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Theses, Dissertations, Professional Papers
Graduate School
1962
Competition and niche segregation in the genus
Microtus
James Ray Koplin
The University of Montana
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6540.
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COMPETITION AND NICHE SEŒEGATION IN THE GENUS MIGROTUS
by
JAMES RAY KOPLIN
B.S. Montana State University^ 1959
Presented in partial fulfillment of the requirements for the degree of
Master of Science
M
ONTANA STATE UNIVERSITY
1962
Approved byg
Chairman, Board \pf Examiner
Dean. Graduate School
Date
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TABLE OF CONTENTS
PAGE
INTRODUCTION
o o o « o r , * o e
S'tS.'bSTÏISn'b o f PX*OLl0m
eu
oo
H i s t o r i c a l Developm ent o f Problem
C o m p e titio n and N iche
. .»
The Study" A rea
o o o o
1
oü
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............................ . . . .
oo=
»
1
..................
. . . . . . . . . .
o o o o û o <
i o o
. >o
o
<> o
5
j.o
Y
o o o
8
T rappxn^ M ethods o o o o o o o o o o o e o o o o o o a o o o o
R^^SULTS
9
o OOOOOOOOOOOOOOOOOOOOo OOOt i OO
T ra p ? x e ld 0 0 0 * 0 0
X
3
o « a c h o o o t > o o o * o o o o o y o o
o o o o o o
Q
.......................
C o m p e titiv e E x c lu s io n P r i n c i p l e
EXPERIMENTAL DESIŒT
e*
30
0 0 0 * 0 0 0 0 0 0 0 0 0 0
l8
0 * 0 0 * 0
2,8
..................................... o * . * . *
I8
Movements and P o p u la tio n D e n s i t ie s . . . . . . . o . . . . . .
31
Sex RatXOS O . O O O O O O O O O O O O . O . O O
i-ti
D i s t r i b u t i o n o f S p e c ie s
B reedxn^ A c tx v x ty
.
DIS CXJS SI02J O O O O o O ,
. . . .
* o. . . . 0 *
.
000
o 00 00
0 .0
00
0 0 0 0 0 0 0
* o . . * * . * . * . *
0 0 0 0 0 0
00
3
^3
C o m p e titio n and O v e rla p p in g N ic h e s . . . . . o o . . . . . * .
U5
I n t e n s i t y o f I n t e r s p e c i f i c C o m p e titio n . . . . . o u o . . . .
U6
E f f e c t s o f I n t e r s p e c i f i c C o m p e titio n
50
.
.
. . . . . . . . „ . .
Removal o f M. P e n n s y lv a n ie u s from th e E x p e rim e n ta l P l o t
Movements D u rin g th e
Removal
P h a se s .
.
* . *
. . . . . . . . .
3l
.
53
* * . * . . * * . . * . *
55
C o m p e titiv e N ic h e E x p l o i t a t i o n and N o n -c o m p e titiv e
N iche O ccupancy
* * .* . . . * .
-iii-
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PAGE
CONCLUSIONS AND SUGGESTIONS . . . . . . . . . . . . . .
SUÎ^^ÆI
a
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o
LITERATURE CITED
o
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......
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. . . . . . . . . .
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c
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w
o
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$Q
6l
LIÇT OF TABLES
TABLE
I.
PAGE
Test of Criteria Used in the Field for Identification
1/7
and Sex Beternixnatxon
U«
Distribution of Trapping Effort and Trap Yield on the
Experimental and Control Study Plots
III.
.............................
. . . . . . . . . . . .
. . . . . . . . o . . . . . . . . . .
30
3^
Average Distance, Av. D, Between Successive Points of
Capture on the Two Study Plots
VIII.
29
Maximum Distance Between Points of Capture^ av. M
^ on
both Study Plots
Vn.
26
Summary of Distribution Analyses for M. pennsylvanicus
on the Control Plot During the Fall Removal Phase . . .
VI.
.
Summary of Distribution Analyses from the Experimental
Plot for the Preliminary Phase
V.
19
Percentage of Trapping Stations and Catches in each
Vegetation and Soil Moisture Type
IV.
.
. . . . . . . . . . . .
33
Comparison of Sex Ratios in both Study Plots During
the Study Phases Listed
...................................
-V -
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1|.2
LIST OF FIGURES
FIGURE
page
1,
Location of Study Plots on the National Bison Range . . , .
2.
Distribution of Trap Sites in Soil Moisture Types on the
Experunental Plot o o o o o o o o o o q o o w q o o o o o
3»
o o o * >o
o o o o o o o o o o o o o o
oow
a o © o q <»o o « p u © PUO o p v
ouoo
« ^ o
. . . n .
10»
22
Distribution of Microtine Captures on the Experimental
Plot During the Fall Removal Phase(9/18/61-11/19/61)
9«
21
Distribution of Microtine Captures on the Control Plot
During the Preliminary Phase (6/23./6l-9,/l7/6l)
8.
llj.
Distribution of Micro tine Captures on the Experimental
Plot During the Preliminary Phase (6/23/61-9/17/6 l)
7»
13
Distribution of Trap Sites in Vegetation Types on the
Control Plot
6«
12
Distribution of Trap Sites in Vegetation Types on the
Expernnental Plot o o o o o o o o o o o u o o o o p o o p
5o
11
Distribution of Trap Sites in Soil Moisture Types on the
Control Plot
L.
10
. .
23
Plot During the Spring Removal Phase (U/l/62-5/19/62) „ .
2k
Distribution of Microtine Captures on the Experimental
Distribution of Microtine Captures on the Control Plot
During the Fall and Spring Removal Phases (9/23/6111/19/61 - L/7/62-S/19/62)
11.
. . . . . » . » » » . » , » »
2S
Mean Maximum Distance Between Points of Capture, Av. M,
Plotted for Successive Captures
-vi-
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36
FIGURE
PAGE
12.
Seasonal Densities of Microtus on the Two Study Plots . .
13»
Graphs of the Percentages of Reproductively Active
39
Females and Juveniles Captured During All Study
Phases on "both Study Plots
lUo
. . . . . . . . . . . . . .
Centers of Activity fo r Microtines on the Experimental
Plot During the Preliminary Phase . . . . . . . . . . .
-vxx-
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1^9
INTRODUCTION
Statement of Problem
Findley (195U) in a review of certain microtine d istrib ution
lite ra tu re noted th a t in
areas "where Microtus pennsylvanicus
(the
meadow vole) occurs by i t s e l f i t i s capable of occupying
most available
h ab itats, including both
wet and
This is TalsoJ
true of M
o montanus (the
montanevole)
dry grasslands . °
.
. « *
^•Where the range of the meadow vole overlaps th at of M. montanus,
a species well-adapted to existence in dry mountain grasslands,
meadow vole i s
the
^ » » forced to r e tr e a t to i t s optimum niche, the hydro-
sere c o m m u n ity F in d le y proposed th at th is niche segregation i s
".
o . due, in part a t le a s t, to competition ^
. o"
An inference
from th is proposal is the hypothesis th at a r t i f i c i a l reduction of a pop­
ulation of meadow voles in a hydrosere w ill re s u lt in movement of mon­
tane voles into the vacated nicheo
The chief subject of th is study i s
a f ie ld experiment designed to t e s t th is hypothesise
I t was conceived
in January, 1961, commenced in June, 1961, and followed through midNovember, 1 9 6 1 , when inclement weather halted a c tiv itie s fo r the w inter.
Field work was recommenced in early April, 1962, and is s t i l l
rn progress
a t th is writing»
H istorical Development of the Problem
To understand the problem a t hand, a consideration of the con­
cepts of competition and niche i s necessary»
Some of the e a r lie s t Ideas about competition and niche are found
-
1-
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in Darwin's ”The Origin of Species."
In a carefully defined application
of Malthus* term 'struggle for existence v,' used
.
. in a large and
metaphorical sense," Darwin included in tra sp e c ific and interspecific
disoperations and predation.
He distinguished in tra sp e cific from i n te r ­
specific relatio n s by referrin g to the d iffe re n tia l degree of severity
between the two--concluding th at the former is more Intense.
f e l t that intrageneric
He also
'stru g g le ' i s more c r i t ic a l than intergeneric.
Darwin explained that the reason struggle between Individuals of the
same species i s the most severe is because ".
.
they frequent the
same d i s t r ic t s , require the same food, and are exposed to the same
dangers."
He also said since
. o . much sim ilarity in habits,
. species of the same genus have
. . . constitution, and .
. stru c­
tu re, the struggle w ill generally be more severe between them
than between the species of d istin c t genera."
.
These statement:: come
remarkably close to the present day concept of niche, and the role of
competition in determining niche exploitation and occupe.ficy.
Joseph Grinnell (191’^, 19?U) developed the term 'ecological
niche.*
He emphasized the r e s tr ic tio n of a species to a p articu lar
set of physical environmental facto rs.
Later, Grinnell
,19^8' stated
th at food and enemies are also orxtical factors of niche.
Elton ( 192 7
suggested th at the place of an animal xn the biotic environment--it 3
relatio n to food and enemies--defines the
“niche® of that ai .. mai ,
Niche is currently viewed as an elaboration of these b a d : pr emises,
r e s tric tio n of a species to a p articu lar niche xs thought to be depend­
ent upon both physical and b io tic facto rs,
as wel. as s tr . - .ra.
physiological, and, in some animals. behavioral ada.p* at .u r
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-3 -
(Kendeigh^ 1962; Lindsdale, 1957)*
That is^ each genome is well adapted
to only a p articu lar niche and in e ffic ie n t u tiliz a tio n occurs in any but
that niche (Mayr^ 19U9)»
Hutchinson (1957) presented a formal analysis of niche concept.
In th is abstract scheme the upper and lower lim its of the
factors de­
lim iting niche were plotted on a m ultivariate coordinate system^ pro­
ducing a n-dimensional hypervolume
species S^„
—the ' fundamental niche® of
Similarly, fo r, species S25. 3^^, . . . ,
volumes ^ 2 ^ N3 ? ° °
the hyper­
are representative fundamental niches.
Several p ractical d iffic u ltie s are associated with the Hutchinsonian
niche.
For one thing, the probability of steady state maintenance must
be considered at each point in N
jj_ rather than the all-or-none existence
im plicit in the assertion.
Also, lin ear ordering for many of the factors
delimiting niche i s not possible.
I f these d if f ic u ltie s coaid be over­
come, analysis of the m ultivariate coordinate ^sterns could be accom­
plished by means of algebraic matrices.
Hutchinson®s concept of niche i s useful, however, fo r i t permits
unequivocal statement of the fa c t th a t the actual set of variables de­
termining the existence of a local population is a subregion of the more
broadly defined fundamental niche peculiar to the species (Slobodkin,
1961) o
Competition and Niche
Thère ex ists much controversy over the meaning of the term “compe­
tition® (Andrewartha and Birch, 195k; Birch, 1957; Odum, 1959)°
Follow­
ing Odum®s suggestion th at general words lik e competition remain broad
in scope, but be precisely defined^ competition is here defined as the
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-u-
more or less excessive demand by two or more organisms for limited com­
mon niche fa c to rs, usually resources such as food and space.
The com­
peting organisms may be of the same or d ifferen t species, but i t is
in te rsp e cific competition that i s of primary concern in this paper
I t is highly unlikely that two species w ill have exactly the same
niche requirements» Indeed, i t is axiomatic from the present-day con­
cepts of species and niche, th at i f two species have id en tical niche r e ­
quirements, then they are the same species—an absurdity.
species may have variably overlapping niches..
However, two
The amount of overlap is
a function of the number of mutual factors delimiting niche and the de­
gree to which these factors are shared—1, e . , the degree of sim ila rity
among the maximum, minimum, and optinram values of the shared X^,,
In
general, however, the closer the taxonomic relationship the more simi­
la r i ty there is in the needs and habits of species.
That i s ,
the most
highly overlapping niches occur in congeneric species ( Darwin , in
Crombie, 19U7)o
The extent to which niches must overlap before the c r ite r ia for
competition are met i s not known, but even sligh t overlap may be guff1cien t.
For example, in cutover areas of Newfoundland where f i r repro­
duction is predominant, competition for food between the d ista n tly
related moose and snowshoe hare may occur (Dodds, I960).
However,
regardless of the amount of niche overlap, unless the r a tio of the combined species populations to mutually shared niche factors Ir- high
enough to re s u lt in an excessive demand for those fa c to rs, competition
w ill be nonexistent (Crombie-, 19L7).
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Competitive Exclusion Principle
The Lotka-Volterra simultaneous d iffe re n tia l equations are
probably the best known mathematical models which have been designed
to predict the outcome of in te rsp e c ific competition.,
These particu lar
equations were derived d ire c tly from the equation of the lo g istic
curve, and were i n i t i a l l y expounded by Gause (l93U)o
They were empir­
ic a lly tested by Gause, and subsequently by others (e. g., Crombie,
19h5s Park e t alo, 19^1; Park, 19L8, 195Ua) <
> Andrewartha and Birch
(195U), in a critique of the Lotka-Volterra equations, questioned the
r e a lity of assigning biological meaning to the proportionality constants
unique to each equation| they also pointed out the ir r a tio n a lity of
giving differen t meanings to dependent variables common to both equa­
tions.,
Andrewartha and Birch suggested th at by ignoring the lim ita ­
tions to the equations, various experimenters have erroneously confirmed
resu lts predicted by the equations, in which case the models may be more
misleading than helpful in the in terpretation of population measurements
and observations.
Nevertheless, the design of equations to f i t specific
experimental population phenomena may re su lt In extremely complex mathe­
matical analyses; and as complexity of models increases ap p lica b ility to
generalized problems is correspondingly diminished (SlonodkiH; 196i),
However, limited modifIciation of general equations, as Hutchinson
(19U7) has done to d iffe re n tia te the outcome of in tersp ecific competi­
tion between two social species from the more general situ a tio n , may be
worthwhile»
Need for some kind of basis from which to work would seem
to ju s tif y use of the Lotka-Volterra equations as models so long as
they are not applied beyond the scope of the models >'Odum, 1959)»
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-6 -
Slobodkin ( I 9 6 I ) , using the Lotka-Volterra equations as a basis,
diagrammetic a lly represented, in several 2- and 3-dimensional models ,
the four possible outcomes of in te rsp e cific competition 0:
(1) and (2) One species or the other w ill always win depending
upon which species has the highest carrying capacity under the conditions
of competitive interaction >
,
( 3 ) Unstable equilibrium in which one species w ill eventually
survive»
When i n i t i a l densities of the competing species are equal
there is no certainty as to which species w ill win, but in unequally
proportioned populations, the species with the greatest i n i t i a l density
wins»
Neyraan et al» (1958) designed a stochastic model to predict
probability of outcome in a competitive system of th is nature,
CU) Stable equilibrium in which some facto r, such as non-specific
predation, prevents the two species from reaching densities necessary to
in it i a t e competition»
Stable equilibrium is not competition according
to the definition used here but is included as '■'potential competition. "
The system becomes competitive when, or if., the lim iting factor is re ­
leased, in which case outcome follows the course of one of the above »
The modified and unmodified deterministic Lotka-Volterra models,
the stochastic model of Neyman e t a l ,, the experiments of Gause and sub­
sequent experiments, such as those of Park e t al» (.19Ll ) » Park '191.8
195Ua, 195Ub), Crombie U9U5, 19U6V, Frank (1952, 1957), and the f ie ld
observations of Diver (l9U0), Dumas (1956), Gilbert et
1952'!,
Lack (19Ù5, 19i|6, 19U7), MacArthur f 1958,), and Ross ' 1957 ! demonstrate
an important concept that has become known as Gausets Rme or.
recently, the ''-competitive exclusion principle*' ( H a r d i n I960):-;
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more
an
ecological niche cannot be simultaneously and completely occ^^ied by
stabilised •populations of more than one species.
Keeping in mind the competitive exclusion principle
the pre­
viously developed concepts of niche and competition , and using the re­
sults of the field study herein reported as a working bae^-s,, it is
practicable to test Findley's hypothesis that interspecific competi­
tion is a factor maintaining nabitat segregation in sympatrro popula­
tions of Microtus spp.
EXPERIMENTAI DESIQSf
Experimental design of the study to test Findley's hypothesis
called for two live-trap grids^ experimental ar^d control, surrounding
small ponds o
habitat
Bordering the ponds was mesic Microtus peongylvar..icus
which in turn was surrounded by xeric M. mortauus habitat. A
control plot was deemed necessary an order to avert masicterprering
factors, other than the reduction of Mo pe nnsyl 'an reus n i m b e r ; which
might induce movements of M. mentants into m-ctc habitat, Microtus
pe rm sylvan icus was chosen as the species for redij.: tc.ot sc.toe conditions
of the experimental design rendered it the more restricted mrcrot^,ne,
its populations berng more or less completely surrounded by M. montanus.
Assumrrg Findley's h}-pothesis to be correct, then, as mentioned
earlier , it was also assumed that removal of M, penr sylvanic as from the
experimental plot would induce M. montants to tn.ade the vacated mesic
habitato
Further, it was felt that habutat chared by the two species
of vole would be a measure of ..uterspec if ic com pet ;tio'
stuc e
t
in the area of tpaiual habitat overlap that compet t;.on lor mutual
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s
-8 -
niche facto rs would exist»
In addition to clarify ing these assumptions, i t was hoped that
the study would answer these questionss
besides affecting d istrib u tio n ,
does in tersp ecific competition impose any significant differences on
other population a ttrib u te s such as density, sex r a tio , age r a tio , home
range, n a ta lity and mortality?
Does Interspecific competition bring
about any sig n ifican t differences in the phenology of these population
a ttrib u te s?
What is the e ffe c t of cyclic population fluctuations on
d istrib u tio n , and other population phenomena?
The Study Area
The National Bison Range, located in the southern end oi the
Flathead Valley of western Montana, is approximately 50 miles north of
Missoula»
The area i s a big game refuge, administered by the Ü» S» Fish
and Wildlife Service»
Grasslands of the National Bison Range are among the int-ermontane
p ra irie regions where the ranges of Microtus pennsylvanieus and
M
» montanus overlap»
Small mammal trapping re su lts from the refuge
indicate the general re s tric tio n of M. pe nr-,sylvanl cu s to mesic., and
M
» montanus to xeric communities (R» 8» Hoffmann and P» L» Wright., per s»
comm» ) »
By virtue of i t s convenient location and because i,t was k. .own
to be inhabited by both species of voles, the Nat,ional Bison Range waa
chosen as the area on which to conduct the experiment to te s t Findley's
hypothesiso
Morris and Schwart.i %
1 9 5 7 characterized the graet»land.z of the
National Bison Range as being esse n tia lly palouse bunch grass p -aigrie ic
composition»
The plant ecology of the palouse praj..ne,
ir the
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.icin.. ty
of Missoula^ has been described by Mitchell (1958)^ and certain charac­
t e r i s t i c s pertinent to the plant ecology of grassland hydrosere vege­
ta tio n in the Flathead V a lle y h a v e been described by Lokeraoen (1962).
Located <>75 miles apart, the experimental and control trap grids
each surround a small pond hydrosere in the midst of the palouse grass­
lands o
Both trap plots l i e on moderate to gradually north-sloping out-
•wash a llu v ia l plains on the north end of the refuge .
The experimental
plot is situated 200 yards south of the Slaughter House and to the west
of the Alexander Basin»
The control plot i s in Alexander Basin .35 miles
north of Indian Springs (see Figure 1 fo r re la tiv e locations of the trap
p lo ts ) o
The combined h ab itats of the two species of Microtus on both
f ie ld study plots exhibit a moisture gradient^ ranging from soils under
standing water to those quite dry to the touch (see Figures 2 and 3 for
a comparison of the d istrib u tio n of moisture gradients between the two
study p lo ts).
Associated with th is moisture gradient is a grassland
hydrosere vegetation.
A sedge-cattail (Carex-Typha) association is
mainly re s tric te d to areas of standing waterji blue grasses (Poa spp.)
primarily to damp so ils (muddy to damp-feeling soils), and the palouse
p ra irie vegetation is found on the d rier soils..
Patches of th is tle and
brush are interspersed over much of the damp and dry moisture regimes
(see Figures U and 5 for a comparison of the distribuai, n of vegetation
on the two study p lo ts).
Trapping Methods
A snap-trap survey of several areas, conducted in late March,
1961, revealed th at the previously discussed Slaughter House and I'ldlan
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-1 0 -
Headquarters
Experimental
plot
Control
plot
1 mile
FIGURE 1
LOCATION OF STUDY PLOTS ON THE
NATIONAL BISON RANGE
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-1 1 "
rift
w
W
D rift
Trap
PONT,
J)
D ^ dry so ils
¥ = wet so ils
FIGURE 2
DISTRIBUTION OF TRAP SITES IN SOIL MOISTURE TYPES
ON THE EXPERIMENTAL PLOT
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.rap
-1 2 .
E
M
0
,-q
1
J?
J)
JD
J)
§
g
^
X' PO MD
c
e-t
C
O
a
8
CO
c5
e
CO
w
PU
D
3
§
H
m
a
É-t
co
o
J)
R eproduced with perm ission of the copyright owner. Further reproduction prohibited without permission.
-1 3 '
D rift
Trap
n?
8
B
B
C
4
Drift
Trap
P
F
P
A - F
A -F
= Carex
= Poa
= Elymus
A~F <= AgropyronFestuca
G
Anrmal grasses
and forbs
R ^ Gat tar. Is
T = Thistles
B - Brush
TR - Cottonwood
FIGURE h
DISTRIBUTION OF TRAP SITES IN VEGETATION TYPES
ON THE EXPERIMENTAL PLOT
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-lU -
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R eproduced with perm ission of the copyright owner. Further reproduction prohibited without permission.
-1 5 -
Springs ponds sa tis fie d the req u isites of the experimental design as
study plotso
The te rra in adjacent to both study plots was grldded with
large bridge spikes, to serve as liv e -tra p s ite markersc
Indivi.daal
spikes were spaced 25 fe e t aparté using a fifty - f o o t s u r v e y o r c h a in
to measure the in te rv a l.
A Jacob“s s ta f f and compass were used to
align the spikes and to measure the 90 degree-angle turns required for
the perpendicular in tersectio n of trap lin es forming the grid.
Trap-
grids on the Experimental and Control study plots covered a to ta l of
2.56 and 3°31 acres respectively.
Pond surfaces, however, reduce the
actual areas trapped to 2 .I4.8 and 3<*26 acres respectively (Figures 2
through 5)Live traps used in the stuc^y were patterned from the design de­
scribed by Mosby (1955), and constructed from one-quarter-inch fib er
board.
Trap doors were made of one-sixteenth inch sheet alu.rai.num.
Traps were baited with mixed rolled oats and peanat b u tter.
Cotton was
provided for nest m aterial.
The study plots were live-trapped from late June through midSeptember, to determine re la tiv e d istribu tio n and popul at. .*,00 composi­
tion of the two species of vole prior to the reduction of Microtus
pennsylvanicus from the Exper1mental p lo t.
Portions of xerrc and mesic
habitat of the Experimental plot were trapped for a 3-day period in
la te June.
Subsequent trappings of the Experimental plot,, totaling
11 days, took place in raid- and la te August, and mid-September.
Control p lo t was periodically trapped fo r a to ta l of 11 days
June through la te July.
The
from la te
Voles were individually marked by ear tagging
with numbered sta in less ste e l fingerling tags, obtained frL-m
R eproduced with perm ission of the copyright owner. Further reproduction prohibited without permission.
he
S alt Lake Stamp Company^ and by toe clipping a f te r the method described
by Baumgartner (I9h0).
This dual marking system proved to be valuable
fo r identifying recaptures with lo s t ear tags or mutilated toes.
No r e ­
captures were obtained th a t had lo s t both the ear tag and additional
toes.
Voles were id en tifie d to species on the basis of rela tiv e d if ­
ferences in pelage and foot color (Hall and Kelson^ 1959)°
distinguished^ using the c r i t e r i a suggested by Davis (1956).
Sexes were
Adult and
juvenile age classes were distingi:iished on the basis of relativ e d if f e r ­
ences in size and pelage ch a rac te ristic of each age class 'Hoffman,
1958)°
The c r ite r ia used in the f ie ld for id en tificatio n and sex deter­
mination were subjected to a t e s t of accuracy.
Id entificatio n and sex
determination of 2U7 accidental and experimental removal vole m ortalities
based on these c r ite r ia were compared to id en tificatio n and sex determi­
nation of the same individuals based upon autopsy analysis.
The re su lts
of th is comparison are seen in Table 1°
The number of trap m ortalities during the warm summer and early
f a l l months was reduced by setting traps only during tl.e night.
nights cooled la te r in the f a l l ,
As the
traps were exposed only dur:ng the day
in order to reduce trap m ortality.
Despite precautions, the etudy areas
suffered a combined trap m ortality of 11% during the prelim.^nary phase
of investigation.
Reduction of Microtus pennsyIvan1cas from the Experimental plot
was begun September l 8 , 1961°
In order to control d r - f t of
Me pennsylvanicus from mesic hab itat outside the Experimental plot^ two
hardware cloth d r if t fences were set across either end of the etreara
R eproduced with perm ission of the copyright owner. Further reproduction prohibited without permission.
-1 7 -
traversing the p lo t.
Each fence was lined with trap s to captwe
Mlcrotns pennsylvanicus d riftin g into the grid area
.see Figj.res 2 and
U (pages 11 and 13) for locations of the d r i f t fences).
By mid-October,
1 9 6 1 , the yield of M. pennsylvanicus was so low th a t traps were l e f t
exposed for a week at a time.
u n til November 19, I 96 I
t ie s for the winter.
Removal of M. pennsylvanlens continued
when inclement weather halted trapping a c tiv i­
During the reduction of M. pennsylvanicus from
the Experimental p lo t,
the Control plot was trapped each weekend from
September 23 through November 11, and also during the spring removal
phase o
TABLE I
TEST OF CRITERIA USED IN THE FIELD FOR IDENTIFICATION
AND SEX DETERMINATION
(Results of the c r i t e r i a compared to re s u lts of autopsy analysis of
2L7 vole m o rta litie s.)
'N,
Per cent
of to ta l
(N/2L?)
M. penn sylv anic us autopsied
lUl
58
M. montanus autopsied
103
U2
Number
M. pennsylvanicus called M. montanus in the fie ld
3
1 .3
M. montanus called M. penn sylvan 1cu.s in the fie ld
2
08
Females called males
6
2.U
Males called females
1
Error in id en tific atio n
5
2..1
Error in sex determination
7
2.8
Error in id en tificatio n and sex déterminai ion
R eproduced with perm ission of the copyright owner. Further reproduction prohibited without permission.
..h
■
a
- 18-
RESULTS
Trap Yield
From June 2 3 , to November 19, I 96 I 3 and from April ?, to May 19,
1 9 6 2 , a to ta l of U^Oii? trap nights, 3>U6l "trap days," and 1.209 "trap
weeks® yielded 1,235 captures of 582 Microtus pennsylx-anlcus, 301
M
o montanus, 26U Peromyscus manicalatus, 1 Mus musculus, 85 Screx vagrans
and 2 Mustela fre n a ta .
A trap night Is defined as the exposure of one
trap for one night) from approximately sundown of one day to approximately
sunup of the next day.
A trap day is the exposure of one trap from dawn
of one day to dusk of the same day, and a tra p week is the exposure of
one trap fo r six or seven days--i. e », from near sundown of one Sunday
to near sunup of the next Saturday or Sunday»
tures were analyzed»
Only the microtine recap­
Table I I presents an analysis of these recaptui'es»
A y ield ra te of olU voles per trap exposure for both plots was obtained
during the preliminary phase of trapping.
Distribution of Species
Figures 6, 7, 8, 9, and 10 show the relativ e distribution of cap­
tures during the preliminary and removal phases.
Table I I I , page 26,
compares the re la tiv e frequency of captures in each vegetation and soil
moisture type.
These figures and Table I I I indicate the relativ e re ­
s tr ic tio n of M
o pennsylvanieus populations to the hydrosere communities
and the complete re s tric tio n of M
o montanus populations to the xeric
grassland communities daring the preliminary phase»
R eproduced with perm ission of the copyright owner. Further reproduction prohibited without permission.
sD
■
O
Q.
C
g
Q.
TABLE I I
DISTRIBUTION OF TRAPPING EFFORT AND TRAP YIELD ON THE EXPERIMENTAL AND CONTROL STUDY PLOTS
■
D
g
o
0
Area
Trapped
Period
Experi-^
Prelim­
inary
ci'
3"
1
3
CD
"n
c
3
.
3"
CD
CD
■
D
O
Q.
CD
Q.
■
D
CD
C/)
C/)
M. pennsylvanicus
F ir s t
Total
Captures Recaptures
M. montanus
_ Peromyscus
Sorex Mus Muste la
F ir s t
Total
Total
T otal
Captures Recaptures Captures Captures
2,233
nights
120
99
89
3h
1
12
11
20
71
30
12
7
22
13
ii
t
31
5
IL
2
6
1
13
22
3
33
197
130
29
53
211
52
28
‘15
iiOO
9/17)
F all
removal
(9/1811/19)
davs
90L
nights
1,057
days
1
L65
weeks
C
a
O
3
■
D
O
Trap
E ffo rt
Spring
removal
3
L
5
3
12
8
llh
Ll
213
72
25
15
26
8
hi
5
279
nights
f k / l /6 2 .
Tlih
5/19/62 ) weeks
Totaio
9
3,L16
nights
2=201
days
1
1,209
weeks
21
11
H
V
O
I
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I
I
(Ç
/>
2
o'
3
TABLE I I (c o n tin u ed )
Area
Trapped
Period
Trap
E ffo rt
M, pennsylvanicus
F ir s t
Total
Captures Recaptures
Peronyecus
Sorex Mus Mustela
M. montanus
F ir s t
Total
Total
Total
Captures Recaptures Captures Captures
8
c5'
3
Control
p lo t
Prelim ­
inary
( 6/ 30-
CD
C
p
.
3"
CD
0
■o
1
c
a
o
3
■o
o
631
nights
27
Ih
là
26
7/30)
F a ll
removal
700
days
hi
11
(9 /2 3 -
11/ 11)
Spring
removal
CD
Û.
(y i/6 2 5 /1 9 /6 2 )
O
c
Totals
CD
(/)
o'
3
Grajjd to ta ls
!\5
days
D
1,
3
L,0k7
nights
■o
I
560
222
157
128
ii3
3 ,k 5 l
days
1,209
70
72
ill
37
weeks
53
8
ii7
5
237
216
8,717
3H 5
85
2li3
72
1
2
1
2
2
21
11
26l|
85
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FIGIîre 6
DISTRIBUTION OF MICROTIIŒ GAPT^IRES ON THE EXPERIMENTAL PLü!
DURING THE PRELIMINARY PHASE 16/2 3/61-9/; ''6 i i
R eproduced with perm ission of the copyright owner. Further reproduction prohibited without permission.
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<k) = Mo p e n n s y lv a n ic u s
3ST = Mo t n o n t a n u a
FIGURE 8
DISTRIBUTION OF MICROTINE CAPTURES ON THE EXPERIMENTAL PLOT
DURING THE FALL REMOVAL PHASE (9/18/61-11 ■•'19^'61 :
■
R eproduced with perm ission of the copyright owner. Further reproduction prohibited without permission.
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0
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o =M
o pennsylvanicus
X =M
o montants
FIGURE 9
o f MICROTINE c a p t u r e s o n t h e EXPERIMENTAI PLOT
DURING THE SPRING REMOVAL PHASE »'U 1 / 6 9 - 9 / 1 9 - 6? i
d is t r ib u t io n
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TABLE i n
C/)
C/)
PERCENTAGE OF TRAPPING STATIONS AND CATCHES IN EACH VEGETATION AND SOIL MOISTURE TYPE
Experimental P lot
8
Prelim inary Phase
Removal Phases
% Type
M. penn. M. mont.
Sampled Captures Captures
CD
% Type
Sampled
F a ll
Mo pehhô M. mont,
'Captures Captures
% Type
Sampled
Spring
_____
M, penn. M, mont.
Captures Captures
3
.
3"
CD
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3
Vegetation Type
Carex
Typha
Poa
Elymus
Agropyron-Festuca
'Scattered annuals
Cirsium
Rosa Syrnphoricarpos
Bare
11
2
10
25
30
10
8
59
15
6
12
0
0
3
0
0
ii
36
a
8
8
50
13
15
0
0
0
11
63
21
8
li
5
0
11
52
5
33
-
-
3
10
50
13
15
0
0
0
11
5
0
11
-
—
-
62
11
ii
81
2
0
W
-
CN
-
1
19
17
ii
0
-
-
S oil M oisture Type
Wet
Dry
16
81i
78
22
0
100
7li
26
85
15
57
li3
71
26
73
27
83
17
CD
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Q.
C
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Q.
TABLE m
"D
CD
(c o n tin u ed )
C/)
C/)
Control Plot
CD
Prelim inary Phase
8
% Type M. penn.
Sampled Captures
3
.
3"
CD
CD
■
D
O
Q.
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a
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3
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O
CD
Q.
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CD
C/)
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Removal Phases
M. mont.
Captures
F a ll
% Type M. penn, M. mont.
Sampled Captures Captures
Spring
% Type M, penn. M, mont.
Sampled Captures Captures
Vegetation Type
Carex
Typha
Poa
ETÿrous
Agropyron-Festuca
S cattered annuals
Cirsium
Rosa Symphoricarpos
Bare
Ik
73
0
“■
—
2
38
11
3
13
0
0
10
0
LL
37
0
12
L
13
1
0
3L
3
1
3
0
7
0
16
8L
73
27
0
100
LI
9
16
2L
-
h
11
L
2
38
-
-
0
50
2
k
13
1
0
3L
~
L3
L6
10
0
iL
0
Li
59
2L
76
-
10
0
66(2)
-
0
-
33(1)
0
0
0
0
0
-
-
0
0
0
0
—
S o il M oisture Type
Wet
Dry
2
98
Ll
59
66(2)
33(1)
(N)
0
0
i
ro
-28-
I f tra p y ield , 1, e .,
the number of voles caught in a given trap,
i s a random function then the d istrib u tio n of captures should not vary
sig n ific an tly from a Poisson d istrib u tio n (Simpson, 15*60) <
. Using the
Chi-square goodness of f i t t e s t with a ,05 level of significance, the
observed trap yield data were compared to a th eo retical Poisson yield
for the same data.
Table IV summarizes the re s u lts of these analyses
for the Experimental p lo t.
Trap yield data from the Control plot during
the preliminary phase were too low to be suitable for distribution anal­
y s is .
In a Poisson series the mean equals the variance so that the
variance divided by the mean should equal unity.
I f the proportion
variance/mean i s sig n ific an tly less than unity, the variance i s less
than that appropriate to a Poisson se rie s, indicating th a t the in d i­
viduals in a population are distributed over the area more evenly than
expected from random scatterin g .
Conversely, i f th is proportion is sig ­
n ific a n tly greater than unity, the variance i s greater than th a t appro­
p ria te to a Poisson series,
indicating a d istrib ution less even (th at
i s , clumped) than th a t expected from random scattering (Andrewartha and
Birch, 195U)=
Table V presents sim ilar d istrib utio n analyses for Mi.rotas
pennsylvanicus on the Control plot during the f a l l remo/al phase.
Not
enough xeric habitat was sampled during the f a l l removal phase to ana­
lyze M. montanus distrib u tio n .
In M
„ montanus clumped d istrib u tio n i s associated with reproduc­
tio n , and random d istrib ution with the non-breeding period in August
(see Figure 13, page UU)-
This is true also in M
,. pennsy 1vancui
R eproduced with perm ission of the copyright owner. Further reproduction prohibited without permission.
TABLE IV
CD
■D
O
Q.
C
SUMMARY OF DISTRIBUTION ANALYSES FROM THE EXPERIMENTAL FLOT FOR THE PRELIMINARY PHASE
(Observed y ield frequencies compared to th e o re tic a l (Poisson) frequencies by the ChiSquare Goodness of F it T est.)
g
Q.
■
D
CD
C/)
C/)
8
M
.
No.
Voles
per
Obs.
Trap FreqSc
Dates
Theor.
Freqs.
Chi-square
and
S ig n ifi­
cance
Variance/ No.
Mean
Voles
( D is tr i­
per
ObSo
bution )
Trap Freqs.
M. montanus
Chi-Square V ariance/
and
Mean
( D is tr i­
Theor. S ig n ifi­
bution )
cance
Freqs.
(O'
6 /2 3 -6 /2 g
3
3"
CD
CD
■
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Q.
8/11-8/13
C
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CD
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CD
C/)
C/)
9 /lh -9 /iW
0
1
2
3
ii
0
1
2
3
0
1
2
3
h
?
6
0
1
3
h
5
13
6
9
3
1
11
6
2
3
17
11
111
9
9
6
0
30
17
13
9
1|
2
10
12
7
2
1
9
8
3
2
1^
21
15
7
3
1
0
lU
23
18
10
ii
2
1.33
s ig n i f i­
cant
not s ig ­
n ific a n t
23.32
very s ig ­
n ific a n t
21.52
very s ig ­
n ific a n t
1.16
(clumped)
1.01
(random)
2 .3 1
(clumped)
.111
(even)
0
1
2
3
ii
0
1
2
3
0
1
2
3
ii
5
6
0
1
2
3
1|
5
3li
5
1
2
-
67
15
1
0
1Ï5
20
31
6.00
s ig n i f i­
cant
9
1
1
1.81
(clumped)
I
ro
—
38"
111
1
0
■
I
.29
not s ig ­
n ific a n t
.95
(random)
110
27
1|
1
0
1
0
“ 89
7U
31
50
1 8 .5 8
13
7
3
2
17
li
0
0
veiy sig ­
n ific a n t
3
1
0
3.79
not s ig ­
n ific a n t
1.75
(random)
0
0
1 .8 1
(clumped)
-3 0 -
TABLE V
SUMMARY OF DISTRIBUTION ANALYSES FOR M. PENNSYLVANICUS ON THE CONTROL
PLOT DURING THE FALL REMOVAL PHASE
Dates
9/23-9/2L
and
9/30-10/1
10/15
and
10/28
11/It
and
11 /I I
No.
Voles
per
Trap
Obs .
Freqs
Theor
F reqs.
0
1
UO
2
h
3
U
U
2
32
22
7
2
0
0
1
2
3
hS
U2
lU
1
3
k
1
0
0
1
2
Bk
Bh
9
0
8
1
13
2
Chi-square
and
Significance
Variance 'mean
(Distri button)
5 hh
significant
1.8?
( clumped)
.68
not s ig n if i­
cant
.98
' random)
1.13
not sig n ifican t
.89
ra/
R eproduced with perm ission of the copyright owner. Further reproduction prohibited without permission.
-3 1 -
except for the August 11-13 trapping period»
The random distribution
of Microtus pennsylvanicus on the Experimental plot in mid-August may
be the re s u lt of summer heat causing a period of rela tiv e in activ ity
at the time»
Movements and Population Densities
Brant (1962), conducted a study to determine ".
. » the con­
stancy with which small rodents occupy specific areas, and the manner
in which grid live-trapping » » » data may be used for estimating den­
s itie s ,
even though the area occupied by an animal may change in size,
position, and in te n sity of use»"
He proposed use of the term "movement
pattern" to refe r to the " » » » actual movements in three-dimensional
space th at an animal makes during any specified period of time » » »
Brant adopted use of the term 'movement pattern" in order to avoid
'■"the ambiguities associated with home ranges » » » »
Furthermore, he
considers the positions of recapture for any animal to be a function,
"in the mathematical sense," of the movement pattern of that animal »
He feels th at i f a trapping technique is standardised and since a.
functional relationship between movements and capture ex ists, then
changes in capt'ore pattern w ill re fle c t changes in movement pattern*
Brant formulated and discussed ap plicab ility of three recap­
ture measures:
1.1) the maximum distance between capture ,u M >/'i the
distance between successive captures, D; and ( 3 ) the time between
captures, t
He concluded that these thr ee measures have a f ut ct.oual
relationship to the movements of grtd-luve trapped animals.,
The maximum distance between captures is defined a: the di^.ta* :e
» » between captures a fte r the second, th ird ,
» » . --..‘n recaptures
R eproduced with perm ission of the copyright owner. Further reproduction prohibited without permission.
”32”
for each animal."
Placing the number of captures
'.
. . on the ab­
scissa of a graph, and the average length of M fo r a number of ani­
mals o o o designated av. M., . o . on the ordinate, a curve can be
plotted th a t w ill show the manner in which av. M. increases with
successive captures
» .
u n til . . . "th is curve becomes asymptotic
to a lin e p a ra lle l to the abscissa ,= . . . "
This asymptote represents
the lim its of movement fo r these animals and i t i s assumed that the
population has a stable movement pattern and th at individual animals
. eventually captured near the lim its
were
. . . " o f th is pattern.
In the event th is curve finds no asymptote, then ".
. . i t seems lik ely
th at the animals were progressively occupying new areas."
The "distance between successive captures, D," xs the sum of the
distances between a l l or a portion of the recaptures of an individual
or a population for a given period of time.
D may also be computed for
each sex and/or each age class of a population.
The value, average
distance between successive captures, av , D, is the sum of the distances
for the individual, population, or age class, divided by a number <K) of
individual observations recorded.
several ways including ‘'( l )
movement patterns . . . .
Brant suggests use of av,. D in
a comparison of the relativ e size of the
of two or more populations;; ’,2 ) a determination
of the changes in movement patterns that may be related to seasonal
changes in density, breeding,, or other factors^ ( 3 ) an attempt to deter­
mine how stable . . .
parameter to replace
movement patterns are; and ( h) . .. . a suitable
“one-half the meari width of individual home
ranges* in estimating the si se of the area sampled by grid live
trapping.
R eproduced with perm ission of the copyright owner. Further reproduction prohibited without permission.
-33-
Brant
ture and the
designated the measure of time, in days^ between
next t j
and the average values,:, av. t .
successive captures, t ,
one cap­
The time between
and av. t may also be computed for individuals^
populations, and/or populational sub-units in the same manner as that
for values of D and av. D.
Values of av. t/2 may be used as an e s t i ­
mate of the number of days a marked animal i s s t i l l available for tra p ­
ping a fte r i t s l a s t capture.
Brant
re fle c ts th a t "the explanations of M, D, and t
seem too abstract unless one struggles a b i t
.
with the problems
in interpreting the recapture patterns . . .
. . may
faced
He provides several
“common sense® examples of these three measurements in order to c la rify
the logic of th e ir use, and reference is made to his paper for further
elaboration and c la rific a tio n .
Brant's suggested uses of M, D, and t are adopted in th is r e ­
port.
Tables VI and VII give tabular analyses of M and D values, and
Figure 11, page 3 6 , presents a graphic analysis of av. M curves.
A
synopsis of t values i s not offered since only the form av. t / 2 of
th is parameter i s used in order to estimate the length of time a
marked animal remained in a study area a fte r that animal was la s t cap­
tured .
Values of av. M are presented in Table VI.
Brant noted that
fluctuations in av. M are related to a loss of indrviduals from the
sample.
For example, values of av. M for Microtus pennsylvanicus
females on the Experimental study plot in Table VII rise for the f ,1r s t
two captures, then drop for the fourth and f i f t h
captures.
Bra,nr,
fe e ls " th is decline in av. M indicates the loss through death or
R eproduced with perm ission of the copyright owner. Further reproduction prohibited without permission.
CD
■D
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CD
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TABLE VI
MAXIMUM DISTANCE BETWEEN POINTS OF CAPTURE, AV. M, ON BOTH STUDY PLOTS
(N re fe rs to the size of the sample. Values above the ho rizo n tal lin e
in the av. M columns are p lo tted in Figure 11, and values below these
lin e s in the av. M columns were averaged and are shown as horizontal
lin e s in Figure 11.)
CD
8
CD
_______
Mo Pennsylvanie IS
3
.
3"
M. montanus __________
CD
Experimental P lo t
CD
"O
O
Q.
C
Q
Capture
°
■
D
O
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D
CD
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2nd
3rd
llh
5th
6th
Mh
8 th
9th
Males
N av. M
33
13
6
9
1
,1
28
liO
13
13
0
0
Female^
N av. M
L5
23
12
5
3
3
n
0
33
3b
23
0
12
8
36
16
Control P lot
Males
N av. M
ii
5
2
0
1
29
6l
30
0
25
Experimental P lo t
Females
N av. M
19
11
6
)4
2
1
,1
ll
26
23
3H
50
75
35
Males
Îî av^ M
ll
I
2
32
31
30
Females
N av. M
31
9
5
2
1
35
13
IS
56
71
Control Plot
Both Sexes
N av. M
8
3
1
ll
28
25
I
00
9
-35TABLE V I I
AVERAGE DISTANCE, AV. D, BETWEEN SUCCESSIVE POINTS OF CAPTURE
ON THE TW
O STUDY PLOTS
(Av, D was determined for each sex and species by dividing the
to ta l number of fe e t between successive capt'ures by the to ta l
number of recaptures (N) during the phase lis te d .
From Brant,
1962,)
Study Plot
and Phase
Control plot
Preliminary
phase
Microtus pennsylvanicus
"Females
Males
Av, D
Av, D
N in fe e t
N in fe e t
17
25
6
58
Microtus montanus
Females
Males
Av, D
Av, D
N in feet
N in feet
-
-
Control plot
Fall removal
phase
25
23
15
25
2
0
7
19
69
10
39
23
33
30
1?
30
27
h6
18
38
13
ih
3
50
2
210
-
-
-
-
Experimental plot
Preliminary
phase
Experimental plot
Fall removal
phase
Control plot
Spring removal
phase
Experimental plot
Spring removal
phase
1
3
262
98
2
93
R eproduced with perm ission of the copyright owner. Further reproduction prohibited without permission.
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60 --
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Maies - Exp. p lo t
•Fem ales - Expo p lo t
“ Females-Control p lo t
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5
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5 60.
3
h
5
Number of captures
0) 20 _.
ü
S
p
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'H 10 0 Maies - Exp. p lot
n
• Females - Exp, p lo t
® Both sexes-Exp, p lo t
^ Roth se^es-Gontr^l p' lot
p
g
r
Number of captures
FIGURE 11
MEAN MAXIMUM DISTANCE BETWEEN POINTS OF CAPTURE, AV, M, PLOTTED FDR SUCCESSIVE CAPTURES
iS o lid horizontal lin e s represent an average of the av. M values below the lin e s in the
av M columns in Table VI. Butted h orizontal lin e s represent asymptotes for each curve.)
I
O
V
jJ
“37 ~
emigration, of individuals with large movement patterns
Be­
tween the fifth and sixth captures two individuals, each with a small M,
disappeared, and this resulted in an increase of av. M..
The disappear­
ance of animals with large values of M may indicate that such animals
leave the trapping area or that they have a lower swvival value than
animals with small values of M (Brant, 1962).
Values of av. M for the two species are graphed in Figure 11,
page 3 6 . Values below the horizontal line in the av. M colums of
Table VI, page 3U> were averaged and plotted as solid horizontal lines
in Figure 11 in order to avoid the confusion of marked fluctuations in
av. M as sample size (N) decreases.
Estimates of av. M values for micro tines on both study areas are
based upon the asymptotes (dotted horizontal lines) of the av. M curves
in Figure 11.
A comparison of asymptotes in Figure 11 reveals that
most values of av. M were less than 60 feet, arid that av. M values for
male Microtus penn sylvan1cu a were higher than av , M values for female
M. pennsylvanicus on both study plots.
Asymptotes in Figure 11 are
higher for M, pennsylvanicus on the Control plot than for
M. pennsylvanicus on the Experimental plot, and higher for M. montanus
on the Experimental plot than for M. montanus on the Control plot.
The
av. M values for M. montanus are biased however, since only two recap­
tures of this species were obtained on the Control plot during the pre­
liminary phase and only a small amount of xerrc habitat on both study
plots was sampled during the removal phases.
The foregoing considerations Indicate that most of the individual
vole movement patterns were stabilized at or below 60 feet
R eproduced with perm ission of the copyright owner. Further reproduction prohibited without permission.
-38-
Table VII^ page 35> presents an analysis of the average distance
between successive captures, av. D, for each sex and species of Microtus
on the two study areas.
This table shows that av. D increased during
the f a l l removal phase for both male and female M. pennsylvanicns on the
Experimental plot while av. D on the Control plot remained essentially
the same for females and decreased for males during the same study phase.
The higher values of av. D for M, pennsylvanicus during the f a ll removal
phase on the Experimental plot are associated with the progressively
smaller trap yields resulting from the reduction of the M. pennsylvanicus
population on the study plot (see Figure 12).
Brant (1962) also noted an inverse relationship between movement
patterns and densities of Reithrodontomys and Peromyscus and some indica­
tion of a relationship for high densities of M. callfornicus.
He con­
cluded that movement patterns of the f i r s t two rodents were inversely
density-dependent, but f e l t i t unlikely that av. D was density-dependent
for M
o califomic-uSo The data in Table VII, page 39> and Fig:.re 11 , page
3 6 , however, indicate the av. D may be Inversely density-dependent for
at least M. pennsylvanicus.
The limited data available for M. montanus xndxcate that av\. D
decreased fo r females and increased for males on the Experimental plot
during the f a ll removal phase,
Table VII shows that during the prelimi­
nary study phase av. D values for female M
- pennsylvan icu s were lower
than av. D values for male M. pennsylvanicus on both study plots..
The
comparatively low av. D values for M. pennsylvanicus d im ,g the pre­
liminary phase and female M. montanus during the f a ll removal phase may
be correlated with increased reproductive a c tiv itie s in these mice
R eproduced with perm ission of the copyright owner. Further reproduction prohibited without permission.
-39-
Control plot
U 0--
o - M
o pernsylvaniens
&
montanus
- Mo
20--
10
June
July
Augo
Sept..
Oct..
Preliminary phase
w
Q
i
o 200
Nov,,
Fall removal phase
Experimental plot
M
o- M
o penn sylvan icus
P-.
■O
#100
M 90
m 80
s 70
:
à
montanus
M.
Ê0
so
ho
\
A
30
\
>
20
O
Q
O
-
10 y
June
July
Aug,
Preliminary phase
Sept,
Oct
^
Nov.
Fall rem^. a: phase
FIGURE 12
SEASONAL DENSITIES OF MICROTUS ON THE TW
O SThDY PLOTS
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H
-Uo—
during the same time (see Figure 13> page UU) »
Brant found a similar
contraction in the movement patterns of female Microtus callfornicus
during breeding periods.
Figure 12, page 39> gives a comparison of microtine densities on
the two study plots during the preliminary and f a l l removal phases of
the study.
The number of animals on each area was computed essentially
by means of the Schnabel method (Davis, 1961).
The "number of marked
animals in the area" column in the Schnabel method table was modified.
In th is column a marked animal was considered no longer available for
trapping av. t/2 days a f te r i t was l a s t captured.
The value av. t/2 is
used in order to make the Schnabel method more r e a lis tic by taking into
account the disappearance of marked animals,,
The value was applied only
to those marked individuals th a t were not removed or found as trap
m ortalities-—i .
e„, those whose la s t recapture need not have been th eir
terminal recapture.
The size of the area sampled was determined for each species of
Microtus by adding a border of width av. D to the perimeter of traps
enclosing the area considered ch a rac te ristic habitat for each microtine..
Figures 6 and 7, pages 21 and 22, show the rela tiv e distribution of
areas fo r which microtine densities were computed„
Figure 12 then^
gives economic density (Allee, _et ad--, 19U9) for each species on the
two study areas.
The M. montanus density for the Experimental plot
during the f a l l removal phase is not shown since only very minimal
sampling of xeric h ab itat was conducted there during this phase.
The
density of M. montanus population on the Experimental plot is assumed
to have remained a t le a st stable through the f a l l Sj.nce in d i''? id u a ls
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—U X —
were often captured in traps occasionally set in xeric habitat to check
fo r th e ir presence^ and since la te r in the f a l l several Microtus montanus
were taken in mesic habitat,.
The sta b iliz a tio n of the M, pennsylvanicus population on the
Experimental plot a t 2 h mice per acre during the f a l l removal phase is
interpreted to mean th at d r i f t of M
o pennsylvanicus from adjacent habitat
was such th a t the method of removal was not e ffic ie n t enough to produce
a lower density.
That is^, the method of removal in the face of th is
d r i f t was only e ffic ie n t enough to maintain a M
» pennsylvanicus density
of 2 h mice per acre.
This also means th at the d r i f t fences and d r i f t
trap s were not completely effective^
No microtines were captured fo r marking during the spring removal
p h a s e s o no densities were computed for th is period.
Five hundred-
sixty trap days on the Control plo t yielded only 3 M. pen n sy lv an icu sa ll
three being recaptures th at had been marked during the previous summer
and fallo
Average trap yield for the Experimental plot during the f a ll r e ­
moval phase was .08; for the spring ran oval phase .09; indicating th at
Experimental plot microtine densities for the two periods were slmilar„
Sex Ratios
With the exception of M. pe nn sylvan icus captures during the f a l l
removal phase, traps on both study plots yielded fewer male than female
microtines.
Table VIII presents a comparison of sex rat i os based upon
i n i t i a l captures from the study plots during the various rtudy phases.
Thirty-three M. pe nnsy1van1 cus and 2U M. montanus were not
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sexed.
—
U2—
Too few microtines were captures on phe Control plot during the
spring removal phase to analyze sex ratios..
TABLE VIII
COMPARISON OF SEX RATIOS IN BOTH STUDY PLOTS
DURING THE STUDY PHASES LISTED
M. pennsylvanicus
Study Area
and Phase
Number
Females
Number
Males
M
o montanus
Number
Females
Number
Male s
Experimental plot
Preliminary phase
65
U7
lt6
3U
Experimental plot
F all removal phase
63
83
18
12
Experimental plot
Spring removal phase
21
6
25
21
Control plot
Preliminary phase
13
7
5
Control plot
F all removal phase
17
20
ih
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10
-L 3 -
Breeding A ctivity
Breeding a c tiv ity was determined by graphing the per cent reprodactively active females of each species of Microtus (see Figure 13)„
A reproductively active female was one with a perforate vagina and/or
v isib le mammae.
The percentages of juvenile (sexually immature) micro-
tin es of both sexes captured are a^so graphed.
Figure 13 shows a f a l l period of reproductive activ ity for
M
o monta:;v.s th at is not found for M. p e n n s y l v a n i c u s This tendency
towards non-overlapping reproductive seasons may be the resu lt of in te r ­
specific competition*
Before offering such a conclusion, however, xt is
f e l t th a t fu rther investigation xnto the phenomenon is needed, since the
phenology of microtine reproduction is known to be highly variable
(Re Se Hoffmann, pers, comm.)■
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-h h -
Microt-us pennsylvanic~as
w
(D
f-1
cd
I
C
m
0)
.5
Ü
£
^J
<n
CD
rH
■
’H
I
g
§
Period of no
trapping
Q
)
" '- D
-4^
Ü
-§
o
a
0)
c
M O N T H S
CD
Ü
u
CD
P-.
^ lo o r
c
CD
Microtus montanus
k
iX
t
803-
8a60
- -
UQ.
20 . .
20
Period of no
trapping
J
J
A
S
0
N
J
D
MO N T H S
F
M
A
M
FIGURE 13
GRAPH OF THE PERCENTAGES OF REPRODUGTIVELY ACTIVE
FEMALES AND JUVENILES CAPTURED DURING ALL
STUDY PHASES ON BOTH STUDY PLOTS
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-U5*
DISCUSSION
Competition and Overlapping Niches
In a comparison of North American and Bermudian avifaunas^
Crowell ( 1 9 6 2 ) found three passerine species common to both the con­
tin en t and the island.
He also found the to ta l avifaunal complement
u tiliz in g the habitats of the three Bermudian passerines to be com­
prised of a fewer number of species than the number of species u t i l i z ­
ing
the respective hab itats of the three North American birds.
From
a comparative gynecological study Crowell concluded th at, due to the
smaller number of avian species in Bermuda, the three species of
passerines experience a lesser degree of in tersp ecific competition in
Bermuda than they do in North America.
Crowell demonstrated th a t a
manifestation of th is reduced competition was the ecological replace­
ment of the North American species missing from Bermuda by the three
Bermudian passerines.
Crowell found th a t ecological replacement of the North American
species missing from Bermuda by the three passerines resulted in slig h t
a lte ra tio n s of b i l l shape and sizes of the Bermudian birds but th at re ­
placement occurred without the acquisitions of new behavioral t r a i t s ,
indicating that niches are overlapping rath e r than discrete..
Examination of microtine d istrib u tio n i s also indicative of
overlapping niches.
That i s , in areas where species of Microt a s
pennsylvanicus, M. montanus, or M. ochrogaster occur alone, they u t i ­
liz e a l l available microtine habitat (Findley, 1 9 5 U ) , a situation simi­
la r to the niche u tiliz a tio n of the three passerine species in Bermuda.
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-L 6 -
However^ in areas of sympatry^, microtines u tiliz e the habitat contain­
ing niche factors for which they are optimally adapted^ a situation
sim ilar to niche u tiliz a tio n of the three passerine species in North
America.
In te n sity of In tersp e cific Competition
Inquiry into the intensity of in tersp ecific competition between
microtines on the National Bison Range leads to a consideration of the
proposal that the in ten sity of competition between Microtus
pennsylvanicus and M. montanus is d ire c tly proportional to the amount
of mutual h ab itat u tiliz a tio n by the two species.
The rela tiv e number
of traps yielding captures of both species of vole is assumed to be
proportional to the in te n sity of in te rsp e cific competition.
Figure 6.
page 21^ shows that of the 130 traps yielding microtine captures on the
Experimental plot during the preliminary phase^ nine {7%) captured both
species of voles.
On the basis of the foregoing proposal., there
appears to have been in te rsp e c ific competition fo r 7% of the area in­
habited by microtines on the Experimental plot during the preliminary
phase.
Figure 7> page 22^ shows th a t none of the traps on the Control
plot yielded any captures of both species of Microtus during the pre­
liminary phase j, indicating th at there was no in tersp ecific competition
for mutual niche resources between the two species of vole on the Control
p lo t during the preliminary phase.
However, Figure 10, page 25, shows
th a t of the U3 traps yielding microtines on the Control plot during the
f a l l removal phase, eight ( 19%) captured both species of vole, indica­
tin g th at there was increased in tersp ecific competition for the area
inhabited by microtines at th at season.
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“U?~
The seasonal variation in m’ltual habitat overlap may be corre­
lated with the seasonal variation in population densities on the two
study plots (see Figure 12, page 39)»
During the preliminary phase
combined microtine species population densities on the Control plot
were comparatively lower than either the fall removal phase densities
on the Control plot or the preliminary phase densities on the Experi­
mental plot.
A consideration of the nature of trap yield i s necessary to
enlighten any conclusions regarding the inten sity of in tersp ecific com­
p e titio n based solely upon the number of traps yielding both species of
Microtus.
The nine traps on the Experimental plot that captured both
species of vole during the preliminary phase yielded 37 captures of 32
individual microtines—20 capt’ores of 20 M. pennsylvanicus and 1’’ cap­
tures of 12 M
o montanusc
That i s , 32 of the 37 microtines captured in
the nine traps were not recaptured in any of the same traps during the
preliminary phase.
A comparison of the proportion of recaptures to
t o ta l number of captures for these 9 traps to a similar proportion for
the 56 traps yielding only M. pennsylvanicus, and the 65 traps yielding
only M. montanus, gives a probability of recapture ra tio of 5 .3"’r i l l , ''l l 8:
52/lU2 ( olUs »914-2 «36) for the respective trap groups.
This r a tio repre­
sents comparative indices of a c tiv ity among the trap groups.
The
reason th a t the index of a c tiv ity for the nine traps yielding both
species of Microtus is the lowest is because these traps are in areas
of re la tiv e ly minimal microtine a c tiv ity .
Similar analyses for the
Control plot during the f a l l removal phase gives 8 traps yielding both
species of vole, 26 traps yielding only M. pennsylvanicus, and 9 traps
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-L 8 -
yxelding only Micro ~bus mon'banns <
» Comparative Indices of a c tiv ity of
^/27°37/63all/26 (*3 3 5 . 5 9 %
0^2 ) are found.
Again, the traps yielding
captures of both species of vole have the lowest index of a c tiv ity .
Plotting the centers of activ ity (Hayne, 19U9) for microtines
captured two or more times daring the preliminary phase on the Experi­
mental plot (Figure II4,) gives one a visual impression of the fa c t that
voles avoided the traps yielding captures of both species.
Further­
more, a lin e connecting the outermost perimeter of centers of a c tiv ity
of M. pennsylvanicus and the innermost perimeter of centers of a c tiv ity
of M. montanus shows that these voles avoided a f a ir ly wide intervening
s t r i p between th e ir respective h ab itats.
The average width of th is
s t r i p i s about UO fe e t (range between 15 and 100 fe e t).
A
comparison
of the average width of th is s tr ip with the av* D values of Table VII,
page 3 5 , indicates th at the movement patterns of the voles were less
than the average width of th is s tr ip .
These fac ts and the comparatively low index of activ ity for the
group of traps capturing both species suggest th a t there was not as
much competition for mutual space as a consideration of only the munber
of traps yielding captures of both microtine species suggests.
A consideration of centers of a c tiv ity for the Control plot is
not presented since too few M. mon tan .s were recaptured during the pre­
liminary phase and not enough xeric habitat was sampled during the f a l l
removal phase to give an unbiased analysis of centers of a c tiv ity for
M. montanus.
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-U9=
——
x"
X
X
• - M
o pennsy 1vaniciis
centers of a c tiv ity
if - M
o mont arras
centers of a c tiv ity
FIGURE lU
CENTERS OF ACTIVITY FOR MICROTINES
ON THE EXPERIMENTAL PLOT DURING
THE PRELIMINARY PHASE
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-5o-
Effects of Interspecific Competition
Although the competitive system between Microtus pennsylvanicus
and M. monxanus has strong distributional consequences, i t does not
seem to have affected any of the population attributes other than the
possible tendency towards non-overlapping breeding seasons (see above),
A comparison of microtine sex ratios obtained in this study with
those from some other studies does not give cause to suspect that in ter­
specific competition has affected this population attribute.
Table VIII,
page 14.2 , shows that the cumulative sex ratio for M, pennsylvanicus for
the entire study was essentially 1 male to 1 female ( l 6 ii maless 171 fe­
males) c and the cumulative sex ratio for M, montanus was 1 male to 1,2
females ( 82 malessllO females),
Getz (196 I) obtained a cumulative sex
ratio for M, pennsylvanlcus population in Michigan of 1 male to 1,20
females [hhS males® 536 females),
Spencer (n, d ,‘f reported a July sex
ratio for M, montanus in Oregon of 1 male to 1,75 females (20 males® 35
females),
Spencer gave no further breakdown on the relative numbers of
male and female M, montanus, reporting rather that
,
of trapped animals were recorded at frequent intervals.
. the sex ratios
At no time was
the I s l ratio of males to females far out of balance,'
In this study no attempt was made to conduct a comparative mor­
phological, behavioral, or physiological analysis between sympatric and
allopatric populations of M, pennsylvan1 cus and M, montanus,
Recently,
Findley and Jones (1962) conducted such a comparative morpho.logical
study between sympatri.c and allopatric populations of M, montanus
M
o longicaudus, and M, mexicanus in New Mexico,
They concluded that
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n
-51-
the White Mountains of Arizona competition between sympatric populations
Microtus mexicanus and M
o montanus has resulted in some color, size,
and cranial divergences that differ significantly from allopatric varia­
b ility of these characters»
Browr* and Wilson (1,956) have reviewed the
lite ra tu re on allied species that are convergent when allopatric and
divergent when sympatric»
They coin the term "character displacement'"
to characterize the phenomenon, and describe character displacement as
the situation in which, when two species of animals overlap geo­
graphically, the differences between them are accentuated in the
zone of sympatry and weakened or lost entirely in the parts of their
ranges outside this zone. The characters involved in this dual d i­
vergence-convergence pattern may be morphological, ecological, be­
havioral or physiological. Character displacement probably results
most commonly from the f i r s t post-isolation contact of two newly
evolved cognate species. Upon meeting the two populations irteract
through genetic reinforcement of species barriers and/or ecological
displacement in such a way as to diverge further from one another
where they occur together.
This study and the studies of Findley f',195U) and Findley and
Jones ( 1 9 6 2 ) indicate that microtines exhibit at least morphological,
ecological, and behavioral displacement characters.
Removal of M. pennsylvanicus from the Experimental Plot
One way to te s t the hypothesis that the reduction of a
M. pennsylvanicus population in a hydrosere community will indu-'e move­
ment of M. montanus into the /acated habitat
to make a comparison of
the preliminary and removal phase trap yields of M. montanus ..o ttie
mesic habitats of the experimental and control study pioio.
During the preliminary phase neither the U21 tiap efforts
one
trap effort is the exposure of one trap for one nighi or o.e day vr one
week) in the mesic habitat of the Experimental plot nor the 101 trap
efforts in the mesic habitat of the Control plot yielded a.y raf.t j.rocr
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-S 2 of Microtus montanuso It Is concluded, therefore^ that habitat segre­
gation was maintained on both study plots during the preliminary phase»
Fifteen hundred and ninety-five trap efforts in the mesic habitat
of the Experimental plot and l68 trap efforts in the mesic habitat of
the Control plot during the fall removal phase yielded U6 captures of
Mo montanus, U5 of which were on the Experimental plot»
The one capture of M» montanus on the Control plot during the fall
removal phase was of an adult male that had been recaptured several times
in the small "island" of Elymus directly north of the pond and west of
the overflow ditch (see Figure S, page lU) and whose center of activity
was in this small stand of Elymus » This vole is believed to have been
frightened into entering the trap in mesic habitat, due south of the stand
of Elymus, since he was observed to leave the base of a large Elymus
plant and scurry about 20 feet to enter this trap as the author walked
past the spot of his departure,.
Upon checking the trap the author dis­
covered his identity; when he was released he rushed back t_. the place
of his original departure»
mid-October.
This tole was captured in mesic habitat in
He was subsequently recaptured In the island of xeric
Elymus habitat; never again in mesic habitat »
In order to compare these captures they were calculated on the
basis of yield per 1,000 trap efforts.
This gave yields of ?8 raptures
of M» montanus per 1,000 trap efforts for the Expei i.meni al plot and 6
captures of M. montanus per 1,000 trap efforts for the Control plot.
statistical comparison of this difference in yields,
square goodness of fit test, gave a Chi-square of
nificant even at the .0005 level.
A
ising the Chiwhich is sig­
It is concluded therefore "nat
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-S 3 reduct-ion of a Microtis pennsylvanicus population from a hydro sere will
induce movements of M
» montanus into the vacated habitat.
There were no captures of M. montanus on the Control plot during
the spring removal phase.
Therefore, no yield comparison between the
two plots could be made for this period.
However, 757 trap efforts in
the mesic habitat of the Experimental plot yielded 83 captures of
M
o montanus, giving added indication that M. montanus will invade the
habitat of a reduced population of M. pennsylvanicus.
Movements During Removal Phases
I t is d iffic u lt to draw any conclusions regarding the nature of
movements of M. montanus in mesic habitat, based upon their captures in
mesic habitat, since so few of the captures yielded living animals.
Twenty-two of the 26 captures of M. montanus during the f a ll removal
phase and a ll 83 of the captures of M. montanus in mesic habitat during
the spring removal phase were obtained as mortalities.
Table I I , page 19, shows that there were l5 recaptures of
M
o montanus during the f a ll removal phase and two recaptures during the
spring removal phase.
Analysis of these captures shows that foicr of
the f a ll removal phase and both of the spring removal phase recaptures
were in mesic habitat.
Both spring recaptures and two of the four f a ll
recaptures were obtained as mortalities, leaving only two of the fail
removal phase recaptures available for future recaptures.
In addxtion,
during the fa ll removal phase four M. montanus were Initially captured
arid marked in mesic habitat.
None of these six M. montauu- , available
for recapture, was ever recaptured, indicating that movements were
probably exploratory in nature.
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-ShFigure 10^, page 25.., and Table UI^ page 26^ show that most of
the Microtus pennsylvanicus captures on the Control plot during the
f a l l removal phase were in the comparatively xeric habitat adjacent to
mesic habitat that had been occupied daring the preliminary phase
(Figure ?, page 22, and Table I I I , page 26), indicating a seasonal shift
of the population from mesic habitat to adjacent comparatively xeric
habitatc
Figure 8, page 23, and Table I I I , page 26, do not show this
sh ift so clearly for the Experimental plot, since a much higher pro­
portion of mesic habitat was sampled on the Experimental plot during
the f a l l removal phase.
Nevertheless
the majority of captuires of
M
o pennsylvanicus, during the fa ll removal phase, and a ll captures of
M
o montanus (obtained mostly after mid-October) during the f a ll were
in mesic habitat adjacent to xeric habitat, and Figure 8, page 23, shows
that only the perimeter of traps bordering the xeric habitat yielded any
captures of M
« montanus,
The majority of M
o pennsylvanicus captures
after early October were obtained in the perimeter of traps bordering
the xeric habitat.
I t was not until during the spring removal phase
that any 11o montanus invaded the very mesic ^Figure 9, page 2k':
Movements of M. pennsylvan^eus on the Control plot to the adja­
cent xeric habitat was maiviiy the distante of one row uf traps on either
side of the mesic habitat.
Table VII, page 35. shows that av. D values
for M
v pennsyIvanicus on the Control plot during the fa il re mo uil phase
were about 25 feet, a distance l^ss than the asymptotes for the av. M
curves in Figure 11, page 36., i.ndi.cat:i,ng that the f a ll movement;, to the
relatively xeric habitat were well with,in the normal movement pattern:.,
and this f a ll movement into xeric hab,„tat was probably not much fart.her
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-S 5 than about 25 feet from the nearest mesic habitat„ and represents
nothing unusual.
The avc D value for the four marked Microtus montanus invading
the mesic habitat on the Experimental plot during the f a l l removal
phase was 105 feet, and the av. D value for the two marked M. montanus
invading mesic habitat during the spring removal phase was 95 feet.
Both of these values are much greater than the av„ M curve asymptotes
graphed in Figure 11, page 36, indicating that these movements were a ll
well beyond the normal movement patterns of the animals involved»
These animals may have been attempting to shift their movement patterns
and their actions may represent the fact that most of the M
» montanus
captured in mesic habitat were attempting to establish new centers of
activity»
In the absence of suitable recapture data, however » i t is
impossible to interpret any M
» montanus movements into vacated
M
» pennsylvanicus habitat, other than to say that existing results sug­
gest the movements were exploratory in nature»
Exploratory movements
are, in the final analysis, a necessary prerequisite to the establish­
ment of new centers of activity, and further work may show that indi­
viduals of M
o montanus may establish themselves in vacated
M
o pennsylvanicus habitat»
C o m p e titiv e Niche E x p l o i t a t i o n and
N o n -C o m p e titiv e N iche Occupancy
The s tu d y d e s ig n e d t o t e s t F i n d l e y ’*s p r o p o s a l t h a t t h e m a i n t e n ­
a n c e o f h a b i t a t s e g r e g a t i o n I n s y m p a tric m i c r o t m e p o p u l a t i o n s is due
i n part t o i n t e r s p e c i f i c c o m p e t i t i o n , r e s u l t e d i n an a c c e p ta n c e o f t h e
h y p o t h e s i s t h a t th e r e d u c t i o n o f a M» p e n n s y l v a n i c u s population in a
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•“5 6 -
hydrosere will induce the invasion of Microtus montanus into the hydrosere.
An evaluation of the nature of trap yield during the investiga­
tion indicated, how everthat the two microtines avoided rather than
competed for mutually shared habitat., and that the movements of
M. montanus into mesic habitat were of an exploratory nature, resulting
in the conclusion that some reciprocal avoidance mechanism such as
interspecific intolerance may be operating to prevent, or at least re­
duce, competition for mutual niche resources.,
A similar experimental
study was conducted by Teal (1958) on habitat selection of two species
of fid le r crabs (Uca) which inhabit adjacent salt water marsh habitats.
One species, U. pugilabor, inhabits sandy fla ts, while the other,
U. pugnax, inhabits muddy, marsh grass-covered substrates.
Teal found
that one species tended not to invade the vacated habitat of the other,
indicating the complete absence of active competition in these sympatric
congeneric -species.
The absence of interspecific competition in
closely related sympatric species does not mean that competi;t::on in the
past has to be ruled out as a factor bringing about habitat segregating
mechanisms (Odum, 1959).
The studies of Svardson (19U9) on turds and
of Harris (1952) on rodents, showed that the nature of habitat segre­
gating mechanisms in at least some vertebrates are behavioral.
These
studies demonstrate that a number of bird species and two subspecia^ of
Feromyscus maniculatus, P. m. gracilis and P. m. b a i r d i possess innate
behavioral mechanisms for the recognition of habitats conta'rr.ing the
niche factors conducive to their existence.
In l i ^ t
of the concepts of n-atural selection and on the hasi
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-S7-
of the results presented and discussed in this study^ and the niche
utilizatio n models of Mayr (19U9) and Svardson (19U9), i t would appear
that natural selection favors the most efficient exploitation of a
nicheo
The role of interspecific competition is viewed here merely as
one of the many possible factors delimiting the N-dimensional hyper­
space of a specieso
This view places the same significance upon the
role of interspecific competition as i s placed upon the role of any
other factor delimiting niche., such as food^ space, temperature, light.,
moisture, etc»
I t is known that these other factors exert their most
limiting influences on population density levels mainly when a new form
of one of these factors is f i r s t encountered by a species, e. g ., during
major climatic change, etc. 'Allee, et
19L9; Lack, 19$h)°
These
limiting influences are selective in nature, serving to *fit'J uhe genome
of a species to the newly modified hyperspace.
The significance of the reciprocal depressing effects of the ex­
perimental mixed species culture studi.es of Cromble ' 19LS, 19U7) ,
Gause (193U), Park f 19L8î 195La, b). Park, et
' 19L1), Neyman, et al.
(1957)> Frank (1952, 195^), and others is in their resemblance to what
takes place when formerly allopatric congeneric species f i r s t come into
a situation of sympatry, i . e . , they represent models suggestive of the
fact that recently sympatric allied species undergo competitive riiche
exploitation.
The relative absence of interspecific competition reported
in th is study and the relative or complete absence of interspecific com­
petition reported in the studies of Cade (I960),, Lack .1945
-946„ 1947) ,
MacArthur (1958), Pitelka (195i), and others, for established sympatric
congeneric species are viewed as models suggesting that established
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-5 8 -
syropatrically allied species enjoy non-competitive nicne-occupancy»
Field studies demonstrating significant int-erspecific competition be­
tween sympatrically allied species should consider the possibility
that these species may have recently come into a situation of sympatry.
CONCLUSIONS AND SUGGESTIONS
On the basis of the foregoing discussion,, i t is concluded that
the c rite ria of interspecific competition, as set forth in the Intro­
duction, are not met by Microtus pennsylvanicus and M. montanus on the
National Bison Range.
Rather, i t appears that a mechanism of in te r­
specific intolerance maintains a s tr ic t habitat segregation between the
two species.
The exact nature of this mechanism can only be speculated
on and a behavioral laboratory study, such as the one conducted by
Harris (1952), is suggested in order to solve the problem.
The limited data available for M. montanus movements into mesic
habitat suggest that these movements were of an exploratory nature.
Before concluding that M. montanus will not establish movement patterns
in mesic habitat, however, i t is suggested that the selective removal
of M. pennsylvanicus from mesic habitat be continued, in order to see
whether individuals of M, montanus will in time come to in'habtb mesic
habitat in the absence of M, pennsylvanicus.
F i n a l l y , e x a m in a tio n of the centers of acta 1 ty in Figure lU,
page U9> s u g g e s t s t h a t M. p e n n s y l v a n i c u s w i l l establish movement, patterns
i n x e r i c h a b i t a t , s i n c e some o f t h e M. penn syIvan, ic ur center,^ of a-: tiv lty
a r e i n Elymus v e g e t a t i o n .
a lso
sh ifted
Microtus pennsylvani.cus ^r. the Control p_.^t
their a c t i v i t i e s to r e l a t i v e l y xeric habita.!: d.j,r.ing tne fali.
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-S 9 removal phase»
Therefore? the reciprocal reduction of Microtus montanus
population is suggested to determine whether individual M. pennsylvanicus
will come to occupy xeric habitat.
SUM
M
ARY
A field study designed to te s t the hypothesis that habitat segre­
gation in sympatric populations of microtines may be due to in ter­
specific competition was in itiated in June? 1961? and carried on through
mid-November, 1961.
Field work recommenced in April? 1962? and was
s t i l l in progress at this writing.
Control and experimental live trapping plots were located and
established on the National Bison Range in two areas with sympatric pop­
ulations of M. pennsylvanicus and M. montanus.
Preliminary investiga­
tion demonstrated that under natural conditions M. pennsyIvanieus was
restricted mainly to mesic communities and M. montanus to xeric communi­
tie s.
Experimental reduction of M. pennsyIvanicus numbers induced the
movements of a significantly larger number of M. montanus into the mesic
habitat of the Experimental plot than were found in mesic habitat, on the
Control plot.
Examination of the respective centers of a c tirlty of
M. pennsylv anicus and of M. montanus during the preliminary phase of
investigation and the nature of M. montanus captures in the mesic habi­
t a t of the Experimental plot indicate that the c rite ria for in te r ­
specific competition, as defined and discussed in the Introduction , are
not met.
There appears? rather, to be an interspecific intolerance
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-6 0 -
mechanism operating to maintain the habitat segregation.
A consideration
of some other field studies on interspecific competition indicates that
interspecific competition is more or less absent in most sympatric con­
generic species.
Indeed, if a species is optimally to utilize its niche,
then it seems logical that interspecific competition would, of necessity,
become minimal or non-existent.
These considerations do not rule out the possibility of the occur­
rence of interspecific competition in the past.
On the basis of results
in this study, the reports of other studies on interspecific competition,
and in the light of the concepts of natural selection, the proposal is
made that interspecific competition may function as the agent of natural
selection to adapt the genomes of congeneric species, first coming into
a condition of sympatry, for optimal utilization of their respective
niches.
Models of competitive niche exploitation and non-competitive niche
occupancy in sympatric congeneric species are discussed.
Suggestions are made to determine the nature of interspecific in­
tolerance mechanisms, assumed to be responsible for habitat segegration
in sympatric species of Microtus. Reciprocal removal of M. montan is to
determine whether M. pennsylvanieus will move into xeric habitat, is
also suggested.
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LITERATURE CITED
-6l-
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LITERATURE CITED
Allee, W
o Co, A. E. Emerson, 0. Park, T, Park, and Ko P. Schmidto
Principles of animal ecology» W. Bo Saunders, Philadelphia*
Andrewartha, H. G», and L. Go Birch. 195U»
The distribution and abundance of animals.
Chicago, 111.
Univ. Chicago Press,
Baumgartner, L. L. 19^0.
Trapping, handling and marking fox squirrels.
U«Uiil+-U50.
Birch, L. C. 1957»
The meaning of competition.
19h9o
Jour. Wildl. Mgt.
Am. Nat. Uls5-l8=
Brant, D. H. 1962.
Measures of the movements and population densities of small rodents.
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Brown, W, L ., and 0. E. Wilson. 1956»
Character displacement. Jour. Syst. Zool. 5§U9-6U»
Cade, To J.
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Ecology of the peregrine and gyrfalcon populations in Alaska.
California Publ. Zool. 63s151-290.
Univ.
Crombie, A. C. 19Ü5.
On competition between different species of graminivorous insects.
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Further experiments on insect competition.
London (B) 133§76-109.
Proc. Roy. Soc.,
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Interspecific competition.
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1962.
Reduced competition among the birds of Bermuda.
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Ecol
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Darwin, C. 1859»
The origin of species by means of natural selection, or, the pres,
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Davis, Do E.
1956.
Manual for analysis of rodent populations.
Baltimore.
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3
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I960.
Estimating the numbers of game animals. ^ Manual of Game Investi­
gational Techniquesÿ H. S. Mosby (ed.), Edwards Brothers, Ann
Arbor, Michigan.
Diver, G. 19U0.
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“’The New Systematics,*' Jo Huxley (ed.), Oxford University Press,
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Dixon, W
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1957»
Introduction to s ta tis tic a l analysis. McGraw-Hill, New York, 2nd.
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Dodds, Do Go i 9 6 0 .
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in Newfoundland. Jour. Wildl. Mgt. 2Ls52-60.
Dumas, P. G. 1956.
The ecological relations of sympatry in Plethodon dunni and Plethodon
vehiculum» Ecol. 37si;8U“U95.
Elton, Go 1927 .
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-------
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Gilbert, 0», T. B. Reynoldson^, ar-d J. Hobart» 1952»
Ganse^8 hypothesis| an examination» Jour» Animal Ecol» 21*310-312»
Grinnell, J .
1917.
The niche-relationships of the California thrasher»
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Hall, E„ R»,and K» R» Kelson»
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1959»
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Hardin, G» I 96 O»
The competitive exclusion principle»
Soi» 131:1292-1297°
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An experimental study of habitat selection by prairie and forest
races of the deermouse Peromyscus maniculatus» Contrib» Lab» Vert»
Biol,, No, 56, 53 PP»
Hayne, D, W, 19U9»
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amm, 3011-18»
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The role of reproduction and mortality in population fluctuations
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Concluding remarks»
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. 19L7.
Darwin's finches.
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