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Experimental studies of competition among four species of voles Hawes, David Bruce 1975

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E X P E R I M E N T A L S T U D I E S OF C O M P E T I T I O N FOUR S P E C I E S OF  AMONG  VOLES  by David A., U n i v e r s i t y  A thesis the  Bruce  Hawes  of California, Berkeley,  submitted i n p a r t i a l requirements Doctor in  19  f u l f i l m e n t of  f o r t h e degree  of  of Philosophy  t h e Department of Zoology  We  accept  this thesis reguired  The  University  as conforming  to the  standard  of British  November,  1975  Columbia  In  presenting  this  an a d v a n c e d d e g r e e the I  Library  further  for  shall  agree  thesis  in p a r t i a l  fulfilment  of  at  University  of  Columbia,  the  make  it  freely  that permission  s c h o l a r l y p u r p o s e s may  by  his  of  this  representatives. thesis  for  available for  financial  written permission.  ZoologyDepartment  The  of  University  of  British  2075 W e s b r o o k P l a c e V a n c o u v e r , Canada V6T 1W5  January 16, 1976 Date  by  the  is understood gain  Columbia  for  extensive  be g r a n t e d  It  British  shall  requirements  reference copying of  Head o f  that  not  the  I  agree  and  be a l l o w e d  that  study.  this  thesis  my D e p a r t m e n t  copying or  for  or  publication  without  my  ii  ABSTRACT The c o m p e t i t i v e voles, B.  £le£hrionomys  lgngicaudus,  were  studied  Species were  and i n  the  ^aoperi,  vicinity  in five  a s c e r t a i n e d by  t o one  other.  trapline  (Cricetidae:  Rodentia),  o f V a n c o u v e r , B.C.,  sampling.  species  on  the  defined of  Canada. habitats  IJicrotus three  occurrences  basis  of  oregoni,  o r two h a b i t a t s and d i d n o t  Although  species  vegetationally distinct  the b a s i s of s u b j e c t i v e l y  predictable  four  Microtus  u b i g u i t o u s l y , whereas t h e other  restricted each  among  M. t o w n s e n d i i  occurrences  occurred  on  interactions  orecjoni  species  were  overlap  with  were p r e d i c t a b l e  h a b i t a t s , they various  were n o t  environmental  parameters. Overlapping studied  by  or  means  of  experimentation. utilize  overlap  M.  atypical,  experimental  mark  populations  and  lgngicaudus  but  removal  b e t w e e n C.  a seasonal  contiguous  methods  could  be i n d u c e d  to  habitat  by  that habitat.  The  o f M. o r e g o n i a n d M.  component, being  were  recapture  apparently  gajDeri  of v o l e s  not  suitable, from  oregoni  was f o u n d  greatest during  the  and  to  have  non-breeding  season. jjicrotus significantly to  be  a  affect  function  introduction occupied  oregoni  only  of  and  M. t o w n s e n d i i  each o t h e r , of  M.  effect  substrate topography.  M. t o w n s e n d i i by  and t h i s  were  oregoni  into caused  an  area  reductions  found was  to shown  Experimental previously both  i n the  iii  population  s i z e of  habitat.  Subseguent  the  H.  M.  gregoni  and  to the i n t r o d u c t i o n  c r e g g n i p o p u l a t i o n was  restricted sites  excluded  to e l e v a t e d areas.  disclosed  in? i t s r a n g e  that  A  M.  o f M.  from  survey  gregoni  of  tcwnsendii,  flat of  and  utilized  habitat  14  different  H.  townsendii  p o p u l a t i o n s were n e g a t i v e l y c o r r e l a t e d  and  height  water t a b l e  of  the  plot  species composition. explain  populations ft. o r e g o n i  character was  hawks and  individual  those  also  the  affected  owls  did  not  vole species.  sympatric  s m a l l e r than  that  displacement  observed:  populations  significantly  winter  P r e d a t i o n by  c o e x i s t e n c e o f t h e s e two  Apparent  J3»  above t h e  and  M.  body  with in  of  M.  other  gregoni  weights  in  townsendii  were  populations  of  Oregoni. These r e s u l t s gregoni  by  t h a t M.  interference  appears to escape relationship  indicate  this  is  t o w n s e n d i i competes  competition,  interference  hypothesized  by  with  while  M.  oregoni  burrowing.  A  similar  between  M.  gregoni  and  h • Igngic^udus. Additionally, limitations The  imposed  relevance  relation America.  to the  of  specializations by  of each  these s p e c i a l i z a t i o n s  these  specific  distribution  of  findings  v o l e s p e c i e s and are  discussed.  i s discussed in  micrctine rodents  in  North  iv  TABLE OF CONTENTS ABSTRACT .. . ... .......... .... .............................. i LIST OF TABLES  i  v i  LIST OF FIGURES ........................................ v i i ACKNOWLEDGEMENTS  .......................................viii  INTRODUCTION ..........* . .. .............................  1  METHODS  ,.M.M«<MMM>  4  1. Study a r e a s ....................................  4  2. L i v e - t r a p p i n g p l o t s ............................  6  3. L i v e - t r a p p i n g procedure  ........................  7  4. T r a p l i n e s ......................................  10  . .. ,.. M>fV'^M<>> N  M  M  5. V e g e t a t i o n a n a l y s i s 5.1.  .. 11  H a b i t a t d e f i n i t i o n s .....................  5 . 2 . H a b i t a t parameters 6.  ......................  Hater l e v e l s  12 14  15  RESULTS . .* ............................................ .  16  1. G e n e r a l c o n s i d e r a t i o n s .........................  16  1.1. T r a p p a b i l i t y and r e l i a b i l i t y o f demographic d a t a .......................  16  1.2. H a b i t a t o c c u r r e n c e o f s p e c i e s ...........  18  1.3. C o r r e l a t i o n o f v o l e s w i t h s u b s t r a t e parameters  .............................  23  1.4. M i c r o t i n e c y c l e s ........................  26  1.5. Home range ..............................  30  i o no r e................ 2. C o e1.6. x i s t eRne cp er o doufc tM. g p n i and M. t o w n. s, e.......... n d i i .... 3 45 2 2.1.  P r e - e x p e r i m e n t a l c o n d i t i o n s a t Ladner ... 4 2  2.2.  F a c t o r s d e t e r m i n i n g s p e c i e s c o m p o s i t i o n . 44  V  2.3. D r a i n a g e - a l t e r i n g e x p e r i m e n t  ............  47  .................  49  2.5. G o o s e L a k e C o n t r o l ......................  50  2.6.  Habitat utilization  ..  51  2.?.  Competition  .................  59  ..................  60  .......................  65  2.4.  I n t r o d u c t i o n experiment  2.8. C h a r a c t e r 2.9.  Role  coefficient  displacement  of predators  3. S e a s o n a l i t y o f o v e r l a p and 3.1.  4.1.  b e t w e e n M.  oregoni  C. g j E f i e r i - . . . . . . . . . . . » . . . . > . . . . . . . . . . . . . .  67  Boad K p l o t  67  4. L i m i t a t i o n s , H»  patterns a t Ladner  placed  oregoni  ............... o n H. l o n q i c a u d u s  by  ..........,.......................  Goose l a k e experiment  ...................  DISCUSSION  69 69 75  1. DI en ft ie nr ia tc it oi no n o of fc |o .m p oe rt ei gt oi no in a .. . . .,.g . . . . . . .. . 2. n. d . C. a p.,.p.e.r . i .•. .....  7758  3. I n t e r a c t i o n  o f M. o r e g o n i  a n d M. t o w n s e n d i i  ....  79  4. I n t e r a c t i o n  o f M. o r e g o n i  a n d M. l o n g i c a u d u s  ...  82  5. P r o b a b l e i n t e r a c t i o n o f M. t o w n s e n d i i a n d M. l o n g i c a u d u s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  84  6. G e n e r a l i t y o f r e s u l t s  87  ..........................  SUMMARY AND C O N C L U S I O N S  92  LITERATURE  95  APPENDIX  CITED  .....,,^..,.,..,....,...,......,...,...........103  LIST Trappability microtines  OF  TABLES  estimates  f o r live-trapped  N u m b e r s o f s m a l l mammals c a p t u r e d i n e a c h h a b i t a t i n s n a p - t r a p l i n e s .................. B e l a t i v e abundance o f sampled h a b i t a t s  small  mammals  i n  Correlations among microtine species a b u n d a n c e a n d s u b s t r a t e p a r a m e t e r s .......... S t a t i s t i c a l c o m p a r i s o n o f home r a n g e e s t i m a t e s between c o e x i s t i n g s p e c i e s  size ........  S t a t i s t i c a l c o m p a r i s o n o f home r a n g e s i z e estimates f o rspecies occupying two o r more g r i d s .................................. Comparison sustainable  o f minimum s u r v i v o r s h i p r a t e s by e a c h s p e c i e s .................  Rates of successful establishment of i n t r o d u c e d fl. t o w n s e n d i i a t L a d n e r G r i d 1 ... Statistical comparisons of microtine h a b i t a t u t i l i z a t i o n p a t t e r n s ................ Independence and s u b s t r a t e Estimation  of microtine distribution w e t n e s s .......................  of competition  coefficients  vii  LIST 1.  Sampling  2.  Trap plot  3.  placement  5  at  Goose Lake  Habitat distribution determined by snap-trap  5.  Minimum plot  estimates  population  Sex  and  species  comparisons  size  among l i v e - t r a p p i n g  Contours o f water l e v e l s  13. 14. 15.  16.  a t Road K ,.  Minimum population enumerations at Goose L a k e C o n t r o l p l o t ..............................  9.  12B.  p l o t s ...........  enumerations  Comparison  12A.  of microtines as s a m p l i n g ...............  a t Ladner  8.  10. 11.  Experimental 8  Population  7.  FIGURES  localities  4.,  6.  OF  of l i t t e r  o f home  localities  size  27  28 29  range  ............  among s p e c i e s  21  32  ........  37  1 ......  45  C o n t o u r s o f w a t e r l e v e l s a t L a d n e r G r i d 2 ...... Contour r e p r e s e n t a t i o n s of v o l e abundance a t L a d n e r G r i d 1 ..................................  48 56  Contour representations of M. t o w n s e n d i i a b u n d a n c e a t L a d n e r G r i d 2 .....................  57  Contour abundance  58  a t Ladner  Grid  representations of M. o r e g o n i a t L a d n e r G r i d 2 .....................  Comparison populations  o f body weights between two o f M. o r e g o n i . . . . . . . . . . . . . . . . . . . . . .  64  Cumulative r e m o v a l o f M. o r e g o n i f r o m G o o s e L a k e E x p e r i m e n t a l p l o t .........................  70  D i s t r i b u t i o n a l p a t t e r n s o f M. l o n g i c a u d u s a t Goose Lake Experimental plot before and a f t e r r e m o v a l o f M. o r e g o n i  71  Pattern of microtine North America  89  species  diversity i n  viii  ACKNOWLEDGEMENTS First, Taylor, and  I wish  t o thank  f o r showing  of the project.  offering  many  I.  McT.  Krajina,  offered  manuscript. , his  interest I  unpublished Much o f  Likewise,  The o t h e r  L.  Fisher, and  t o Dr.  J.  P.  D. ,.  H.  V.  J .  on  the  Pearson f o r  project.  Krebs  data.  and  commented 0.  conversation  also  Bunnell,  f o r lending  me  and a d v i c e , and f o r His  provided  coworkers,  Mary  access  some  to  the of  field  the  staff of  was  of  made  possible  t h e U.B.C. R e s e a r c h  Transport  allowed  use  by  the  Forest. of  the  A i r Base.  Data  t o Mr.  work  was f a c i l i t a t e d Centre  Steve  at  Borden  by h e l p from  U.B.C.  I  am  f o rh i s advice  the staff of particularly  and  assistance  programming. I  of  and c r i t i c i s m s .  read  C.  Redfield,  the Ministry  Biology  manuscript  data.  a t Ladner  grateful in  t o Dr.  research  the  D.,  t o unpublished  Data a n a l y s i s the  and  Mary  and g u i d e l i n e s a ta l l  and encouragement o f t h i s  Jim  cooperation  H.  J.  i n my  read  F.  I am a l s o g r a t e f u l  access  and  fields  Cowan,  f o r enlightening  providing Taitt  Drs.  advice  am i n d e b t e d  traps,  suggestions  comments  m e m b e r s o f my c o m m i t t e e ,  Dr.  and enthusiasm  She c a r e f u l l y  useful  Chitty,  supervisor,  interest  p r o v i d i n g many h e l p f u l  stages  my  gratefully  acknowledge t h e N a t i o n a l Research  Canada f o r s u p p o r t  and f u n d i n g o f t h i s  research  Council through  ix  grant  No. A - 3 7 6 2  graduate  to  Dr.  M.  s c h o l a r s h i p s to myself.  at  U.B.C. a l s o  and  research  assisted  T a y l o r and through The D e p a r t m e n t o f  financially  She d i d  not  involved  i n  shouldered  her share  every  the  type  the  other of  manuscript,  aspect  the  field  and  difficult  i fnot impossible without  snap-trapping  her f o r her enthusiasm  fruitful  discussions  Additionally, sustained  Zoology  by p r o v i d i n g t e a c h i n g  t o acknowledge t h e help  trapping  to  post-  assistantships.  Most o f a l l , I wish Myrnal.  J.  and we  her unfailing  me a n d my  had  of  but  the  work;  both  and  I am for  concerning  wife, was She livebeen  indebted the  this  patience, concern, this  the  would have  her help.  r e s o l v e throughout  she  project.  programs  advice,  o f my  project.  and l o v e  project.  many  have  1  INTRODUCTION The  present  formulation kinds  of  of  the classic  competition  diversity  theory  methods  principle  insufficient  ecologically the  similar  hypothesis  Therefore, especially ecologists The  are t o understand  present  study  Columbia:  townsendii macrurus Booth.  Voles  sufficiently is  (1954), that  how  and  of  species  found  i n  competitive  of  that suggests  that  than  i t  might  be.  competition,  are  needed  i f  n a t u r a l communities.  rodents serpens  for  two  partition  interactions  i n  southwestern  Merriam.  lonqicaudus  gapperi  cascadensis  reasons. history  resources.  They that  are i t  Also, Findley  of c o m p e t i t i v e r e l e a s e , has  i s t h e agent causing  Microtus  Microtus  i n morphology and l i f e they  famous  coexistence  effects,  Clethrionomys  chosen  using evidence  competition  the  investigating  (Bachman),  and  were  similar  not obvious  fruitful  Microtus oregoni  Herriam,  of  allocation  a failure  microtine  townsendii  out, the  (1934)  looks at the competitive  of  s o many  f o r lack of d i v e r s i t y , but  cases  studies  mechanism  among f o u r s p e c i e s British  less  points  patterns  resource  organisms,  field  i t s  f o rthe  explain  are there  Cause's  Specifically,  i s  more  "Why  (1959)  which had been t h e backbone  can account  to  Hutchinson's  (1969)  since  of  communities.  exclusion  Miller  does not account  and  natural  As  by  question  exclusion principle,  experiments,  is  i s inspired  animals?"  competitive of  study  suggested  habitat restriction  of  2  microtines. Grant series of  (1969,  of  space.  pp.  94-95)  is  a  that  In  general  of  paper o f  mechanism  At the  why t h i s  Although  competition, of  the  start  competition  s e r i e s . Grant  interaction  the  my  the  restriction  (1972,  for  space  and  states  species,"  s o , and a s k u n d e r  what  s e l e c t i v e f o r c e s upon  thesis  questions  emphasis i s  competition  of  my s t u d y ,  and  first  little  no g u a n t i t a t i v e  o c c u r r e n c e s of  on  the  investigating  the  effects  geographically  of  were:  If  so, howls  the  categorized?  To answer t h e s e  and s a m p l e t h e  on  Vancouver. these  four  micro-geographically of  guestions,  small  of  Are  habitat  estimate  information,  was a v a i l a b l e  vicinity  species  allopatric?  explicitly,  the  posed  sympatric  specific  information,  voles in  guestions  parameters,  is  a  factors.  virtually  habitats  performed  interspecific  o c c u r s and what a r e  interactants.H  habitat  of  phenomenon among r o d e n t  it  environmental  The  final  1972)  showed t h a t h a b i t a t  a result  now "We may i n g u i r e  involvement  and  the  that  1971b,  suggests that "competitive  conditions  the  1971a,  experiments  some m i c r o t i n e s i s  for  the  1970,  a  each s p e c i e s best I  had  series  mammal  to  of  define  substrate  populations  with  respect to each. Next, limitations species features,  and  I  tested  to  the  the  habitat  specific  distribution  effect  of  hypotheses and  factors  physiognomy,  concerning  abundance such as  behavioral  of  vole  vegetational and  social  characteristics carried  out  field  of the competing species, experiments  whenever  and  predation.  possible.  ii  METHODS 1.  Study  Areas  T h i s s t u d y was c o n d u c t e d a t t h r e e U.B.C. B.C.;  Besearch t h e U.B.C.  University Ladner  of  ranging  Research  British  i n  Forest  (formerly  Haney) ,  vicinity  of the  Vancouver,  B.C.; a n d  1) .  i s an a r e a o f about  Western over  i s dominated  by  55  Jem , 2  Hemlock most  Zone  of  western  (Krajina,  the  U.B.C. -  hemlock  (Tsuga  stages and edaphic h a b i t a t s a r e  U.B.C. E n d o w m e n t L a n d s  hectares  isolated and  the  a p p r o x i m a t e l y 30 t o 900 m e t e r s .  vegetation  Successional  sites:  present. , The  6 90  from  the  i n  B.C. , ( F i g .  i n theCoastal  The c l i m a x  Ridge  i n  Columbia  elevation  heterophylla). also  Maple  U.B.C. R e s e a r c h F o r e s t  i ssituated  1965).  at  Endowment L a n d s  A i rBase i n D e l t a ,  The  It  Forest  separate  of  from  mature  (40-70  Ladner subject habitat fields.  to  years  forest  approximately  thet i p of Point  communities  i n theCoastal  The sampled  (Pseudotsuga  at  other natural  i s situated  (1965).  forest  i s an a r e a o f  by u r b a n  Douglas-fir  communities  old)forests  Grey.  I t i s  Vancouver,  Zone o f K r a j i n a  were a l l  dominated  relatively  by D o u g l a s - f i r  menziesii). A i r Base periodic  consists  i s a  flat,  flooding  almost e n t i r e l y  diked  area  by  brackish  of  regularly  originally  waters. mown  The grassy  Figure Sampling  1.  localities  6  2. Microtines was  marked  Live-trapping Plots  were l i v e - t r a p p e d  grid).  Each  stake,  and  Longworth  were p l a c e d  forest  within  plot,  September  1970,  was  continued  plots  expanded plot  to at  were  (GLC) p l o t  plot  apart.  was  a  labeled  was  stake. established i n  along  western Road  this  i n  approximately  hemlock  K  plot  a 12 b y 12 g r i d .  i n the  w a s a 10 b y April  1971,  Trapping  weekly  was  intervals  l o c a t e d i n t h e U.B.C. R e s e a r c h  three In  a  o f Goose L a k e .  field 10  a t approximately  b y 10 g r i d  weekly  intervals  week i n t e r v a l s July  with until  Goose  Lake  1972, i n a  grass  107 m  plot.  I t  placement  o f t r a p s such  elevation.  t h e t r a p s s e t 6.10 m November  1974, a t  i n 1972 and 1973, a n d a t t w o  1973, I e s t a b l i s h e d t h e Goose Lake  Control  Forest  i n 197 4.  (GLE) a p p r o x i m a t e l y  stream  The  was e s t a b l i s h e d i n J u l y  I t was s a m p l e d r e g u l a r l y  approximately  a  km  (usually  a n d Kempson, 1949)  p l a c e d 9.14 m a p a r t , b u t  sedge dominated  This  to  1.6  plot  November 1974.  Control  Plot  (RDK),  Originally,  a p p r o x i m a t e l y .1.5 km s o u t h  or  (Chitty  an a r e a o f 4 0 - y e a r - o l d  Forest.  at this  Two  and  Each  with a uniquely  a one-meter r a d i u s o f each  i n  of stakes  plot  until  live-traps  a t 305 m e l e v a t i o n ,  grid  the  p o i n t was m a r k e d  t h e Road K p l o t  U.B.C. R e s e a r c h 10  plots.  with a r e g u l a r arrangement o f p o i n t s  sguare  One  at five  0.5  km  consisted  south of  of a  Experimental  the  Goose  somewhat  that a l l o f a grassy  was s a m p l e d . , I n t h e r o u g h l y  field  Lake  irregular adjacent  triangular  area.  7  bounded on two s i d e s riparian I  vegetation  p l a c e d 59 t r a p s  Control  plot  supplementary in  by  (Fig.  established Air  trappings  on t h e  (LG1)  are  r o a d s and l a r g e  trapped  at  p l o t s are  listed  March in  the  3. At a l l whenever start traps  grids,  was  baited  m a t e r i a l as n e e d e d .  three  toes,  marked  two  number  toes  rapidly,  tags,  fields  10 by  were  a  10 g r i d s  1 km a p a r t  drainage  21,  Lake for plot  of  of  at  1975.  were  Ladner  stakes  and a r e  ditches.  (LG2) ,  7.62  separated  These p l o t s  (occasionally  were l e f t not  m by  were  three-week  Trapping dates  for  all  Procedure in  actually  session,  usually  with o a t s ,  processed,  place  and  taking  place.  in  the  prebaited  and  At  nesting  animals  By c l i p p i n g a front  were  bleed,  and,  identification.  up  foot,  c o m b i n a t i o n s c o u l d be o b t a i n e d .  permanent  the  released.  and one from  u s u a l l y d i d not  the  evening,  and s u p p l i e d w i t h  by t o e - c l i p p i n g .  from h i n d f e e t  sufficient healed  ha,  except  in grassy  The n e x t m o r n i n g c a p t u r e d  removed f r o m t h e t r a p s , were  0.45  Appendix.  traps  of each t r a p p i n g  Mice  by  Goose  Grid 2  Live-trapping  trapping  were s e t ,  the  Ladner  two-week .. i n t e r v a l s until  and  side  Goose L a k e E x p e r i m e n t a l  1973,  about  several  intervals)  one  simultaneously  and  were b o t h  The p l o t s  and  1973. 1  They  This plot  trapped  i n September  Base.  apart.  2).  were  Grid  roads  and e n c o m p a s s i n g a p p r o x i m a t e l y  J u l y and A u g u s t , Ladner  logging  unlike  to a The ear  Occasionally  Figure Trap placement i n r e l a t i o n Experimental plot.  2. to h a b i t a t  a t Goose Lake  9  additional original  toes  were  number  The  could  species  animal.  lost, be  and  but i n nearly  trap  For aicrotines,  location  I also  number,  ectoparasites,  and r e p r o d u c t i v e  recorded  position  of the  scrotal  pigmentation.  or  imperforate  size  of  the  whitish  were  recorded  noted  the  weight, the occurrence  were,  for  testes  the  data.  males,  and  the  or  nipples  ( i n c l u d i n g the presence under  closed  the  or  abdominal  absence  I recorded  an o p e n o r  molt  reproductive  versus  presence  For females,  tissue  individual's  External  scrotal  f o r each  o f any  vagina,  mammary  cases  ascertained.  identification  data  a l l such  a  pubic  of  perforate symphysis,  or absence of  skin),  and  obvious  pregnancy. Two used.  variations  To  minimize both  detained set  i n  traps  the traps  long  as  during  as  during  In  nights  and l o c k e d  were  every  the  fioad  trapping of  the  summer,  From long  were  animals  were  time of trap  mortality, I  trapping  approximately  rates  being  set  at  shrews.  Four  was  t h e day t o  to s i x ninety-minute  to  was  done  dawn  and  No d i f f e r e n c e b e t w e e n t h e  K p l o t was a l s o u s e d  minutes during  as done  November  evident. as a shrew  f o r a n o t h e r s t u d y . (Hawes, 1 9 7 5 ) , t h e t r a p s ninety  only was  and c o l d , t r a p p i n g  traps  capture  procedure  weather, and then  open a t dusk.  daytime and n i g h t t i m e Since  length  the mildest  the daylight hours,  checked  plot  the  described.  when  standard  and t h e p o s s i b i l i t y  necessary.  overnight March,  o f my  avoid  periods  trapping  were  checked  mortality  of  c o n s t i t u t e d one  10  day's e f f o r t  and  were counted  4. The  small  sampled  by  was  of  set  lines  traps  The  evening. of  The First,  were of  Second, the microtines,  the  line  did  with  distances.  Therefore,  that  be  traps  placed  i n c r e a s i n g the  range,  one  capturing the  large  likely  I  would  With  this  unit  length  necessarily sampled  number o f set  sampling of  nights at  and  as  traps  habitat.  two  had  were  be  the  traps to  within  increase  w i t h i n the  traps  advantages.  out  in  a  i n each  function  of  bait  species  over  reguired  normal  passages.  an  animal's  the  p r o b a b i l i t y of  line  capture be  catch.  voles  stereotyped the  most  concerned,  sampling . period.  set  technigue,  heayy  attracted to  animals'  and  encountered.  primarily  in capturing  line  sections  always a v a i l a b l e at  success  traps  morning  after a particularly  to  Each  intervening  several  was  with  setting  variable,  to  close  by  l e a s t every  they  were  baited  meters apart.  trapline) i s expected to  linear,  habitats  effort  appear  expect  animal  two  which I  number o f  would  that  to  were n e a r l y  even  not  trapping  sampled of  traps  animals  these  e a c h t r a p l i n e was  close spacing unsnapped  of  checked  habitat being  points along  By  one  consecutive  length  period.  Museum S p e c i a l m o u s e t r a p s  spaced  traps  The  uniform  of  trap  Traplines  I standardized  for three  days.  one  mammal p o p u l a t i o n s  peanut b u t t e r . lines  as  home  Third,  made i t l e s s microhabitat.  rate  (catch  per  a direct,  but  not  density  in  the  11  Reproductive  data  were  snap-trapping. , Snap-trapped field, the  animals  i n conjunction  were a u t o p s i e d  and r e p r o d u c t i v e o r g a n s were v i s u a l l y  a i d of a  absence size  collected  hand  lens.  of visual  tubules  of the seminal  females,  the  Testes  i n the caudal  vesicles  condition  size,  for  t h e number morphology  the  ovaries  absence  of  reproductive those  data  in  the  corpora  lutea,  and t h e presence  were noted. same  manner  or  External as  were  f o r live-trapped animals.  Vegetation  Vegetation  i s t h e most  measure i n r e l a t i o n  Two  distinct  ecologists. mosaic  considers  distribution are  commonly  approach,  continua  of  attribute  herbivores.  used  by  plant  i s to consider habitat variation  pioneered  variation  overlapping  to the  environmental  vegetational types;  approach  second  "plant  The f i r s t  Analysis  obvious  approaches  of discrete  sociology The  mammary t i s s u e  were t a k e n  5.  to  contained  or large f o l l i c l e s ) ,  secretory  For  including  of  albicantia,  or  males.  of embryos o r p l a c e n t a l s c a r s , t h e gross  corpora  with  e p i d i d y m i s , and t h e  size  they  the  presence  and  (whether  i n  inspected  the  were r e c o r d e d  of the u t e r i ,  with  i n  by  which  may  i s  Braun-Blanguet  ordination nature  this  (Bray  as  a  o r may  the  as a plant  (1921,  1932).  and C u r t i s ,  1957),  complex not result  pattern  of  i n distinct  communities."  The  basic  differences  between  the  two  approaches  12  concern plant  the  assumptions  sociology  ecologist  is  approach,  considered  associations occurring  are  within  biotic  subjected can  and to  then  advanced  explained,  work I  with  (Anderson,  have  used  advantages.  theory,  but  plant  since  has  most  been b a s e d  habitats  Krajina's  (1965,  biogeocoenoses,  and  Each h a b i t a t  to  of  mice t o  be  had  biogeocoenosis,  to  be  defined  available insure  as  be  was a  a large within  The  and then  vegetation  computers  a function  of  and these  some e x t e n t ,  may  be  superior  better  in  work  sociological  for in  practice, in  British  principles.  Definitions p r i m a r i l y with  biogeoclimatic to  the  i t .  habitat. extensive 3)  1  2)  zones  for a  Krajina s  too  reference and  following criteria:  enough area  generally  sampling.  ordination  measured and  botanical  plant  according  plant  unreliable,  methods t o  were d e f i n e d  in sufficiently  adeguate  the  on  the  processes  the  of  as  is  1969)  contained  the  both  Habitat  Sampled  aid  Ordination  of  of  1971).  sociology  5.1.  to  In  analysis.  techniques,  i t s  Columbia  ecological  the  the  reliable;  studied.  statistical  each has  especially  and  the  In  assessment  judgement i s c o n s i d e r e d  mathematical  my  subjective  and  are  methodology.  p h y s i c a l c o m p o n e n t s m u s t be  measured components In  the  accurate  them  rigorous  be  the  defined,  approach, subjective the  underlying  small Each  and Most  or  1)  population  smallest  unit,  discontinuous  habitat  accessible  had  to  be  tracts  to  importantly,  the  13  habitats  had  to  vegetational recognized  be  d i s t i n g u i s h a b l e on  characteristics.  as  Grassy  The  five  Fields.  These are  graminiferous,  types.  The  first  vegetation.  c o n s i s t s of  is  not  type  evident  occur.  the  microtine  regard  of  flooded  not  areas  feet  by  understory. of  the  logged  by  of  are  years.  of  are which The  fields,  often  mown f o r  where  hay.  succession  mowing found  fields,  successional  vegetation  Hemlock Zone  4 and This  10  years  habitat  affected or  lakes.  shrubby vegetation,  and  (woody p l a n t s  plants  areas sampled  streams  4  d i f f e r e n c e was types  two  that  s t a b l e , as  even  of  in  usually  and  does  between  I chose  to  habitat.  Herbaceous  Western: between  4 years,  i n t h e s e two  shrubby  The  are  fairly  significant  These  environment  levels  be  one  Riparian., the  appears to  drained  are  and  a f t e r 3 or  poorly which  catch  high).,  succession,  i n winter,  When n o  Regrowth.  of  flat,  a f t e r 3 or  them as  dominated  or  were  herbaceous,  well drained  c h a n g e s become o b v i o u s  consists  stages  by  fields  successional  second  floral  chosen  Grassy  early  The  of  habitats  dominated  temporary,  partially  basis  follows:  usually  second  the  are  less  abundant  were a l l i n t h e  seasonal  have a  than ,in  15 the  subzone had  been  sampling.  i s defined  Riparian  stages  dry  ( K r a j i n a , 1965), a n d p r i o r to  by  forest  as  that  portion  v a r i a t i o n of  habitats  are  of  water  dominated  v a r i a b l e herbaceous  plant  cover. Douglas-fir  forests.  These are  forests located  in  the  14  wet  subzone o f the Coastal  All  forests  sampled  (Pseudotsuga  dry  western  1965).  and  by  were r e l a t i v e l y  1965).  Douglas-fir mature  of  These a r e f o r e s t s  the  Coastal  These f o r e s t s  hemlock  significant  (Krajina,  dominated  Hemlock F o r e s t s .  subzone  (Krajina,  Zone  stands  old.  Western the  were  menziesii)  50-110 y e a r s  Douglas-fir  (Tsuga  lateral  were  located  i n  Western  Hemlock  usually  dominated  by  or,  i n  of  red  cedar  heteroBhvlla),  seepage, by  western  . Habitat  Parameters  Zone  areas  (Thuja  plicata). , 5.2. All during  habitat  August  vicinity  1974.  out  light  intensity,  and p l a n t  area that  Four square clipped  growth dry  were  was c l i p p e d  At each  soil  or chosen  point  varied  habitats,  Material the current  from year*s  66°C  weighing t h e dried  material.  was  incident  i n  The  size  of  with  habitat. 1 m  2  was  a n d 0.5 m  2  was  years'  growth, and t h e  for  at  a quadrat  previous  was e s t i m a t e d  days  the  habitats,  biomass o f the l a t t e r three  i n  composition,  i n forest  and r i p a r i a n  from  estimated  pH, a n d t h e biomass o f t h e  of vegetation  fields.  was s e p a r a t e d  clippings  randomly  and species  were c l i p p e d  regrowth  i n grassy  weight  measured  were measured o r e s t i m a t e d .  meters  i n  cover  humus t y p e ,  herbaceous layer  clipped  Points  were  of the trapline sites.  laid  the  parameters  by d r y i n g  a drying  the  oven and  15  6. At  Water  Ladner, I t r i e d  between  Levels  t o measure  the plots i n the winter  differences  take  technique  i t t o the ground  vegetation.  extremely  the  Since  hard,  I  using  the s o i l  had  measured  the yardstick.  dented  the  observed  soil  of  surface  water  ( i f water  seeped  called  saturated).  the  into  Unsaturated  procedure  January  29,  were b e i n g  were  the  heel  soil  was  stake,  t o compress  both  plots a  water  constant was  was z e r o ,  then I  then  o f my b o o t , a n d saturated  with  soil  'dry'  was c a l l e d  among  degrees o f  was  and no  dryness.  out a t Ladner Grids  a heavy r a i n f a l l .  1 and  2  24, 1975.  Both  dates  On F e b u a r y  24,  1975,  a l s o m e a s u r e d o n 12 a d d i t i o n a l g r i d s  o r had been trapped  was  the s o i l  1 9 7 4 , a n d on F e b u a r y  were two days a f t e r levels  was c a r r i e d  value  was t o  each  force  The  the depression,  was made t o d i f f e r e n t i a t e  This  i n  standing  I f that with  or not the  water  surface  the  whether  attempt  enough  of  no t r o u b l e i n a c h i e v i n g  The d e p t h  with  only  table  ground.  t o measure t h e water l e v e l s  compression.  on  of  a wooden y a r d s t i c k a n d , o n e f o o t s o u t h  press the  I used  drainage  m o n t h s when t h e w a t e r  was g e n e r a l l y a t o r a b o v e t h e s u r f a c e simple  i n  which  b y C. J . K r e b s a n d c o w o r k e r s .  16  BESULTS 1. 1.1.  My  General  T r a p p a b i l i t y and fieliability Demographic Data  live-trapping  effect  of  trapping  expense of trapped,  on  however.  captured  procedure the  maximizing  parameters  increases  method a g a i n s t  a  such  as  that described  by  five  (1974) h a s  species  intensive (the  sum  alive  the  trappability  f o r the  live-trapping upper l i m i t s  .72  to  species. .77,  which are  species. his  1974),  Krebs et  than  the  trapped  a l .  population  procedure,  periods  index  more  trappability  the  more),  occurring  this  sum  of  the  known  to  be  I  at  calculated each  of  my  I ) .  These i n d i c e s are  biased,  but  may  compare  townsendii  for  suffice  M.  J . Krebs M.  to  estimates  orggpni  of  of  ranged .82  ranged  Hilborn's estimate  recaptured  of  animals  or  indices for  to  d i v i d e d by  a l l  to  (1969).  according  Hilborn's  for  the  i t i s appropriate  to Hilborn's estimate  estimates less  of  done a t  calculated trappability  Microtus  H o w e v e r , C.  experience  proportion  (Table  comparable My  demographic  trappability,  methods.  .83,  errors i n estimating  microtines  plots of  population  Using  trap  the  the  number o f c a p t u r e s  three  minimize  more i n t e n s i v e t r a p p i n g  possible captures  for  trapping  voles  technique. of  number o f  of  to  of  proportion  {Hilborn,  my  designed  was  the  test  of  This  the  as  was  population.  Since  decrease  Hilborn  Considerations  for  from  .80  from this  .51  for  to this  ( p e r s . comm.), r e p o r t s t h a t oregoni  individuals  are  in  less  17  Table Trappability  Species  estimates  Plot  I for  •N  tagged  C  animals.  P  E  max  E  . mm  C.  gapperi  RDK  7  25  32  .78  M.  longicaudus  GLE  14  77  1 20  .64  .53  M.  oregoni  RDK  28  209  345  .61  .53  M.  oregoni  GLC  129  11 2 7  1467  .77  .72  M.  oregoni  LG1  1 84  900  1780  .51  .38  M.  oregoni  LG2  17  95  166  .57  .46  M.  townsendii  GLC  17  91  109  .83  .76  M.  townsendii  LG1  52  286  372  .77  .68  M.  townsendii  LG2  134  660  917  .72  .60  N = number  of  C = sum o f  captures  P = sum o f E E  max  animals  possible  = C/P  . = min  (C-2N)/(P-2N) ' '  captures  :  .61  18  trappable  than  Overall, to  be  my  C.  results  at least  trapping  f o r M.  roughly  regimens.  gapperi  those  recaptured  for  M.  the  other  trappability indicating  infrequent,  individuals  of  The of  given  f o r C.  the  I I and  and  M.  riparian  to  population of  the  i s and  a stream.  of  Lake M.  stream  longicaudus regrowth  I I I .  i s  as  lower  than low  plots  as by  Species  These f i g u r e s limited i s  measured  to  as  meters  captured  r a t e s per  are  100  m  show t h a t t h r e e  of  i n habitat restricted  western found  occurrence to  grassy  hemlock  forest,  in  only  two  regrowth. were  captured  However, l a t e r Experimental  and  seldom  used t h e  riparian  dweller  especially  in  dependent  at  grassy  sampling  plot)  was  habitat,  for  the  the  capture  longicaudus  a  intensive  longicaudus  individuals  essentially  lonqxcaudus  (Goose  M.  of  habitat,  gapperi i s restricted  M.  adjacent  use  townsendii  longicaudus  Two  area  i n Table  Microtus  habitats,  interpret  g a p p e r i and  expressed  four microtine species are  C.  substantially  numbers o f  the  fields,  more  trappabilities  I  i n each  trapline  3) .  of  appear  species.  of  (Fig.  individuals.  M. . t o w n s e n d i i  Habitat Occurrence of  and  i n Table  are  marginal  trapping effort  trapline,  and  estimated  species.  estimates  1.2.  oregoni  longicaudus  these  townsendii  comparable t o those The  and  M.  showed on  grassy that  of  fields  the  same  that  the  the  presence  area.  Microtus  also  utilizes  h i g h e r e l e v a t i o n s and  on  Table I I Numbers o f s m a l l mammals c a p t u r e d i n e a c h h a b i t a t i n s n a p - t r a p lineso E f f o r t i s e x p r e s s e d as m e t e r s o f t r a p l i n e , s p e c i e s as numbers o f i n d i v i d u a l s .  Grassy Fields EFFORT  HABITAT Riparian  Regrowth  Douglasfir  Western Hemlock  4035  2893  3040  3030  4555  15 2  3  13  10  3  15  6  0  0  0  0  0  0  0  0  0  20  15  75  SPECIES M i c r o t u s oregoni M. l o n g i c a u d u s M. t o v n s e n d i i  170  C. g a p p e r i Peromyscus  maniculatus  Zapus t r i n o t a t u s Sorex vagrans  0 21 5  " 26 1  155  1  •. 0  99 0  4  14  6  8 9  15 0  17 4  1 0  1 0  1 0  0  2  5 0 0 0  obscurus  29 0  S. c i n e r e u s  0  5 0  S. b e n d i r i  0  0  S. p a l u s t r i s  0 0  1 0  Ochotona p r i n c e p s Eutamias amoenus  0 0  1 2  0 0  0 0  E.  0  0  1  0  1 0  0 0  0  0 0  S.  Neurotrichus  gibbsii  tovnsendii  Mustela  erminea  Scapanus o r a r i u s  0  0 2  Table I I I R e l a t i v e a b u n d a n c e o f s m a l l mammals i n s a m p l e d h a b i t a t s . D a t a a r e e x p r e s s e d as a n i m a l s c a p t u r e d p e r 100 m e t e r s o f t r a p l i n e ,  HABITAT Grassy Fields  Douglasfir  Western Hemlock  Riparian  Regrovth  .37 .05  .10  .43  .52 •  .20  .33 .00  4.21  .00  .00  .00  .00  Clethrionomys gapperi  .00  .00  .00  .00  .44  Peromyscus m a n i c u l a t u s  .52 .12  2.47 .03  5.12  Zapus t r i n o t a t u s  1 .73 .90  2.17 .00  Sorex vagrans  .72  .03  •  Sorex obscurus  .00  .26  .50  .37  Sorex c i n e r e u s  .00  .17 .00  .30  .00  Sorex b e n d i r i  .00  .00  .03  Sorex p a l u s t r i s  .00  .00 .  .00  Neurotrichus  .00  .03 .00  .03 .00  .09 .02  .00  .11  .02  .10  .07 .00  SPECIES Microtus oregoni Microtus longicaudus Microtus townsendii  others  gibbsii  1  3  .03  .00 .46  .  .07 .00  .13  .04  o  Figure Habitat distribution snap-trap sampling.  3.  of microtines  as  determined  by  "M t  s:  M Cg  M.o.  Hi  M.o.  mi M  HL  m  M.I. c.g. Grassy  Fields  M.t.  e.g.  M.t. 15533  Riparian HABITAT  s  Regrowth  e.g..  M.t.  Ml,  Douglas-fir  c.g.  Western  Hemlock  22  rockier  soils.  fields)  only  It  peripherally  Microtus in  oregoni  that i t occupies  represented in  removed  from  other  the  different were grassy  fields;  These  the  combinations  and  M.  poorly  animals  hemlock  M.  C.  far be  abundant,  forest. three  species  and  M.  are  with each  five  longicaudus and  were  must  not  of  h a b i t a t s , 30  p o s s i b l e , yet  gregoni  adults  hemlock f o r e s t  although  are  oregoni  Although  fourth overlaps  Microtus and  species  only  four  townsendii  i n regrowth  gapperi  in  in and  western  forest.  The  live-trapping  voles  similar  Plots  set  in  fields  townsendii.  One  M.  oregoni  rarely,  M.  implies interact hemlock),  plots  revealed  to t h a t demonstrated  Ji*  while  western  oregoni,  JI. o r e a o n i  riparian;  hemlock  of  i n western  to occur:  grassy  three  breeding  Among f o u r s p e c i e s o c c u p y i n g  overlap  found  forests,  tracts  while  (e.g.  other  four-species system,  segregated,  others.  the  habitats.,  habitats.  succeed  Thus, i n t h i s  from  five  M.  habitats  transiently.  hemlock  Hence,  apparently can  spatially  a l l  extensive  residents.  or  other  differs  i n western  captured  in  occupies  and,  plot  longicaudus  t h a t M.  caught (GLE)  M. was  longicaudus  directly. c a u g h t M.  by  The oregoni  the  a  distribution snap-trap  only  set  M.  alongside  townsendii  a stream  caught  near  and  townsendii  M.  only and  plot C.  set  gapperi.  lines.  oregoni  throughout the  of  and caught  the  plot,  stream.  This  occasionally  i n forest  (western  23  1.3. Small were  C o r r e l a t i o n of Voles Parameters  mammals w e r e  estimated  Theoretically, the  four  sampled  at  i f the  microtine  a  and  total  of  distribution species  factors,  factors.  Correlation coefficients  of  abundance  hypothetical  causal  they  for factor  c o m p u t e d , some s p u r i o u s occur  by  chance;  interpreting The M.  these  best  correlation species  be  correlations  are  correlations  with  which  explain  Microtus  cover,  and  an  of  number of  with  per  cent  found  i n treeless  found  them  With  58  would  the  those an each  coefficients  be  should  i s  expected  use  of or  i s  absolute more and  to  caution  in  number  of  i t  (r =  may  than  discussed  with  both  (|p|  <  remaining .5).  than of  Only  .3,  the  and  observed  below.  plants a  correlations (biomass,  negative  implying that an  from  The  cent  abundance shows  cover,  This  cause-and-effeet  greater  9 per  .707).  stem  weak  value  s p e c i e s ) , and  areas  shrews  the  unlikely.  herbaceous  tree  between  relatively  townsendii  measures  of  any  s p e c i e s and  same c o n d i t i o n s . . a  a l l  listed  three  of  correlated with  microtine  number  the  between  are  by  any  were computed between  one  spurious,  relationship  variation,  abundance of  (Table I V ) .  hence,  the  preferring  thus  locations.  results.  and  may  twenty-six  be  correlations  correlation  townsendii  parameters  limited  should  each  Substrate  substrate  and  are  estimated  index  then  with  abundance  M.  per  with cent  correlation townsendii  of  i s  herbaceous  Table IV Correlations  among m i c r o t i n e s p e c i e s Microtus oregoni  a b u n d a n c e and s u b s t r a t e Microtus townsendii  parameters.  Microtus longicaudus  Clethrionomys gapperi  M.  oregoni  1 .000  M.  townsendii  -0.175  1.000  M.  longicaudus  -0.236  -0.088  1.000  -0.213  -0.063  -0.109  1.000  0.233  -0.156  -0.016  0.010,  Zapus  -0.211  -0.095  -0.046  -0.117  moles  -0.050  -0.092  -0.159  -0.054  0.011  0.707  -0.186  -0.006  0.298  0.21 8  0.307  -0.288  0.354  0.379  -0.112  -0.201  -0.383  -0.198  -0.311  0.359 -0.194  Clethrionomys Peromvscus  shrews Incident  light  Biomass h e r b , fo t r e e  cover  cover  shrub  cover  0.376  -0.255  -0.173  fo h e r b ,  cover  0.388  0.466  -0.096  0.007  -0.207  -0.080  0.454  0.046  0.285  0.299  -0.413  spp.  0.420  0.429  0.004  -0.288  # woody s p p .  -0.003  -0.385  -0.022  -0.012  f f  moss c o v e r  pH  # herb.  .  -0.187  25  plants. Microtus same f o u r  measurements  correlated.. cent  oregoni  abundance shows with  w h i c h M.  A d d i t i o n a l l y , M.  shrub cover.  H e n c e , M.  townsendii  oregoni oregoni  same h a b i t a t s a s M . t o w n s e n d i i ,  c o r r e l a t i o n s with  abundance i s  i s c o r r e l a t e d with i s most  but i s a l s o  the  per  abundant i n t h e found  i n shrubby  habitats. Microtus and,  longicaudus  negatively,  species  i s  with  found  herbaceous cover  pH,  per  indicating acidic  associated  the other  moss  cover,  and  light  Hence,  lacking  abundance shows  the  Microtus  this dense  species.  c o r r e l a t i o n s with  per cent  i s f o u n d i n mossy  the canonical  and  19 p e r c e n t  explain  vole  estimated  combined  correlation  tree  cover,  forests  with  of the t o t a l  distribution  estimated,  parameters rather summary,  oregoni,  to  southwestern intimately  variation.  B.C.  failure  to  to the parameters to optimal  Either  which  values  were  of  the  t o maxima o r m i n i m a .  voles be  The  possible explanations.  are responding than  species  and a b u n d a n c e on t h e b a s i s o f t h e  are not responding  or they  between v o l e  i n d i c a t o r parameters accounts f o r  p a r a m e t e r s has two  species  In  tree cover.  habitats with  incident  soil.  abundances  the  gapperi  cent  cent  open  that the species  However,  only  per  i n  Clethrionomys. low  i s c o r r e l a t e d with  are  found,  with  microgeographically Microtus  c o e x i s t s , with  oregoni  the exception allopatric  overlaps,  each of t h e other  three  and  of i n  hence  species.  26 The  distributions  predictable but  were  substrate  on not  of  the  these  b a s i s of  tendency  rodents  to  have  undergo  have suggested  (1974)  a ..priori, my  study  population  i s not  that  to  townsendii This  can  i t was  for  at  townsendii  undergo  the  (1974).  moderate  oregoni typical Both  with  that Krebs  and  populations cycles.  microtine cycles the  results.  per  These  minimal. s i g n s of  encountered  ones.  their  population  a l l microtine  absent from  population  sporadically  some f i e l d s  Population  G r i d 2 v a r i e d by  Ladner  and  plots,  variation  at  species  H.  either  Control plot  M.  longicaudus  while  estimates a f a c t o r of  d i d not  microtine cycles described  numbers  trapping  Lake  for  (Fig. 4). Microtus  in  prevalent  affect  was  appearing  Myers  estimated  renowned  showed d i s t i n c t  abundant i n s i m i l a r  three  single,  periodic  have been  species  snap-rtrapping;  fl.  definitions,  to undergo four-year  concerned  however, appear M.  i s so  considered  fluctuations  fluctuations. during  tendency  be  been  regular,  Myers  Only  be  Cycles  long  This  effects,  b a s i s of  Microtine  fluctuations.  se,  the  to  parameters. ,  Microtine  Although  were f o u n d  subjective habitat  p r e d i c t a b l e on  1.4.  should,  vole species  were e n c o u n t e r e d  i n the oregoni  the  Boad  ( F i g . ,6).  snap-trapping showed K plot At  L a d n e r , M.  to  i n Krebs  and  regularly lines.  little ( F i g . 5)  appear  In  and live-  year-to-year or  at the  oregoni  Goose  showed  a  Figure Population  estimates  t - U t  M.  m o 9 •  M.  4. at Ladner  oregoni tovnsendii  plots.  Minimum  Population  Minimum Population Index  Minimum Population  to  28  Figure Minimum p o p u l a t i o n  5.  enumerations  a t Road K  plot.  <! oo  CM  8+  ONDJ 1970  FMAM.J J A S O N D J 1971  FMAMJ J A S O N D J F M A M J J A S O N D J 1972 1973  FMA 1974  Figure Minimum p o p u l a t i o n  6.  enumerations  a t Goose Lake  Control  plot.  ON CM  40 T  c o Z5  a  =3  E  i—i—i—i—i—i—i—i—i—i—i—i—K  J A S O N D J FMAM J J A S O N D J F M A M J J A SON 1972 1973 1974  30  decline is  d u r i n g the  correlated  populations  second year  with  a  (Fig.  4).  s p e c i e s may a c c o u n t later  (Sections Few d a t a  state  of  dramatic  trapping, increase  Population  for  this  but in  this M.  townsendii  interactions  d e c l i n e and w i l l  decline  of  be  two  presented  2.2-2.8).  were c o l l e c t e d c o n c e r n i n g C . g a p j a e r i ,  little  the  except  that  local  abundances of  so I  this  can  species  seemed s e a s o n a l l y s p o r a d i c . ,  1.5. The e l l i p t i c a l Hoffmann of  (1975)  ellipse  model o f  obtain  captured . eight  grids.  distribution  home r a n g e  was used t o  individuals  trapping  Home Range  This  of  model  capture  b a s e d on t h e i r  Koeppl,  home r a n g e  strictly  of  normality  only at the  is  discrete  borders  captures, of  hold f o r  assumes  points,  a  three  of  the  i.e.  grid  and i n t e r p r e t a t i o n  t r a p s p a c i n g tend  estimates  than  (Hayne, effect  1950; of  this  For example,  limit is  size  the  livenormal  probability  technique the  assumption  to  1954; should  produce  grids Faust, be  recorded  values recordable  Larger  or et  do  Additionally,  c o m p l i c a t e d by  sizes.  smaller  Stickel,  grid  bivariate  trap stations.  different  with wider  do  the  met b e c a u s e c a p t u r e s c o u l d be  points,  slightly  estimates  in  and c o m p u t e s a  and  dispersion.  my s t u d y .  not  size  o r more t i m e s  The n e c e s s a r y a s s u m p t i o n s u n d e r l y i n g not  Slade,  grids  g r i d s and  larger closer al.,  considered  home  for  being grids range  trap spacing 1971). when  The one  is  31  interpreting  the  results.  S e v e r a l c o n c l u s i o n s c a n be drawn from size of  estimates variance  of  each s p e c i e s  revealed  variance  of  sex  plot  the  (Table  effect of  was u s e d t o  on  of  the  In  the  cases  and  v o l e s occupying the  same  Goose  Ladner,  significantly  Lake  Experimental  larger  home r a n g e  of  for  A t Road K,  at  larger  and  o r e g o n i and M. t o w n s e n d i i d i d n o t  the  analysis  species  M. o r e g o n i  Similarly,  range  A two-way a n a l y s i s  effects  except  s e x was s i g n i f i c a n t .  significantly. M.  test for  all  C. gapperi  A one-way  (p < . 0 0 0 2 ) .  home r a n g e s i z e o f  V).  7).  home  t h a t males h a v e s i g n i f i c a n t l y  home r a n g e s t h a n do f e m a l e s of  (Fig.  the  C. gapperi home r a n g e  did  not  the sizes  differ  home r a n g e s i z e s o f  differ.  plot,  s i z e s than  However,  M. t o w n s e n d i i  at had  d i d M. o r e g o n i .  i  1  I  Table V  |  I I | \  S t a t i s t i c a l c o m p a r i s o n o f home r a n g e s i z e e s t i m a t e s between c o e x i s t i n g s p e c i e s . Probabilities given are that observed d i f f e r e n c e s are a t t r i b u t a b l e t o random e v e n t s .  J I I I  i  Locality  Species  S|>.  Prob.  Sex P r o b .  j  |  RDK  M.o.  and C . , g .  .68  .086  |  |  GLC  M.t.  and M . o .  .027  .0078  I  |  LG1  H.t.  and M . o .  .49  .01  |  i  .  ; Similarly,  test  a two-way a n a l y s i s o f v a r i a n c e was  the e f f e c t s  of  grid locality  and sex on t h e  i used home  to  range  Figure  7.  S e x a n d s p e c i e s c o m p a r i s o n s o f home r a n g e s i z e a m o n g live-trapping localities. S t a t i s t i c s performed on log-jQ t r a n s f o r m e d d a t a ; mean a n d 9 5 ^ c o n f i d e n c e limits are plotted.  5.  n=5  1  8  10  t I  22  29  2  1  15  16  5  5  1  2  10  6  4. 9  3."  CD N  1  CD CD „ „ C CN  03  DC  O M. townsendii  E  +  o  -i-  CO  d*  9  o  CD CO  E o o  X  •© M. oregoni  -—  1.+ © C. gapperi  0,  Road K  IGoose Lake Control  Ladner Grid 1  Ladner Grid 2  33  size  of voles  only  part complicated by d i f f e r e n t  i — "  (Table VI).  •  ;  This part of the analysis grid  i s the  sizes. 1  ;  I  Table  I  VI  J 1 | J  S t a t i s t i c a l c o m p a r i s o n o f home r a n g e s i z e e s t i m a t e s f o r s p e c i e s a t two o r more g r i d s . Probabilities given a r e that observed d i f f e r e n c e s a r e a t t r i b u t a b l e t o random e v e n t s .  I  I  Species  Grids  1  J3« 2£«a2iii  I  J I I  Sp. Prob.  Sex Prob.  j  RDK a n d GLC  .0001  .0001  |  M. o r e g o n i  GLC a n d LG1  .065  .0001  |  j  H. t o w n s e n d i i  GLC a n d LG1+2  .0005  .0015  |  I  H. t o w n s e n d i i  LG1 and LG2  .56  .018  |  *  ;  •  i  Microtus  oregoni  home r a n g e  sizes  The  K plot  and  Boad  has wider  similar  at  Goose  versus Boad than with  i slarger  home  Control  ranges  confidence  high  Control  degree  Microtus  M.  (p  .0001).  mean home r a n g e s i z e  of the total (103%  versus  area  plot  produce a t Boad  of the plot  than  3 3 % f o rmales and 3 3 %  The d i f f e r e n c e  implies  statistical  that  to a greater  C o n t r o l home r a n g e s . of  larger  <  so e s t i m a t i o n b i a s c o u l d  t o the existence of larger  forest-inhabiting  t o have  t h e Goose Lake C o n t r o l  are underestimated  a r e t h e Goose Lake the  than  The e s t i m a t e d  percentage  Lake  K was f o u n d  a t Goose Lake  19% f o r f e m a l e s ) . K  Boad  trap spacing,  results.  K i sa larger  than  at  This,  degree coupled  significance,  home  range  the  lends  sizes  i n  oregoni.  oregoni  was  found  t o have l a r g e r  home  range  34  sizes  a t Goose Lake  level  of  Control  significance  than  at  (.065) i s j u s t  (.05).  S i n c e Goose Lake  Control  Ladner  grids,  i s towards  Lake  t h e bias  C o n t r o l home r a n g e  difference  t o be  Microtus  tendency size.  sizes;  larger  Hence, t h e  at  observed  Finally,  home r a n g e  2. range  size  t o i t s environment  the  can  density,  range  but comparisons  a function  two  species  Ladner. home r a n g e  sizes  the  species  same  differences  life  were  from  of  Goose  Lake  t h a n M. o r e g o n i . a r e compared statistically  at  parameter  an  animal's 1968).  Nest s i t e s , may  area  (Jewell,  at  social  affect  indicate  w a s f o u n d b e t w e e n home r a n g e  at  that  t h e minimum a r e a i n  factors  habitat  grid  statistically  history  among s p e c i e s  o c c u p y i n g t h e same  However,  This  (McNab, 1 9 6 3 ; S c h o e n e r ,  and other  had  o f M. t o w n s e n d i i a t  indicator  of t h e minimal s u r v i v a l  No d i f f e r e n c e  Control  i s  support i t s e l f .  interactions,  is  the  this  c a u s e d by  different  o f an a n i m a l must i n c l u d e  animal  size,  size  i s a relevant  c a n s e r v e a s an e c o l o g i c a l  which  than  consider  Lake  relationship  Grid  home r a n g e  I  o f t h e expected bias  Ladner  The  value  s i z e s than a t Ladner.  1 was n o t s t a t i s t i c a l l y  adaptation  grid  the  u n d e r e s t i m a t i o n o f Goose  Goose  Grid  that  the test  i s a smaller  Ladner  Home  above  therefore,  home r a n g e  i sthe reverse  significant.  however,  significant. townsendii  significantly  Ladner;  that i t 1966).  sizes  Eoad  home  K  of the or  at  M. t o w n s e n d i i h a d l a r g e r When home r a n g e  at different  sizes  of  localities, a l l  significant,  and  some  35  striking size  differences occurred.  o f M. o r e g o n i  mean home r a n g e If Ladner  a t Road  size  smaller  home r a n g e  home  first.  species  a t t h e same p l o t t e n d  range  estimates  appears  times t h e  sizes indicates better  Since  habitat  nine  range  a t Ladner.  appears t o rank  size  mean home  K i s approximately  last.„  range  For example,  Goose Lake s e c o n d ,  size  estimates  to  be  a  and Road  K  o f two d i f f e r e n t  t o be more s i m i l a r  o f t h e same s p e c i e s  habitat,  than  at different  more i m p o r t a n t  home plots,  determinant  than  species.  1.6.  Reproduction  One o f t h e m o s t i m p o r t a n t of  a  species  strategy  accurate  of  reproduction.  presumably  environment.  forces  i s  i s an  Thus,  a c t i n g upon  reproduction; ecologically compared  adaptive  the  biology  fittest therefore,  significant.  reproduction  to  Darwin  With these  i t s  can  (1859)  maximize  tenets  i s  species  that  successful  strategies  points  be  evolutionary  One o f t h e b a s i c  they  the  reproductive  and  reproductive  among  history  reproduction  ecological  since  when  life  response  i n  the population.  modern o r g a n i s m i c are  of  of the  An a n i m a l ' s  differences  indicators  animals  aspects  are  i n mind, I have and  localities  studied. Results  are  analysis  of  apparently  estrous  presented  data  was  f o r female not  simple.  i n d i v i d u a l s had open  voles Most  only. pregnant  The or  vaginae and, i n the  36  later  stages  females not  of  pregnancy,  had open p u b i c  a l l pregnant  ascertained  whitish  sub-cutaneous  deemed  to  animal  was  The  had open  from  external  tissue.  per  cent  size  data  and  Lactation  was  by t h e p r e s e n c e  Live-trapped  females  of were  was o p e n o r i f t h e  f o ra l lfour  confidence  Figure  8.  and  g a p p e r i do n o t d i f f e r  species  limits  3.25,  3.0  statistically  and  litter  3.5, r e s p e c t i v e l y ) .  larger  than  those  the  longicaudus,  in  M. t o w n s e n d i i , h a s a n a v e r a g e l i t t e r  i s significantly  and  are presented i n  T h r e e o f t h e s p e c i e s , M. o r e g o n i , M.  (averaging  which  Many  lactating.  average l i t t e r  species,  symphyses.  vaginae.  be r e p r o d u c t i v e i f t h e v a g i n a  ninety-five  C.  pubic  symphyses, b u t were n o t pregnant,  animals  easily  open  The  size  size fourth  of  5.4,  of the other  three  species. The  litter  agreement 1954)  size  with  a n d o f 3.2  another  1962).  effort  by  obtained,  search  a  this  of  (Colvin  T h e mean l i t t e r  than  could reflect  species.  (3.25)  the estimate New  error  difference  Estimates  of  reproduction  of the literature.  Colvin, of  1970).  M. t o w n s e n d i i  York  size of of  i n  4.28  (Patric,  d u e t o my  small  reproductive  3.8 a n d 4.0  f o rlaboratory-raised and  i s i n  (Cowan a n d A r s e n a u l t ,  o f C. g a p p e r i f r o m  geographic  respectively,  M. l o n g i c a u d u s studies  i s much l o w e r  subspecies  or  o f 3.11  ( G a s h w i l e r , 1972).  This discrepancy  sample s i z e ,  f o r M. o r e g o n i  the estimates  3.5 f o r C. g a p p e r i for  estimate  were  M. o r e g o n i a n d No  intensive  were f o u n d  i n a  Figure 8. Comparison of l i t t e r s i z e among s p e c i e s : s i z e and 95f° confidence l i m i t s .  mean  litter  LITTER  -f-  Clethrionomys  M. oregoni  M. longicaudus  M. townsendii  -4-  SIZE  -4-  o  -f-  CO —I  38  Reproductive that  i s  effort,  channeled  the  into  s i n g l e comparison  However,  many  size,  such  one  and  variables  sex  made among  be known b e f o r e  c a n be  ratio  of  data  be known  made.  Other  concerning  offspring,  1954).  litter  size  Of  an  accurate  than  litter at  and  first  survival  breeding  these  most  species.  nestling  of the population  (Cole,  energy  i s perhaps the  c a n be  must  the proportion  time should  have  that  available  f a c t o r s as a g e - s p e c i f i c f e c u n d i t y , age  reproduction, rate,  of  reproduction,  important  c o m p a r i s o n among s p e c i e s  amount  a t any  factors,  seasonal  I  breeding  freguencies. The  comparative  species  i s  together, interact 1954). most  To of  best  as in  the a  this  reasonable The  a  model  subtle  by  and  a  unexpected  each  a l l factors  parameters  simple  reproductive  by  manner  model  can (Cole,  in  which  parameters are included.  are l a c k i n g f o r c e r t a i n parameters,  i s  deterministic size  population  the proportion  Lactation  considering  often  of  f o r three 100  Males are disregarded  to  put f o r t h  reproductive  aim, I developed  in population  January.  used  various  effort  I  made  guesses.  starting  month,  assessed  the relevant  Hhere e s t i m a t e s  changes  reproductive  estimate  cf  i n Microtus  and  calculates  years  lasts  months)  four-month-old in  the  number  of  approximately  from  females i n  model.  of the females that the  (36  monthly  For  each  were  lactating i s  litters  produced.  21 d a y s  (Hatfield,  39  1935),  so  An  the  employed  additional  considered. mortality 1940)  Microtus  that wild  for  fecundity  the  According  of  Microtus can  begin  of  only  one  month. they  are  that  these  very  first  a  of  signs of  successful  (1956),  do  not  The  age  (Krebs  The  model  f o r such  0.5  for  account  f o r Microtus, I assigned  third  first-  and  a relative  second-month  month a n i m a l s ,  begin  to  and  of  of  first  with time and  1.0  of  Myers,  variation.  value of  animals,  a value  found  necessarily  phase of p o p u l a t i o n c y c l e  fecundity of  age  not  are  with  to  an  breeding  year  Therefore,  species  Greenwald  to vary  not  first  weaning, at  reported  does  at to  at  most  has  1974).  age  months o l d .  pregnancy.  been  an  age  reproduction and  also  reach  the  soon a f t e r  two  Hanson,  model, t h a t a g e - s p e c i f i c  to Greenwald  around  and  of  i t i s reasonable  U s u a l l y , however, they  until  by  this  measure  breeding  breed  followed  of  also  comparison  (Leslie  Therefore,  a  reproduction.  a  microtines rarely  purposes  i s essentially  agrestis,  fecundity  f e c u n d i t y i s reduced.  assume,  i s conservative.  o f a g e - s p e c i f i c f e c u n d i t y was  age-specific  indicates  which  factor  For  and  estimate  zero  a value for  of  mature  animals. Clethrionomy.s  g a p p e r i d i d not  pattern,  f o r I recorded  calendar  year  of  the  onset  (May  to August).  age-specific  of  an a  no  appear  i n s t a n c e of b r e e d i n g  animal s 1  birth.  This situation zero  can  the  same  during  the  A l l overwintered  f o u r month p e r i o d of  fecundity to  to follow  be  f o r the  reproductive modeled first  by  before  activity setting  f o u r months  of  40  life.  Since  would  not  after  breeding reach  t h e end  lasted  only  reproductive  of t h e  breeding  four  months,  maturity i n the season  in  juveniles model  until  they  were  which  born. The given  number  month c a n  product  of the  expected  (assumed  survival  factor of  to  was  because of  exposure, difficult and  to  mortality  per c e n t  (Hoffmann,  includes  sources  estimate,  I  nestlings  though  the  the 0.5  the  nestling  i s egual  maximum  to  The  significant  I n my  mortality  The  per  survival  sum of  litter  nestlings  the  (Krebs  mother),  and  magnitude  model, s i n c e not  magnitude of t h i s  The  realistic  cent  age.  factor may  be  of  to  factor  had  apparent no  basis  among s p e c i e s an  35  Hoffmann's  the I  this  30  this  included i n  and  Myers,  of  a t a minimum o f  make i t t o w e a n i n g  c a r e and  more  on  the  The  number o f  or  i n  abandonment i s unknown  1974).  50  a  total  number  month.  give  estimated  that  females  expected  (directly  but  1958).  to the  for a l l four species).  p o p u l a t i o n i n any  was  of  actually  born  proportion of  the  a l . ,  assumed  changing  for  competition, or  nestling  the  size  litter  reproduction.  et  times  the average  estimate,  Batzli  group  the  age-  predation  sibling  group as  the  included  monthly  any  times  i s  the  age  during  reproductive females  by be  classes  born  estimate  to  born  i n t h a t age  number o f f e m a l e s  females  for  e s t i m a t e d f o r each  of  multiplied  a l l age  1974;  be  t h a t s h o u l d be  number o f a n i m a l s  The  number b o r n  lost  then  fecundity times  species.  over  animals  proportion  specific  litter  of  important  even  aspect  41  of  the reproductive Progression  multiplying  strategy  from  the  one  month  calculated value the  age  as explained  of three  itself  above.  number  i n  the  the population  values  the  until,  the population  The  by  i n age group one i s  i s adjusted  years,  population.  that  done  age g r o u p by t h e  that  number  was  The model i s r e c u r s i v e ;  hypothetical  survivorship rates  i n each  survivorship rate  same a s t h e s t a r t i n g  maintain  The  species,  another  and e n t e r i n g  group.  o f t h e monthly end  to  number o f a n i m a l s  monthly s u r v i v o r s h i p r a t e nth-plus-one  of the d i f f e r e n t  of  can s u s t a i n  at  i sthe minimum  and  s t i l l  a r e compared i n T a b l e V I I .  Table VII C o m p a r i s o n o f minimum s u r v i v o r s h i p s u s t a i n a b l e by each s p e c i e s . Species  Grid  C. g a p p e r i  SDK  .9111  3*  GLE  .8413  M. o r e g o n i  GLC  .8481  M. o r e g , o n i  LG1+2  .8735  fi. t o w n s e n d i i  LG1+2  .7563  ifiSaicaudus  These into  r e s u l t s show t h a t  reproduction  tolerate can,  rates  than  a much g r e a t e r  i n t h e sense  Survivorship  M. t o w n s e n d i i  do a n y o f t h e o t h e r mortality rate.  of Gagil  and S o l b r i g  puts  more  species,  Microtus  energy  and  can  townsendii  ( 1 9 7 2 ) , be s a i d  t o be  42  r-selected  with  Conversely,  respect  C.  reproduction,  gapperi  mainly  and  cannot  tolerate  the  other  species.  K-selected Microtus other  with  by  as  the  far  virtue  of  a  roughly  to  M.  microtines studied. less  low  C.  r a t e as  gapperi  the  can  other  longicaudus  intermediate  energy  numbers o f  mortality  Therefore,  and  other  puts  great  respect  oregoni  and  to  litters,  can  be  any  said  three  are  the  of  to  be  species.  similar  between  into  to  each  other  two  species.  2.  Coexistence 2.1.  Two  were  set  October  at  used  and  1973.  for field  third  grid  to  Dr.  C.  J . Krebs  his  control  known a l i v e , for  Krebs's  captured  in  M.  townsendii  Grid  1  and in  Ladner  Populations  were  1975,  2,  were  1973.  Traps  t r a p p i n g commenced i n monitored  1974.  i n March  Grid  September  until  September  Ladner  From these  without  September two  1973,  grids  were  experiments. of  and  and  Conditions at  A i r Base  trapping ceased  1  oregoni  prebaited  until  Because  Grid  Ladner  Ladner  out  manipulation until  M.  Pre-experimental  grids,  established  of  plot  shortage be  allowed  at  Ladner,  a  Grid 2  such  G r i d E.  traps, I could  used  has  Ladner but  of  as me  a  to  are  E,"  given  trap  Data  expressed  I used  period  an  data  for  number  from  Ladner  minimum  currently  the as  as  a  However,  unpublished  i n f o r m a t i o n i s not Therefore,  establish  control.  use  "Grid  not  number  available of  indicator  mice of  the  43  population  size.  variables,  such  three  as  the  grids are  M.  2  high  The  winter.  ponds  the  up  winter.  to 5 inches  exclusively  in  the  portions of the M.  and  To  this  Ladner  Grid  ditches October. in  the  of  to the  grid  drier,  during  the  summer  drainage  growing  The of a l l  and  and  Ladner  2  contained  oregoni  also  dry  and  during  very  i t  of  I  M.  similar  was was  Grid  few  led  to  oregoni  in  both  2.  more a b u n d a n t  the to  the  would  on  almost  raised,  i n order  improved  wet, during  captured  the  and  grids  fairly  flat  only  drier  hypothesis survive  the  drainage  of  expand  i t s  plot.  make t h e  The  two  to Ladner  ground  i f  of  E.  contained  M.  the  oregoni  hypothesis see  1  Grid  more  2 plot,  winter.  To  was  vicinity  r e g u i r e s dry  utilization  of  that  oregoni  2  1  both  hummocky  E  Ladner Grid  test  Grid  Ladner of  t h a t M.  plot,  for Grid  Grid  species composition  observations  average  deep c o v e r i n g l a r g e a r e a s  Grid  1  better drained  that  a running  random  experimentation,  L a d n e r G r i d 2 was  Krebs s and  Ladner  1 was  of  4.  topography  much o f  The  after  populations  Ladner G r i d  topography  and  i n Figure  differed. ,  the  used  density, while  low  townsendii.  with  I  effects  population size  evident.  at  the  the  d i f f e r e n c e s between Ladner  relatively H*  of  before  shown  were  oregoni  weather,  both  Several Grid  as  minimize  indicator  populations, these  To  I  dug of  patterns  season,  so  a  1974, were n o t  no  other  series  of  completing altered  drainage them  until  by  late  v a r i a b l e s (such  as  44  vegetation)  should  have  been  altered  by  the  habitat  manipulation. The  absence  puzzling. the by  of  Therefore,  from  the hypothesis  Ladner G r i d  means o f an i n t r o d u c t i o n e x p e r i m e n t . and October  released  into  obtained  from  1 0 , 1 9 7 4 , 85 i n d i v i d u a l fl. t o w n s e n d i i  were  Ladner  located  approximately  Ladner,  each  comparing  same  the  To t e s t  plots,  of  vegetation  eleven  A i r Base.  Composition contained  grid  mainly  contained  only  level influenced species  hypothesis,  R i v e r , were compared  similar  grid  t h a t water  this  Serpentine  of Ladner  drier  were  A i r Base and  Species  wetter  were measured  The r e l i a b i l i t y the results Points depth  surface.  east  released animals  at  measurements of water Ladner  and  with the species  three  at  composition  plot.  plots.  water  Ladner  Determining  suggesting  Water l e v e l s  years.  The  i s an a r e a  the  of fourteen  Serpentine  1.  10 m i l e s  while  composition. levels  River  Factors  M. o r e g o n i ,  Grid  two l o c a l i t i e s :  M. t o w n s e n d i i  was t e s t e d 28,  Serpentine  At  of  Between August  River.  2.2.  1 was  t h a t some a t t r i b u t e  h a b i t a t made i t u n s u i t a b l e f o r i f . t o w n s e n d i i  1974,  at  M. t o w n s e n d i i  of the technigue  a t Ladner  regarded  measurement The  (Fig. 9).  results  on F e b u a r y  as of  c a n be e x a m i n e d  G r i d 1 f o r t h e two 1  three  inches  level  below  given  the  are essentially  seems t o have been  by  separate  'dry are arbitrarily  o f t h e two y e a r s  The water  25, 1975, f o r a l l  a  soil the  slightly  45  Figure  9.  Contours of water l e v e l s at Ladner G r i d at o n e - i n c h i n t e r v a l s .  1.  Contours  Key:  Dry Sites; Raised, no contours plotted  Wet Sites; Saturated Soil  Very  Wet Sites; Standing Water \ or deeper  inch  deep  45A  46  higher and  in  pools  1975,  each  on  the  as  of  trapping  analysis  the  was  deviation  of  the  abundance.  This  significant  (to  one  between the  0.795).  appears  an  new  index  of  mice d i v i d e d by  was  in  effect.  A  water  mean  water  level,  analysis 0.01  level,  and  the  indicates  level)  abundances of  the  and  (as  a  vole  measure  analysis  reveals  M.  oregoni  M.  townsendii of  the  i s  distribution  of  substrate.  other  conclusions  affected  79  abundance  competition  is  68  abundance and  combination The  that  an  are,  cent  can  be  of  level  the  (r  =  the  being  28 the  regression  variation variation  accounted  therefore,  by  negative  for  by  of of a  factors. that  interspecific  factor limiting  Ladner, but  mediated  of  most  for only  Multiple  cent  per  important  voles at  and  per  three  water  two.  vole  of t o p o l o g i c a l  variation, the  a  species  the  f a c t o r of  D.B.C  the  was  per  observed  the  standard  that  correlation two  was  i n d i c e s of  d e v i a t i o n accounted  more s i g n i f i c a n t  among  number  the  i t s standard  of  be  canonical  the  heterogeneity) cent  the  BMD:06M r o u t i n e a t  level  to  abundance  (using the  the  Hater  technique  hummocks  q u a n t i t a t i v e comparison  plot,  program  on  same p a t t e r n o f  day.  number o f  done  Computing Centre)  and  the  simple  same  species  calculated days the  My  precise to allow  measured For  overall,  i s perceived.  sufficiently grids  hut  the  that  abundance  this  relative  and  competition  wetness of  the  47  2.3. The  d r a i n a g e - a l t e r i n g experiment  failure. seen  Water l e v e l s  on  the  before  the  control  correlated A similar Grid  2  drop by  tolerance oregoni.  oregoni  by  could  moderate  M.  as can  to near  drop  be  zero  population in  townsendii  a  ( F i g . 10) .  oregoni  numbers  population.  population at  r a t e of  as  of  in  Ladner  the  experiment  townsendii of  on  this should  populations  of  (Cuterebra  sp.)  and  to  botfly from  have  M.  of  influence in a  infestation  which  to respond could of  be M.  greater than  does  townsendii  was  lower,  control are under  to  and  due  the  the  by  grid.  drainage  to a  townsendii  i s paralleled  repeated  September  decrease.  response  the  been  p a r a s i t i s m seemed  experiment be  August  appears  1,  hypothesized  Grid  have  been a m e d i a t i n g  If. o r e g o n i  apparent  could  r a t e of b o t f l y  r a t e s of  botfly  oregoni  increasing  have  Ladner G r i d  failure  experiment  M.  similar  M.  from  observed  The  results  of  a high  factors.  H.  levels plunged  townsendii  p o p u l a t i o n did not  experiment  in  M.  numbers  or  At  the  2 to  The  a  a s Jf. t o w n s e n d i i  to  The  oregoni  i n c r e a s e i n the  t h a t was  competition,  11*  water  began.  interference  Botflies  absent,  M.  suffered  in  townsendii,  1974.  of  lowered,  2 was  observed.  parasitism  H*  plot  w i t h an  was  caused  experiment  increase i n the  The  B.  population of  the  at Ladner G r i d  were s u b s t a n t i a l l y  i n computer-drawn contours  However, just  D r a i n a g e - a l t e r i n g Experiment  number at  a similar  of  Ladner increase  Therefore,  inconclusive. better conditions.  the This  Figure  10.  C o n t o u r s o f w a t e r l e v e l s a t L a d n e r G r i d 2: at one-inch i n t e r v a l s . Key as i n F i g . 9 .  Contours  1974 Before Drainage  1975 After Drainage  49  2.4. The Grid  Introduction  results  1 are  of  presented  the  i f  weeks a f t e r  their release.  rate or  of  they  all,  30  of  themselves was  a  was  not  success due  One  85  i n the  to  a deficiency  was  ii*  townsendii.  Ii*  townsendii  September  remaining  50  50  new  its  animals i n the  suggests  of  communication the  preceding of  that  these  Grid  these born  only  s e l e c t i o n of  1  other  between  5 appeared  The  with  55  animals;  capture  the  to  capture  of of  months. animals H.  f a c t o r such  settling  unmarked  to resident  from  surrounding  townsendii  been r e s i d i n g i n i t .  physical  In  introduction  animals,  more a t t r a c t i v e a f t e r o t h e r had  females  townsendii  new,  immigrants.  dispersing  some s o c i a l or  Of  twelve  the  established  M.  of  i n s i x months c o n t r a s t s  1 field  species  indicative  for  must h a v e been  in  introduction  the  Ladner  might have been  immigration  own  appearance  1975.  four  habitat.  in  the  Ladner Grid  of  captured  that  fields  absence  were  March  deemed  animals.  experimental  to released  and  found  (35.3%)  addition  juveniles  The  sudden  were  River  In  1974,  animals  was  animals  i n the  Ladner  plot at least  phenomenon f o l l o w i n g  the  be  no  Serpentine  the  at  between m a l e s and  Thus, the  showed t h a t  unsuspected  experiment  and  Animals  difference  released  plot.  and  No  experiment  i n the  establishment  animals  the  VIII.  were c a p t u r e d  successful  between Ladner  introduction  i n Table  successful  Experiment  as  members  Perhaps t h i s  the of i s  pheromone-mediated  a l t e r a t i o n of p l a c e s by  found  habitat  dispersing  important voles.  50  -  1 —  Table Hates of  a.  VIII  s u c c e s s f u l establishment of introduced t o w n s e n d i i a t L a d n e r G r i d 1.  Origin  Sex  Serpentine  Hales Females  29 40  10 14  Ladner  Males Females  10 6  3 4  83  30  # Released  Totals Releases Successes r— T ' 1 ' 39 1 13 | I r4 1 46 1 17 I I i,.„ i. . , i  Hales Females  16  1 f~  Serpentine  1 ~i  69  !  1 i  4.  #  Successes  X*  =  .058  X  •=•  .282  ,  - . .,...  i —  Ladner  7  j  1  23  2  | 1  i  i. .  2.5. The was  p o p u l a t i o n of  subjected  population itself. trapping  of  M.  Although data,  Goose L a k e  Control  M.  at Goose Lake C o n t r o l  oregoni  to a natural,  of  heterogeneity only  i  I of  townsendii  unplanned, experiment spontaneously  I  was  collecting  had  not  been  the  utilization  Apparently,  but  the  plot.  the  when  standard  guantifying  i n v a s i o n of  the M.  a  established live-  environmental  H e n c e , c o m p a r i s o n s c o u l d be  patterns of  plot  made  species. townsendii  had  an  effect  51  similar  to  portion  of  invasion, used  that the  and  species  the  at  To  the  wetness of first  were  grouped  done  to  z  25  of  sguares  means o f  calculated f o r the  treatment)  value. the  of  some  the  two  Ladner patterns, the  I  did  utilization  and  a vole  the  other  species  trapping  with  to the  by  number o f  row-sum  hypothesis  two  distributions.  with  was  prior i s that  to no  the  the  of  2  freedom.  calculation  the  exists  or  less  results  are  tested the  null  The  .01  for  of  difference of  X  chi-sguare  captures  A probability  significant.  sguare  table;  (no  was  increasing  degrees  zero  This  f o r each  contingency  compared  stations  t r a p s each.  reliability  appropriate  statistically  the  four  2 X 25  null  the  of  were d i s c a r d e d  i n Table  at  mouse c a p t u r e s  The  considered  For  a  and  sguares i n which the  presented  analysis,  Then, the  by  species  substrate.  values.  was  between was  into  of  the  in later analysis.  compare  patterns  two  interaction  within plots,  statistical  statistic  X  type  the  The  after  Nevertheless,  Patterns  to  improve  were compared  either  one  utilization the  of  plot.  s h o w up  vole species  relative  Any  the  changed  habitat utilization  analyses,.  the  value  of  Utilization  relative  compare  the  b y ffl. o r e g o n i  localities  Habitat  of  expected  utilized  introduction.  coexisting populations  two  p a t t e r n s between  In  experimental  portions  assess  types  the  d i f f e r e n c e s between the  2.6.  two  plot  different  significant  of  IX.  second  type  of  analysis, I  52  Table IX Statistical  comparisons  of m i c r o t i n e h a b i t a t  u t i l i z a t i o n patterns. STATISTICS  TREATMENTS Ladner G r i d  df  1:  M. oregoni control winter  with  M. o r e g o n i summer  24.50  24  M. o r e g o n i control winter  with  M. o r e g o n i e x p t ' l winter  75.67  24  with  M. t o w n s e n d i i — expt'l winter  286.4 .  24  19.81  19  79.82  21  38.63  24  45.99  24  78.43  24  **  24  **  •  M. o r e g o n i e x p t ' l winter Ladner G r i d  2:  M. o r e g o n i control winter  with  M. o r e g o n i control winter  with  M. t o w n s e n d i i control winter  with  M . towns e n d i i control winter  with  M. o r e g o n i summer M. t o w n s e n d i i — control winter M. t o w n s e n d i i — summer M. t o w n s e n d i i — e x p t ' l winter  Goose L a k e : M. oregoni with before i n v a s i o n  M. o r e g o n i after invasion  M. o r e g o n i after invasion  M. townsendii — , : :— after invasion  ** *  ,,-4-u  with  108.6  P r o b a b i l i t y of independence l e s s  than  P r o b a b i l i t y of independence between  0.001  0.01  and  0.001  53  hypothesis that the wetness of the three wet,  substrate.  categories and  above  very  the  half  measured  table, inch or  of c a p t u r e s  traps  the  each  The  expected  The  results  relatively during  wet  season,  oregoni  the  second  wet  time  of H.  season  utilization before  the  sites  were  inch  below  the  of  The  the  one-  numbers  p r o p o r t i o n of  total  number  observed  squared  of these  and  divided  w i t h two  i n Table  of  number  results  of by  (X )  was  degrees  of  2  X.  introduction  experiment  are  Before  the  introduction  and  oregoni  a b u n d a n c e sho»s a  weak  sites.  M.  In  the  townsendii  oregoni  trap sites.  i s strongly correlated The  wet  following  abundance i s c o r r e l a t e d  season,  with dry  M.  trap  introduction  correlated  the  season,  with dry  M.  the  presented  of  correlation  After  are  straightforward.  the  between  chi-sguare s t a t i s t i c  results  as  the  n u m b e r was  inch  summed.  times  sum  one-half  winter.  calculated  The  dry,  s t a n d i n g water  were t h e n  category  number. ,  measurements:  table,  had  the into  one-half  sites  were  expected  with the  The  water  difference  the  level  of  classified  were more t h a n  and  wet  independent  were  d u r i n g most of t h e  values  in  freedom.  winter  very  deeper  and  compared  sites  was  sites  water  f o r each category  captures. captures  on  i n c h above  and  Expected the  Dry  frequency Trap  based  wet.  between o n e - h a l f water  capture  and  abundance  M.  introduction  and  very  oregoni  sites.  during i s  the  strongly  Hicrotus townsendii  w i t h wet  pattern of  w i t h wet  dry  at  wet  sites.  during  the  o f fl. t o w n s e n d i i d o e s  this  wet not  54  Table Independence o f m i c r o t i n e  c a p t u r e s and s u b s t r a t e wetness,  Ladner G r i d 1 M. o r e g o n i . f i r s t w i n t e r : Observed Expected Pond 36 49.0 Vet 153 167.3 Dry 219 191.7 X = Prob. = 2  X  Ma,  o r e g o n i , summer: Observed Expected Pond 25 37.3 Vet 153 127.5 Dry 133 146.2  8.52 0.014  M.  o r e g o n i , second w i n t e r : Observed Expected . Pond 13 39.0 Vet 103 133.3 Dry 209 152.7  X = 10.35 Prob. = 0.0058 2  M.  t o w n s e n d i i , second w i n t e r : Observed Expected Pond 63 39.2 Vet 167 134.1 Dry 97 153.7  X = 44.91 P r o b . .= 0.00001 2  Ladner G r i d  X = 43.38 P r o b . = 0.00001 2  2  M.  oregoni. f i r s t winter: Observed Expected Pond 33 35.3 Vet 26 31.7 Dry 13 5.0  M.  o r e g o n i , summer: Observed Expected Pond 24 27.9 Vet 26 25.1 Dry 7 4.0 X" = Prob. =  X = 13.97 P r o b . = 0.0012 2  M. . townsendii f i r s t winter Observed Expected Pond 63 69.6 Vet 71 62.5 Dry 8 9.9 X = Prob. = 2  2.16 0.34  M„  t o w n s e n d i i , second w i n t e r : Observed Expected Pond 60 51.2 Vet 392 394.2 Dry 60 66.6 X = Prob. = 2  2.17 0.34  2.86 0.24  M.  t o w n s e n d i i , summer: Observed Expected Pond 52 46.1 Vet 39 . 41 .4 Dry 3 6.6 = Prob. =  2.85 0.24  55  differ  significantly  difference wet  i n the  seasons before  the  during  i s  M.  of  occupied  most o f t h e  M.  occupied  presence to  be  of  those  of  plot;  change  from  observed  Grid  dry  trap sites  season, expand agree  no  out  oregoni  in  i s  M.  the  highly  oregoni  12)  i s that  and  the  changes i n  introduction,  M.  the i n t r o d u c t i o n ,  ground,  oregoni however,  apparently, i t i s the  the  wetter  the  substrate  oregoni. the  as  habitat-altering  striking  nor  nor  as  experiment.  clear-cut  summer.  the  water  1,  M.  However, utilized  level  oregoni  was  are  townsendii  of  or  drier  the  plot  significant  portion  of  the  changed.  wet  i s evident. plot  a  at  abundance i s c o r r e l a t e d  only during the  the  as  f o r wetter  does i t s u t i l i z a t i o n  between the  across  experiment  Microtus  preference  preference  with the  and  profound  the  after  drier  to  Ladner  the  1.  time,  after  drier  produced  significant  and  with  results  not  winter  was  ( F i g u r e s 11  introduction  a t any  before  not  2 are  show any  expansion  at  of  between  The  coexisted.  townsendii t h a t causes  the  substrate  they  Before  the  results  does not  Grid  only M.  Ladner Grid  did  oregoni.  M.  season.  introduction  difference  the time  u n s u i t a b l e f o r H. The  plot  the  townsendii  behavior  it  after  general conclusion of  dry  and  a.  presence  the  p a t t e r n s of  as  The  that during  utilization  significant, townsendii  from  i n the  pre-experimental  season. Microtus dry  season.  results  at  During oregoni These Ladner  56  Figure  11.  C o n t o u r r e p r e s e n t a t i o n s o f v o l e abundance a t L a d n e r G r i d 1. Contours r e p r e s e n t p r o b a b i l i t y of capture.  K E Y :  P = Probability 0.0 < P <  0.2  0.2 < P < 0.4 0.4 < P < 0.6  0.6 < P < 0.8 0.8 < P < 1.0  of  Capture  56A  M. oregoni  M. oregoni Control  M.  Winter  oregoni  Experimental  Winter  Summer  M.  townsendii  Experimental  Winter  Figure  12A.  C o n t o u r r e p r e s e n t a t i o n s o f M. t o w n s e n d i i a b u n d a n c e Ladner Grid 2 . Contours represent p r o b a b i l i t y of capture. Key as f o r F i g . 1 1 .  57A  Control  Winter  Experimental  Winter  58  F i g u r e 12B. C o n t o u r r e p r e s e n t a t i o n s o f M . o r e g o n i abundance a t L a d n e r G r i d 2. C o n t o u r s r e p r e s e n t p r o b a b i l i t y o f capture. K e y as f o r F i g . 11.  Control  Winter  Summer  59  2.7. Using  the  experiment  townsendii  M.  oregoni  vole  results  (LG1)  M.  Competition  and  (GLC),  oregoni  in  directly  measures  The estimate  most  size  M.  (extending recruits  oregoni  problem  population winter.  A linear  and i s used  significant  one  function  of  This  the  procedure  coefficient  i n  this  oregoni Instead  of  procedure  would  and  during  the  15 u n t i l 15) was  i s to  have  of using  non-breeding  control,  fitted  estimates  to of  equation  a  been  control  seasons;  the second  first  the  was t h e  population  non-reproductive  the  season  appearance  of  a function of the population  the  points  M. o r e g o n i fits  i n preference  data  logarithmically  of  15.  was  Untransformed  a  I a l s o assumed t h a t t h e  April  a t November A curve  a  November  around  only  the depression i n  competition  not appeared.  treatment.  from  of  oregoni.  considered  of  as  invasion  effect  H. t o w n s e n d i i . ,  I compared two c o n s e c u t i v e was  introduction  containing  the numerical  w h a t t h e p o p u l a t i o n o f M.  experimental  well,  of  difficult  Kj. t o w n s e n d i i  first  size  previously  Specifically,  the  o n H.  planned  spontaneous  was e s t i m a t e d  numbers  townsendii  plot,  plot  I estimated  increase  had  a  the  the  s p e c i e s on t h e o t h e r .  n u m b e r s o f M.  M.  from  from  into  Coefficient  during  the p o i n t s from to higher  f i t  linear  transformed  data.  because i t suggests  representing  order  equations This  a constant  the  the  control  both  plots  polynomials. better  than  observation mortality  i s  rather  60  than  a constant  rate of  Hence, two  parameters appear s u f f i c i e n t  population level The  on  during  0.1191  mice  decreased The  the  especially Myers  day,  Ten  i n the  i f the  cyclic  were  XI).  one  localities.  At  !•  on  i s not  The  population  noticeably  .574  another  smaller  at  rate  the  of  Control  day. are  important  that  the  i n d i c a t o r of  that  the  weaker changes  ±  at  mortality  of  .143  at  and  are  in  eight  at  and  competition  L a d n e r and  differently  Character M.  at  while  different  from  -.116  the  two  the  depressant  i s apparent,  two,  Krebs  rodents  the  two  the  which  Ladner  data,  a significant  oregoni  of  decrease.  I t appears, therefore, that  significantly  2.8.  r a t e of  a l l microtine  these  at  Ladner M.  populaton  method  the  obtained  estimated  affect  is  From  Goose L a k e .  the  the  Goose Lake  s p r i n g , and  to  estimates  ±  effect  at  this  population  have a s c r i b e d  (Table  the  predict  same f u n c t i o n a l p a t t e r n i n t h e  assumption  was  the  until  second  townsendii  that  i n mid-November i s an  coefficient  species  of  1 decreased  of  species.  at  slope  to  season:  0.0866 mice per  for this  Goose Lake  .180  the  while  r a t e of  population  (1974)  operation  and  Ladner G r i d  operated The  non-breeding  assumptions  size  expected  years.  at  major  factors  15,  per  at  population the  the  November  population  mortality.  two  effect  at Goose  of  Lake  zero.  Displacement  oregoni  individuals  at than  Ladner other  consists populations  of of  61  Table XIA E s t i m a t i o n of c o m p e t i t i o n c o e f f i c i e n t s at  Ladner.  CONTROL SERIES Trap P e r i o d  Number o f d a y s  4 , 5 6  T  8 9 10 11 12  .  Predictive  1  equation:  5  M. o r e g o n i p o p u l a t i o n  6 20 34 75 96 110 125 138 4  75 76 69 66 66 70 59 63 53  E x p e c t e d pop. = I n i t i a l  pop. -  0.1191(days)  EXPERIMENTAL SERIES T.P.  Days  M.  24 25 26 27 28 29 30 31 32 33 34  -2 13 27 49 58 91 98 104 110 117 125  • 73 68 65 51 41 37 41 41 40 37 32  o.  Mean a l p h a = 0 . 5 7 4  M. t .  19 19 22 26 26 26 34 40 41 49 55  A J J e u  ocu  o. 73.00 71 .21 69.54 66.92 65.85 61.92 61 . 0 9 60.37 59.66 58.83 57.87 M.  95$ c o n f . = I 0.143  Alpha 0.537 0.206 0.61 2 . 0.956 0.958 0.591 0.484 0.480 0.446 0.470  62  Table XIB E s t i m a t i o n of competition c o e f f i c i e n t s  at  Goose L a k e ,  CONTROL SERIES Number o f  Trap P e r i o d  oregoni  equation:  Expected pop.  population  31 32 31 32 31 29 29 28 28 24 25 23 23 23 22 19  3 17 . 47 59 66 73 80 88 94 100 108 117 130 136 145 1 58  15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 Predictive  M.  days  = Initial  pop.  -  0.0866(days)  EXPERIMENTAL SERIES T.P.  Days  43 44 45 46 47 48 49 50 51  1 13 29 65 87 106 122 136 150  .  Mean a l p h a =  M.  o.  34 33 32 30 28 25 22 22 22 -0.116  M.  t.  0 1 2 3 7 5 6 12 10 95% c o n f .  Expected • M . o. 34.00 32.96 31 . 5 8 27.46 26.56 24.91 23.53 22.31 21 . 1 0 = ±  0.180  Alpha  -0.040 -0.210 -0.647 -0.206 -0.018 0.255 0.026 -0.090  63  !•  oregoni sergens that  weights that  of individuals  the  M.  oregoni  significantly This  i s  (November  juveniles  does  oregoni  at different from  lighter  difference  months  M.  I have s t u d i e d .  t o March) not  per  noticeable  the  cent  the shows  consistently  data.  and  (Fig. 13).  in  the  when t h e i n g r e s s o f  affect  20  were  are  those from Goose Lake  especially  of  times of the year  Ladner  than  Comparison  winter  light-weight  Overall,  smaller  than  Ladner  those at  Goose  Lake., To  demonstrate  attributable demonstrate of  the  If  show  same  between  small  size  Ladner  of Ladner  genetically, Body  body  rather  weights  periodically.  and M.  or  responsible  the  number  of  plausible  as l i k e l y M.  i s  s e , one  presence  must  absence for  the  possible be  proven  alternative  effectors.  o r e g o n i were c a u s e d  or other edaphic f a c t o r s ,  t h e same f i e l d s  weight  p_er  c o m p e t i t i o n cannot  of Ladner  soil,  townsendii  differ  not  Since  eliminated  vegetation,  the  are  H o w e v e r , t h e most  size  differential  displacement  unlimited,  townsendii inhabiting  Microtus  be  be  the smaller  climate,  to  change.  the e f f e c t o r .  weight  other than the  species  i s nearly  explanations can  M.  factors  weight  hypotheses be  that  this  character  competing  observed  to  to  that  trends. weights  Such did  Goose Lake  would  be  i s not  not  by  then  expected the  case.  significantly  populations.  Thus,  the  oregoni serpens i s hypothesized to  than environmentally, induced.  of  Seasonal  microtines changes  are such  known as  to  change  are described  by  Figure  13.  C o m p a r i s o n o f body w e i g h t s b e t w e e n two p o p u l a t i o n s o f M. o r e g o n i . Mean and 95% c o n f i d e n c e l i m i t s o f v o l e s c a p t u r e d a t Goose L a k e C o n t r o l p l o t and L a d n e r G r i d s 1 and 2 a r e p l o t t e d .  64A  FEMALES 30  CO  E  n=  T  75  5 8  1 9  2 8  57  2§ -  81  32  38  42  51  51  66  28  54  24  73  82 83 64 21 19 90 94 48  * \I  CD  20-  +  15-f  |  l  l  I  CD  "CD  10  f  H  J  F  1  1  M  A  H  1-  M  J  -  J  *  \  *  H  A  S  O  N  D  MALES n= 51  30 T  CO  66 41  20 T  20  2 4  2 2  f  t  f t  sz "CD  48  8 8  25  £ 03  CO  5 2  15 --  10  H  J  33 2 9  39 2 2  25  34 51 11 84  20  H  M  A  M  J  •jj- Goose Lake Population •J Ladner  65 54  22  t i l l I* * *  h  F  65 65  Population  J  A  h  S  H  O  N  1  D  65  Iverson for  and  in  Turner  the  associated 1952;  and  showed  with  yet  peak  clear  Ladner  The  the  Ladner  density  and  weight Goose  and  Goose  had  body  Lake of  townsendii  i s common a n d  it  i s rare  spotty  in  2.9. Predation in  can  a community  Cramer  and  others).  1972;  this  either  take  a  species  than  of  enough  for  a l l  resources During of  raptors  the  the  1961;  difference oregoni  be  relative  At  while  at  to  between  the  townsendii.  Paine,  fioughgarden  or  species  Ladner,  Goose  Lake  prey  to  have  Feldman,  a  1969;  1975;  predators  proportion  biomass  species  Harper,  occur,  greater  the  1966;  and  to  coexisting  must be  surplus  and must  of  some  reduced  of a v a i l a b l e  1974).  winter  foraging  to  number o f  significantly  (Addicott,  population,  Predators  phenomenon  others,  Lake  M.  ubiquitous,  Hole of  (Slobodkin,  For  a consistently  distribution.  increase  Hay,  M.  population  weights.  of  appears  abundance  had  Krebs,  population  in  reasonable  difference  usually (Chitty,  neither  changes  population  and  are  C h i t t y , 1962;  earlier,  lower  accounted  microtines  than the  M.  and  of  cyclic  consistent  account f o r the  distribution  of  were  weights  C h i t t y and  mentioned  population  only  Ladner  1962; As  Also,  body  populations  evidence  population  the  Higher  1957,  others).  abundance. higher  were o b s e r v e d , and  analysis.  Kalela,  1964;  (1974)  of at  1973-74, I o b s e r v e d Ladner.  The  high  numbers  most numerous o f  these  66  were Marsh Snowy, these ii«  Owls.  During  r a p t o r s preyed  townsendii  Westham M.  Hawks, R o u g h - l e g g e d  Island,  oregoni  (C.  J.  i s the  pers.  M.  oregoni  that  preferential M.  from  oregoni  described Short-eared  lines,  and  rostra. on  the  the  b a s i s of  as I  cent, the JJ.  the  the  virtual exclusion  i t occurs  the  the  the  of  molar be  67  the  (Hall  tooth  hypothesis at  i s  Ladner  139,  rate  can  pellets easily the  and  null  M. of  townsendii  per  of  1959). fallen  i n the  skulls  or  bone. 91  per  occurred cent a in  in  were  binomial a  one-  t h a t r a p t o r s do  m u s t be  non-selective  side  t e e t h have  significant  hypothesis  separated  lingual  61,  the  counting  Kelson,  which  87  of  by  be  samples approximate  d i f f e r e n c e i s not  on  the  (mainly  vicinity  r i d g e s remain  or  snap-trap  raptor pellets  160 - M i c r o t u s  Assuming the  high  of  River  standardized  r i d g e s on  Microtus, of  Hence, the  a  on  more s u c c e s s f u l  townsendii  made e v e n w h e n t h e  tailed  If  of  species  number o f  which  prey  be  immediate  contents  Microtus  this  preferentially  of  the  from  distribution, test.  M.  setting  townsendii.  townsendii.  on  habitat  Serpentine  Hence, the  Owls)  snap-strapped  pellets,  at  collected  can  M.  habitat  earlier, I  i m p r i n t of  were  to  Since  coexist with i t .  upper  Identification out,  similar  with  T h e s e two  that  in  species  predation to  hypothesis  only  comm.), i t a p p e a r s t o  analyzed  third  tested the  in this habitat.  Simultaneous lines  I  and  on fl. t o w n s e n d i i .  similar  than  allows  time  Owls,  preferentially  and  in  Krebs,  this  Hawks, S h o r t - e a r e d  not  accepted.  predation  were  to  67  account would  f o r the coexistence be  necessary  population  below  limitation. No  to  I  show  an  i n  survivorship  at Ladner  Lake.  the raptors at  also,  being  absent  Finally, 1971)  indicates  periods  populations  at  work  predators and low  Ladner  winter  high  by  raptors  coexistence 3.  does  The H.  Road  oregoni  species  and  the  plot  C.  gapperi  occurred  regularly. which  most l i m i t i n g  during  was  i n the p l o t ,  Clethrionomys  to  and  s p r i n g . , The  seasonal  skewed  ratio  sex  only  on  for  the  study  of  oregoni  for  H.  the  Although  oregoni  individuals,  again  appearance of  suggest  M.  sympatry.  gapperi  disappeared  the  predation  account  or  Plot  were i m m a t u r e m a l e s , a p p e a r e d fall  vole  density during  Therefore,  intended i n  The  Ladner.  Road K  K  season. 1966,  S e a s o n a l i t y o f o v e r l a p between a n d C. g a p p e r i 3.1.  breeding  i n c r e a s i n g i n numbers  appear at  seasonal,  abundances.  numbers.  o f t h e two s p e c i e s  Goose  1964,  population  not  between  mainly  (Pearson,  are  were e i t h e r  raptor  the  resource  predator-free  vole  prey  high  limits  discernable  were  predator-vole  a relatively  voles  was  the  that  i t  n o r do I t h i n k i t l i k e l y .  Ladner  most  maintaining of  predation  then  determined  of  of decreasing  species,  and a t r e l a t i v e l y  during  detailed  that  show t h i s ,  populations :  two  otherwise  cannot  difference  of these  was  the  late  captured  nearly a l l of  a t t h e Road in  both  K  plot  winter  species  t h a t t h e Road K p l o t  or plus  i s  i n  early the  marginal  68  habitat of  a  s e r v i n g as a d i s p e r s a l population  oregoni, size  throughout reason The  trapping  results  western the  lateral than  with may  and  western  which layers,  leaf  melanization, The  proportions gapperi  the  result  same  were  (RDK)  plot  Douglas-fir Since  showed  soils  held  Road  forests  together  K  mats o f  by f u n g a l h y p h a e ,  Both  no  a n d 3)  other  i n almost  egual  oregoni  and  more f r e g u e n t l y on t h e f i r s t  type  of s o i l .  a preference  swordfern  had t h i c k  occurred  plot.  categories:  with .  swordfern;  of s o i l  tree.  some m e l a n i z a t i o n o f  which  This  f o r two s p e c i e s  (Hawes,  the  snap-trapping  three  associated  2)  captured  obtained  of  i s a f f e c t e d by  t h e dominant s p e c i e s of  associated  type  predominates  vegetation  two t y p e s the  of  Snap-  conditions or sub-story  were  no  plot i s  correlated  and  litter  first  showed  this  hemlock.  vegetation  gapperi  friable,  and  i n  in  western  forests.  were  on t h e s e c o n d  species  i s  were d i v i d e d i n t o  m u n i turn) ;  undecomposed  population  abundance i s  than  i n the plot  Microtus  H o w e v e r , i n many a r e a s  hemlock  soil  members  ( F i g . 5).  understory  soils,  (J?°AXstichum  than  tree  a l s o r e v e a l t h a t C.  soils  soils.  stable  low,  predominated  hemlock f o r e s t s .  acidic  mineral  a  s h o w t h a t C. g a p j a e r i u s u a l l y  be m o r e i m p o r t a n t  1)  habitat.  s e e p a g e a n d i s more t y p i c a l  Soils  C.  oregoni  dominant  soil  of  results  M.  f o r superfluous  another  most o f t h e s t u d y  obscure.  plot  i n  i n c o n t r a s t , maintained  The  in  residing  sink  of  result  contrasts  shrews  for a different  1974).,  M.  soil  where type  with each  on t h e  69  4.  Limitations  P l a c e d on M.  4.1. The excluded  by  M.  removal of  M.  When  plot  the  occupy  the  along  a  predicts be  oregoni  removal of  by  into  the  the  the  plot  November  28,  10 as  the  few  oregoni  that  M.  dispersers  The  then  the  July  tapered  captured  significant  Jj.  to  only  hypothesis will  longicaudus  forests  were  24,  for  removed  1973.  By  off, indicating  that  been until  numbers o f  removed  M.  and  removed.  Very  O c t o b e r 3,  1974.  oregoni, fields,  probably began  to  November  28,  traps.  the  1973,  found  population  animals  p o p u l a t i o n had  compared  14,  and  Although  the  in  absence  the  removals  of  was  oregoni  The  successfully  beginning  were  plot.  area.  captured  distribution  1973,  of  ( F i g . 14).  from surrounding  i n the  oregoni  months  oregoni  date,  M.  p o p u l a t i o n was  1973,  most o f  the  M.  was  a stream.  expansion  vacated  experimental  I found  to  approximately  appear  trapped,  adjacent  the  actively  Goose Lake E x p e r i m e n t a l  narrow s t r i p  T h e fl. o r e g o n i  After  first  being  the  longicaudus  population  M.  the  by  w h i l e M.  followed  from  tested  was  plot  that  by  Experiment  longicaudus  was  from  was  entire  M.  that  oregoni  longicaudus  oregoni  Goose Lake  hypothesis  M.  with  October  (experimental) of  M.  M.  longicaudus distribution  3,  1974,  between  inclusive  distribution  oregoni  until  appears  of  M.  t o be  December ( F i g . 15).  longicaudus expanded  in  70  J  Figure  14  D  Cumulative removal of M. oregoni Experimental p l o t .  f r o m Goose L a k e  70A  paAOiuay jeqiunN  71  F i g u r e 15. D i s t r i b u t i o n a l p a t t e r n s of M* l o n g i c a u d u s a t Goose Lake E x p e r i m e n t a l p l o t b e f o r e and a f t e r removal o f M. o r e g o n i . A.  B e f o r e removal:  J u l y 7, 1973 t o November 28, 1973.  Key o 0 captures  © 3 captures  o 1 capture  (|| 4 captures  © 2 captures  (n) n captures  71A  After  removal:  December 14,  1973  t o O c t o b e r 3,  1974.  72A 9  73  comparison of  M.  oregoni,  change H«  with the  this  i n the type  longicaudus.  essentially adjacent  (control)  apparent  both  to the stream  control  captures  i s the  were  recorded  explain  most  i s  control  over  the center of the plot;  two  juveniles,  to  did  adjacent  not r e s u l t grassy  Since fact  of  reasoning  reproductive either  o f H.  as  The many  captures this  grassy  of these  were  soon a f t e r o f M.  appears t o have  i n expansion  i s  may  longicaudus.  i n the  h o w e v e r , most  area from  molting  longicaudus  been  minimal,  of the population into  the  field. of t h i s  experiment  However,  t o expand c o u l d failure on  were n e g a t i v e , t h e  appeared i n the c o n t r o l  importance.  lpngicaudus  some i n t e r n a l  expansion  oregoni  the r e s u l t s  minimal  twice  and t h e  Thus, t h e response  t h a t n o H. l o n g i c a u d u s  of H«  pelage.  of  F i g . 2).  of  of which disappeared  t h e r e m o v a l o f M.  and  composed  were r e c o r d e d  in  subadult  with  a  immediately  a greater p o r t i o n of the year;  A number o f c a p t u r e s  into  without  distribution  of the plot  distribution,  of the observed  both  presence  experimental,  ( c o m p a r e F i g . 15  distribution  the  occurs  The  and  t o t h e edge  experimental  i n  expansion  of habitat u t i l i z e d .  restricted  than  distribution  have  the  been  i n the population.  of the i n d i v i d u a l s  established the  themselves  disappeared  from  area.  sufficiently  p r o d u c t i v e and had  failure  attributable I reject  the b a s i s of the apparent  success  plot  good  captured.  along  the  Hence,  the  access  to  of to  this  line  health  and  Juveniles stream  population the  i s  field,  or was but  74  apparently The that for  avoided basic  interference limiting  the  i t even  conclusion  i n the  reached from  b y fl. o r e g o n i ecological  a b s e n c e o f ]f.  i s not  this  directly  distribution  of  H.  oregoni.  experiment  i s  responsible longicaudus.  75  DISCISSION  1.  Definition  Competition topic  that  has l o n g  community. the  Lotka  number o f  individuals  resources.. of  ideas  similar  received (1925)  of  species  were Gause  simple  and V o l t e r r a  limited  by  eguations  was termed  and  "Gause's principle"  principle."  Roughly, t h i s  s p e c i e s c o u l d occupy the would  always  be s l i g h t l y  would e l i m i n a t e  it,  (1922),  that  order  persist.  to  Interest (1958)  or,  each  in  concept  to  the  species  of  dimensional hypervolume, this  in  must  a distinct  a natural  studies attempting to  guantify  occurring competitive  systems  community.  followed.  other  and  Grinnell niche  as  in  an N-  application  A large  niche parameters  two  Hutchinson's  niche  (1958)  no  b e c a u s e one  of  and M a c A r t h u r ' s  Lotka-  that  phraseology  ecological  these  exclusion  the  by  the  experiments  than  was reawakened  the  the  indefinitely  have  by  systems.  "competitive  more e f f i c i e n t  competition  reformulation  from  proposition states  same n i c h e  finite  showed t h a t  biological  of  given  for  competition  or the  a  others,  derived  laboratory  that  numbers  in  competition  to  a  scientific  theorized  exist  experiments,  competition  the  and t h e  ( 1 9 3 4 ) , and s u b s e q u e n t l y laboratory  organisms i s  (1927)  could  c o u l d be s u c c e s s f u l l y a p p l i e d  Volterra  of  a t t e n t i o n from  each s p e c i e s t h a t  The c o n c e p t o f  of  Competition  species that could c o e x i s t ,  environment,  use  among  of  of  number  of  naturally  76  Patterns ecological the  of  communities.,  perceived  presence  Cole  f o r even one  (1960),  and  How  ecological  to  have  from  found  among  out,  that  The  mere  species i s  were a r r a n g e d  (1960) , S l o b o d k i n  in  know, h o w e v e r ,  d i f f e r e n c e s would  pointed  be  competition?  differences  another,  can  d o e s one  i f niches  Hardin  others  utilization  pattern results  of  sufficient, respect  resource  randomly exist.  with  Thus,  (1961) , K i l l e r  Gause's  not  as  (1967),  principle  i s  demonstrated?  Two  encountered.  The  untestable. How  then  definitions first are are  i s  mechanistic, whenever  has  Darwin  (1954),  Birch  Jaeger  (1972).  states  t h a t two  any  one  and  utilize  short  The  by  Klomp other  common  supply  Clements  (1957),  or  commonly  s t a t e s t h a t two  been accepted  (1859),  d e f i n e d and  are  they  sufficiently  definition e.g.  of competition  competing in  i s competition  to  or  be  of  i n a corresponding  biologists, Park  (1967),  definition  more o r g a n i s m s c o m p e t e i f an  species results  This  (1939),  Miller  i s a functional  which  limiting.  Shelford  (1961),  species  resources  generations and  more  and which  increase  decrease  in  in the  others. The because  first inherent  competition.  in  Since  same b i o l o g i c a l latter  definition  the  i t two  i s , i s  an  object  definitions  phenomena, e i t h e r  definition,  perhaps,  proposed  will by  are  logically (resource) applied  purer of to  the the  suffice;  however,  the  MacArthur  (1972),  i s  77  preferable with  to  the  ecological  has  i t i s e a s i e r to  been shown t o  patterns  observational,  i s  distribution  of  by  to  approach  Cody's  c o m m u n i t i e s and  The  a  demonstrate  resource  first,  primarily  the  Examples  (1968)  to  regular,  within  niches).  Brown and  related  methods.  niches  of  are  two  be  demonstrate  (overdispersion  rodent  because  data.  Competition utilization  former  Leiberman's  resource of  study  non-random  of (1973)  this  base  type  grassland study  of  of bird  desert  communities.  The  second  method,  demonstrate  differences  the  or  presence  Connell rodents  absence  primarily in of  experimental,  i s  to  utilization  caused  by  resource other  (1961) f o r b a r n a c l e s  and  species. Grant  For  (1972)  for microtine  h a v e shown t h a t h a b i t a t d i f f e r e n c e s can  competitive Wilson,  interactions.  1956)  examples  of  environmental The  and this  sufficient  to  partially  answered  occurrence  of  coexisting  ecological approach  displacement  release which  result (Brown  ( F i n d l e y , 1954)  take  advantage  from and are of  heterogeneities.  guestion  experimental  Character  example,  allow  the  of  what  coexistence of by four  pair.  necessary  or  m i c r o t i n e s p e c i e s can  be  considering study  investigations  species  factors  of  the  p a t t e r n of  habitat  in  light  of  the  interactions  of  each  species the  are  78  2.  Interaction  Clethrionomys sympatry  only  Clethrionomys this did  the  sense  resource.  to  be  i t  distributional  vegetation  t h a t i t can  evergrowing food  of  and  cheek  Microtus  authors  1971;  vegetation,  rarely  of  only  44  accounting Hence,  not  species  has  been  seeds. per  cent  f o r an M.  qualitatively  In  of  additional  oregoni in  the  diet.  and  that  of  and  t o be  green diet  40 C.  per  M.  of the  oregoni  This  was  abrasive  species,  by  Olson,  gapperi  Batzli  mainly  of  90%),  and  accounts  Clethrionomys, cent  The  several  1969;  (at least  material  having  limited.  comprised  of  suggests  coarse,  reported  grasses  contrast,  gapperi  revealed  similarly  Fleharty  especially  this  study  Microtus  1973)  K-  Intensive  teeth.,  are  1965;  be  season.  amount  consume.  Hiewe,  green  the  plant  to  e n a b l i n g C.  g a p p e r i and  teeth,  (Zimmerman,  Pitelka,  forest  non-breeding  in  the  microtine species;  tactic  in than  in  found  r o o t e d cheek  limited  poorer  was  o v e r l a p b e t w e e n C.  has  habitat  herbaceous  also  in  vole  sparsest  other  hemlock-cedar  Clethripnomys is  gapperi  found  association.  poorer was  hemlock f o r e s t .  confined to the  i t  the  were  more n u m e r o u s  habitat  to the  gapperi  habitat  i n much  life-history  i n western of  hemlock  f a r the  The  contained  relation  section  that  by  C.  Jf. o r e g o n i  i t appeared  a significant  largely  was  microtines.  in  succeed  one  and  and  western  Clethripnomys  selected may  the  gapperi  other that  gapperi  in  habitat,  o f Jf. o r e g o n i a n d  (Drozdz, probably  for seeds 1967). differ  79  C. would as  g a p p e r i consumes a  be  expected  Peromyscus  have  been  Griffin,  I  and  ideal a  the the  and  Levins  one  with  York, these  two  correlated  results  such  species  not  between t h e s e does  i t  (Kirkland  did  and  show  two  a  species,  exist.  some c r i t i c a l  (1964,  that  1967)  would  generalist  Perhaps  combination  eating  from  favor  or  and  visa  Levins*  two  of  which  of  Forest.  and one  exist  I t may  determine  be  local  Peromyscus  seems  Clethrionpmys i s  both  other and  within  species  genera.  habitats  the bounds  possible  the boundary  generalist  the  The  Clethrionomys i s absent  U.B.C. R e s e a r c h r e s o u r c e base  by  on  specialist  versa.  ideas.  foods preferred  Clethrionomys predominates  replacement  have t h e o r i z e d  C l e t h r i g n o m y s , M i c r o t u s , and MacArthur  generalist,  which  by  seeds,  seed-eating rodents  antagonism  affected  conditions  to test  Habitats  my  of  Peromyscus.  over  situation  mutual  i s  MacArthur  species  although  a  ClethrionomY§  ecological  I n New  negative correlation  suspect that  Microtus  with  t o be n e g a t i v e l y  1974).  significant  t o compete  maniculatus.  shown  high proportion  to  two  of  guantify  conditions  by  in  for  specialist  species.  3. The coexist either  Interaction s p e c i e s M. more  of  the  of  M.  o r e g o n i and  o r e g o n i and  often other  and  at  M.  M.  townsendii  t o w n s e n d i i were found  higher population densities  coexisting  species  pairs.  to than  Several  80  conclusions  about  be  reached.  M.  townsendii  At  Ladner,  were  introduction  o f M.  in  of  numbers  part  the  oregoni  can  oregoni  and  apparently  caused  a  grid.  reduction the  These observations  affect  one a n o t h e r ,  of  M.  townsendii  to  flat  indicate  and,  displace  t h e b e t t e r d r a i n e d and t o p o l o g i c a l l y  hence,  Jj. oregoni  varied portion of  Ladner Grid  1 indicates that the difference i n substrate  vital  the  to  Similarly, invading H.  oregoni  townsendii  with  correlation  M.  oregoni  competitive  At  no s i g n i f i c a n t Ladner,  wetness,  was a l s o a  that  fl.  of  that  townsendii  townsendii  excludes  M.  upon t h e was  not  was,  and  presence  of  from  these  competes  with  the f l a t ,  oregoni  presumably  escape  hairs,  varied,  two-  habitats.  M.  this  interference  s h o w s some m o r p h o l o g i c a l  and  oregoni  emerges  h a b i t a t s by i n t e r f e r e n c e , then  eyes  impact  the  dimensional  species  i s  townsendii  b u t M.  function  pattern  suggests  M.  i s  interaction.  topography  by i n t e r f e r e n c e , e x c l u d i n g i t f r o m  dimensional If  had  The  the  where t h e  habitat  M. , t o w n s e n d i i . observations  of  Lake,  population.  correlated this  outcome  a t Goose M.  An  compete.  The f a i l u r e from  M.  and i t s e x c l u s i o n from  species adversely  definition,  of  two s p e c i e s  t o be n e g a t i v e l y c o r r e l a t e d .  townsendii  M.  of these  abundances  shown  of the experimental  that by  the i n t e r a c t i o n  short  that  are  fur  with  usually  by  M.  from  oregoni  burrowing.  c h a r a c t e r s , such  little  as  differentiation  associated  with  of  twocould The small guard  subterranean  81  animals. Microtus not  permit  The  observed  Ladner  oregoni easy  adaptive voles  character  response  effective  Tunnels might which  M.  rhizomes).  Microtus  are  claimed  be  local  populations,  then  burrowing  availability also,  determine  tunnels  predation.  commonly o b s e r v e d Forest. refuge  The from  competitive  small  species In which  general, two  the  provide  Ladner  an  Smaller be  more  for  M.  magnitude be  M.  were  of  limiting  quantitatively,  refuge  at  tubers,  with  food  competition.  or  O.B.C.  gregoni  turn  weasel  from  weasel  weasel s i g n  the  gregoni of  oregoni  i n Finland  interacting  a  and  in  might  would over  fl.  were  Research provide  confer  a a  townsendii.  predation  nor  i t s  estimated.  s p a c e a p p e a r s t o be  species  as  availability  or  ermiuea)  which  could  other  f o r M.  I f food  outcome of  of  advantage  than  (i.e. roots,  winter  be  tunnels  weasels,  specificity  these  at  which  should  supply  1972).  (Mustela  both  However, n e i t h e r t h e  by  could  could  Weasels  a food  qualitatively  activity  to  in  townsendii.  oecongmus p o p u l a t i o n s  (Tast,  either  size  which  townsendii,  limited  rhizomes  townsendii.  interpreted  M.  would  townsendii.  also provide  and  Eriophorum  i s  M.  gregoni,  body  burrows,  i s u n a v a i l a b l e t o M.  to  study,  presence of  smaller  at excluding  i n M.  smaller  in this  to the  dig  a  i t s burrows  much l a r g e r  displacement  have  examined  would  v o l e , and  passage f o r the  populations  populations  i s a small  the  compete., V e r t i c a l  major resource separation  i s  for a  82  necessary It  condition  f o r coexistence o f these  i s not a s u f f i c i e n t  habitats  condition,  (e.g. D o u g l a s - f i r  separation i s possible,  Possibly  M. t o w n s e n d i i  limit  i t to  available in  Sect.  4.  two  The  of  grassy  maintaining  did  into  will  be d i s c u s s e d  a n d M.  that or  further  the  not  a n d M. o r e g o n i ,  was d o n e to  and r i p a r i a n .  i n regrowth,  riparian  to  result  hypothesis  of  i n  were No  but the  habitat  was  i s  (Conley,  many h a b i t a t s r a n g i n g  This not  o f M.  that  the  from  1971).  of  result  of that  implies  important  i n  longicaudus. habitat i s itself  appears u n l i k e l y  In  oregoni  population  expansion  habitats occupied  localities.  meadows  a  habitat.  competition  longicaudus  variety  geographic  longicaudus  regrowth  adjacent  the unutilized  u n s u i t a b l e f o r M.  subalpine  characteristics  primary . p r o d u c t i v i t y  the habitat restriction  However,  other  where  i s present.  r e m o v a l o f a p o p u l a t i o n o f M.  meadow  interference  wide  some  detail.  experimental  population  work  M. l o n g i c a u d u s  longicaudus  the  regrowth),  M. o r e g o n i  longicaudus  i n two h a b i t a t s :  e x a m i n e d i n more  that  M.  or experimental  limitation  H«  high  o f M. o r e g o n i  species,  to coexist  a  of  This limitation  Interaction  intensive  from  only  i n  5.  The found  and  has b e h a v i o r a l  habitats  water.  however, because  forests  vertical  two s p e c i e s .  i n  by t h i s  New . M e x i c o  view  of  species at  i t  occupies  I n Nevada i t i s found i n  wet g r a s s y  swales  to  sagebrush  83  and  Symphoricarpus  species  desert  ( L i n s d a l e , 1938).  i s m o s t common a l o n g  also  found  i n  thickets  salt  water  marshes  (Bailey,  has  mountain  meadows t o s a g e b r u s h  that  In British  forest some  among  of  variety  the  The  other  that  the were  species.  the  (Krebs  from  i t not  for  The  to  4000  species from  i s  found  meadows  the  presence  passive,  but  on  grass  a  great  to boreal  forest  grassy  of  and  h a b i t a t a t Goose other  microtine longicaudus  M. o r e g o n i  H. l o n g i c a u d u s  of  parallel,  the to  other  i n these  o f H.  some  longicaudus possibly  voles.,  interaction some  Hence,  for  innate habitat preferences, with  meadows  utilize, indicates  Local populations  to competition  lgngicaudus  in  i n t e r f e r e n c e o f M. o r e g o n i .  resource.  findings  feet,  i t i n h a b i t s sedge and  outcompetes  t o have e v o l v e d  and  species usually occurs i n  occupy t h e grassy  be  presumably  report  variable  t h a t t h e e x c l u s i o n o f M.  by a c t i v e must  (Dalguest,  (1965)  i s extremely  of h a b i t a t s , especially  I have found  response  marshes  and H i n g a t e , H S ) .  could  exclusion  appear  islands  species  undetermined  II*  many h a b i t a t s r a n g i n g  p o p u l a t i o n s o f M. l o n g i c a u d u s  not caused  habitats  in  the  sea l e v e l  habitats, ranging  wide range  which  is  coastal  i s  Washington  The  I n t h e Yukon t h e of  In  desert t o  subspecies.  alpine tundra  Lake  i n  but  as i n f r e s h and  C o l u m b i a , Cowan a n d G u i g u e t  edge h a b i t a t s from  meadows.  to  recorded  1936).  t h e h a b i t a t o f PI. l o n g i c a u d u s  differs  mountains,  and woods a s w e l l  species  1948).  been  streams i n  I n Oregon t h e  extent,  of  H. o r e g o n i the  and  conclusions  BH  about  the  found by  M.  t h a t M.  oregoni  interference.  which  M.  If  Biparian  a  n  d  species  M.  townsendii oust  Microtus longicaudus habitats.  collected  to  M.  longicaudus  is  thin  and 5.  question  Interaction  found  i n grassy  from  this  fields  of  reproduces high  at  Ladner)  that  a  to  M.  areas  i n some,  two-  the  thin  but  soil.  not a l l ,  data  I  way  longicaudus of  relevant  i s found  fields,  habitat. that  high  were  expect  not that  o n l y where the  primary  faster  or  caused  and  unpredictable.  M.  townsendii  townsendii  M.  Since  and M.  townsendii  t o g i v e i t an  soil  and  over  a  longicaudus  great  This species nests  not  excludes  i t  shows a  number  in flat,  (as i t can  mortality, and  above  year.  advantage  heavy  the ground  of wet  townsendii  part of the  great  density independent i s  i s  Microtus  predation i s extremely  floods,  longicaudus  advantage  a longer i s  M.  possibly  productivity.  potential  where by  M.  Microtus townsendii  appear  where  of  unanswered.  reproductive  situations  habitat in  longicaudus  o f how  remains  adaptations  major  rocky. Probable  interact  longicaudus  then  hypothesis, areas  M.  similarly  the  have  soils.  from  found  I  essentially  rocky  oregoni  this  i n logged  the  interact,  also  M. The  and  although  test  with  i s  interact  M.  was  compete  found,  two  presumably  regrowth  i s  interaction.  habitat,  h a b i t a t with t h i n  the  2£S32Ili  would  does not  longicaudus  dimensional  H*  Oregoni-M. t o w n s e n d i i  such  habitat  a in be as i s  (Hilborn,  85  1971)  and  can  successfully  raise  litters  under very  wet  conditions. Microtus species. lacking  It in  glands  often  the  probably of  environment  as  and  much a s  formed mice  by  leaf  use  use  territory  the  M.  by  of  well  on  townsendii  appear of  surface.  piling  over  the top  surface are  runways i s probably  to  utilize  animals which  e r r o r of  traits  of  may  disadvantageous  M.  of  plant  the runway. community  have  The  results  make  that  a  a  given  becoming  used worn  When a c a n o p y of  the  from  i s  runways,  view.  to  existed remain  have  This  resources  familiar safe  from  are  beneficial  other  habitats.  to  runways  within a  More i m p o r t a n t l y , t h e to heavy c r o p p i n g  before,  generations.  that in  in  proven  previous  restriction  s p e c i e s r e q u i r e s abundant food  may  are  anti-predator device.  townsendii  r e p r o d u c t i v e r a t e and  they  t o have been  none  the  be  and  an  where  areas  are  flapperi.  indicate  hidden  Many o f  high  but  voles,  soil  runways a l l o w s  C.  may  the  the  which  species.  trial  fields  and  social  f u n c t i o n as  interaction to t h i s  a  glands  ( Q u a y , 1968) .  the  grassy  be  attacks,  generations  into  litter  of  oregoni,  by  distance the  of  inch  and  predators  The  as  to hip  a social  a d d i t i o n to c r e a t i n g cover  habitual  in  an  M.  aggressive  social  again  foraging  habitual In  over  developed  have  more a t t r a c t i v e  runways  appears  1 i n t r o d u c t i o n experiment  moderate degree of  over  well  significance  Ladner G r i d  The  also  longicaudus,  focus  additional the  possesses M.  These  of  townsendii  means short  resiliency  m u s t a l s o be  of  great.  86  Otherwise,  the  systems could  tendency be  Microtus more x e r i c habit. to  be  restrict  severe  might  a  times  increased  humidity  water  balance  kangaroo r a t s  Nielsen,  1950,  limiting, to cope  microtines. to  be  One  individuals during  probably or  likewise M.  be  xeric the  contained be  townsendii  would  i n the  has  systems,  be  role  in  and  found  overall  water  i s  less  able  be  more  and  Schmidt-  subterranean  thereby M.  curtailing townsendii  O.B.C. R e s e a r c h i t i s  at a strong  Forest  reasonable competitive  habitats. a lower and  can  food.  reproductive rate, apparently  1946).  superior competitor little  been  where  Therefore,  may  nesting  humidity  reported finding  nest  i n  of water d e p r i v a t i o n  females,  conditions (Hall,  expected under  crucial  effects  weather.  longicaudus  surface  a  than  i n s e a s o n a l l y dry  c o n f i n e d t o runway  under f a i r l y  dry  in their dry  runway  stress  surface  townsendii would  expect  assume t h a t M . , t o w n s e n d i i  Microtus  M.  (1974)  water  (Schmidt-Nielsen  for lactating  especially  to  burrows has  situations  stress  Hilborn  dead  disadvantage  not  In  would  most s e v e r e  reproduction.  to  1952).  water  i t s  within  plays  surface-nesting  with  of  greater than  this  of  also suffer  result  absolute humidity  to five  activities  disadvantage.  townsendii  h a b i t a t s as  The two  a  to  survive  Hence, i t would  i n habitats that  Microtus  oregoni  t o have a c o m p e t i t i v e advantage  these c o n d i t i o n s .  i s  were would over  87  6. The  accuracy  improved  by  the  are  similar  t o those  and  of  a n a  the "  localities  would  habitat  the  study  aggressive  i s  the  only  Peninsula  of  of food  lacking  both  1 9 6 5 ) . ft among  restrictions  and requirements  The e x a c t  from  which  laboratory same  may  relationship  be i m p o s s i b l e  observe  aggression species  of  of the  on w a t e r b a l a n c e  and i n t e r f e r e n c e c o m p e t i t i o n  i s difficult to  i n and  the  between  should at  also  present.  field,  field  rodent  and  and  aggression  can  disagree  1972).  Generality findings  on t h e  patterns  habitat  that  microtine  i s  utilization  resources  limitations.  e x p e r i m e n t s on t h e (Byrne,  the  (Krajina,  (Cowan a n d G u i g u e t ,  habitat  i n v e s t i g a t e d , a task  results  of  f o r some o f t h e s p e c i e s .  behavior  behavior  by  Parts  vicinity  i s d e s i r a b l e , a s i s d e t a i l e d work  physiological  be  and  and v e g e t a t i o n  Vancouver  differentiate  preferences  Further  anomalies.  have c l i m a t e s  townsendii  the  can  I s l a n d , a n d many o f t h e i s l a n d s  Sechelt  fl.  of  Vole  Vancouver  results  experimentation  distribution  o f Georgia  comparison  be  field  Y e t fl. t o w n s e n d i i  2££<JOJli  voles  of these  of  Strait  islands, 11-  and completeness  Peninsula,  in  1965).  Results  further  investigation Sechelt  Generality of  i n  can  other  animals.  I  competition  i s mainly  competitive  have  be  ascertained  localities concluded f o r space,  interactions  i s  and  only  by t e s t i n g  these  on  other  species  that,  among  microtines,  and  that  determined  the  outcome  mainly  by  of  of the  88  structure the  of  species  method  the  h a b i t a t and  involved.  of  be  the Space  be  to  present,  or  of  be  species at  which  light  of  North  America  species  that  appear  t o c o e x i s t on  overlap  (Hall  and  trends  areas of regions  Kelson,  ( f i g . 16).  varied topography of  major  f o r e s t or  and  north.  3)  i n the  More areas  for  speciation for  species three may  as  isolation  genetic  stock.  necessary  for  a v a i l a b l e range of  competing. subdivide; i f area,  must  possible.  be  my  American  the  A  number o f  Maser and  map  of  microtine  of  Storm,  The  microtines  results.  basis  must  range 1970)  shows in  ( i . e . m o u n t a i n o u s r e g i o n s ) , 2)  in  change  more d i v e r s e  map  1)  such  prairie-montane  1)  A  and  to  of  isolation  climatic  the  ecotones,  mountainous  opportunity  for  opportunity  divergence is  (Mayr, regimes  along  forest  increased  subseguent  occur  as  coexist in  greater  a result  Geographic  speciation  to  appears  structures  geographically  reasons: exist  geographic  greater  may  the  are  this  geographic  Microtines are  vegetational  tundras-boreal  beyond  different  showing the  1959;  species  that space  small  f o r North  continental  three  found  i s difficult  separation  diversity in  go  of  m o s t common  competing  f o r which v o l e s  sufficiently  vertical  looked  I have  attributes  i s the  among  coexist within a  habitats with  pattern can  are  separation  conclusions  resource  i s a resource  microtines either  My  however, as  proximal  behavioral  division  1974).  generalization, to  Spatial  resource  (Schoener,  the  regarded 1966).  may  of as 2)  increase  A the  Figure  16.  P a t t e r n of m i c r o t i n e s p e c i e s d i v e r s i t y i n North America Data o b t a i n e d from range maps p u b l i s h e d i n H a l l and K e l s o n (1959) and Maser and Storm (1970).  89A  90  number 3)  of  species  A greater  result  of  found  variety  of  regions the  of  exception  of  northern  California,  i s probably  third  factors i f  mountains the  were  are  an  increase  species  the  of  to  vegetational habitat  change  diversity  inherent i n range may  exist  species  be  along  diversity species these  and  species  probably  in  these  both  appears  to  be  diversity  of  microtines.  small  a  line  herbivores  than  i n the  f a r north  would  this have  than  would  i s not  seen;  the  highest  America.  along  lines  attributed or  to  of  major  increased  to inaccuracies  marginal  overlaps  ingress of animals of  sufficient  replacement. condition  This  competed  but  and  important  Alternatively,  s i d e s of a  first  second  diversity  a net  and  more  Alaska  be  the  the  would  regions,  by  a  one  i n North  can  The  In f a c t  and  maintained  as  with  glaciated,  were  diversity  maps.  exist  separate,  regions.  species  area.  Washington,  importance.  number o f c o e x i s t i n g m i c r o t i n e s Increased  may  Oregon,  mountains  Yukon  geographic  diversity,  recently  in  i n warm, a r i d  mountains of  habitats  diversity that  a small  climate.  difficult  climatic  of  high  of minimal  diversity,  show l e s s  and  c o a s t a l area  factor  habitat  distinct  varied topography  Since  expect,  within  result  primarily  from  Habitat permitting  i s expected  i f  for  physical  microtine species  diversity  space. The in of  the  trend  north  towards increased  runs counter  increasing  species  to  the  diversity  g e n e r a l l y observed as  one  pattern  moves away f r o m  the  91  poles  (Mac&rthur,  species  decreases  response  to  However, t h e number  northward  the decreased  Nevertheless, unstructured  1972).  more tundra  from  the treeline,  habitat  species  of  diversity  coexist  on  microtine  probably of the  the  as a  tundra.  relatively  t h a n c o e x i s t on t h e s i m i l a r l y monotonous  prairies. The the  trend  results  allocation would  seems r e a l  o f my s t u d y .  be v a l u a b l e  habitats  are u t i l i z e d ,  the  mechanism  t o k n o w how n o r t h e r n  with  habitat  exists,  Some o t h e r  on t h e b a s i s o f  must assume p r o m i n e n c e i n n o r t h e r n  compare  more  but i n e x p l i c a b l e  then  increased  those  overlap  i n  the  division occurs.  i t i s important overlap.  zone:  habitats I f  more  to determine  resource  latitudes.  microtine  temperate of  of  I t  communities whether  i s  more  finer,  habitat  or  overlap  what f a c t o r s  allow  92  SUMMARY AND 1)  Microtine rodents  separated species not  mainly  studied are  overlap  microtine  burrowing. related  to the  Vertical  M.  M.  weights  Although  the  appears  to  or  other  too  two of  two  four  h a b i t a t s and habitats  vegetation.  The  overlaps  do are  fourth to  some  three. competes  with  this  the  high,  M.  oregoni  by  interference  by  competition the  M.  oregoni  a  necessary  i s  directly  h a b i t a t : where cannot  dig.  condition for  species. M.  did  best  are  the  These  escapes  oregoni  townsendii  evidence  Vancouver  Three of  u b i q u i t o u s l y and  a p p e a r s t o be  than  be  of  outcome o f  i s  these  high  of  p h y s i c a l c o n f i g u r a t i o n of  A population of  other.  oregoni  separation of  vicinity  i n one  townsendii  table  coexistence 3)  the  Thus, the  water  region  and  only  basis  occurs  Microtus  interference,  body  the  with each of  2)  the  found each  on  species  i n the  habitat differences.  with  recognizable  extent  by  CONCLDSIONS  i s  living  d e n s i t y had  other not  explained  exclusively significantly  populations compelling,  by  the  of  M.  this  in  a  lower  oregoni.  difference  phenomenon o f  character  displacement. 4) raptors voles  Preferential did  not  predation  occur,  nor  noticeably l e s s i n areas  Hence,  predation  account  f o r the  by  hawks  coexistence  for  prey  species  was  s u r v i v o r s h i p of  of  high  and of  one  M.  owls gregoni  raptor i s and  resident  densities.  insufficient M.  by  to  townsendii.  93  5  )  Microtus  townsendii  that  appear t o c o n f e r  and  competitive  factors  such  runway  .high  surface  a  Microtus  number  of  i s  M. o r e g o n i  of  localities,  i s apparently  between  M. o r e g o n i  i s b e l i e v e d t o be m e d i a t e d  and decreases  Aggressive  separation i s  i n the  i n the breeding  form  Although  competition This  of  as being  of microtines.  f o r space.  observations  from  a  i n that i t but  active  C. g a p p e r i  by  the  i n t e r a c t i o n s are important  hypothesized  an e x c e p t i o n ,  mainly  the  not involved i n  and  components, as a r e mechanisms f o r avoidance  be  on  seasonally  excluded  competition  interaction  coexistence  i n  of  t h e species t o water  probably  minor  habitats  high  t o areas  nesting  succeed  relatively  Spatial  a  restriction.  The o v e r l a p  8)  and  dry h a b i t a t s .  release a t other  from  habitat 7)  to  h a b i t a t s by i n t e r s p e c i f i c  interference  to established  the species  habit  habitats  Specifically,  aggregate,  to subject  longicaudus  shows c o m p e t i t i v e  this  The  h a b i t a t , but appears  6)  to  M. t o w n s e n d i i  i n seasonally  others.  to limit  productivity.  i n some  of a c t i v i t i e s  tendency appear  specializations  advantage  i n  restriction  allows  flooded stress  as  rate  primary  competitive  disadvantage  systems,  reproductive  shows b e h a v i o r a l  among  hypothesis  of competitive  of  occupancy necessary arctic  season.  i s The  presence  of  life-history interactions. of  distinct  f o r geographic microtines  may  m i c r o t i n e s a p p e a r s t o be i s  i n  agreement  with  i n t e r a c t i o n s within other  taxa,  94  and  appears  to explain  species diversity  much  of  i n temperate  the  pattern  North  America.  of  microtine  95  LITERATURE  CITED  Addicott, J. 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Mammal. 46:605-612.  of  103  APPENDIX  Trapping  Dates f o r  all  Grids  Boad K 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51  October October October November November Febuary March March March April April April May May May May May June June June July July July July August August September September September October October October October November November November December January January Febuary Febuary Febuary March March March April A pril April April May May  Plot 18, 24, 31, 14, 28, 20, 13, 20, 28, 12, 20, 24, 3, 11, 18, 24, 31, 8, 17, 29, 7, 16, 23, 31, 9, 19, 1, 18, 30, 10, 17, 24, 31, 13, 22, 29, 11, 24, 30, 10, 19, 28, 6, 16, 30, 9, 16, 23, 30, 10, 17,  1970 1970 1970 1970 1970 1971 1971 1971 1971 1971 1971 1971 1971 1971 1971 1971 1971 1971 1971 1971 1971 1971 1971 1971 1971 1971 1971 1971 1971 1971 1971 1971 1971 1971 1971 1971 1971 1972 1972 1972 1972 197 2 1972 1972 197 2 1972 1972 1972 1972 1972 1972  52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102  24, Hay flay 28, June 4, June 11, June 18, November 25, July 3, July 10, July 17, 24, July July 31, August 7, August 16, 27, August September 4, S e p t e m b e r 10, September 25, October 1, October 10, October 19, 26, October November 2, November 13, 26, November December 5, December 14, January 1, 13, January 20, January 27, January Febuary 3, Febuary 11, Febuary 17, 24, Febuary 3, March March 11, March 17, March 24, April 1, April 8, April 15, April 23, April 29, May 7, 15, May 23, May 30, Hay June 6, 23, June 29, June July 6,  1972 1972 1972 1972 1972 1972 1972 197 2 1972 1972 1972 1972 197 2 1972 1972 1972 1972 1972 1972 1972 1972 1972 1972 1972 1972 1972 1973 1973 1973 1973 1973 1973 1973 197 3 1973 197 3 197 3 197 3 1 973 1973 1973 1973 1973 1973 197 3 1973 1973 197 3 1973 1973 1973  103 104 105 106 107 108 109 110 111 112 113 114 115 116  July 13, 1973 July 2 3 , 1 973 3 0 , 197 3 July August 9, 1 9 7 3 August 18, 1973 August 26, 1973 September 2, 1973 S e p t e m b e r 2 9 , 1973 October 15, 1973 2 8 , 1973 October November 1 8 , 1973 1974 22, Febuary March 2 4 , 1974 April 12, 1974  106  Goose Lake 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51  July August August August August September September September September October October October October November November December January January January January Febuary Febuary Febuary Febuary March March March March April April May May June June June July August August September September October November November November December January Febuary March March March April  Control 25, 1, 7, 16, 28, 4, 11 r 17, 25, 1, 10, 19, 26, 7, 18, 2, 1, 13, 20, 27, 3, 11, 17, 23, 3, 11, 24, 31, 9, 22, 7, 23, 6, 23, 30, 12, 5, 18, 3, 30, 14, 2, 15, 28, 14, 19, 10, 1, 17, 31, 14,  1972 1972 1972 1972 1972 1972 1972 1972 1972 1972 1972 1972 1972 1972 1972 1972 1973 1973 1973 1973 1973 1973 1973 1973 1973 1973 1973 1973 1973 1 973 1973 1973 1973 1973 1 973 1973 1973 1973 1973 1973 1973 1973 1973 1973 1973 1974 1974 1974 1974 1974 1974  52 53 54 35 56 37 58 59 60 61 62 63  April May May June July July August September October October October November  30, 11, 24, 13, 2, 25, 16, 5, 3, 16, 29, 15,  1974 1974 1974 1974 1974 1974 1974 1974 1974 1974 1974 1974  107  Goose Lake 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33  Ladner  Experimental  July July July August August August August September September October October November November November December January febuary march March March April April May May June July July August September October October October November  7, 24, 30, 5, 9, 18, 27, 3, 30, 14, 22, 2, 15, 28, 14, 15, 10, 1, 17, 31, 14, 30, 11, 24, 13, 2, 25, 16, 5, 3, 16, 29, 15,  1973 1973 1973 1 973 1973 1973 1973 1973 1973 1973 1973 1973 1973 1973 1973 1974 1974 1974 1974 1974 1974 1974 1974 1974 1974 1974 1974 1974 1974 1974 1974 1974 1974  1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35  Grids  % and 2  October 11, 26, October November 8, November 21, December 19, December 19, January 29, Febuary 19, March 5, 20, March April 2, April 18, May 3, May 16, June 7, 25, June 18, July August 7, August 28, S e p t e m b e r 18, October 2, October 18, 18, October November 13, 28, November December 12, January 3, 15, January 14, Febuary Febuary 21, 27, Febuary March 5, March 12, 20, March March 21,  1973 1973 1973 1973 1973 1973 1974 1974 1974 1974 1974 1974 1974 1974 1974 1974 1974 1974 1974 1974 1974 1974 1974 1974 1974 1974 1975 1975 1975 1975 1975 1975 1975 1975 1975  

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