  obtaining  t h a t food  was  common  in  an  rates  individuals  these  results  to  this  decline  and  the s m a l l e r  members  of  during  prevented  growth  larger  interpret  i n short supply  interference competititon  the  negative  d e c l i n e , while  rates.  populations.  adeguate  a limiting  share  of  the  factor during  this  period. Dispersal  is  populations.  In  i n c r e a s e d , and condition period  behavioural voles  these  than  were  lighter  than  interactions  resident between  during  tendency populations.  terms of  lifetime  capture)  and  (duration  the  was  to l e a v e at the  animals  lifetime  was  i n c r e a s i n g and  of  life  to  the  between  d i s p e r s a l t e n d e n c y had  i s unknown.  that  increased  among  each o t h e r  first  were  and  a  heritable  the h e r i t a b i l i t y  in  last they  non-dispersing Litters  differentiated  d i s p e r s a l tendency.  w h e t h e r or n o t  this  smaller  distributed  non-randomly d i s t r i b u t e d .  and  based  led  Even among t h e  lifetime  maternally  suggest  I f littermates dispersed,  lifetime  or  I  L i t t e r m a t e s resembled  to  but  reproductive  r e s i d e n t s and  non-randomly  respect  townsendii.  subadults  summer.  peak p o p u l a t i o n s and  of  Dispersers during  large  same age.  still  townsendii  in  voles.  mature  d i s p e r s a l tendency.  tended  both  dispersal  were more o f t e n  reproductively  Dispersal the  summer,  subadults  of subadults  members o f  peak  Hicrotus  were r e s i d e n t s u b a d u l t s .  becoming  disperal  the  peak  I conclude basis  in  in  with that M.  i s genetically  225  Avian vole sex  predators  populationsof t h e prey,  losses  in  they  were s e l e c t i v e  impact  provided  even  showed  though  peak, and d e c l i n i n g  that  juvenile  i n declining  estimated  preweanling  was l o w e s t  preweanling The  increased  voles.  in  failed  immigrants  into  presumably  because  a  trap  trappability pitfalls. than  live-traps  they  used of  a  size  caught  suggest  t h a t these  earlier  than  earlier  before  were  and  was ones.  i n peak  populations Higher  showed t h a t  they  o f demographic are  entered  low  used,  in  class  because  the  enter  only i n p i t f a l l s in pitfalls  live-traps,  social  heirarchy.  were s o c i a l l y  and  because  growth  rates  in live-traps.  v o l e s reached  voles  upon t h e  i n l i v e - t r a p s and  had f a s t e r  and l a t e r  some  Many new  presumably  c l a s s was d i f f e r e n t  of  parameters  live-traps. to  enumeration,  slower-growing  in  However,  populations.  failed  faster-growing  direct  highest  c l a s s o f v o l e s was d e p e n d e n t  in  Voles caught  voles f i r s t  they  populations  Minimum s u r v i v a l o f a s i z e of  juvenile  t o enumerate a l l members o f any s i z e  dispersed before  study  i n the decline.  I t a l s o showed t h a t e s t i m a t e s  individuals  the  populations.  use o f l i v e - t r a p s and p i t f a l l s  may be i n a c c u r a t e i f o n l y  type  declining  m o r t a l i t y d i d not occur  concurrent  live-traps  survival  and  vole  survival  p o p u l a t i o n s and l o w e s t  actually  this  in pitfalls,  increasing  and  f o r t h e s i z e and  o f preweanling  numbers o f j u v e n i l e s were c a u g h t  enumeration  on t h e  o f the predators.  estimates  increasing,  impact  f o r the majority of  populations,  the t o t a l  in  h a d no s i g n i f i c a n t  they c o u l d not account  This study  Large  Although  declining  underestimated  survival  apparently  sexual  therefore  I  maturity dispersed  d o m i n a n t and c o u l d g a i n  access  226  to  live-traps.  

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