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Fates of disappearing individuals in fluctuating populations of Microtus townsendi Hilborn, Ray William 1974

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FATES  OF  DISAPPEARING POPULATIONS  INDIVIDUALS  OF  MICROTUS  IN  FLUCTUATING  TCJJNSEfiCI  by RAY B.A.,  A THESIS THE  WILLIAM  Grinnell  SUBMITTED  IN  REQUIREMENTS DOCTOR  in  OF  the  HILBORN  College,  1969  PARTIAL  FULFILLMENT  FOB  DEGREE  THE  OF  OF  PHILOSOPHY  Department of  Zoology  We a c c e p t  this  t h e s i s as  required  THE  UNIVERSITY  OF  JULY,  conforming  to  standard  BRITISH 1974  COIUMEIA  the  In p r e s e n t i n g an a d v a n c e d the  this  degree at  L i b r a r y s h a l l make  I f u r t h e r agree for  scholarly  by h i s of  thesis  this  written  it  freely  British  available  for  that permission for extensive  p u r p o s e s may be g r a n t e d  for  It  financial  gain  permission.  The U n i v e r s i t y o f B r i t i s h V a n c o u v e r 8, Canada  Columbia  the  requirements  Columbia, reference  copying of  I agree and this  shall  that  not  for  that  study. thesis  by t h e Head o f my D e p a r t m e n t  is understood  Depa r t m e n t  Date  fulfilment of  the U n i v e r s i t y of  representatives. thesis  in p a r t i a l  or  copying or p u b l i c a t i o n  be a l l o w e d w i t h o u t my  i  Abstract  Advisor: Fates natural  of  disappearing  populations  evacuated  areas  individuals  with  of  the  454  tagged  Most  due  tc  dispersal.  found  on  one  the  declining  numbers  increase  in  techniques, during were  the  dying  second long  f i r s t in  the  the  used  a  very  good  of  from  going  (minimum  measured  period.  It  declining  is  other  by  of  number of  of  animals  the  actual  was  was  cf no  above that  individuals  and  during were  A computer  It  period  concluded  constructed  numbers.  were  the  numbers  was  remains  There  individuals  was  Eaney,  numbers  numbers  traps.  the  populations  causes,  into  representation  near  unknown  technique  estimates  found.  out  in  individuals' the  219  radicactively  declining  were  decline  peak  two  marking  conducted  during  as  by  Forest  many  but  rate,  declining  trapping  of  period  period  nests  without  the  periods  and  were  and  for  live-trapping  carcasses.  study  increase  areas;  by  locate  Research  remains  either  brief  of  of  was  few  their  r e l i a b i l i t y index  study  during  period  of  such  dispersal  distances  model  Two  to  year  Columbia  determined  habitat,  disappeared  three  from  were  townsendi  tags  that  this  during  individuals  radioactive  British  Wehrhahn  marginal  disappearance  observed; were  of  C.F,  Microtus  and  during  B.C.  of  individuals  University  Dr.  to  the  moving  simulation test  the  concluded  that  known  to  be  population  alive) size.  i i  TABLE  Abstract TABLE  OF  CONTENTS  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ,.  OF C O N T E N T S  LIST  OF F I G U R E S  LIST  OF  i i  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  TABLES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . •,............ .  vi  , . . , , . . . . . . . . . . . . . . . . . . . . . ...  1  INTRODUCTION MET HODS  6  Method  Of  Heather  Trapping  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  9  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  10  Estimation  Of  Numbers  Simulation  Results  Of  Estimation  Of  Recovery  Of  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  13 15  Parameters  Method Tests  11  Simulation  Tagging  Of  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  Model  Radioactive  RESULTS  i v V  ACKNOWLEDGEMENTS  The  i  . . . . . .  28  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  40  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  43  The R a d i o a c t i v e  For  Natural  . . . . . . . . . . . .  43  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ... . . . . . . . . . .  46  Population  Fluctuations  Calculation Survival  Of  Rates  Immigration  Tagging  Populations  Grids  Survival  Technigue  p And Q . . . . . . . . . . . . . . . . . . .  And I m m i g r a t i o n  Rates  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  64  Recruitment  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  67  Radioactive  Tagging  68  Removal  Breeding  Experiments  Movements  Between  P  And Q  59 61  Of  Grids  50  . . . . . . . . . . . . . . . . . . . . . . . . . .  Intensity  Onto  46  Results Grid  Grids  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Z  72 74  i i i  Grids  A And B  DISCUSSION  . . . . . . . . . . .  Reproductive Survival Removal  . . .. . . . . . , . . . . . . . . . . . . . . . . . ... . . . . . . . . . . . . .  78  Intensity  Rates  . . . . . . . . . . . . . . . . . .  79  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  80  Experiments Between  Nature  Of  The D e c l i n e s  Causes  Of  Disappearance  APPENDIX  I  APPENDIX  II  CITED  And R e c r u i t m e n t  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  Movements  LITERATURE  77  Grids  And Immigration  81  . . . . . . . . . . . . . . . . .  81  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  82  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  87  ...  .  .  .  .  .  .  .  .  .  .  .  .  .  . V  .  >  .  95 99  . , . . . . . . . . . . . . . . . . . . V . . . . . . . . . . . . . . . . . . . . . . . . . .  100  i v  LIST  Of. F I G U R E S  Figure  1.  Aerial  Photograph  Of  The  Figure  2.  Effect  Of  Population  Figure  3.  Effect  Of  Survival  Figure  4.  Effect  Of  Trappability  Figure  5.  Effect  Of  Variance  Figure  6.  Effect  Of  Reduction  Figure  7,  D i s t r i b u t i o n Of  Figure  8.  Simulated  Hicrotus  Figure  9.  Numerical  Changes  Figure  1 0 . Body  Weight  Figure  1 1 . Body  Heights  Figure  1 2 . Number-Of  Figure  13. Proportion Of  Figure  1 4 . D i s t r i b u t i o n Of  Study  Size  Bate  Of  On UNA  Factor  On G r i d s  Immigrants Adult Body  16  . . . . . . .  18  . . . . . . . .  20  . . . . . . . . . . . . .  22  And J o l l y .  On HNA A n d J o l l y .  Population. P,  24  . . . . . . . . . . . . . . . . . . .  32  . . . . . . . . . . . . . . . . . .  37  Heights  . . . . . . . . .  On F e n c e d  Grids  Females  And Z.  On U n f e n c e d  Capture Onto  ....  Q,  Capture  F i r s t  . . . . .  O n MHA A n d J o l l y .  Trappability.  At F i r s t  7  On UNA A n d J o l l y .  Trappability.  At  Areas  P  A n d Q.  48  Grids.  51  .Grid...,  54  . . . . . . .  62  Lactating. On G r i d s  66 P  And Q.  .  84  V  LIST  1.  Results  Table  2,  Hypothesized Fates  Table  3.  Population  Parameters  Table  4,  Indices  Reproductive  Table  5.  Recoveries  Of  Table  6.  Results  Removal  Table  7.  Movements  Table  8.  Rates  Table  9.  Characteristics  Of  Of  Simulation  TABLES  Table  Of  Of  OF  Of  Of  HNA A n a J o l l y .  Baaioactively On  Grids  P  Q.  Voles.  . . . . . . . . . .  Intensity.  Experiments Grids  Tagged  And  P  And  Individuals.  The  60  69  . . . . . . . .  73  . . . . . . . . . . . . . . . . .  75  Z.  Disappearance. Of  41  ..  On G r i d Q.  39  65  Radioactively-tagged  Between  . . . . . . . . . .  88 Phases  Of  M.  townsendi.  ...  94  ACKNOWLEDGEMENTS  No this  thanks  project.  great  deal  Anderson, Charles other  of  vole  forth  assistance  Dr.  Chitty  a l l  and  the  put  with  and  Janice  Pat  Wilson,  Irene  did  forests  Dr.  The  provided  Data  LeDuc,  of  Paul  Starr,  Dr.  Anderson,  considerable  impede  C. J . and  time  Krebs,  readable  radioactive  isotopes.  Dr.  to  Anderson, Tom  Krebs  a l l  provide of  areas  and  reading  English.  other  Computing  computer  Pierre  Dr.  f i e l d British  provided  the  Alan  Powell,  time.  computer  Kleiber,  progress James  particularly  into  into  Rob  provided  times.  to  with  Columbia  Stander  best  comparisons  study  Jeff  their  Dr.  Charles  B r i t i s h  Judith  and  University  Centre,  a  Judith  Haney  the  provided  Wingate,  Redfield, and  in  Taitt,  for  and  B i r d s a l l ,  i t  Janice  me  support.  Mary  data  of  assisting  Wehrhahn  some  times.  Borden,  those  financial  LeBuc,  Anne  rain  Biology  Judith  converting  assisted  C . F.  Redfield,  different  Alan  LeDuc,  Dr.  Forest  Stephen  Janice  and  University  the  Casselman  thesis.  Krebs  various  The  at  a l l  Robertson,  Research  Webb,  for  Fraker,  into  at  and  much  provided  James Pam  f a c i l i t i e s .  B i l l  a l l  Suzanne  Columbia  too  Boonstra,  Dr.  assistance  Centre,  C.J.  populations.  Yorgue,  ventured  be  i n t e l l e c t u a l  Krebs  Sullivan,  B i l l  Df.  Rudy  Birdsall, Ralf  would  of  and this  Redfield,  Dr.  Dennis  this  manuscript  Mr.  W.  Rachuk  1  INTBODOCflOH To a  understand  species,  constant density that  one  in to  that  numbers, its  Among  more  and  have  long  have  been  1942).  or  been  last  voles  two-  of  to  characterized decrease  in  by  lower  the  and  estimates  of  Notable  papers) 1969,  ,  and  and  North  America.  his  Krebs  et  vole  the  outbreaks  years deal  Vole  (Elton of  work,  on  many  populations generally  increase  reproduction,  and  are  periods  reproduction.  of  crude  and  are  populations  done  of  studies,  reproduction  are  because  abundance.  numbers,  Periods  populations.  and  been  declining  and and  of  exceptions  1973).  and  number  rates  Krebs  good  has  survival  density  a  data,  survival  declining  of  occur Species  numbers  and  thousands  cycles.  large of  scientists,  years  that  of  Vole  its  population  perhaps  in  of  increasing  high  estimates  increasing  Europe  four-year by  Despite  body  a  periods  dramatically  f i f t y  return  numbers.  studied,  of  generally  changes  declining  for  i s  that  study the  abundance  that  Cricetidae).  described  substantial  can  their  to  the  processes  observe  during  interest  a  the  and  fluctuate  the  population  intensively  been  of  determine  one  and  (Rodentia;  periods  al  level,  lemmings  undergo  other  observe  increasing  regularly  intensity.  a  numbers  that  of  reliable  manipulate  noticeable  species  in  that  and  have  Within  yielding  et  of  species  voles  on  in  periods  are  factors  former  fluctuate  they  of  can  fluctutes  between  the  the  have  of  of  are  few  survival  studies  indices  co-workers al  and  Host  work  there  rely  only  reproductive  Chitty (Krebs  for  (1952  1966,  gathered  data  and  Krebs from  2  capture-recapture Berkeley, study  California  near  aicrotus  density,  of  and  rates  juvenile  effort  and  from  recruited  to  previously. four  are  the the  The ratio  times  populations.  during  increases  survival  was  survival rates,  rates  and  population  I  1960,  rates  must  that  epidemic  during  hypothesis 1954,  has  the  Krebs  to  The  interested and  in  Krebs  theories periods  of was in  rejected  1966).  effect  both  the  There  on  the  of  young four  The  responsible  several  weeks were  than  11%  about  is voles  populations  in  higher  and  female  reproductive population  changes  explaining  is  survival  and  growth  in  vole  changes.  (1966)  decline.  and  reproductive  fluctuations  these  numbers by  of  declines, i n  an  lactating  populations was  four  males,  lactating  changes  explain  of  females  Indiana  in  lactating,  juvenile  the  same  disease  been  adult  survival  the  declines  rates  number  the  during  summarized  females  increasing  explain  1967)  during  of  higher.  about  conventional  survival  Chitty  20%  are  females  Hale  hypothesis  numbers  (1952,  rejected  animals  any  had  in  than  am p a r t i c u l a r l y  Chitty  was  about  to  of  the  of  rates  higher  declining the  of  number  Reproductive  enumeration  measures  index  five-year  direct  of  basic  a  and  of  Percent  and  near  pchrogaster  survival  the  study  Microtus  used  percentage  success.  estimated  cn  measurements  females.  survival  two-year  californicus,  They  survival,  of  a  Indiana,  their  parameters:  juvenile  survival  about  flicrotus  Bloomington,  population  and  on  during  £ennsy1vanicus.  measure  index  studies  in  have the  survival. repeatedly  reduction  earliest for  the  in  theory death  of  (Elton  1942).  This  workers  (Elton  194 2 ,  evidence  that  neither  3  shortages  of  food  nor increased  f o r t h e d e c l i n e s (Krebs al,  1969),  other  but  1966, K r e b s a n d DeLong 1 9 6 1 , K r e b s  there  are advocates of these  e c o l o g i s t s working  Pitelka,  1964).  For  theories  offered  to  realistic  are  those  p r e d a t i o n r a t e s can account  with  small  mammals  example, P i t e l k a explain  t h e o r i e s among (Pearson  1966,  s t a t e s "... o f a l l t h e  microtine  attributing  et  cycles  causal  the  most  significance  to  p e r i o d i c c h a n g e s i n q u a l i t y and q u a n t i t y o f f o o d . " Another  set  aggressive  of  hypotheses  behaviour  cause  a  proposes  that  social  reduction  in  survival  populations o f high d e n s i t y . There a r e s e v e r a l this  idea.  Christian  have s u g g e s t e d stress  which  hypothesis has  that reduces  densities  survival  was o r i g i n a l l y  r e p r o d u c t i o n . , A second (I960,  r e c e n t l y been m o d i f i e d by K r e b s e t a l ( 1 9 7 3 ) .  increasing lower  f o r aggressive  rates.  of  social  p r o p o s e d by L i d i c k e r  A  third  and  (1962).  high density populations a r e individuals  According t o  behaviour  hypothesis  during  aggressive He s u g g e s t s more  considering  behaviour  He h a s s u g g e s t e d  with the  has  been  that individuals  from  prone  to  from l o w d e n s i t y a r e a s , and s i m p l y  crowded a r e a s . peak  1967), and  populations produces high d e n s i t y populations  survival  importance  (1964),  to physiological  proposed by C h i t t y  this hypothesis, selection  from  and  lead  in  v a r i a t i o n s of  (1950) a n d C h r i s t i a n a n d D a v i s high  and  dispersal  than  move i n t o  less  t h a t i n d i v i d u a l s may. d i s p e r s e  p o p u l a t i o n s because i t i s s e l e c t i v e l y  advantageous  f o r them t o do s o . The  a b o v e h y p o t h e s e s c a n be  separated  into  two  groups  4  based situ  on  the  prediction  mortality.  study  grid,  as  Predation which  be  die  on  weasel lack  of  changes and  in  In  food, in  increased  these  and  changes death:  declines.  changes  in  should  Stress,  predispose of  which  area  individuals and  by  which or  die  from  because  that  "increased  in  situ.  s t r i f e ,  by  lack  require  that  should  adequate  food,  the  proximate  cause  and  to  disease  of  food,  or  losses  be  regularly  in  and of  natural  might  with through  a l l  occur  of  individual  genetic  Christian  caused  stress,  as  stress,  of  losses  death  may  distinguishable  die  not  there  weasels  increase  not  weather  that  would  do  on.a  dispersal.  eaten  are  in  or  entirely,  remains  of  individuals  also  situ  the  of  predict  interactions,  lack  are  probability  predation  individuals  v i a b i l i t y in  death  by  their  w i l l  but  the  Individuals  suggest  v i a b i l i t y  mortality,  death.  declines  in  is  the  generally  movement,  situ  from  contrast,  (1964)  about  theories  aggressive  predation...",  in  death  but  v i a b i l i t y  Davis  genetic  situ  grids,  k i l l s .  to  disappear  l i t t l e  make  mortality  dispersal  w i l l  the  situ  opposed  and  die  w i l l  In  they  genetic  death,  but  agents  of  lead  to  death  in  in  situ  death.  In  my  study  of  dispersal  townsendi  populations  fates  disappearing  individuals,  to  in  of  d i f f i c u l t to  separate  individuals about simply  the  I  determine from between  number  dispersed.  of  used  and  three  adjacent  fates  to  The  which  have  Death  i s  movement  areas have  provides not  Microtus  determine  which  studies.  study  individuals  situ  methods  previous  emigration.  in  died  the  teen  very  d i f f i c u l t  of  marked  information but  have  5  A  second  individuals area.  and  to  measure  his  of  A with Geiger  whereas should  of  methods section.  a  This  Myers  and  type  are  being  method  remove that  rate  of  change  in  technique  was  Krebs  (1971),  done  thesis  I  to  the  to  mark  by  Krebs  consider  the  fate  of  of  been  large  number  An of  and  individuals,  recover  should  be  recovered disperse  their  or  to  Barbour  are  the taken  a al  while  i t  method and  is  to  then  use  Animals  study by  a  et  carcasses.  on  with  follow  individual  alternative  to  which  an  individuals  movements  used  1954;  following  small.  their  widely  (Godfrey  very  regarding  discussed  and  in  dispersal  in  The  has  follow  is  area,  predators  recovered.  be  used  dispersal  marker  hypotheses  enumeration  the  the  by  this  to into  in  of  relation  and  individuals  i s  situ  be  mortality  measure  of  in  marker  probability  tag  in  w i l l  a  populations.  part  measuring  individuals  The voles  In  This  the  not  a  control  studies  of  radioactive die  is  this  way  radioactively  which  change  of  dispersing  the  that  experiments  number but  immigration  voles  radioactive  limited  is  in  is  measure  in  counter.  1969),  habitat  dispersal  individuals^  third a  and  dispersal  these  disappearing  measuring  assumes  coworkers.  results  of  area  empty  tendency  continued  and  an  method  into  dispersal used  from  This  dispersal  method  small  purpose  techniques.  regulation  relation rates.,  mammal of  the  this  to An  of  numbers  my e s t i m a t e s  of  analysis  two  work  i s  included  was  to  examine  in  of  the  in  in situ  census methods  r e l i a b i l i t y  of  my  6  MfTH'Ogg  T h i s s t u d y was c a r r i e d Columbia  Research  The f i v e  monitored  this  7.6 and  during  study  areas  was c o v e r e d b y a c h e c k e r b o a r d  (ft,B,P,Q,  m (25 f e e t ) a p a r t . G r i d s A a n a B were i r r e g u l a r l y  shaped,  the  generally  poor  townsendi  appearance  of  these populations t o  the  trapping  winter for  of  six  1971-1972, weeks,  when  fluctuations  snow  depth  and d u r i n g t h e l a s t s i x every  week. G r i d s P, Q, and Z were l o c a t e d 200 m f r o m  in  a  and  s u b s e q u e n t l y seeded 9  the  determine  months o f t h e s t u d y , when t h e y were o c c a s i o n a l l y t r a p p e d  x  from  T h e s e p o p u l a t i o n s were t r a p p e d e v e r y o t h e r week,  during  prevented  by  t h a t t h e s e g r i d s were p o o r q u a l i t y M i c r o t u s  h a b i t a t . I monitored  numbers.  in  monitored  i f v o l e s moved i n t o m a r g i n a l h a b i t a t d u r i n g t h e i r  except  located  t o J u n e 1974. The l o w numbers o f M i c r o t u s  suggested  townsendi  They were  o l d c o a s t f o r e s t , and were  on t h e s e g r i d s , a n d grass,  of trap  f i e l d s on a n o l d h o m e s t e a d , were s u r r o u n d e d  t o 40-year  1971  third  Z),  placed  20-year  in  and  sites  abandoned  May  British  t h r e e - y e a r s t u d y a r e shown i n F i g . 1.  e a c h c o n t a i n e d a b o u t 90 t r a p s i t e s .  two  of  F o r e s t , l o c a t e d n e a r H a n e y , B.C., 30 m i l e s  e a s t of Vancouver.  Each g r i d  out i n the U n i v e r s i t y  50-acre  grid,  g r i d s . f i and  B,  a r e a w h i c h had b e e n l o g g e d a n d c l e a r e d i n 1968,  and  grid  w i t h n a t u r a l g r a s s e s . G r i d P was a Q  12  was 10 x 10. G r i d s P and Q were t h e  l o c a t i o n s o f t h e r a d i o a c t i v e t a g g i n g e x p e r i m e n t s , a n d were r u n from  J u n e 1972 t o May  1974. The p o p u l a t i o n s  were  trapped  the u s u a l f a s h i o n except f o r the r a d i o a c t i v e t a g g i n g .  in  7  Figure  1.  Aerial British  photograph  of  the  Columbia Research  study  areas  Forest,  in  near  the  University  Haney,  B.C.  of  9  Grid  Z  was  triangularly two  sides  shaped,  by  individuals  captured  following  served  as  a  immigration  The et  al  at  each  left in  and  (1969).  40' m f r o m  grid  Q,  and  used  processed  method  station  and  where  sign  cotton  were  two  method  Each  anywhere  on  of  in  other  as  Myers  1974;  it  was  bounded  on  experiments.  A l l  described and  Krebs for  below,  (1971).  and  Grid  evacuated  area  was  identical  to  that  used  was  live-trapped  with  Longworth  was  covered  in  during  third  mortality,  general  vole  Traps  Using  the  each  were and  check  in  again  in  overnight these  and  have  could when  week  set  on  measurement  Z of  the  the  next  only,  to  able  to  Krebs traps  live  traps  were  locked  open  when  not  board.  Traps  were  heavy of  be  the  trap  found.  Whole  set.  the  by  These  Trapping  exception  techniques, been  a  vicinity  trap  second  areas, with  activity  in  every  set  schedule.  each  morning  were  bi-weekly  trapping  following the  a  the  placed  days  area  on  of  above.  heat.  June  dispersal.  trapping  placed  traps  to  Tra£j3in<j  use,  noted  1973  fences  were  the  J u l j  continuously  permanently  for  from  about  mouseproof  removed  Method Of  trapped  on  station oats was  grids  A and  afternoon, . checked afternoon,  morning. prevent I  During  locked the  mortality  avoided  follow  and  these  and done B  as the  open  summer,  from  the  a l l  trap  populations  year  almost  round. Upon and  f i r s t  numbered  capture  each  fingerling  vole  fish  was tag  tagged in  the  with  a  right  lettered ear..  The  10  following  data  location  on  testes,  or  or  to  recorded  weight;  the  at  sex;  abdominal;  imperforate,  symphysis  weighed  also  grid;  scrotal  perforate pubic  were  and  each for  for  nipples  capture:  males,  medium, or  open,  nearest  gram  specially  of  vaginal  small,  s l i g h t l y with  position  females,  closed,  species;  or  open.  the  o r i f i c e  large,  and  Mice  were  constructed  spring  scales. Few usually the  voles be  their  recognized  grid.  number  lost  They  was  directly  onto  program  to  from  were  recorded.  tags,  their  Data  and  recorded  cards,  standard  and  sex,  retagged,  computer a  ear  and  such  individuals  weight, the  in  f i e l d  format  change  were  converted  numerical  location  appropriate  the  then  and  could  by  ^n  punched  a  for  on  computer subsequent  analysis.  Weather The  weather  throughout  the  in year  six-week  dry  spell  averages  well  over  Harch take  and place  i s  the  throughout  water  the  nests  between  fallen although  per by  of  at  least in  logs  the 10  during  a  although  some  year,  and  months clumps  Snow  1971-1972  four  to  begins  in  growth  may  table  i s  the  there  is  of on  wet  Precipitation  growth  water  is  usual  Plant  the  stumps.  of  B.C.  August.  year.  July,  grass and  Haney  and  Because  most  their  spaces  winter,  f a l l .  for  make  the  over  of  exception  July  inches  grids  voles"  the  during  usually  during  v i c i n i t y  with  100  surface on  the  the  at  running  year,  the  the  surface,  is  infrequent  trapping  had  or  to  in in be  11  s u s p e n d e d on interfered  grids  A  and  B  as  noted  w i t h t r a p p i n g on g r i d s P, Q,  above.  Snow  never  and Z.  E s t i m a t i o n Of Numbers  Several  mark-recapture  s i z e have b e e n methods  use  used the  by  ecologists  proportion  sample t o e s t i m a t e  (1966),  of  avoids  Krebs  (1966)  the  and  use  period  Chitty  et  known t o be  alive  i s and  not  a  Krebs  new use  of  the  relative  merits  method, t h a t p r o p o s e d by J o l l y  as  during  a  in  population  s i t u a t i o n s i n which  o f one w i d e l y (1965),  They u s e t h e  s i z e . Although the  e s t i m a t i o n methods a r e a l m o s t u n i v e r s a l l y u s e d . compares  and  Mierotus,  (MNA)  technique i t in  Chitty  a l (1969) have  populations  as an index o f population  o f enumeration  biology,  and u s u a l l y assume a l l  Krebs  of  These  i n d i v i d u a l s i n each  use o f e s t i m a t i o n t e c h n i q u e s .  minimum number o f a n i m a l s sampling  marked  population  years.  p r o b a b i l i t y of capture.  attempted complete enumeration which  f o r many  the population s i z e ,  i n d i v i d u a l s h a v e an e q u a l Phipps  methods o f e s t i m a t i n g  This used  section  estimation  compared  with  the  MNA method o f e n u m e r a t i o n . Digital  computers  opportunity t o test  have  provided  estimation techniques  researchers through  the  Monte-^Carlo  s i m u l a t i o n . M a n l y (1970) a n d C a r o t h e r s  (1973a,b) have done many  simulations of estimation techniques,  p r i m a r i l y t o examine t h e  robustness  including Jolly's.  of  various  examined t h e r e l a t i v e rates  and  estimators,  importance of population s i z e ,  probability  of  capture.  Carothers  Manly  survival simulated  12  populations capture not  with  (trappability),  seriously  populations generally In used  bimodal  (no  an  births  effort  my  different  parameters.  of  MM:  and  the  Jolly  be  or  deaths),  to  examine of  of I  trappability  of  work  Krebs at  estimated the my  unmarked  (1966), the  University  expected studies.  error  in  in  the  the  having  five  the  a  index  as  of  voles,  I  wide  variety  of  believed  a  parameters  in  MHA  I  affecting  the  estimates  between  sampling  trappability,  the  simulation  After for  five  (1969) ,  and  runs  I  of  recent  unpublished  model, the  and  species  Columbia.  simulation index  and  not  consider.  of  British  BHA  shall  variance  al  of  of  are  rate  parameters et  I  was  closed  survival  animals.  Krebs  parameters  merits  only  results  populations  important  trappability,  five  the  examined  size,  his  to  estimate  simulated  populations  populations  intervals,  from  open  so  natural  potentially  those  that  Carothers  the  population  estimated  found  to  studies  sampling  to  susceptibility  ;  simulated  p r i o r i  of  biased. However,  applicable  in  distributions  Microtus  U t i l i z i n g I  have  Jolly  these  estimated  estimator  in  13  The  Simulation  The was  model  sampled  Model  simulated on  thirty  simulated  population  the  time  which  was  at  sampling  i t  interval  an  pseudo-random  number  was  that  less  The  than  capture and  size  changed.  changes the  in  birth  deaths  the  the  rate,  during  multiplying size,  of  MHA  the  age-dependent  (1970)  population  parameters.  the from  by  population a  normal  and  at  that with  an  the  nearest  i n  I  am  w i l l of  This  population  new  trappability  and  distribution  with  of  a  the  size.  by  mean  by  and simply  population The  from  a l l  accurately  survival  rate.  that  any  affect  The  mean  trappability  zero  and  a  as was  deviation  number  my and  specified  standard  of  size,  integer.  were  pseudo-random a  determine  births  model  i n d i v i d u a l s  product  of  seriously  trappability  Each  to  confident  not  J o l l y  population  selected  age-independent  results  the  rates  time.  the  determined  death  a  capture.  population  numbers  were  randomly  each  (0<x<1)  of  the  specified  The  were  population  adding  rate.  At if  calculate,  starting  to  estimates  of  the  down  survival  about  were  the  distribution  sampling,  the  and  stored.  probability  which  in  "captured'*  to  interval  birth  died  variance  determined  death  population  Manly«s  After  time  f i n i t e  used  size:  were  uniform  parameters  the  alive a  was  capture,  was  individual's  population and  a  size :  individual  its  captured  from  index.  variable each  of  individual  Three  that  individuals  conclusions  last  of  For  probability  rounding  individuals  From  occasions.  drawn  each  the  and  represents  population  information  estimate was  a  of  (drawn standard  14  deviation  of  one),  to  the  calculated  trappability  to  i f  one,  This  or  i t  method  skewed  was  of  was  less  high  than  when  and  population  greater  assigning  distribution  relatively  mean  than  zero  i t  mean  one,  was  individual  the  trappability. i t  set  was  equal  trappability  variance  of  trappability  If  the  set  equal  to  zero.  produced  trappability  was  close  to  a was  zero  or  one. Chitty  and  have  a  have  already  Kempson  lower  unmarked  (1949)  probability been  mice  was  animal  caught,  the  factor.  unmarked  For the  If  mice  each  calculated  the  was  difference  between  Thus  of  population  the  error  1%  overestimate.  and  the  good  Jolly  index  of  period,  estimated would  size,  and  an  variance were  relative  also  the  of the  a  of  the  would  the  of and  one,  animals  estimate  error  201  error  of  by  mice.  size  actual  calculated,  consistency  the  marked  actual  .07  Once  to  Jolly  the  new  equal  number  indicate  of  as  of  each  divided  was  the  and  error  which  factor.  relative  dividing  -.2  estimate the  by  and  The  individuals  was  actual  index,  mice  multiplying  factor  the  untrapped  trappability  trappability  of  The  in  reduction  out.  calculated  size.  same  UNA  the  a  by  reduction  printed  estimates  than  trappability  the  sampling  and  an  the  that  Reduction  by  i t s  had  population,  capture  calculated  trappability  reduction  of  caught.  individual's was  suggested  in were  of  the  into  the  population underestimate indicate  the  MNA  index  provided  measures.  a  a  15  !§§al£§ 9.1  An  examination  increasing,  results  as  in  ten  is  high  each  the  and  long  time  apply  only  The  to  when  trappability  Figures runs.  in  constant, of  The of  2  to  each  6  for  No  basic  of  marked  while  the  was  overestimate,  which was  of  s i m p l i c i t y , size.  sampling obtained  i f  survival  MNA  which  takes  follow  programs.. w i l l  used  in  a l l  of  be  a  should If  few  useful  than  estimate  varied to  examine  obvious runs  only  was  simulation  a  the  that  were  a  were wide  range  interactions they  exist.  population  sampling of  held  size  interval,  .03,  and  equal  individuals.,  drawn the  the  over  trappability  be  the  of  parameters  per  unmarked  always  on  i s  .8  may  less  the  made  of  and  results  was  i t  variance  depending  no  been  occurs  index  MNA i n d e x  runs  conclusions  J o l l y  revealed  stable  have  this  results  to  thirty  sampling  the  of  rate  trappability  to the  the  the  although  .6,  for  a  respect  high.  one,  of  index  Thus  attempt  survival  when  intense  is  trappability  MNA  low,  was  For  with  generally  exception  taken,  parameters  Several  would  present  parameters,  100,  model  series  except  values.  between  the  relatively be  Fig.,8.  of  s t a b i l i z e .  to  in  populations  major  are  later  with  populations  for  trappability to  stable  results  samples the  and  estimators  are  run  same  periods.  the  i l l u s t r a t e d  presented  Although periods,  of  decreasing,  differences, a l l  Simulation  from  these  actual  either  an  parameters.  figures.  population underestimate In  a l l  cases  The size,  or  an the  ure  2.  and  The Jolly  birth of  effect  of  population  estimate  rate  i s  of  . 2 ,  trappability  is  .03,  as  error  Jolly  intervals.  the The  population  trappability  trappability. for  size  marked  is  the  size. is  .6,  bias  of  Survival standard  mice  mice.  x represents  the  The with mean  the for  have  95$ the  UNA  the  MNA  i s  . 8 ,  deviation  unmarked  estimate  triangle  on  the the  same mean  confidence index.  17  IOOT  50 cr o  cn cn LU  0  4i  -50  100  1  0  1  —  1  1  1  1  1  ——  500  POPULATION  :  1  1  1  1000  SIZE  ure and is  3.  The  Jolly 100.  effect  of  estimates  survival of  population  Trappability  trappability  i s  .03,  trappability  as  marked  survival  rate.  rate  is  .6,  unmarked mice.  on  the  size.  bias  Birth  the  Population  standard mice  of  size  deviation  have  rate  HWA  i s  the always  of same  1.0  -  19  200  o on  100  LU  0 4  f  i nn -I—i—i—i—i—i—i—i—i—•—i—i—i—i—i—i—i—i—i—i—• 0 25 50 75 100  % SURVIVAL  20  Figure  4.  and is  The  effect  Jolly 100.  trappability  estimates  Survival  deviation same  of  of  is  of  as  the  population  .8,  trappability  trappability  on  marked  birth is  .03, mice.  bias  size.  rate  the  MM  Population  size  i s  unmarked  of  .2, mice  standard have  the  X  CD O  ERROR  cn CD  cn CD  O  o CD  CD-|—I—|—I—I—I—H—I—I——H—I—I—I—t—I—i—•—' '  cn o  cn  CD  are of  5.  The the  effect MNA  Population  and  size  trappability trappability  of  is as  i s  variance J o l l y 100, .6,  marked  of  trappability  estimates survival  unmarked mice.  is  of .8,  mice  on  the  bias  population  size.  birth have  rate the  is  .2, same  % !  cn  I  J N ,  I  I  ERROR I  CO f\) ^  — • ro co  JN,  cn  C D O C D C D C D C D O O O O O O.  col  ure  6.  The  individuals population birth of  rate  effect on  the  size. i s  trappability  of bias  reduced of  Population  . 2 , is  the  MNA  size  trappability .03.  trappability  is is  and  Jolly  100, .6,  of  unmarked  estimates  survival  standard  i s  of . 8 ,  deviation  25  100 T  50o  LU  0  -50-  100 j  '  H  1  1  0  1  1  1  1  • 1  1  .5 TRRPPRBILITY  1  OF  NEW  MICE  26  MNA  index  has  Fig.  2  size  less shows  between  10-15%  high  (less  than  of  results  1000.  and  the  MBS  50),  parameters  used,  than  this the  Fig.  3  1.0.  The  poor  survival  much  as  a  15%  below  of  MNA  100% the  were  estimate.  I  Jolly  the  J o l l y  also  At  low  of  the  MNA.  that,  survival  the  of  The  range  is  rate  from  obvious.  With  size  the  MNA  was  size.  The  confidence  rates  never  was  worse  as than  intervals  those  of  MNA  i s  superior  per  sampling  that  less  both  size.  population  results  of  index  survival  than  was  general  the MNA  Jolly  sizes  estimate  for  varying  smaller  these  generally  Jolly  MNA o v e r  while  was  the  of  estimate  population  population  population  of  total  in  true  considerably  a l l  low.  the  results  from  the  estimate  i s  population  conclude  estimate.  and  overestimate actual  at  of  Jolly  estimate  superiority  the  10?  figure  approximation presents  j o l l y  that  Jolly  the  varying  variance  good  to  The  index  the  higher from  to  the  and  conclusion  .2  than  25  considerably  give  variability  than  .9  the  J o l l y  interval. Fig.  4  presents  from  .15  to  Jolly  and  MNA  the less MNA  Jolly  MNA  the  s t i l l  through error  were  estimate  than was  .95.  When quite was  actual  results  good,  quite  be  but  high,  population  of  of  varying  trappability  considerably  estimation can  the  the  calculated  less  below and  size.  .5  the MNA  However that  1  than  .5  v a r i a b i l i t y index  was  the  variance  of  Jolly,  population  Table  trappability  greater  the  than  various (see  was  the  later).  parameters  both of much of and the  27  Fig.  5 shows  trappability,  the  results  from  .01  .4  . 5 ,  to  the  variance  in  . 5 . , When  the  variance  of  i s  has  individuals  accumulated  classes  to  trappability earlier.  The  distribution is  the  this  robustness  smaller  6  reduction  in  .11.  and  the  MNA  sensitive  to  for  factor  of  the  these of  population variance to  in  the  and  method a  MNA  of  of  J o l l y  1.0  described  nearly  uniform-  these  runs  estimates  index  changing  to  usually  the  has  trappability l e f t .33,  changes  the .25,  in  both  Both  and  represent  .17,  the  right  marked  points  .15,  J o l l y  parameter  are  The  of  .20,  between  they  proportional  individuals.  intervals.  .12,  estimate  values.  30-"40%  indices  With  low  are  with  clearly  parameter.  results  of  better size.  trappability 0.0  feature  and  the  . 5 ,  .25  good  population  MNA  difference  generalizations.  or  major  to  of  following as  The  unmarked  right  confidence  this  produces  equal  l i t t l e  index  wide  From  From  is  .3  the  of the  truncation  results  reductions  reduction  relatively  the  represent  There  in  the  again  trappability  animals.  distribution  limits.  presents  proportional and  both  although  points  unmarked  a  of  confidence  Fig.  of  trappability.  parameter,  hand  owing  variance  in  the  changing  trappability most  and*  of  than  the  Second,  size,  trappabilities  simulations  F i r s t ,  the  Jolly  the  survival  trappability.  these  MNA  lower  index  make , is  always  index  in  representing  index  i s  quite  between  Finally  MNA  I  the  than  sampling MNA  index  .5  and  as  actual  insensitive periods, is  the  to and  sensitive differential  28  trappabilities  of  marked  Estimation  Of  Parameters  Having  examined  parameters, species  I  of  of  size,  on  based The  consider data,  data are  Tilden  and  the  the  five  from  the  following  control  of  population  size  have  two  data  and  &  sets  of  high  from  period  of  parameters  of  low  and  in for good  numbers  a l  there  each  for  population  index  are  most  and  density  are  high  and  least  accurate  size  used  I  will  californicus  1969) ;  Microtus at  changes  in  populations,  I  species.  poor  and  work  were  survival,  and  MNA  One  is  the  other  survival.  Both  accurate  and  set  when  when t h e y  are  survival low.  Size  population  obtained  from  the  tested  input  to  indicated  model  pennsylyahicus  these  the  have  five  unpublished  Since  rates  numbers  et  (recent  and  as  on  which  Microtus  (Krebs  Jolly  The  of  Microtus  Microtus  Columbia).  survival  of  Population  five  accumulated  my e s t i m a t e s  the  A.  the  simulation  sources:  orggoni  British  period a  of  1966) ;  grid  Microtus  University  is  varying  the  species  (Krebs  and  set  in  bias  for  townsendi  a  of  from  them  Populations  MNA.,  ochrogaster,  from  them  relative  individuals.  Natural  effect  used  Microtus  used  unmarked  For  the  estimated  voles,  estimation  and  that  MNA. the  the  Although  model*  this  in  is  the  not  a  I  simulations  used  the  is  that  estimates  being  results  from  the  serious  problem,  simulations because  MNA  29  is  always  close  population  the  actual  population  size.  s i z e has s u c h a s n a i l e f f e c t on MNA  my c o n c l u s i o n s were  to  Also,  (Fig.,2), that  would n o t be d i f f e r e n t i f t h e p o p u l a t i o n s  sizes  higher.  B. S u r v i v a l R a t e s The s u r v i v a l r a t e s p r e s e n t e d survival  rates,  papers),  this  individuals  as  a r e - the  calculated  minimum  released  by  survival at  a  Krebs  rate  (1966  i s  two-week and  the  later  number  of  sampling period divided i n t o the  number o f t h o s e i n d i v i d u a l s known t o be sampling p e r i o d ,  minimum  alive  the  following  (eg, caught i n that period or l a t e r ) .  C. T r a p p a b i l i t y I method  used f o u r methods o f e s t i m a t i n g t r a p p a b i l i t y . was  to  u s e t h e r a t i o o f t h e number o f a n i m a l s  during a trapping animals  known  The  period divided  to  by  the  minimum  work. The s e c o n d method was t o p l o t  the  which an i n d i v i d u a l  of t r a p p i n g p e r i o d s individuals periods  were  constrained  that not  the individual were  known  considered.  number  to A  Indiana  o f , trapping  was c a u g h t a g a i n s t  t h e number  was known t o be a l i v e . be  alive  regression  t o pass through the o r i g i n ;  was t h e e s t i m a t e  of  be a l i v e . T h i s i s t h e method t h a t h a s b e e n papers from t h e i r  in  caught  number  u s e d by K r e b s e t a l (1969) a n d o t h e r  periods  first  l e s s than was  three  calculated  the slope of t h i s  o f t r a p p a b i l i t y . The t h i r d  Any  line  method was t o t a k e  t h e a v e r a g e o f number o f t i m e s c a u g h t d i v i d e d by t h e number o f  30  times have  known been  methods  to  alive  have more  produce  obviously  been  than  two  nearly  biased  caught  only  denominator,  and  second  individuals  f i r s t  and  denominator.  A l l  these  to  individuals  to  the  third  included seemed  except  in  a  caught  the  p r i o r i  (  be  less  LeDuc  trappability  and  females  for  Hicrotus  difference.  For  this  the  Fraser third 4  data  Biver  methods  of  are  biased,  for  input  the  was  that  slightly.  methods  from  grid  .08  estimating Although  used  used  I to  the  in  the  distribution  differences  between  conclusions  drawn  from  the  third  past of  by  that  model  Krebs,  trappability  a l l  of  from  was  capture  to  and  the  and  Fen  in  presented no  males  have  way  this  model.  However,  very used  trappability  to  later.  in  fourth  only I  the Fig.  .76.  agrees  methods  Lower  and  HNA  these  seasons.  using  third  me  l i t t l e  the  was  calculate  allowed  changes  found  methods  i t  Krebs  between  sexes  not  method  method.  seasonal  the  because  similar  This  method  the  weight  were  trappability  effects  method  simulation  and  have  between  recognize  numerator  third  and  fourth  trappability I  the  method  Serpentine  the  includes  include  trappability  of  difference  the  combined  the  estimate  and  last  a l l  and  the  examined  I have  are  numerator  methods  fourth  townsendi,  the  they  method  to  these  disproportionate  than  in  known of  trappability.  have  C near  but  the  in  and  of  biased  reason  .84,  the  The  differences  Valley,  method  shows  and  both  give  f i r s t  (1974),  in  For  times.  three  f i r s t  third  methods  the  A l l  capture  calculation  to  and  last  that  in  and  individuals  results,  the  once  few  a l l  periods.  since  individuals  of  for  identical  upwards  the  alive  with  the  examine Also  effect  the the  differences  31  between  these  methods  trappability  was  my  techniques  of  D.  variance  of  method for  of  of  variance  of  detailed of  i t .  a  trappability  as  of in  population  size  Variance  .025 of i f  to  this  I  in  for  variance was  trappability individuals  use  of  as  a  7 do  large  shown  presumably  by  to  usual very  prove  of  is  i s not  a  t h i s  (Krebs  et  data  on  from was  the  should  of  have  use  with  of more  I  5),  of have  trappability  for 7.  continuous  variable,  but  the not  entire and  I  within  graphic I  between  appeared  as  as  instead  of  populations. the  range  of  demonstration  wanted with  a  to  see  very  low  captures., a  the  calculated  grid  high  data.  distribution change  so  individuals  periods  the  'Pig.-,  more  numerical  number long  a  sample  in  consistently  provides  standard  presented  1969),  low  estimate  for  (Fig.  of  did  an  third  unimportance  affects  a l  the  a  better  distribution C  be  method,  MNA  value  the  by  formula  rough  Trappability  values  than  taken  the  and  a l l  Fig.  a  from  trappability  the  i t  trappability  .0**5, b u t  there  the  instances.  obtained  considering  define  7.  as  might  grid  changed  sample of  is  on  Fig.  values  presenting  those  i f  the  calculated  The  the  The  fractions,  presented  these  This  but  townsendi  series  greater  recommend  in  was  was  and  distribution  Trappability  much  not  trappabity  animal  Trappability.  Microtus  would  trappability.  analysis,  variance  be  individual,  applied.  binomial  used  each  observation,  was  I  trappability  individual  s t a t i s t i c a l  and  estimating  estimating  each  the  lower,  probably  Trappability  Variance trappability  would  minor  These  peak  in  32  Figure  7,  The  distribution  townsendi Beach,  on  grid  British  C,  of on  Columbia,  the  is  the  an  was  captured  number  of  known  to  than  three  plot.  be  trapping alive.  Serpentine  from  Trappability individual  trappabilities  number  May of  at  periods  Individuals  trapping  periods  Fen  1971  to  trapping  least in  known are  not  near  Crescent  periods  an  to  Microtus  December  once,  which  for  divided  1973.  in  which  by  individual be  alive  considered  in  the was less this  250 200 150 100 50 0 0  .5 T R R P P R B I L J TY  34  the  the  If  left  these  hand  not  the  appear  distribution  First  known  be  possible  alive  Fig.  7  are  known  to  be  a  of  of  bimodality  of  E.  Trappability It  has  population authors  are  should  that  be  a  trappability there tried  is to  unmarked that 70  in  short  of  and  extreemly only  how  of  i f  and  the  my  I stress  graphically  who  were  is  not  of  as  individuals  are  of  fractions  methods that  examine  these  such  range, by  the  several  i t  p l o t s ,  a l l  twice  presents  animals  sensitive  low  or  trappability  l i f e ,  useless.  once  I  periods,  the  appeared.  i l l u s t r a t e s  number  time,  Marked  and  Dnmarked  been  suspected  more  susceptible  rarely  (Fig.  that  7)  in  in my  the  individuals species, less  in  ratio for  to  the  i s  Fig.  very 7  is  p o s s i b i l i t y  Individuals some  individuals  trapping is  traps.  a  This  data  gives  but no  species  individuals  Presumably  of  of  that  the there  an  1963)  marked  to  and be  of that  (1956,  mice,  appear  once  of  indication  Tanaka  a  some  distribution  trappability  several  in  others;  implies  trappability. of  than  proportion  trappability,  unmarked  trappable.  that  there  enters  bimodality  estimate  percent  C,  peak  trappability.  bimodality  any  many  to  caught  two  of  such  have  estimation  are  trying  suggesting  population  or  duration  plots  no  Appendix  estimate  for*a  such  method  be  large  demonstrates  the  alive  and  but  grid  one  f o r ... t r a p p a b i l i t y  limited, only  to  a  only to  i t  for  are  for  begin  Second,  possible  usually  recaptures  animals.  7,  my c a l c u l a t i o n s .  there  to  Fig.  individuals  would  in  of  points. to  of  non-trappable  trappability and  side  and found  30  to  individual  has  35  entered for  a  trap  the  older  i t  cotton  rats  i s rat  (Hall  individuals  are  individuals  (Joule  The  f i f t h  areas 1971  I  to  1974),  and  and  in  trapped  every  on  In  G)  discontinued, but  was  I).  August,  During  two  the  grids  trapping  of  prebaiting,  to  seem  to  avoid  Kempson  G  to  place.  objects  resumed  and  trapping  restarted,  58  marked  reduction  the  would  voles animals  on  factor  i s  an  58/108=  on  of  In  grid .54.  July Fraser  areas  (Grid  grid  (Grid on  February, prior  on  the  that  to  the  grid  for  because  that  voles  (Chitty  and  technique  continued  to  stay  animals  caught  on  during  compared G.  the  f i r s t  of  the  mice.  Hy  Krebs  the  1974,  occurs  documented  estimate  as  from  numbers  common  of I  study  the  other  weeks  unmarked  captured  the  trapped  a  had  grid  of  remained  on  this  Two  years  environment  number  provide  were  these  well  i s  populations of  of  traps  is  estimate  delta  Four  their  trappability  relative  the  the  prebaiting It  to  caught  was  two  both  G.  the  ratio  subordinate dominant  To  1974).  prebaiting  the  number  in  I..No  If  numbers,  reduction  new  than  socially  for  one  placed  l e f t  in  in  on  were  Grid I  shown  trappable  experiment.,  continued  for  1949) ,  the  Island  control  mouse-trapping. similar  on  than  weeks  (LeDuc  trapping  are  two  they  similar  been  socially  following  was  has  less  simulation  trapping  years  resumed  resumption  traps  two  were  was  the  1973,  are  individuals.  Westham  fiiver.  rats  It  1974).  the  unmarked  1973  again.  that  Cameron  the  July  so  trappable  performed  were  do  younger  less  of  to  that  parameter  trappability parameter  l i k e l y  108  estimate (  at grid  week  of  relative  When g r i d with  in  G  was  on  Grid  of  the states  36  t h a t H i c r o t u s townsendi  seems t o h a v e t h e l o w e s t  trappability  of unmarked i n d i v i d u a l s o f a l l t h e s p e c i e s he h a s s t u d i e d . F o r this  reason  a  reduction  factor  e s t i m a t e f o r t h e o t h e r s p e c i e s , and estimate  f o r Microtus townsendi.  an unmarked v o l e o f t h e traps,  clearly  size  smaller  of  .66  i s u s e d as a r o u g h  .50  is  used  as  a  rough  This i s the p r o b a b i l i t y  normally voles  captured  will  have  a  that  will,  enter  much  lower  trappability. E a c h o f t h e f i v e p a r a m e t e r s was high  populations  then run f o r the low  i n e a c h s p e c i e s ; t h e p a r a m e t e r s and  of the s i m u l a t i o n s are presented  in  Table  m a j o r d i f f e r e n c e s b e t w e e n s p e c i e s . MNA is  2-10%  low.  Jolly  l a r g e a v a r i a n c e as Fig. giving Both size.  The  Jolly  survival rate  There  i s 10-2071 low  are and  MNA  population  very  low  Jolly  i n d e x , and  Jolly  are very  accurate  indexes  of  estimate  of s u r v i v a l r a t e ,  and  (from MNA)  are  equally  good  estimate. population  the  minimum  indexes  of  densities,  where  the  Jolly  estimate  the  except becomes  e r r a t i c , e i t h e r method i s q u i t e s u i t a b l e f o r a p p r o x i m a t i o n numbers  as  cycle,  a c t u a l s u r v i v a l r a t e . I t i s c l e a r from t h i s f i g u r e t h a t at  no  MNA.  8 presents a simulated microtine  and J o l l y  results  e s t i m a t e s g e n e r a l l y have a b o u t t w i c e  actual population size,  MNA  1.  and  w i t h i n t h e range of parameters encountered  of  i n natural  mouse p o p u l a t i o n s . My  f i n d i n g s a r e i n complete agreement with those  (1970) and v o l e s and  Carothers  (1973b).  t h e e f f e c t s o f open  of  Manly  Lower t r a p p a b i l i t i e s o f u n m a r k e d populations  do  not  seriously  37  Figure  8.  Simulated in  Microtus  numbers.  squares  The  solid  represent  represent  the  population  MNA  the  line Jolly  index.  is  going actual  through  a  fluctuation  population  estimate,  and  the  size,  the  circles  38  SAMPLING PERIOD  Table as  1.  Results  densities  Species  of  simulations  ar.d s u r v i v a l  .  Population Si7e  fjicrotus Cdliiornicus • ^Icrotus  and  using low  Survival Rate  209  .77  15  .6B  1 CO  estimated  p a r a m e t e r s from  d e n s i t i e s and  survival,  Trappability f!enn Variance  .77  . Xictot^s cre^ion i  'icrotas t c w^d i  20  .62  60  .  15  .63  70  . 80  fi 200 30  species  and  .0U2  .70  .80  ..026  .011  .82  .039  Each  species  e r r o r i s averaged  Relative ?1ean  Variance  i s run  froo  two  for  Error Bean  Jolly Variance  -. 12  .003  -.OH  .001  .003  -.00  .020  -. 10  .00tt  -.05  . CCS  -.20  .00K  -. 05  . 029  -.07  . 00 1  -.02  .C01  -.18  .007  -.09  .011  -. 1 1  .0011  -.OH  -.12  .008  -.10  .013  -. 12  . 009  -.09  . CC8  - . 18  .006  -.06  .017  .66  .50  high  simulation  - . 16  .66  . 85 .70  Microtus. of  .66  ao  . 90  of  variance  . Reduction Factor  . 85 .71)  ^icrotus  .039  five  ilrror  .C05  40  affect  my  results.  estimation  in  two  Kikkawa species  capture-recapture in  intensive  on would  experiment  I  compensate  for  Radioactive I  would their  in  a  be  radioactive taken  possible  to  for  that  were  good  I  use.  marked  and the  of  of  Further unmarked „  simple  estimators  the  Jelly  census  and  accurate  very  to  estimate enumeration  picture  of  the  populations.  this  by  fates  were  once  low  factor  Several  while  of  more  amount was  or  would poor  not  expected  of  minimized., tags  Table for  detection  die  in  2  Voles or  situ.  study  but  It the  there  areas,  summarizes  animals  of or  factors  predation, the  situ  weather,  these  on  in  recovered.  to  where  dispersed  poor  to  predation  radioactive  be  of  vulnerable  by  that  mostly  one  determine dying,  skeletons  food,  by  to  animals  individuals  weakened  "be  method  that  carcasses  predators  would  different  as  s t r i f e  a  tagging  assumed  death  that  generally  expected  I  tag,  by  individuals  that  vole  radioactive  recovered  intraspecific  was  my  reasonably  and  disappeared.  predisposed  is  mice  that  concluded  supplement  assumptions  feel a  as  of  to  size  Refinements  limiting I  such  and  parameter  T_a_gcjinc|  used  animals  were  the  population  helpful  performed.  provide  fluctuations  of  population  mammals,  response  be  unnecessary.  techniques  small  programmes  trap  individuals  seem  of  estimates  sampling  experiments  (1964) s t u d i e d  so the  disappearing  reasons. different  radioactive  isotopes  have  been  used  for  Table  2 rates of r a d i o a c t i v e tags  Hypothesized Cause Of Disappearance  F a t e Cf  for postulated  Carcass  '  ultimate  S a t u r e Of Recovery  causes of  Tag  Predation  disappears  nc s i g n except weasel k i l l s , which ar G identifiable as s u c h  Dispersal  disappears  no  Starvation  in situ  death  found with skeleton  iE  situ  death  found with skeleton  in situ  death  found with skeleton  in situ  death  found with skeleton  'Agonistic  encounters  Lowered v i a b i l i t y due t o g e n e t i c changes  sign  disappearance.  42  marking being  small the  mammals:  most  suitable  because  iridium  and  studies  of  months,  are  in  50  The  wire  with  the  I  and  the  section  tag  placed placed  checked losses were  the  to were  not  a  around wire  then  preferred  ah .2  the  trapping  noted.  retagged,  wire epoxy  tag  2  ,  expensive;  form,  core  lengths, tags tag  i t s  glue.  about  s i x  and  are  was  a  wire  and  per  of  ,4  to  mm  1 minute  mark the  the  1000 ' s with  proved to  in  wrapped  contact  method  cm.  coated  then  used  only  purchased  covering  area  This  about  tantalum  producing  the  for  microcuries of  so  isotopes  of  which  200  ear  and  took  8  of  cm  on  1  considered  to  make  be and  tag.  on  make  wire  platinum,  placed  Each  and  the  activity  fingerling  with  Q were  the  in  .75  i s  half-life  tantalum  into  coated  on  a  mm c e n t r a l  entire  per  have  of  P  on  become  the  tantalum  longer  have  the  $2.00  no gold  used  was  is  and  h a l f - l i f e ;  wire  satisfactory. about  I  iridium,  long  available  with of  gold,  Cobalt  Both  layer  cut  were  Grids  was  mm  The  tag  quite cost  spools  lengths  digit,  kind.  consists  diameter.  i t s  readily  .1  voles.  of  expensive.  cm  a  these  frequent.  tantalum  this  moderately  cobalt,  monitored  in  the  techniques.  In  addition  right  left sure  ear  ear.  At  the  these  the  every tags  mouse,  a  capture  to  described the  in  normal  radioactive  tag  the  was  mouse  were  s t i l l  intact,  and  any  that  lost  their  radioactive  tag  losses  were  quite  Individuals but  of  manner  rare.  ;  43  Method  Of  Tags  were  staked  out  Recovery detectable  in  square  making  every  location  hide  ten  without solid am  any  rock,  tags  that  Haney,  and  eight  tags  grids  on  the  Of  did  not  every  third  the  delays  in  of  radioactively  periods  was  tagged  ,?7  non-radioactively  (215  off. have  tags  near  been  meter  on  is  the  of  surface,  I  grids.  An  recovery  of  the  the  a l l  feet  the  lost,  around  them two  the  of  co-workers  find  tags  study  and  a l l  at were  perimeter  individuals  with  of  home  Technique  assumptions a  about  radioactive  Between  riot  of  July  this tag  technique. on  and  1972  radioactively-tagged tagging  the  on  The  other  average  surviving  individuals), individual  while  surviving  vole  July  1973,  control  Owing  to  individuals  probability  between  the  a  to  survival.  isotope,  untagged.  individual  tagged  each  had  I  During  from  through  through  detection  Tagging  of  was  were  a l l  were  one  to  were  grids  grids.  radioactive  study  able  m band  placing  delivery the  to  10  survival.  individual  effect  throughout  i t s  of  known  number that  within  occasions  found  fallen  Badioactive  was  affect  have  the  two  recovered  any  of  slowly  detectable  method  were  walked  were  detect  a  assumption  to  I  was  a  edge  The  the  study  and  tags  have  m the  7  traveling On  searched  to  tested  I  for  also  I  grid.  a l l  I  on  known  Tests  One  that  was  I  Tags  since  about  squares;  grids  trouble.  are  study  ranges  the  the  check  recovered. the  that  on  on  confident  additional  a  sure  tags  foot  25  at  of  trapping  p r o b a b i l i t y of was  .78  a  (250  individuals). l i t t l e  or  As f e l l  no  as  an  vicinity  the  tags,  of  heavier. tags,  To  I  period.  of  is  of  individuals  the  two  weeks and  respect  time  to  disappeared  lost  this the  I  to  left  decayed  bi-weekly  number  of  sampling  8/549 that  or  .015.  to  their  of  not  their  a l l an  tag  attacked  the  it  fifteen the in  During  carcasses,  to  and  after  animal  skeleton  of  the  lost  the  I  was  previous each  percentage  situ,  i t s  tagged  carcasses  at  different  study.  During  the and  summer, had  winter  the  fungal  carrion  usually  in  sampled with that  trapping trapping  recovered.  weeks  1.5%  tags  animals  be  s i x  each  random  would  about  average  of  tags  capture.  in  the  by  that  their  Since  caught  the  being  during  lost  the  last  died  been  and  of  for  radioactive  expected  .75%  since  of  alive  was  are  higher  have  have  about  underestimate  and  loss  periods  would  individuals  when an  of  animals  tags  bit  tissue  disappearance  trapping,  non-radioactive  a  disappearance,  that  lost  be  has  occasionally  radioactive.tag  known  Thus,  disappeared  slowly;  them.  rate  the  tagged  to  the  tags  throughout  reduced  to  of  the  quite  and  with  minor  number  that  out  to  tagging  tags  happens  seemed  owing  disappearance  and  This  isotope, the  radioactive  rate  probably  is  assumed  year  loss  radioactive  the  had  this  rotted,  ear.  assumed  Because  prior  I  the  prior  bi-weekly  period,  the  radioactively  This  period.  above,  estimate  divided  of  survival.  the  product  number  but  that  on  animal»s  well,  radioactive  suggests  effect  mentioned  off  tags  This  To times  test of  carcasses  growths  beetles  completely  carcass  had  rapidly  eaten  a l l  flesh  in  a  few  days.  in  f i e l d  was  not  the  taken  by  a  underestimation it in  apparently this  reduce  study. the  Only  the  proportion  Loss  to of  tagged  carcasses  carcass  was  scavengers in  infrequently,  summers, of  the  This  frequency  occurred During  of  recovered.  scavenger. of  one  situ I  death,  ignored  the  rapid  rate  carcasses  taken  by  of  i t  I  left  presumably would  cause  but  since  as  a  decay  scavengers.  factor should  46  RESgLTS  Population 1 Inequations  I  wish  to  explain  t o w n s e n d i p o p u l a t i o n on  and  the  t h i s study, I  two  the  these  have  combined  m a l e s and  females f o r analyses  females,  but  two  (Chitty  the  workers  calculated  Q.  of  the  Because t h e  g r i d s seemed  a l l  s i n c e I am  i n s u r v i v a l between  1952,  data  for  to  be  of  most  population seperatly  i n t e r e s t e d i n very and  my  analyses. seperated  parameters. for  I  males  and  l a r g e - s c a l e changes  analysis  d i f f e r e n c e s i n s u r v i v a l b e t w e e n m a l e s and  the  throughout  K r e b s 1 9 6 6 ) , have  parameters  periods,  Microtus  demographic  g r i d s were t r e a t e d i d e n t i c a l l y  Previous  also  fluctuations  g r i d s P and  a t t r i b u t e s o f t h e v o l e s on same,  2  G r i d s P A nd  showed  f e m a l e s were  that  slight,  I have combined t h e s e x e s f o r p r e s e n t a t i o n i n the t a b l e s . Such combining  of  data  i s only reasonable  when t h e c h a n g e s one  interested  i n are of a l a r g e - s c a l e g u a l i t i t i v e  n a t u r e , and  more s u b t l e q u a n t i t a t i v e c h a n g e s . When t r a p p i n g was June  1972  ,  no  Hicrotus  g r a d u a l l y c o l o n i z e d the area of  1972,  and  reached a high of  entire  reproduced  first  year  (1969)  for  during the  l a t e summer  fall  voles  s p r i n g of  Hicrotus  in they  by  July  1973.  and and  spring,  During  the p o p u l a t i o n maintained  g e o m e t r i c r a t e of i n c r e a s e averaging  al>  begun  were p r e s e n t , b u t  of the s t u d y ,  no b r i e f d e c l i n e i n t h e  not  townsendi  during the f o l l o w i n g winter 115  is  18%  1973  bi-weekly.  as r e p o r t e d  pennsjj 1 v a n i c us,  and  as  There  the a was  by K r e b s  et  found  in  147  Microtus  townsendj  (Krebs  minimum  number  animals  and  Q.  From  rapidly  for  level  73  of  constant  the  in  of  1974,  June,  I I  have  are  low  the  the  of  as  two  to  of  30;  ten  presents  alive  on  population, i t  then  when  P  declined to  a  approximately  they  individuals  the  grids  recovered  remained  mid-April, of  been  the  at  declined the  very  beginning  ended.  questions control  periods  of  1)  the Hhy  declining  voles  during  characterized  by  between-period  about  grids:  data  into  did  the  numbers?  the five  their  differences  population  in  other  2)  voles what  periods?  periods  that  within-period survival.  The  follows.  1972  II  July  and  III  September  IV  December  V  April  used  have  the  June  had  F i g . ,- 9  Numbers  combine  I  I  low  study  two  to  July,  disappearing  are and  a  two  the  chosen  believe  periods  If  on  fates  s i m i l a r i t i e s  to  u n t i l a  when  during  have  115 i n  consider  fluctuations  the  then  com.).  known  November.  reached  w i l l  disappear  of  weeks  late  from and  were  high  six  sharply,  I  of  pers.  wanted different  July  August to  to  May  increase  1973  decline 1973  March  increase  1974  peak  1974  examine  period  1973  November  1973  and  to  to  changes  definitions  decline  in  reproduction,  to  match  the  I  would  breeding  seasons. Having  selected  the  periods  for  pooling  data,  I  48  Figure  9. (a)  Minimum  grids Winter  P,  Q.  number (b)  months  of  of  Number Nov.  of -  animals  known  animals  trapped  Feb.  are  shaded.  to  be on  alive  on  grid  Z.  49  MICROTUS  GRIDS  TOWNSENDI  100  t v i w W W  ^ w w w w w w w  WWWWWV WWWWNW N N N N N N N Jw  25  WWWW\)(W\W^ wwv^w\/www^ W \ Y \ \ N \ 7 W W W ^  W \ / . \ \ W / , N N N N N W  WSAW-.'-wNNNW^ WiNwN'WSWWW \\KKWWWWWW \-/wWWWWW\^ \/.WWWWWSW^  a 5-  ^WWWWWWW W W W W W W W V SWWWWWWWWWWWWSWW* w w w w w w w v W W W W W W W V WWWWWWWV W W W W W W W V WWSWWWWW^ W W W W W W W V W W W W W W W V V W W W W W W W W W W W W W " W WSWWWWWW^ 1  a  LU CD  M  1  I'M M  1  GRID  -  101 5  Z  H I II I I  REMOVAL  W W W W W W W W \ W W W V W W W \  N N N N N N  15  w w s w  W W W W W W W W WWWW\ >. W W S W W WWWWWWWW wwwwwwww \\\\\\\vWWWW W WW\WWW W\ W W W W W W W W W W W W W W W W w w w \ )\ \ \ \ \ \ \ \ \ V W W W V WWWWN wwwsw WWW\lWWWW\ \ W W W W ,WW\W\ sWWWW www\Kwwww www\Kwwww W W W W W W W W W W W W W W W W \WWWW wwww . WWW\|VWWW\ vwsww sWWWW vwww\ \\\\s\\\ \\\\\\\\  WWWWWlftWW* WWWWW/sWW;  CO  cm  A^. |  ^ ^ ^ ^ ^  50  Li_ O  P+Q  N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N W W W W W W W V N N N N N N N N N N N N N N N \ W \ W W W W N W W N N N N N N N N N N N N N W W \ N N N N N N \ N N N N N N N N N N N W N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N ' W N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N W N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N  W W W W W W W W V W W W W W W W V W S W W W W S W SSNNSNS W S S S S S S S VSSSNS WSSSSSSS WSSSsAsSSSSSSS  H—f—l—{—I—I"  0  J  1972  S  N . J  M  M  J  1973  S  N  J  M  M  1974  50  calculated  the  relevant  Calculation  of  of  rates  survival  numbers  population has  in  already  small  parameters been  mammals  for  each  described. involves  period.  Calculation  a  number  of  losses  due  i  assumptions, to  death  particularly  from  Calculation  losses  Of  Survival at  time  t  and  others  trapped being  Survival rate  w i l l have  subsequently  comparison  are  that  before.  I  shall  individual  born  and  43  grams,  prep.).  Q.  has  are  in  i s  on  Also,  i s  our  the  the  a  area  or  of  at  a  and  distribution  to not  of  an  grid,  entered  or traps  10  as  any  shows  the  individuals  weights  adult  newly-captured  Fig.  adult  in  trapped.  immigrant  individuals of  rate.  of  the  any  an  fates  gchrogastgr  previously  as  be  townsendi,  been  newly-trapped  definition  two  number  had  Krebs to  survival  Hicrotus  elsewhere.  new  the  large  but  alive  t+1.  f a i l i n g  of  immigrants grid  time  Microtus  never  grid,  individual  dispersed,  recruit  born  an  individual  that  the  the  number  Rates  pennsj;ivanicus,  define  separate  that  trapping  d i s t r i b u t i o n of  which  to  calculation  either  individual  A large  any  have  born  on  on  died  of  that  were  newly-captured  P  that  californicus,  individuals  weight  alive  Mi££2%1 s  animals  body  probability  be  feature  caught  wishes  dispersal.  equivalent  with  Microtus  These  the  s t i l l  considered  voles  is  to  one  A;nd I s i i g r a t i o n  assumed  A s t r i k i n g  and  due  i f  weighed vole is  on  grids  more (Krebs  skewed  to  than in the  right.  It  i s  d i f f i c u l t  to  determine  i f  these  heavy  animals  are  51  Figure  10. Distribution  of  body  f i r s t  for  grids  Data  capture from June  1972  to  weight P May  and  of  Hierotus  Q.  Both  1974.  townsendi  sexes  at  combined.  FREQUENCY o .  cn CD  C D  m o  C D  O  P  T  -<  C D  cn C D '  O  C D  n rn  oo  Ln  C D  I  53  immigrants, of  body  but  weights  immigration fenced Very 13%  few  grid,  individuals  these  with  This  suggests  that  area  as  do  greater  the  I  between  have  animals  animals  of  individuals  data  capture in  Fig. that  capture.  I  and  more  than  grids  P  and  response but  I  voles  40  proceed are  assumption, immigration  on  the  the  I  rate,  w i l l  may  factor  I  be  w i l l  i t  of  to to  these  recognize  make  of  affecting that  in  consider  that order i t s  fenced animals born  grid),  than  and  During  40  of g  However,  rapid  40  on  point  proportion  between  due  a  net  the  more  reach  due  level.  cut-off  growth.  period  i s  fenced  .001  were  on  weigh  slow  the  elsewhere).  proportion  assumption  and  of  com.).  most  higher  w i l l  in  difference  must  a  a  capture  unknown  (born  for  capture.  inside  then  (born  differences N,  the  arbitrary  gain, grid  at  capture  recruits  during  grid  immigrants. but  an  data  Chi-sguared.test  traps  area,  f i r s t  individuals  This  to  immigrants  the  f i r s t  some  as  grid.  different  respond  weight  that  g at  or  unfenced  during periods  fenced  Q.  g on  as  assume  Q and  40  the  which  pers.  than  g  taken  (Boonstra  more  40  few  Q  these  weighed  chosen  than  presents  into  f i r s t  unfenced  as  area  at  at  11,  and  d i s t r i b u t i o n  g  g  on  11  an  the  40  40  body  in  at  than  than  born  suggest  P  an  look  more  animals  c l a s s i f i e d  rapid  P  s i g n i f i c a n t l y i f  to  Fig.  grids  31% on  c l a s s i f i e d  periods  i s  capture  weighed  on  weighing grid.  to  voles  data  they  this  prohibited.  newly-tagged  shows  of  f i r s t  N adjacent  compared  f i r s t  test  at  was  grid  of  a  at the  growth,  g before  f i r s t  unfenced animals  grids weighing  immigration  onto  d i f f e r e n t i a l  trap  fenced heavy this to  populations, newly-tagged is  a  dubious  estimate  consequences  in  the the  54  Figure  11. Distribution Microtus  of  body  townsendi  the  permission  four  month  period  on  of in  weights fenced  Rudy the  at  grid  f i r s t  M.  Boonstra). spring  of  (data Data  1974.  capture used taken  for with  from  a  55  20  T  >CJ  :z:  UJ ZJ  a  UJ u_  0  Jl  10  BODY  n.  20  WEIGHT  R 30 OF  40  50  NEWLY  60 CAUGHT  70 MICE  80  56  discussion. It  is  different the  well rates  differences  This II.  The  males  separating that  a If  avoided  good  than  than  40  weighing  is  correlation  no  rate  immigration  rate  s i g n i f i c a n t l y  that  during  weeks  different  grids  (except  With  g  were  of  during  to  and  of  my  and  at  examined  Q in  sexes.  Appendix  heavier  at  crude  method  on  the  grid,  I  at  f i r s t  gained  of  believe  capture weight  there  newly-tagged  voles  correlation  of  periods than  I,  III  are  zero  (r =.25 2  disappearance  other  adults  the  are  and  moving  and  rate and  p=.009), adults  moving  P  should  rate  (increase)  more  individuals  declining  disappearance  a  weighing  grids  newly-tagged  periods  be  weighing  for  elsewhere,'and  are  rapidly  should  voles  disappearance  during  period  I  between  reorganization, there  adult  high  g  have  the  adult  greater  grow  very  weights,  born  social  for  the  so  grams  newly-tagged  40  areas  respect  of  however,  c o r r e l a t i o n between  P  3.1  40  significant  except  capture  born  that  number  between  voles  1969),  grids  given  body  If  of  a1  of  than  grid  number  female  adequate.  lower  between  than  is  et  for  animals  more  the  at  g and  periods  immigration  from  so  and  f i r s t  average  g cut-off  capture  more  an  females,  on  at  presented  only  p=.30).  2  good  to  40  There  (r =.04,  during  is  born  40  g.  weighing  is  body  than  correlation  less  The  weight  animals  individuals and  i n  immigrants  single  male  1952, Krebs  were  capture  that  (Chitty  distribution  f i r s t  Q  documented  in  be  a  adult  numbers..  and  adult  IV  (peak)  suggesting are  into  moving  the  study  V).  study  areas,  I  have  assumed  that  the  57  number  of  roughly  equal.  in  immigrants This  relatively  disperses into  to  the  will  another  making  anywhere  at  against  i t .  situated higher  The  The  poorer  present as  the  The  is  individuals increased the  to  approximately per  grid,  individuals  move  25  and  this  argument.  mouse  "sign"  areas  that  would  to  the  the  to  that  that  we at  i f  to  be  I  are h a d ..a  general,  nearby  rate  get  have  in  to  but  arguments  produced  exchange  Q appeared  and  grids  should  emigrated  the  two  f i e l d  have  of  cannot  the  they  the  not  make  argument  that  or  surplus  calculation  accordingly;  look  habitat, than  P and  diversity  grid,  i s  move  areas.  as  many  even.  situated  In  the  the  same  land.  other  emigration  rate  that  should  assumptions,  assumption  young  areas  dispersers  adjacent  good  of  grids  weak  emigration  argument  relative  case,  of  the  be  located  individual  individual that  should  areas  every  recognize series  emigrants  study  us . n e v e r t h e l e s s  relatively  a  for  an  simplifying  f i r s t  producing  potential  a  Let  production  other  I  of  for  another  calculated  a l l .  in  area,  this  number  habitat,  requires  the  the  reasonable  areas.  rate  treat  without  seems  uniform  study  emigration  and  into  lbs  of  of  our  area.  placing  oats  every  It  evidence  areas  compared  trapped  immigration  a  grid  This  traps  bait  food  area.  no  being  trapping  putting  the  i s  balanced  study by  whole  into  not  of  additional  There  within  the  produced  this  are  process  effect  move  against  may  in  of  and  other also  i s  encourages  could  the  be  due  boards  traps. week  have  those  nearby.  Taitt  to on  used bait  encouraged  runways  with  I  as  d i f f i c u l t more  and  to  test  or  other  on (pers.  similar com.)  58  has  not  increased  densities  similar  grasslands  could  tested  be  nearby, grid  I  but have  As  by  in  stated  the  Fraser  i t  done  earlier,  losses  included  both  the  relative  equality  of  calculate  the  important  in  i£  1  §ilM»  minimum  emigration actual  rate.  complementary  mortality  number  of  in  situ  River  Delta.  grid  and  The  food  a  to  hypothesis  untrapped  individuals  of  from  areas  live-trapping  a  'net  This rate,  net have  and  on  proportion  i n  f i e l d  emigration.  the  deaths  (defined  deaths.  and  immigration  the  rate  immigration  of  losses  calculated  survival  quality  •sacrificing*  death  number  calculating have  high  so.  generally  these  a  requires  have  separate  adding  snap-trapping  since not  by  these  areas.  rate*  which  previously)  used  rate, to  can and  This  that  survival  we  losses,  deaths  survival  been  Assuming  emigration, total  of  studies  is  occur i s  plus and  calculate  the the its the  59  Survival  Bates  Table and  summarizes  3  Q by p e r i o d .  survival  rate,  survival III,  During  and low  Then  the animals  This  poor  Microtus during low  remained  1973-T974. low  In  until  then  there April  the  end  of  was n o t  a  Survival  remained  high  (.90),  population populations pers.  com.).  throughout  general  increased on  grids  P  and  at Q  area the were  1973). rate.  populations survival  f a l l  as  same  winter  (.83), time  declining  of  remained of low  the  inside  AA,  level,  period  was  of  remained  and  second  of  (.72)  high  and  dropped,  This  the  i n the vicinity  the spring  unfenced  and  Survival  I,  and V.  rapid  to i t s former  phenomenon  rapidly  very  com.).  again  throughout  and on nearby  a  poor  the  the study.  was h i g h  enclosed  very  recovered  the survival  survival  areas  trapping  pers.  II  the end of June, at  adult  periods  periods  P  adult  Minimum  and peak  phenomenon  reported  on grids  minimum  rate.  survival  (up t o  (Anderson  weeks,  the  i n decline  to disappear  also  period  f o r eight  and  rapidly  was a g e n e r a l  tgjwnsendi  as  i n increase  .60)  & co-worker  parameters  survival  of the study,  started  areas.  to  (.58  year  survival  this  presented  t o .88)  (.84  increased  study  i s  and the net adult  the f i r s t  population  my  Survival  was h i g h  and IV,  the population  the  f i r s t . fenced  where  the  that  the  (Anderson  c Table  •  I 1  |  1  3.  1  II  7  —  T  Per iod  P o p u l a t i o n p a r a o e t e r s f o r £icrctus t o v n s o n d i on g r i d s p and q.  I r.eaP. | No. Of I Ada I t s  T  1 1 18.  r  _.—  Hinlnuo I Net | Adult Juvenile | Adult | s e c r u i t n e r.t I I n m i g r a t i c n I A d u l t Survivall Survival IRate P e r Rate Per I Rate 1 |Adult Adult  •*ean i I n d e x Of R a t e Of I J u v e n l l e I n c r e a s e Surv i v a l ?or I 7 Keeks  1  . 18  1  . 31  2.7  1  .13  1  - .97  I  .81  +  -.22  | 17.9  1 1  0.9  1  -13  I  .11  1  -72  1  .58  I  .93  • 1  .87  1  1  1  1  3  !  .1"  6. 2  I  ."5  I  .06  !  I 30.0  |  -.01  3.9  I  • 15  I  .05  1  .93  1  .88  I  I 31 2  1  -.29  2. 3  I  . 29  I  .10  1  . .70  1  .60  I  i  —  HI I  IV  61  lSli3£lii°£  I  define  number by  the  g at  2£l2 S l i ^ s  of  to  of  time.  Immigration the  population. During  Harch  During  f i r s t  dropped  off  for  the  and  there the  trapping  period.  periods the  and  pulse  immigrants period  are  II,  which  of  the  of  onto high  rates  were as  grids  P  recruitment  is  that  weakest  this  in in  Q in  June  these  came  the 3  this  40  Table  3.  the  came  i n .  period  II,  i t  then  III,  IV,  Fig.  12 each  captured  during  period, of  to  Fig.  12.  The  and  July  1973 it  overlaps survival,  obscure  the  pulse  of  followed  seems  newly-tagged  period.  Up  during  tend  weight  than  week.  basis  and  divided  into  but  per  the  study.  periods  were  areas,  heavy  during  the  captured  Table  in  of  During  on  g,  animals  two-month  seen  these  45  immigrants  defined  in  of  as  more  given  individuals),  4  shown  and  is  immigrants  since  immigrants  assumption  immigrants  a l l  10  weighing  periods  immigrants  of  and  June  study.  than  of  Half  1973,  immigration  obvious  that  number  July, I  less  than  winter  trapping  the  period  16 i m m i g r a n t s  and  (28  more  rate  f i r s t  Bay,  of  trapping  voles  only  high  rest  were  presents  the  two  s t i l l  and  any  weighing  1973,  A p r i l ,  was  June  for  immigration  during  immigration  V  rate  voles  adult  low  of  the  £  previously-^tagged  The  was  end  And  immigration  newly-tagged  number  the  P  a  likely voles  62  Figure  12. Number and in  Q.  of  immigrants  Immigrants  weight.  Winter  per  are  trapping  defined  months  of  period  as  Nov.  new -  to  voles  Feb.  are  grids above  40  shaded.  P g  63  J  . S  1972  N  J  M  M  J 1973  S  N  J  M  M 1974  \  64  Intensity  Of  Table by  Breeding 4  period.  I  percentage  summarizes  several  use  indices  of  adult  number  of  pubic  symphysis  traps.  Host  and  females  was  April  and  were  percent  of of  in  sample  Except  1973-1974,  animals  level had  throughout  l i t t l e  effect  the  of  sizes  were  of  females  year  the  or most  breeding  thus  giving  population  with  birth  the  indices  did  not  began  in  show  that  shows  the  13  large  nipples,  reliable  at  i n  a  the  growth  which  index  of  winter  of  f a i r l y  intensity  i n  spring  Fig.  hiatus  the  nipples,  females  breeding  These  medium  and  large  when b r e e d i n g  period.  i s  intensity intensity,  occurred;in  lactating  study, the  number  1974.  with  the  or  Intensive  every  for  upon  of  breeding  medium  of  study  time  females  reproduction. the  the  February  adult  number  of  reproductive  with  January.  breeding  larger  and  only  to  of  pregnant,  during  the  and  females  because  open,  November  1973  females  obviously  breeding  f a l l ,  occur  four  indices  uniform  of  breeding  rates.  Rgcruitment Table  3  recruitment the  (1966 from  and birth  the  of  by  previously:  as  rate.  number  divided  shows The  number  index  has  papers)  f i r s t of  index  number  later to  index of  newly-tagged  used  and  The  of  the  defined  as  voles  four  weeks  by  Krebs  past  survival  weighing  and  sub-adult  the  recruitment  voles  is  lactating in  estimate  s u r v i v a l ,  survival  females  been  capture.  juvenile  juvenile  of  to  of  juvenile  newly-tagged  the this  the  young  rate  is  less  than  voles  defined 40  g  at  u 4.  T a h i P  Table  Period  II  III  I n d i c e s o f c e c r o d . u c t i ve i n t e n s i t y f o r a d u l t (>43 g.) I - ^ - ° i , c o n s i d e r e d s e p r a t e l y a t each s  I  n  d  i  l  u  a  l  3  Number Of »emal*s~~ Lactating  55/204  - 27?  24/103  - 23:  6/  43 - - 14'  7/ 56 V  v  1 .r  1 4/ 68 - 2 07.  .  a  r  a  Number C f Females Pregnant  Number C f F-emales With. O p e n Symphysis'  3.6 -  1 0 - 5 ?  9  -  7 -  18?. 9 .!• K>":  9 -  9 •'  3 -  T"  9-16?  c - 11  20 - 29?=  4 - 6 ?  7  N u m b e r Of Litters In Traps  3 - 1 % 0 - 0* 2-5%  4 1 - .U-  capture  66  Figure  13. Proportion  of  that  lactating.  were  shaded.  adult  females Winter  (>43  g.)  months  on of  grids Nov.  -  P  and  Feb.  Q are  67  _ J CD ZD CD i—i  cn Y— CE u_ h— o CJ (X  NNNNNNNNNNNNNNY NNNNNNNNNNNNNNY NNNNNNNNNNNNNNY  N W W W W W W W W W N W W W W W  75  \wwwwwww  NNNNNNNNNNNNNNN N W W W W W W W NNNNNNNNNNNNNNN N W W W W W W W NNNNNNNNNNNNNNN NNNNNNNNNNNNNNN NNNNNNNNNNNNNNN' NNNNNNNNNNNNNNN' NNNNNNNNNNNNNNN NNNNNNNNNNNNNNN NNNNNNNNNNNNNNN NNNNNNNNNNNNNNN NNNNNNNNNNNNNNN' NNNNNNNNNNNNNNN NNNNNNNNNNNNNNY NNNNNNNNNNNNNNN' NNNNNNNNNNNNNNN' NNNNNNNN" NNNNNNNN  11  i—i CO UJ cn _ J  ct: o_  ex  21 LU  NNwNNNN ANNNNNNN -NNNNNNNN ANNNNNNN NNNNNNNN KNNNNNNNN NNNNNNNN KNNNNNNNN NNNNNNNN NNNNNNNN NNNNNNNN NNNNNNNN NNNNNNNN NNNNNNNN ANNNNNNN NNNNNNNN NNNNNNNN KNNNNNNNN ANNNNNNN ANNNNNNN ANNNNNNN ANNNNNNN ANNNNNN ANNNNNN ANNNNNN ANNNNNN  NNNNNNWNNNNNNN  1—  o  KNNNNNNNN NNNNNNNN KNNNNNNNN NNNNNNNN NNNNNNNN NNNNNNNN KNNNNNNNN KNNNNNNNN KNNNNNNNN NNNNNNNN NNNNNNNN NNNNNNNN KNNNNNNNN KNNNNNNNN KNNNNNNNN NNNNNNNN  WNNNNNNNNNNNNNN NNNNNNNNNNNNNNN NNNNNNNNNNNNNNN NNNNNNNNNNNNNNN NNNNNNNNNNNNNNN NNNNNNNNNNNNNNN NNNNNNNNNNNNNNN NNNNNNNNNNNNNNN  NNNNNNNNNNNNNNN NNNNNNNNNNNNNNN NNNNNNNNNNNNNNN NNNNNNNNNNNNNNN NNNNNNNN\NNNNNV  25  NNNNNNNN A N N N N Y " NNNNNNNNNVNNNN NNNNNNNNN\NNN ' NNNNNNNNNNVAN/N  NNNNNNN ANNNNNN ANNNNNNi NNNNNV ANNNNY , ANNNNN^V ANNNNN* ANNNNN  NNNNNNNNNN\N NNNNNNNNNN  I I I II  0  J. S 1972  N  J  M  H I'l 1  M J S 1973  N  J  M. 1974  68  f i r s t than  capture 40  g in  divided the  survival  exception  of  rate  was  during rate  high  decline was  adulthood. decline that  and  summer during  low  period  II  IV  was  V;  that was  i n  almost  peak  during  Tagging  were  study  and  During  each  balanced,  signifying  4 tags  were  recovered  of  loss a  due t o  number  voles  that  died  the  number  the on the  that  and III, the  to  i s  rate  in  and  survival  (.13  reached  .15)  during  clear  from  Table  from  period  III  rate.  those  during  3 to  Survival  s  two  periods,  increase  period  grid.  the fate  of  in  weasel  nests  is  calculated  mortality,  disappeared,  the  and  number  immigrated  Dividing  disappeared  recovered  of  immigration  that  were  situ  voles  between  number  voles  i n  death  of  that  the d i f f e r e n c e and  I  radioactively-tagged  these  no s i g n  Assuming  disappeared  15 o f  showed  period  immigrated.  Only  were  that  proportion  Recruitment  IV.  219 v o l e s  skeletons,  recovered  the  Results  disappeared.  bearer, The  this  period  with  recruits  recruitment  higher  of  these  identical  times  II.  more  index  study  period  It  !  the change  than  or  IV.  due t o  III  carcasses  period  The  periods  low  growth  was t h r e e  later  very  weighing  the  period  few of  population  were  voles  time.  this  i n  recruitment  During  during  but during  but  ll^jg§Stiye  that  the two i n c r e a s e  (.60)  reproduction  of  throughout  decline  and peak  change  number  at  high  Recruitment  the  period  was  period  so  the  population  juvenile  the  by  the  their  former  (Table  as  of i s  as tags  5).  follows,  and a  number  emigration  the number  gives  11  and  voles  are that  the,number  of  that  died  by  proportion  of  Table  5. R e c o v e r i e s  of r a d i o a c t i v e l y - t . v g g e d i n d i v i d u a l s .  ON  Period  Sun her Of aadioactivelyTagged Individuals Disappearing  p r o p o r t i o n Of Disappearance Due t o Death  . 17  37  I V  V  Total  . 67  11  .53  uo  . 60  91 219  .75  Estimated Nuuber Of Ceaths  6.3  Ca r c asse s Or Skeletons Found  1 15?  So S i g n (unknown Fate)  Weasel Kills  2 30*  0  01  16%  0  07!  2U. 8  10  U0%  5.8  0  0%  1  17*  0 OS  2  85  3  123  0  35!  1  1*  . 21.0 63. 3 129.7  2 15  U  11  0*  4 6 7=  70  d i s a p p e a r a n c e due t o d e a t h , E x c e p t f o r p e r i o d of d i s a p p e a r a n c e  due  Multiplying  number  the  to  death of  the  actually  between  .53  radioactively-tagged  d i s a p p e a r e d by t h e p r o p o r t i o n gives  was  I the proportion  of disappearance  In  radioactively-tagged  period voles  V,  for  disappeared;  period.  voles  died  on  the  the  number o f  or  skeletons  (in situ  (unknown f a t e ) ,  death),  and r e c o v e r i e s  percentages i s given  91  proportion  of  that  68.3  during  this  voles  classification  types;  no s i g n a t s i t e  i n weasel nests.  carcasses  of  recovery  Each o f  these  i n T a b l e 5.  From t h e 130 r a d i o a c t i v e l y - t a g g e d died,  that  period.  i n each  i n T a b l e 5, I d i v i d e d d e a t h s i n t o t h r e e  death  voles  radioactively-tagged  t h a t d i e d , a n d t h e number o f r e c o v e r i e s given  that  example,  area  S i m i l a r c a l c u l a t i o n s were made f o r e a c h  From t h e e s t i m a t e d  voles to  d i s a p p e a r a n c e due t o d e a t h was . 7 5 , s o I e s t i m a t e d radioactively-tagged  .75.  due  e x p e c t e d number o f r a d i o a c t i v e l y - t a g g e d died.  and  voles  30 (23%) o f t h e t a g s were r e c o v e r e d .  estimated  t o have  A l m o s t a l l i£ s i t u  death occured during  t h e s h a r p d e c l i n e i n t h e summer  (period I I ) . In s i t u  d e a t h a p p e a r s t o have b e e n a n u n i m p o r t a n t  mortality periods  factor  during  ( I I , a n d V)  show  any  other  striking  period.  mortality  agents  died i n s i t u the vole  during  in  were v e r y  t h o s e two p e r i o d s .  nest.  In  a  few  nests  in  bundled an  situ  different  Individuals  were f o u n d a s c o m p l e t e s k e l e t o n s , a l o n g  r a d i o a c t i v e and normal e a r t a g s n e a t l y  1973  The two d e c l i n e  differences  m o r t a l i t y r a t e s , which suggests t h a t there  of  with  that both  inside  a  additional.  71  non-radioactively-tagged nest.  This To  that  were  not  at not  part  and  Q  seen  of  for  that  I  and  the  i t  up  forest.  in  should  is  that  only  have  was  P  to  grids,  study.  and  I  grassland  1.  There  an  endless  a l l  one  was  in  the  of  5%  not  of  what  P  and  acres  I  of  searched  the  nearby  constitutes be .used  recovered the  that  surrounding  should  habitats,  tags  areas  20  the  acre)  of  grids  forest.  of  v.  grassland,  approximatly  5%  the  surrounding  near  determine  tags  number  types,  of  in  period  searched  (one  to  these  recovered  large  supply  estimate  of  during  occurred  5%  the  the  dead  classified  grassland,  This  rare  three  The  d i f f i c u l t  found  I  Q into  approximatly  Assuming  ended  and  the  the  surrounding  although  caution. V  Fig.  also  happened on  of  grids  nearby,  plantation,  nearby  of  in  the  forest,  end  was  mortality  what  plantation.  plantation 20%  nest  recovered  the  forest as  of  determine  were  areas  type  vole,  from  period  number  that  habitat  are  Searched  Number  Tags  with  of  tags  summarized  below. Habitat  Percentage  Grassland  20%  16.8  Forest  5%  4.2  Plantation  5%  4.2  Thus  i t  recovered the few  tags  no  tags  were  not  predator  searches, very  appears  long  which  I  very  they  were  strongly I  suggests  that  (more  present  clumped  nests).  distances  unlikely  in  recovered most  than  in  that  i f  would  these  areas,  distribution no  tags  animals  one-half  Expected  mile),  provided  (eg,  during  either or  have  in  a  these  dispersed were  taken  72  by a v i a n p r e d a t o r s . recovered  Even  i f  some  of  these  i t m i g h t have been d i f f i c u l t  o f m o r t a l i t y . I n any case,: my e s t i m a t e s  tags  had  been  to determine  the a g e n t  of i n  mortality  situ  r a t e s would n o t have been a f f e c t e d .  Bernoval E x p e r i m e n t s Trapping  was  Grid Z begun  on  weeks o f p r e b a i t i n g . I n t h e voles and  were  removed,  from  grid  first  Z i n J u n e , 1973, a f t e r two  trapping  periods  two 56  a d e n s i t y e q u i v a l e n t t o t h a t on g r i d s P  Q a t t h e same t i m e .  removed  grid  Z  Fig. 9 in  shows  the  number  of  voles  each t r a p p i n g p e r i o d . T h i s  initial  c l e a r i n g - o f f i s n o t shown i n F i g . 9. T h e s e d a t a a r e s u m m a r i z e d in Table P  and  6. T h e number o f a n i m a l s Q,  captured of  the  number  of  decline  very  few  from  grids  disappearing voles that  on g r i d Z a r e a l s o p r e s e n t e d .  removals the  and  that disappeared  disappearing  During  t h e two  periods  i n d i v i d u a l s showed  ( 0 . t o . 0 5 ) , b u t d u r i n g t h e i n c r e a s e a n d peak  p e r c e n t a g e was much h i g h e r  dispersal  (the  number  of  (.18  to  .24).  An  were  up as  periods  index  of  r e m o v a l s d i v i d e d b y t h e number o f  i  disappearing  v o l e s f r o m g r i d s P and Q) i s g i v e n , and shows t h e  same t r e n d : l o w d u r i n g t h e d e c l i n e s , h i g h e r period  IV,  and  highest  known d i s p e r s a l r a t e , individual next  alive  defined  as  the  probability  peak  that  on g r i d P o r Q w o u l d be c a u g h t on g r i d 6.  Other  I I , when no t a g g e d v o l e s were c a p t u r e d  rate i s nearly constant dispersal rate  the  d u r i n g t h e i n c r e a s e p e r i o d I I I . The  trapping period i s given i n Table  period  during  (.020  to  .026).  (as m e a s u r e d by i m m i g r a t i o n  This  than  on g r i d  Z the during  Z,  suggests  t o evacuated  an  this that  areas)  Table  6.  Results  Of  Removal  Experiments  Grid  Z.  Number Of T a g g e d Removals  Total P r o p o r t i o n Of Number Disappearance From G r i d s P+Q O f R e m o v a l s From G r i d Z F o u n d On > Grid Z  Probability Of D i s p e r s a l Frcn.. G r i d s P And QTc G r i d Z  Period  Number Disappearing From P And Q  I  134  II  126  0  .00  2  III  33  6  .18  13  . 0 14  IV  55  12  . 22  37  .030  V  106  5  .05  28  .020  .0  74  is  a  constant  experiment areas  on  can  with  and  P  other  was  one  Between  far,  grids  I  move  have  used  dispersal number of  to  of  of  any  grid  the  P  1)  could  accounted  The grids  was  (.02  to  same  of  .04)  pattern  from  the  loss  from  loss,  from  complete  Krebs  agreement  (1971) ,  (.10 during  would  by of  to  P  and  more  move  to  dispersal  as  grid  Also  as  7  are  the  other  index  two  between to  increase  during  by  on any that  grids.  movement  periods,  periods.  of the  alive  area  I  rates  disappearance  due  was  presents  shown  of  25  i t  an  individual  measured  the  direction,  an  decline  to  individual  Q than  grids  movements  and  of  study  an  other  Table  disappearance  peak  grid  this  that  some  proportion  known  one  In  between  that  records  individuals,remaining  to  in  period.  .1-2)? d u r i n g  the  from  Assuming  nearby.  by  the  from  grid  movements  2)  for  moving  area  probability  and  for  in  removal evacuated  of  capturerrecapture  grids.  habitat  proportion high  of  lifetime.  another  areas  period,  the  their  movements the  two  are and  differently  move  to  similar  trapping be  no  to  rates  known  to  proportion  l i t t l e  Hyers  individual  between  l i k e l y  movement,  one  Q;  the  dispersal  The  Krebs.  combined  moved  from  but  density.  Grids  throughout  more  that  results  by  of  of  sizeable  These  work  considered  individuals  to  a  populations  have  and  grid  no  for  experiments  unpublished  So  independent  demonstrates  populations.  Hovements  was  Z  peak  similar  recent  on  grid  account  increasing declining  factor,  between and  This  immigration  i s  low the into  Table 7. Movements between g r i d s P and Q.  Period  Number Moved • Between G r i d s  I  13  II  2  III  P r o b a b i l i t y Of Movement From One G r i d To The Other In Any Trapping P e r i o d  P r o p o r t i o n Of Less Accounted For By Mc verne nt  .018  . 10  • .006  . 02  4  .013  . 12  IV  .2  .004  . 04  V  4  .016  .04  .  76  evacuated in  Table  the P,  areas 6.  Since  available and  grids  as  vice  given  grid  should  be  the  number  of  two  points  l i k e l y  the  attempted  to  estimates (Table of  of  7)  voles  shape  and  the  the  should  ratio  these  each  g  40  of  represent  5%  or  of  the  cf  dispersal  40%, area,  be  If  it  estimates  of  a  which  not  are  in  of of  P  and  then  was  on  of that  this  agreement.  these  that P  and  not the Q  are  two  these  two  constant.  The  of  grids  with  by  immigration  dispersal  consistent that  have  3),  ratio  unreasonable,  suggested  I  between  the  these  between  assume  based  grid  between  distance  I  of  multiplied  (Table  available  not  basic  If  estimate  from  dispersal the  shown  from  unknown,  Q  suggest  the  have  movements  is  and  portion  emigrants  dispersal  rough  of  small  movements  phenomenon,  percentage  would  were  grids  (1971)  emigrating  constant.  estimates  this  values  function  on  14%  known  function  based  same  two  a  this  into  of  probability  this  provide  about  other.  of  dispersal  measures of  to  estimate  over  estimates  the  a  individuals number  Because  and  for  Krebs  only  actual  constant.  them,  and  the  some  is  Myers  Q represents  grassland  versa,  by  P  and  habitat  Q for  for  instance  the  geography  two  estimates  77  G r i d s A And  B  Grids J u n e 1974. from  A and The  that  B  (see F i g . 1)  cover  ground,  had  enough l i t t e r the one  g r a s s on  townsendi  i n which the  throughout  May  years  their  trapped  on  and  captured  two  separating the  to  B.  1973, Z.  study  to  fall  thick  f i e l d s o f g r i d s P and  Q  but  to  zero.  marginal  declined During going  townsendi  were  between  the  that the  50  grids (one  pulse m of A  of  woods and  B  individual i s  k i l o m e t e r from g r i d P to a n o t h e r  a r e a ) . I b e l i e v e t h a t the sample s i z e i n t h e s e f o r any  range  Q were  with  from  moss  that  voles appeared  I t seems u n l i k e l y  in  Microtus  1971,  Microtus  coincides  data  is  f i r m c o n c l u s i o n s about r a t e s of d i s p e r s a l  habitats,  but  large-scale dispersal into this two  grids lived  when g r i d s P and  five  which  of  of  networks.  fallen  A l l of these  have d i s p e r s e d one  insufficient into  two  runway  would a c t a s a m a j o r b a r r i e r t o i m m i g r a t i o n known  i t , lacked  t h e r e were 10 t o 15  s p r i n g had  of the study,  g r i d s A and  immigrants to g r i d  g r i d s g r e w v e r y low  on t h e s e  i n t o the  fluctuations,  August  different the  g r i d s . Numbers r e m a i n e d i n  by  to  throughout  an e x c e p t i o n a l l y  initiated  t h e summer and  l a s t two  two  voles constructed  d u r i n g t h e w i n t e r , and  through  quite  1971  f o r p o t e n t i a l runways.,Most  w h i c h had  t r a p p i n g was  t o w n s e n d i on t h e s e  the  was  a l a r g e amount o f moss m i x e d w i t h  s e c t i o n o f g r i d B,  When  grids  these  t o provide cover  Hicrgtus  cover,  two  f r o m May  on a l l t h e o t h e r g r i d s a t Haney and  V a n c o u v e r a r e a . The the  on t h e s e  were t r a p p e d  i t  i s clear that there  marginal  p o p u l a t i o n d e c l i n e s o b s e r v e d on  habitat  g r i d s P and  Q.  was  during  no the  78  D1SGDS5I0N  This  study,  University studies have  of  of  to  of  compare  townsendi in  other  the  the in  causes  this  populations, Indiana  work  literature  the  on  processes three  experiments  each  of  that  and  of  affect  these  I  papers.  lemmings  used  my  in  to  work  three  on  For see  this  of  of  marked  directed  growth  rate.  the  and  complete  study  into  changes  i n  Krebs's set  Myers  me  of  data  review  from  the  (1971) .  to  divide  distinct These  birth  to  Although  the  determining I  are  of  numbers.  individuals.  processes  to  three  survival  at  microtine  the  allowed  marked,individuals,  population  concerning  largest  and  species my  primarily  Krebs  townsendi  determine  are  a  wish  of  declining  Microtus,  I  fiicrotus  results  the  from  First  the  results  I and  those  of  hypotheses  reproduction,  survival  population  in  Microtus  survival  i n  changes  and  are  iy  and  Hicrotus  during  available  combine  processes  trapping,  of  voles  cycle  the  the  detailed  other  discuss  at  togBgendi.  dynamics  represents  recent  techniques  l i f e  i t  data  in  to  compare  because  summarized  then  f i r s t  lemmings.  of  time  s i m i l a r i t i e s  here,  and  those  survival  w i l l  capture-recapture  The  and  relation  I  voles  same  the  the  presented  with  the  Microtus  population  study  reduced  of  discuss  of  general  in  of  data  species  l i t e r a t u r e ,  experiments  well  the  to  at  represent  dynamics  therefore,  between  undertaken  Columbia,  population  chosen,  studies  others  B r i t i s h  the  differences  and  the  causes  w i l l  discuss  because  they  a l l  79  i^£rodiiGti^e High voles  frequently  Keller  ochrogaster  case is  and  of a  during  Krebs  (1970)  evidence  increasing  breeding  period  earlier  during  during  f o r i n  winter  measure..  of  study  i t  i s  1974.  It  i s clear  not  a  impossible  defined  changes earlier)  population period  IV  during rates  that  were  bi-weekly  the  drop  period  I  to  a  be t h e  1966).  There  during  the  1974 than  i t d i d  1973, but the low density  during  variation  of  intensity declined  t o determine  very  rate  when  d i f f i c u l t  i n  to  at t h e end o f t h e  breeding  i n reproductive the population  rate  ceased  i n  intensity  was  fluctuations  same  between  i n  these  growth  period  rate rate II,  rate  A l l of the difference  increase  reduction  i n recruitment  and decline  and immigration  important.  The i m m i g r a t i o n  population  similar  increased  I  •to.wja'se'tfdi^-  were  I v.  Microtus  this  breeding  a l  primarily  season  i n the spring  population  was due t o t h e  period  et  began  i n recruitment  growth  i s  of  season  reproductive  factor  i n  (Krebs  The  i n Microtus  The  populations  study.  the  that  found  this  significant  observed  Krebs  breeding  of  1957, Krebs  populations  longer  spring  populations  populations,  a  1 9 7 3 made  Because  showed  season.  the decline  the increase  (Kalela  pennsylvanicus  , j  no  increasing  reported,  i n MicrotiasJ • c a l i f  increase  the  been  during longer  Recruitment  rates  and Microtus  reproduction result  And  reproductive  have  1971).  Intensity  i n  period  juvenile  a n d minimum  two  III  periods,  dropped occurred  from  i n  and peak  recruitment  adult and  survival yet  the  .14 to - . 0 1 .A  between  but survival  (as  increase  and immigration  80  also  changed  also  found  factor  that  causing  increasing changes  period  The  and  a  it  during  Bates  The  high and  this  study  are  Krebs  et  increase  those  the  of  but  reported  in  Both  were the  was  and  low  well  survival  literature  the  (eg.  population  IV  decline  (. 1 5 ) .  low  High  recruitment of  most  and were  and range  Chitty  work  .72  and  and  declines on of  for  peak in  Micigotus. .82  for  declining  peak  populations  decline  populations  more  Microtus of  and  the  rates  and  observed  within  of  However,  during  during  increase  pennsYlvanicus  and  previous  survival,  declines  the.  (.29)  major  period  increase  survival  study  the  1974).,  populations  this  (1973)  characteristic  during  minimum  was a  period  is  al  numbers.  explain  populations  adult  rate  stable  peak  Hyers  et  from  higher  populations  biweekly  Microtus  Indiana,  of  not  during  report  During  .59.  do  Krebs  change  period  survival  peak  ,86  to  consistent ; with  and  averaged  recruitment  rate  (Krebs  (1973)  populations.  in  increasing  adult  populations,  periods.  rate  was  lemmings  al  a  declining  Survival  averaged  to  recruitment  and  two  population  than  recruitment  voles  change  recruitment  V  peak  these  numbers  in  declines^  in  between  severe ochro^aster  microtine Phipps  than at  declines  1966).  81  Removal  Ex£eriments  The Myers  only  and  loss  from  grid  was  during voles  comparable  Krebs a  (1971).  control  declining on  an  area.  removal  the  previous  found  dispersal  Chitty  while few  experiment  an  and  on  grid  movement  phase,  and  l i t t l e  onto  Phipps  adjacent  2  into  with  empty  movement  and  of  lowest trapped  grid  into  by  removal  (1966)  moved  an  a  control  agrees  done  proportion  populations  individuals  much  Between:  Movements  Grids  was  the  empty  both  these  area  during  during peak  and  between  reported  in  literature  in  And; • I m m i g r a t i o n populations  any  intensive  live-trapping  microtine  fluctuation.  common among  Microtus  (Krebs  tagged  heavy  recruitment, for  to  known  the  were  phases.  'Movements  the  that  increasing  area that  experiments  reported  studies,  increase  decline  during  evacuated and  two  due  populations.  declining, The  They  grid  highest  removal  increasing  and and  quantitative studies Also  such  townsendi  Krebs  body have  have  and  weight not  declining  way.  that  been There  other  individuals  in  populations.  been  few  complete to  be  Microtus have  any  a  appear  coworkers  calculated  have  followed  movements  than  his  not  ignored  more  species newly  calculating  "immigration  rates"  82  ijslssg 21 Iks. S§£lisg§ Chitty  (1955)  declines  based  timing  the  of  decline slight  in  on  has  defined  three  the  rate  population  decline.  Type  numbers  recovery  declines  are  »  during  more  not  over; the  abrupt,  and  are  Type  M declines  are  with  winter  and  early  36  recorded  classified type a  H,  peak  13  type  period  instances, with  followed  no  of  of  peak  G,  H one  high  period  over  rapid  5 type  M,  (Krebs  feature  of  microtine,fluctuations.  two  years,  1952,  Krebs  recovery During occur  or  from a  end  et  al  period,  spring  decline"  has  been  occurred  in  Vancouver  area  months,  II  at  well  other  but  the  short  the  onset  of  1974).  losses  by  rapid are  preceded  by  recorded population Grid) .  may  reach for  1974). this  spring  rapid  may for  may  "spring  {1969)  numbers  rapidly  or  (Chitty  The  populations  in  one  numbers  a l  decline  decrease usually  et  low  phenomenon.,  declining  Krebs  A  variable  r e p r o d u c t i o n . .This  townsendi The  18  d e c l i n e •..  Myers  of  are  have  into  there  a  with  period  documented  and  and  season.  another  remain  consistent  Microtus  (LeDuc  survival  Krebs  year,  (1974)  several  i s  G  associated  Control  from  Type*  breeding  Populations and  possible  increasing  decline  immediately  is a  a  a  literature  Tilden  decline  1969,  period  the  two  a  recover  peak  poor  of  may  in  very  following  an  gradual  one  Myers  are  the  a  decreases  the  there  1966,  low  the  the  and  season.  than  declines  from  of  at  from  but  period  numbers  more  and  Most  with  breeding  during  declines  microtine  decrease  years,  no  Krebs  of  involve  population  beginning  numbers  more  recovery  numbers,  declines  or  intervening  spring.  and  declines  occur  by  very  of  types  in  and the  involve one  compensated  or for  83  by  recruitment Fig.  Q  on  a  14  and  presents  monthly  these  of  changes,  the  presence  of  late  increase  and  during were I  the  many  just  heavy  weeks  later  caused  by  general heavy  a  survived  the  were of  or  spring  in  of  the  during  i n i t i a l  during the  the  findings  Hhat observed in  is in  as  neither  other  the  this  voles  categories are  of  described  decline  during  the  Fig.  14  there  peak  and  period  that  eight  (see  3),  and  table  ever-reached  recovery IV, so.  of  and few  i s  of  the  from  the  during  the  individuals  was  high  lack  declines,  relationship and  lemmings?  as  and  is  relatively  body  weights  heavy  animals  consistent  with  workers.  study  and  high  peak  (1974),  was  V)  the  pattern  This  distribution  and  very  individuals.  period  presence  are  and  individuals  of  1973,  the  (period  increase,  subsequent  end  individuals  peak  the  in  heavy  during  weight  Nevertheless,  the  in  P  the  heavy  that  survival  Few  1974  meaningless.  at  and  Myers  few  Note  g)  grids  individuals  very  few  poor  rates.  and  decline  very  classes  decline  the  (>50  voles,  heavy  decline.  for  distribution  in  Krebs  with  mid-summer  weight  that  by  peak, and  growth  the  phase  of  early peak  weight  numbers  combination  mid-summer in  large  distributions  in  reviewed  there  body  decline  Changes  individuals  to  poor  weight  population as  late  prior  the  basis.  characteristic of  immigration.  by  general  Neither  Chitty  recorded  occurred  the  between  for  during  the  of  two  attributes decline  (1955). any  the  The the  winter  or  of  f i t s  rates type  declines  declines into  of  the  decline  M declines, early  I  but  spring.  84  Figure  14. Distribution months  of  obviously  of  Hov.  body -  Feb.  pregnant  weights are  females  on g r i d s  shaded. not  Eoth  P and sexes  included.  Q.  Sinter  combined,  £8  86  The  f i r s t  the  second  early  decline  level  to  go  that been  period  in  The.  since  started  have  decline  summer.  decline,  in  rapid  and  only  rates  on  (.83)  fenced  grid  on in  grids the  to  area,  where  usual  spring  from  occurred  at  was  similar  most a l  the  the  (1969).  population  differences Indiana  of  to  was  loss.  the  was  the:  high  declines  and  did  not  months  spring  resemble  before  the  a  spring  population  to  normal  a  decline,  for  also  the  grid  i s  breeding  by  i t  was  V  rate  of  observed  would high  period  decline  observed  on II,  very  The  adjacent is  similar  b r i e f ,  except second  Indiana  a  of  peak.numbers,  decline  and  those  decrease. by  Krebs  had  i t  decline  in  numbers.  and  that  observed  low  V  decline  rapid,  period  low  this  period  season.  very  period  that  a  The  during  reduced  was . q u i t e  declines  AA  spring  decline, that  such  immigration.  typical  in  to  spring  suggest  survival  preceded  the  late  f i r s t  the  declined  between  spring-summer rate  It  of  during  declined  numbers  end  occurred  population  declines  . low  and  two  (.90)  The  mid-summer,  unfenced  Vancouver  1974).  during  extensive  not  recovery  et  by  Q was  (LeDuc  the  the  and  areas  P  began  nearby 0  occurred  decline  recovery,  possible  v  second  survival and  period  breeding down,  II  by  The  Krebs and major  observed  None quite  of as  in the  high  a  87  £§2i§§§ 9.1! 2i§^£l§S£^Ii£§ The  rates  summarized 1,0  by  minus  the  dispersal  is  death  and  rate  estimated  period minimum the  (from  the  the  total  chose  minus  estimates estimate rates  into  method  The  of  sum  P  for  agreed so  in  I  feel  situ  death  two  rate  and  skeletons,  a l l  would  the  of of  in  situ  known  during worsts  are  death  in  situ  reduce data  II. death  be  number  are  of  loss  is  due  to  in  situ  carcasses column  i s  losses.  I  grid  Z  or  immigration a  more  of  direct  immigration  assumptions of  (see  dispersal  of  by  estimating  reasonable.  in  Table  to  earlier  60S as the  8 i s  capture,  estimate  that  from  methods  failure  a  or  the  estimation  f i n a l  recovered  s t i l l  and  are  disappearing  situ  calculation  unimportant.  Assuming i s  in  Q because  other  presented  could  were  the  since  probably  tag-losses period  tags  rate  a  The  unknown  removals  presented  scavengers,  factors  the  3.  of  The  of  rate  skeletons  and  probably  by  the  as  of  and  involved  carcasses  three  of  proportion  sample, s i z e s ,  Loss  However,  Table  P  estimate.  rate.  from  instead  is  rate  rate  details),  i t  total loss  dispersal  the  disappearance  The  the  grids  with  The  disappearance.  of  Q  of  rate.  although  and  8.  recovered  greater  dispersal,  section  dispersal,  voles  between  grids  is  rates  involved of  results this  the  movements  6)  rate  immigration  known  survival  Table  total  Table  immigration  radioactively-tagged times  in  causes  At  detect of  of  carcasses  or  situ  suggest  estimate  the  insignificant  these  estimate since  or is  death  that  underestimate, carcasses  minimum  removal  in  most,  a  4 095  skeletons  50%  except  low,  at  during  Table  8.  Rates of disappearance i n M i c r o t u s townsendi. T o t a l i s ( 1 . 0 - m i n i m u m 2 week s u r v i v a l r a t e ) . D i s p e r s a l i s i m m i g r a t i o n r a t e f r o m T a b l e 3. In s i t u d e a t h and unknown c o l u m n s a r e f r o m T a b l e  Period  Total  Dispersal  In  I  .16  .13  .005  .025  II  .42  .14  .113  .167  III  .13  .06  .000  .070  IV  . 12  . 05  ..006  . 064  V  .40  .10  .008  .292  -  Situ  Ceath  5  U n known  89  period  II,  during  mortality. somewhere those  The  rate  Table  in  V  are  either  have  situ  voles  I  have  no  of  accounted  in  situ  presented  in  for  death  Table  almost is  8,  made  this  raptors mouse  traps  weasels  had  large have  predation  I  few  causes  unknown  that  left V  with  and  a l l  probably  and  been on  in  only  from  of  study:  in  caught.  In  grids  and  the  a  twice  not  my  population  on  in  Table  2)  dispersal  I  did  not  predators  were  years  study  areas  (but  weasels  were  period  on  unfenced  nocturnal  captured II  the  was  and  grid  in one  eleven  grids,  unlikely due  f i e l d  suggests  indicating  seems V  of.  Vancouver,,  morning,  period  rates,  very  which  i t  -the  were  owls,  present  up  predation.  nests  three  near  go  either  to  avian  during  addition,  high  or  two  were  very  traps.  weasel  than  made.  predation  due  of  location  in  of  rates  loss  hawks  one  Q  (outlined  entering  from  be  increased  increased  single  weasels  P  to  Only  a  p o s s i b i l i t i e s ;  more 3  can  to  indices  observations  another  caught  two  tags  voles  At  causes  measuring  but  of  dispersal  weredue  of  this  attributed  p r e d a t i o n . . . The  important).  numbers  when  the  sighted  IV.  been  decline  am  study,  during  period  but  systematic  be  during  be  dispersing  take  could  can  radioactive  Haney,  raptors  or  generalizations  II  The  method  No  during  I  were  infrequent.  at  period  period  of  the  death  following  indicate  V.  Recoveries  work  the  dispersal  during  the  would  values  unknown.  period  losses  that  rate  death,  calculated  during  if  8  during  of  period  or  the  situ  values.  decline  are  in  actual  between  From The  which  that they that  to  increased  AA  (located  90  less  than  this  200  period. It  is  changes  may  due  rate  stress,  of  survival with  of of  these  causes.  due  the to  situ  the  there  very  in the  a  yet  there  good  to  reduced  or  very  occurred  summer,  and  extreme  survival by  also  contributing  a  populations also  infested  the  summer  botflies  monitored with of  are It  period  during factor at  botflies  1973.  I  have  II.  this to  t h e . same and no  showed  A l l  of  with that  no  were which  near  for  these i t s  at  Poor Haney  of  poor  hot  dry  some  sort  shortages for  the  may  have  but  other  Vancouver  similar  explanation  this  frequently  survival;  time  have  survival  weeks  responsible  low  that  or  the  unusually  period,  The  period.  six  Voles  the  may  unfenced)  possible  were  V.  during  and  consistent is  the  period  population,  an  of  suggests  t h i s .  The  during  none  v i a b i l i t y  and  seems  untenable.  two-week  during  data  be  period,  a  increased  death  in  agents  histories.  factor.  during  in  the  (fenced  temperatures  parasitized been  the  mortality  in  grids  period  in  genetic  it  that  II  doubt  peak  poor  density  to  period  to  no  situ  appears  mortality  or  but  in  appears  change was  six  some  changes  reason  throughout  through  survival  during  gradual  II  be thus  genetic  i s  on  It  food,  predation,  not  to  of  mortality to  different  extrinsic water  or  survival  reguired.  shortage  predation  death  high  is  reduced  individuals  imply  widely  of  would  occurred  survival part  in  supply, from  stress,  increased  caused  but  mechanisms  went  that  starvation,  period,  explanation  there  loss  predisposed  food  to  agents  Similarly,  maintained  vulnerability  that  declines  other  have  and  unlikely  high  Some  away)  possible  movements  above  m  the  decline decline  were in in  91  period the  V.  I  have  introduction  explanation entering  that  traps  Krebs declines  situ  only move  in  from  animals,  not  move  long  may  survive  home  distances,  to  Krebs  close  to  while  story,  requires  the  poor  without  a  underlying and  causal  suggest  most  distant  are  lost  locales.  mythical  confirmation  This  lemming by  the  new  direct i s  the  that  empty  on  for  either  by  period  They  Chitty  dispersal  dispersal  traps.  cause  that  long-range  habitats  of  agent  (demonstrated  cross  consider  periods  non-trappable  V . I  seasonal  not  (1971),  as  a  ..brief  relationships  and.suggest  occurred  and  be  two  single  declines,  for  to  the  and  and  colonize  uncomfortably and  between  investigate  and  V  very.  They .would  Myers  ranges,  stopping  in  the  period  causal  a  mortality  to  the  lOng-range  II,  with  the  II  not  explanation  their  period  i s  the  or  described  only  during  rejected.dispersal  period  remaining  left  considered  to  there  during  situ  measurement)  in  factors  that  mortality In  have  fluctuations.  in  am  fluctuations..  mortality  disappearance  and  agent  areas.  that  (1966),  mortality  dispersed  unrelated  the  occurs.  to  evidence  Phipps  work,  (1973)  microtine  microtine  and  this  every  evacuated  a l  and  different  decrease  out  individuals  similar  determining the  to  on  et  phenomenon  of  ruled  of  voles other  presumably way,  a  few  suggestion  i s  march-to-the-sea experiments  on  dispersal.  The is  not  idea  that  new.  encourages  long  small  Murray  mammals  (1967)  distance  may  disperse  suggested  dispersal  one  through  long  distances  mechanism  that  aggression  and  92  t e r r i t o r i a l i t y . that  dispersal  (1960)  acted  proposed  disperse. results of  Lidicker  the  The of  that  study  has  that  declines  was  to  a  about  with  types the  similar  animals  f a i l  method  has be  of  experiment of  explore and  period  makes in  might  V.  with  One  or  not the by  setting  require trap,  live  moisture,  of  to the  the  light  Myers  a  this  traps,  capture but  p i t f a l l  i s  would  be  to  via  exit  monitor  die  in  Myers  situ.  individuals the not  that  This method  of  voles,  and  Krebs  dispersing  trapping. fences  If  would  A second  force  of  doors,  population  numbers  to  declines  anticipated.  Krebs,  was  1973  changes  during  d i f f i c u l t ,  trapping  of  information  the  of  summer  be  further  large  because  would  townsendi.  decline  number  assumed  Hicrotus  intensive  large  be  demographic  would  fences  of  to  of  future  not  the  permitted  and  for  data  animals  trapped,  d r i f t  areas  in  animals  makes  in  the  method  when a  but  . More  provide  emigration  exits  technology  would  of  note  should  decline  part  radio-monitoring  into  normal  or  i t  fluctuations  suggested  p i t f a l l  s o i l  Howard  individuals  interpret  cause.  disappearance  decline,  suggested  individuals  sort  of  the  been  present  (1971)  and  response  cautionary  brief  experiments  of  use  a  common  the  dispersing to  to  to  mortality:  phenomenon  shut  were  a  i f  populations  either  but  have  of  long-term  that  then  would  agents  causes  to  enclosed and  the  seasonal  Several  caused  trap  d i f f i c u l t  provided  determine  associated  of  p o s s i b i l i t y  mechanism,  selection  nature  the  proposals.  of  useful  considered  regulatory  group  unknown  studies a l l  a  my e x p e r i m e n t s  above  This  as  (1962)  Such and would  high possible  an some not  r a i n f a l l in  this  93  study.  Badioactive-tagging  might  also  individuals, al  (1969)  may  Table  9  provide  clues  although  the  well is  a  phases  of  phases,  increase,  used peak  periods  summer  of  Microtus  of  the  by  The  w i l l  of  declines.  the  as a  table  and  decline,  help  of  of  island  disappearing by  I  Krebs  et  period  the  in brief between  defined  II  of  as step  table  summer  decline  I a  brief  towards  a  different on  further,  different between  as  underway  this  differentiate  IV a  the  currently  the  four  decline.  period  f i r s t  attributes work  population  summer  periods,  and  other  the  study.  brief  represents  differences  hopefully  this  increase  the  small  discussed  attributes  from  f i l l  a  islands.  demographic  w i l l  on  fates  effect"  decline,  comparing  declines these  of  III  the  small  townsendi.  townsendi  decline.  on  V as  This  Microtus  particularly spring  and  population  about  townsendi  peak,  period  a  'Jfence  summary  I  decline.  description phases  occur  Microtus  period,  of  the  with types  the of  causes  - a b l . . 9. C h a r a c t e r i s t i c s  Phase  Increase  of the phases of ^ i o r o t u s t o v n g e n d i .  Rate Of P o p u l a t i o n I n c r e a s e Per 2 week P e r i o d  Possible Duration I n Weeks  Tise  1«-52  *  .in-.18 °-°  recline - .22  1  B  e  a  r  Of  a  o  tear  u  n  d  F a l l And winter  Baximua -Body weight During.Period  Recruitment Rate  .8U-.87  70-80 50-60  10 Cr S e r e  S p r i n g And G'.i f.'ne r  i)0-50.  6-8  Mid-suaser  U0- 50 Bate P e r 2  .•15  .38  .29  .60  .53  95  LITERATDBE  Barbour, R.W., movements, Car£hO£his  CITED  B.J. Harvey and J.W. H a r d i n . 1 9 6 9 . Home and a c t i v i t y of the eastern worm amoenus amoenus. E c o l o g y 50: 470 - 476.  range, snake,  Carothers, A.D. 1973a. Capture-recapture methods a p p l i e d p o p u l a t i o n w i t h known parameters. J. Anim. Ecol. 125-146. Carothers, A.D. Jolly-Seber  1 9 7 3 b . The e f f e c t s o f estimate. Biometrics,  C h i t t y , D. 1 9 5 2 . 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K e m p s o n . 1 9 4 9 . P r e b a i t i n g s m a l l mammals a new d e s i g n o f l i v e t r a p . E c o l o g y 3 0 : 5 3 6 - 5 4 2 , D. a n d E. Phipps. 1961. A d e c l i n i n g Anim. E c o l . 30: 490-491.  vole  population.  Chitty, D. a n d H. Chitty. 1 9 6 2 . P o p u l a t i o n t r e n d s among the v o l e s at Lake Vyrnwy, 1932 - 1 9 6 0 . Symp. Theriologicum, Brno, 1 9 6 0 , 67 - 7 6 .  96  Chitty, D. a n d E. P h i p p s . 1 9 6 6 . S e a s o n a l changes i n s u r v i v a l i n aixed populations of two species of vole. J . Anim. E c o l . 3 5 : 3 1 3 -3 3 1 . Elton, C. Press,  1942. Voles, Oxford.,  mice  and lemmings.  496 pp.  Clarendon  Christian, J . J . 1950. The adreno-pituitary system and p o p u l a t i o n c y c l e s i n mammals. J . Mammal. 3 1 : 2 4 7 - 2 5 9 . Christian, J . J . and D.E. Davis. 1 9 6 4 .E n d o c r i n e s , and p o p u l a t i o n . S c i e n c e 1 4 6 : 1550 -1 5 6 0 . Godfrey, G.K. 1 9 5 4 .T r a c i n g f i e l d with a Geiger-Muller counter. Hall,  voles (Microtus Ecology 3 5 : 5-10.  D.O. 1974. Reducing the maturity bias i n populations: an example with cotton rats his£idus.) J . Mammal. -55: 4 7 7 - 4 8 0 .  Howard, F. i960. Innate individual vertebrates.  behavior,  estimating (Siamodon  and environmental dispersal Amer. M i d i . H a t . 6 3 : 152-161.  of  J o l l y , G.M. 1965.Explicit estimates from capture-recapture data with both death and immigration-stochastic model. Biometrika, 5 2 : 225-247. Joule, J . and G . N . Cameron. 1974. Field estimation of demographic p a r a m e t e r s : i n f l u e n c e o f (Sigmodon histjidus) p o p u l a t i o n s t r u c t u r e . J . Mammal. 5 5 : 3 0 9 - 3 1 8 . K a l e l a , 0. 1957.Regulation o f reproduction r a t e i n subarctic populations of the vole Clethrionomys rufocanus (Sund.). A n n . A c a d . S c i . F e n n . Ser.~A7 I V . 3 4 : 1 - 6 0 . K e l l e r , B . L . a n d C . J . K r e b s . 1 9 7 0 .M i c r o t u s p o p u l a t i o n b i o l o g y III. Reproductive changes i n f l u c t u a t i n g populations of Microtus ochrogaster and Microtus pennsylyanicus i n S o u t h e r n I n d i a n a , "1965 - 1 9 6 7 . l c o l 7 M o n o g 7 ~ 4 0 l 2 6 3 - 2 9 4 . K i k k a w a , J . 1 9 6 4 . Movement, a c t i v i t y and d i s t r i b u t i o n of the small rodents Clethrionomys glareolus and Apodemus s y l y a t i c us i n woodland. J . Anim. E c o l . 3 3 : 259-299. Krebs, C.J. 1966. populations of 239-273.  Demographic changes Microtus californicus.  i n fluctuating E c o l . Monog. 3 6 :  97  Krebs, C . J , And K.T. DeLong. 1964. A Microtus p o p u l a t i o n s u p p l e m e n t a l f o o d . J . Mammal. 4 6 : 5 6 6 - 5 7 3 .  with  Krebs, C.J.-, B.L. Keller, and R . H . T a m a r i n . 1 9 6 9 . 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Dispersal  i n  vertebrates.  population J . Appl. Ecology  48:  Myers, J . H . and C . J . Krebs. 1971. Genetic, behavioral, and reproductive attributes of dispersing field voles H i c r o t u s Eennsylvanicus a n d M i c r o t u s ochro^aster. Ecol. ionogT~41: 5 3 - 7 8 .  P e a r s o n , O.P. 1 9 6 6 . The prey o f c a r n i v o r e s d u r i n g mouse a b u n d a n c e . J . Anim. E c o l . 3 5 : 217-233.  one c y c l e  of  Pitelka, F.A. 1964. The nutrient-recovery hypothesis f o r arctic microtine cycles. I. Introduction. In " g r a z i n g in t e r r e s t r i a l and marine environments." Black well p u b l i c a t i o n , .pps. 55-56. T a n a k a , R. 1 9 5 6 . O n d i f f e r e n t i a l r e s p o n s e to live traps marked and unmarked s m a l l mammals. A n n o t . Z o o l . J a p . 44-51.  of 29:  T a n a k a , R. small  1 9 6 3 . On t h e p r o b l e m . o f mammal p o p u l a t i o n s . R e s .  trap response types Pop. I c o l . 5: 139-116.  99  APPENDIX  Distribution of recaptures of located i n the lower Fraser Biver May 1 9 7 3 , b o t h s e x e s c o m b i n e d . Number o f Times Captured 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 35 3 51 13 2 1 0 8 16 3 1 243 0 0 0 1  2  3  4  17 16 5 7 2 7 3 1 3 0 0 5  6  Number  I  Microtus townsendi on g r i d ValleyT D a t a ~ f r o n T i a y 1 9 7 1  10 10 4 2 2 2 0  13 5 7 2 2 0 1 0  4 2 5 3 4 0 1 2 0  7  8  9  Of Times  7 7 1 2 1 0 2 0 0 0 10  5 6 3 1 0 0 2 2 1 0 0 11  Known  0 1 1 1 0 0 0 0 1 0 0 0 12  0 4 3 0 3 0 0 0 0 0 0 0 0 13  t o be  0 5 3 0 1 0 0 0 0 0 0 1 0 0 14  1 1 2 1 0 0 0 0 0 1 0 0 0 0 0 15  Alive  1 0 6 1 0 0 2 0 0 0 1 0 • 0 0 0 1 1 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 16  17  C, to  3 0 2 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 18  0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 19  100  AjPPB^DIX  Distribution J u n e 1972 t o  of May  body 1971.  weight  Weight Males 11 - 1 2 1 13 - 14 1 15 - 16 6 17 - 18 3 19 - 2 0 4 21 - 2 2 5 23 - 24 11 25 - 26 15 27 - 28 26 29 - 30 11 31 - 3 2 10 33 - 34 18 35 - 36 15 37 - 3 8 9 39 - 4 0 7 41 - 4 2 11 43 - 44 11 45 - 46 13 47 - 48 6 49 - 50 9 51 - 5 2 11 53 - 54 7 55 - 56 5 57 - 58 3 59 - 60 2 61 - 6 2 4 63 - 64 1 65 - 66 1 67 - 68 0 69 - 70 0 71 a n d m o r e 2  at  II  f i r s t  Females 1 4 5 6 6 14 14 25 19 13 21 22 15 10 8 7 7 6 4 1 4 7 8 4 2 2 2 1 0 1 1  capture,;  Both 2 5 11 9 10 19 25 40 45 24 31 40 30 19 15 18 18 19 10 10 15 14 13 7 4 6 3 2 0 1 3  grids  P  and  Q,  


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