Grant 1972_competition rodents.

Copyright 1972.

All

righu reserved

INTERSPECIFIC COMPETITION AMONG RODENTS 4037

P. R. GRANT

Biology Department, McGill University Montreal,

Quebec,

Canada

The aim of this paper is to integrate the results of the several experiments reported in this series,1 to review other published evidence of competitive in­ teraction among rodent species, and to formulate some general statements.

The experiments were stimulated

by

the lack

of

a rigorous experimental demonstration of competition between species of vertebrates under field con­ ditions. _There is an _abundance of _{indirect evidence and some direct evidence}

(96, 97, 110, 111),

but in contrast to the situation with invertebrates

(38),

experimental

studies with vertebrates have lacked

appropriate controls. Ex­

ceptions to this are Inger & Greenberg's

(76)

study of frogs

in Borneo,

and

more recently Jaeger's

(77, 78)

study of salamanders in eastern North Amer­

ica. The

importance of the subject scarcely needs

emphasizing

at a time when competition theory plays so conspicuous a role in community ecology and evolution analyses

(75,88, 90,96, 98) .

The evidence from other studies o f rodents is reviewed first. Then the experiments reported in this series are discussed. Finally

some generalizations

and theoretical considerations are presented.

FIELD OBSERVATIONS

Contiguous, allopatric distributions of rodent species have been inter­ preted as being the result

of

interspecific competition

( 18, 22, 74, 79, 1 15,

124) ,

but the major evidence for interspecific competition is derived from observations of sympatric species. The evidence falls into two categories:

(a)

two sympatric species exhibit an inverse numerical relationship, and

(b)

two sympatric species

exhibit an

inverse

spatial

re

l

at

i

o

nship

.

In the first category the population numbers of one species rise while those of the other fall. This has been observed and commented upon several times (e.g.

3, 14, 1 7, 49, 56, 61, 70, 91, 92, 128, 129, 138, 146 ) .

In some cases the numerical changes are later reversed, with the common species be­ coming rare and the rare species becoming common. Numerical phenomena of this sort,

and all

the population parameters involved (birth, d'eath, and 1 This is Part V and is entitled Experimental studies of competitive interaction

in a two-species system. V. Summary of the evidence for rodent species, and some generalizations.

79

Annu. Rev. Ecol. Syst. 1972.3:79-106. Downloaded from www.annualreviews.org

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Further

dispersal

rates,

etc),

may

refiect

the outcome of

competitive

interaction in

a

temporarily varying environment, although competition clearly need not be involved (e.g. see

127).

At its extreme, this leads to the extinction of one of the species. Such was observed to occur on Brooks Island in California

(86,

87). Microtus californicus,

a new immigrant to the island, increased in num­ bers at a time when the resident species Mus musculus declined in numbers, eventually to extinction. The authors suggested that the decline of

Mus mus

­ culus was actually caused by

Microtus californicus,

due to competition for food and space. Aggressive interaction was indicated for the latter. This sug­ gestion was supported by the results of work by de Long

(45,

46), who provided evidence for the competitive inferiority of Mus musculus

in

the field on the mainland, and for the vulnerability of Mus musculus to damaging at­ tack from M. californicus in the laboratory (also

86).

Moreover Quadagno

(118)

found that Mus musculus had smaller home ranges in the presence of M.

californicus

than in their absence.

In the second category, two species sometimes occupy different parts of the same habitat for no clearly discernible reason associated with variations in habitat features, and therefore possibly as a result of interspecific interac­ tion

(6, 7,

9,

120).

Small-scale spatial sf:paration of this sort should lead to an increased difference in diets, if one species dominates the other and ex­ cludes it from the best patches of habitat, Cameron

(29)

has found evidence of this with woodrats (Neotoma spp.). In allopatry N. fuscipes and N. lepida have similar but not identical diets. In sympatry N. lepida, but not the domi­ nant N. fuscipes, has shifted its diet to a small extent, as a result of which the diets of the two species are more different in sympatry than in aIIopatry.

Species may also be generally contiguously allopatric, but locally sympat­ ric, and here in the zone of overlap competition between the species prevents one or both from spreading into the other's exclusive domain. Such has been suggested for chipmunk species in western North America

(18,74, 124).

In each of these three independent studies there is evidence from laboratory or the field that one species is aggressively dominant to another and therefore could restrict the movements of the other through aggressive interaction.

Moreover, the variety of habitats oC1;upied by a species is smaller in the presence than in the absence of another species (e.g.

1 1, 1 8, 21 , 52, 53, 92,

95,

125, 146,

other references cited in the Introduction). The appearance of one species in a habitat has been observed to coincide approximately with the decline in numbers or absence of another species normally resident in that habitat

( 1 7, 19-21, 32, 41,

50,

93, 128, 129, 147).

Once again the difficulty arises that the events may or may not be casually related. Apparently con­ vincing explanations involving competition sometimes lose force when other factors are taken into account. This point is well illustrated by the changes in distribution of the arctic hare, Lepus arcticus, on Newfoundland, often quoted as a clear example of interspecifi.; competition

(27, 89a) .

Originally the arctic hare occurred in alpine, subalpine,

and

(lower)

woodland

habitats; but following the iintro

d

uction of

the snowshoe hare,

Annu. Rev. Ecol. Syst. 1972.3:79-106. Downloaded from www.annualreviews.org

Lepus american

us, about

100

years ago, it disappeared from the woodland, which was then solely occupied by the snowshoe hare. The situation on New­ foundland is now similar to the one on the North American mainland. It has suggested to several authors (e.g.

27,

46a, 89a) a simple hypothesis to account for the changes in distribution: the arctic hare, upon arrival in New­ foundland, expanded its range of habitats in the absence of competition from the snowshoe hare, and then contracted the range as a result of competition with it in the woodland. However, the arctic hare is now conspicuously re­ stricted to terrain with ample escape cover from predators (9a). Furthermore the principle predator, the lynx

(Lynx canadensis),

is known to have in­ creased in numbers substantially following the snowshoe hare introduction. Bergerud (9a) has suggested, therefore, that the disappearance of the arctic hare from the woodland is attributable to increased predation there by lynx, whose numbers increased as a direct result of the availability of snowshoe hare as prey. A complex interaction of the three species is indicated, rather than just a simple competitive interaction between the two hare species. Which of the two species, lynx or snowshoe hare, played the larger role in facilitating the habitat

expansion

of the arctic hare is a matter for conjecture. But one additional factor makes the predation pressure hypothesis for the habitat

restriction

more convincing than the competition hypothesis. The arctic hare no longer occupies large sections of subalpine and alpine habitat that are poor in escape cover, and snowshoe hares have not occupied these areas either (9a). In some instances both the distribution of the presumed interactants, with regard to variety of habitats occupied, and their numbers are inversely re­ lated (119, 137, 139-141). Thus, the distribution of numbers in time and in space are complementary aspects of the position of a species in the environ­ ment, and both aspects, according to this circumstantial evidence, are in­ fluenced by the presence of other interacting species.

These dynamic relationships are summarized in two models in Figure 1. In Modell the optima for the two species are widely separated along the habitat axis; in Model

2

they are closer and are nearly identical. The models depict a cycle of reciprocal expansions and contractions, both numerical and spatial, and each cycle is composed of two subcycles. Thus, a given pair of species may progress from Stage

1

to

2

and back again, and only rarely pass all the way from 1 through

2

to 3 and back again. Degree and shape of the overlap segment of the curves depend upon dominance, habitat tolerances, etc. For instance, where a predictable dominance exists, the slope of the curve is likely to be steeper for the subordinate species than for the dominant one, and this may be of use in the testing of a model.

FIELD EXPERIMENTS

Experiments, with or without enclosures, have been conducted only in the last 15 years. Enclosing populations gives control over the interactants as well as other elements such as mammalian predators. One study of interspecific interaction has been made with enclosed populations, and it has given

posi-Annu. Rev. Ecol. Syst. 1972.3:79-106. Downloaded from www.annualreviews.org

MODEL MODEL 2 habitats habitats N N

�

(J

2

\\

fJ 2 \\

WW

3 RW FIGURE 1. Two models of habitat occupancy by competing species. The num­ ber of animals (N) of two spec

i

es, A and :B, is shown in a spectrum of habitats linearly ordered along the horizontal axis.

tive evidence of interaction.

Caldwell (23)

followed the fate of populations

of

Peromyscus polionotus

and

Mus musculus

in a

1

acre field enclosure. Ini­ tially the two species occupied separate areas of the enclosure for no discern­ ible reason connected with variations in the habitat, which suggests an inter­ action between the species. This

is

further supported by subsequent events. P.

polionotus

gradually invaded the area of

Mus musculus,

which declined to extinction in

6

months. However, no aggressive interaction was manifested by animals kept together in the laboratory, so Caldwell concluded that de­ spite the initial spatial separation, the species had competed for food rather than for space. In the laboratory the two species selected the same types of seeds to a large extent

(23).

There wall no control enclosure for the field experiment, but six unenclosed and larger plots which had both species were studied at the same time.

Mus musculus

became extinct on four of these, but persisted for

15

months on the plot most similar in habitat-features to the enclosed area. Without a single-s

pe

cies control, it is not certain that

Mus

musculus

would have persisted in the absence of P.

polionotus,

but that it became extinct in the enclosure at least suggests a competitive interaction. Caldwell & Gentry

(24)

repeated the experiment with essentially the same result, including the initial spatial separation of the species.

Krebs et

al

( 85)

appear to have provided evidence inadvertently of a competitive interaction between

M. pennrylvanicus

and

M. ochrogaster

in

en-Annu. Rev. Ecol. Syst. 1972.3:79-106. Downloaded from www.annualreviews.org

83

closures containing grassland.

M. ochrogaster

became more numerous in an enclosure without its conspecific than in another with it, suggesting an inter­ action between the species when together in the same enclosure.

Vaughan & Hanson

(144)

experimented with unenclosed populations of two species of pocket gophers, using the technique of removing alI residents from an area and then introducing both species to see if they were capable of coexisting in the same habitat. Three areas were used, each

2

to

6

acres in size. One area originally had both species, another had only

Thomomys bot­

tae,

and the third had only T.

talpoides.

Residents were removed, and both species were introduced into each area at high and equal densities. In the following year all the animals were removed, and the experiment was re­ peated for another year with new animals. At the end of both experiments

T.

bottae

predominated in the area which originally had only

T. bottae,

and

T.

talpoides

predominated in the area which originally had only T.

talpoides. T.

talpoides

also predominated at the end of the first experiment in the area originally supporting both species; but at the end of the second experiment only one T.

bottae

was found in this area. Unfortunately no estimates were given of the densities of the two species prevailing before the experiments were carried out so that the above results might be compared, nor were single species controls conducted. The results of the experiments might be exactly those predicted on the basis of different habitat affinities of the species, al­ though interspecific competition could have occurred as well. Vaughan & Hanson also discuss the possible influence of reproduction and dispersal upon the observed changes in numbers. In a later paper dealing with the coexis­ tence of these two species, Vaughan

(143)

gave little emphasis to interspe­ cific competition. [Clough's

(36)

addition experiment is discussed on p.

94.]

The opposite of putting two species together

is

to remove one from an area where previously

it had coexisted with another.

A

shift

in distribution of the remaining species into the area previously occupied by the removed spe­ cies is evidence for a previous interaction between the species; just as, analo­ gously, intraspecific interaction is inferred from the repopulation of an area, from which individuals were removed, by neighbors or more distant members of the same population

(4, 5, 13, 15,25,26,31,73, 83, 107, 109, 132-136,

142).

This technique was first used by Sheppe

(125, 126)

and was later adopted by Koplin & Hoffmann

(82);

Peterson

( 117),

as reported by Baker

(7);

Sheppard

( 123, 124);

and Stoecker

( 137).

Sheppe

( 125)

observed that

Peromyscus oreas

was almost entirely restricted to the floors of ravines in parts of British Columbia, and P.

maniculatus

was present only on the sides of the ravines and in neighboring areas; the two species were present together in a narrow zone of overlap on the lower slopes of the ravines. Having re­ corded no P.

maniculatus

on the floor of a ravine in

2

years, he removed the whole

P.

orear population. This was easy to do because the population was an isolated one. He then recorded

15

P. maniculatus on the floor of the

ra-Annu. Rev. Ecol. Syst. 1972.3:79-106. Downloaded from www.annualreviews.org

84

,. vine in the next 2 years.

This

experiment was performed without a control, so

it is possible that

P.

maniculatus

wo

u

l

d have occurred on the ravine floor

anyway, irrespective of the presence or ab:;ence of P. o

r

eas

,

particularly if the

population level of

P.

man

i

c

ul

atu

s

was higher ill the years following the

removal of

P.

areas. However, Sheppe's

,;onclusion that P.

oreas

had previ­

ously excluded

P.

man

i

cu

l

atus

from the ravine floor is supported by subse­

quent and uncontrolled events. Sometime in the next 6 years P. oreas re-in­

vaded the ravine, because when trapping was next performed in the ravine

only

P.

oreas were captured on the floor (126) .

Sheppe ( 126) also performed the same type of uncontrolled experiment

with Mus musculus and

P.

man

ic

ula

t

us

in

a

ranch

building

where the maxi­

mum abundance of each species coincided with the least abundance of the

other. The removal of Mus musc

u

lu

s

was followed by a small invasion of

that area by

P.

maniculatus.

Again, this might have occurred anyway, espe­

cially as recruits were entering the

P.

maniculatus

population

at this time and

might be expected to disperse. In contrast to the results of Caldwell's study,

there was evidence, from the type

of

wound scars on

P.

maniculatus

individu­

als, of an aggressive interaction between the species (see also King

81).

It is

possible, therefore, that the two species tended to exclude one another from

different parts of the building by aggressive interaction.

As reported by Baker

(7),

Peterson

(11 7)

performed a

similar removal experiment with

cotton

rats

(Sigmodon)

in a

grassland community in Du­

rango, Mexico. The numerically dominant and "spatially unrestricted"

S.

lul­

viventer were removed serially. As the numbers of

S.

/ulviventer declined, the

numbers of

S.

hispidus

increased, and their ranges, previously restricted, ex­

panded. One year later

S. fulviventer

had increased and regained its position

of numerical dominance. These results suggest that

S.

lulviventer

competi­

tively interacted with

S.

hispidus in some way, to the detriment

of

S.

hispidus,

although no controls for this experiment are mentioned by Baker (7).

Koplin

&

Hoffmann

(82)

used the removal te

c

hniq

u

e in a 1lLudy of Mi

c

ro­

tus montanus

and M. pennsylvanicus, but also provided a contemporaneous

control. They

used

two neighboring areas of unequal size as control and ex­

perimental conditions. Both areas had "hydric-mesic" and "xeric" habitats, in

approximately the same proportions, and both contained the

two

species,

wi

t

h M. pennsylvanicus

mainly in mesic and M. montanus solely in xeric

habitats. After M. pennsylvanicus was removed from the experimental plot,

M. montanus was

c

ap

tu

red in both habitats. This contrasts

with the c

om

p

le

t

e

absen'ce of M. montanus from the mesic: habitat on the control area through­

out the study. The authors concluded that M. pennsylvanicus competitively

excluded M. mon

ta

n

us

from the mesic habitat on the control area throughout

the study, and on the experimental area before M. pennsylvanicus was re­

moved. The dominance of M. p

enn

sy

lva

ni

cus

over M. montanus in the labo­

ratory

(103)

supports the conclusion.

The conclusion

may be

correct,

but the (nonstatistical)

comparison

of

control and experimental data is marred

by

the fact that the population sizes

Annu. Rev. Ecol. Syst. 1972.3:79-106. Downloaded from www.annualreviews.org

were quite different on the two areas; hence, the behavior of the populations may not be strictly comparable. Prior to the removal period, M. montanus (like M. pennsylvanicus) was present in much higher numbers on the experi­ mental plot than on the larger control plot. The authors believe this to be due to the grazing effects of Bison bison on the control plot during the study, and to the absence of such effects on the (fenced) experimental plot. Therefore, the failure of

M.

montanus to move from xeric to mesic habitat on the con­ trol plot may have been due to the absence of high-density conditions, rela­ tive to the resources, which might be

e

xp

e

cted to promote dispersal from one h

a

bitat to the other. If this is correct, the movement of

M.

montanus from xeric to mesic habitat on the experimental plot, following the removal of M.

pennsylvanicus,

may have been due solely to an increase

in M. montanus

den­

sity at

this time. Unfortunately, the density of M.

montanus

is not known, because no trapping was done in the xeric habitat on tbe experimental plot at that time. Thus, the hypothesis of Findley

(53),

which predicts the results obtained, is supported but not adequately tested.

A

more recent study of these two species conducted more than

300

km from the Koplin and Hoffmann study area has produced somewhat different results (Stoecker, in press) .

If

one species is consistently dominant to the other in this location it is M. montanus, not

M.

pennsylvanicus, as revealed by laboratory observations in simulated natural habitat. The field evidence for interspecific competition is equivocal.

M.

pennsylvanicus was present in xeric habitat with M. montanus in one year, when the

M.

pennsylvanicus density was high and

M.

montanus density was low, and not in another year

when the populations

were

about

equal. These results can

be

accounted for solely in

terms

of

intraspecific interaction

in the M. pennsylvanicus popula­ tion sufficient to cause movement

into

the xeric habitat at high density but not at low de

n

sity. Stoecker

(137)

made a similar point. Nevertheless, inter­ specific interaction with generally

dominant M. montanus

may also be in­ volved, and it is indicated by a small-scale removal experiment. In one small area

M.

pennsylvanicus was trapped for the first time only after M. montanus· had been removed.

Thus, a comparison of the results of the two studies of these two species leads to the conclusion that if the species compete with each other for space, the outcome varies regionally. This implies that competitive attributes, such as aggressiveness, vary regionally, perhaps in relation to habitat factors (cf.

8).

Although plausible, this need not be so, because aggressiveness also varies in relation to the stage in a population cycle, at least in

M.

pennsylvanicus

(34);

Krebs

(84)

has also demonstrated this with

M.

ochrogaster. Hence, the outcome of competitive interaction between species such as these can be expected to vary at the same location according to the position of one popu­ latio

n

in relation to its

c

y

c

le, and according to the relative densities of the two species.

Sheppard's

( 1 23, 1 24)

stUdy concerned two species of Chipmunks

which are partly sympatric and partly allopatric. Eutamias amoenus

occurs

Annu. Rev. Ecol. Syst. 1972.3:79-106. Downloaded from www.annualreviews.org

mainly in forested areas at generally low altitudes, while E.

minimus

occurs

in alpine meadows and rocky areas at high altitude; there is a narrow zone of overlap in altitudinal ranges where the two species coexist.

S

h

e

p

par

d

took

15

E.

minimus

from a

l

pin

e

habitat and introduced them to

a

forested area from

which 20

E.

amOenus had been removed. From the description

g

i

ve

n of

the habitat

in the area

of

introduction

it appears

to

have

been

suitable

for

either

s

pe

ci

e

s and to have been

more suitable for the introduced E.

minimus

than

was

the habitat in any neighboring area. On two subsequent

trapping

occa­

sions, first three and then one of the introduced animals were captured. Ten

E.

amoenus

were introduced into an

are:a from

which 14

E. minimus

had

been

removed, and another 10 were

introduced into

an area in which 14 r

e

s

i­ dent E. minimus

w

e

r

e

allowed

to remain.

None of

t

h

e introduced E.

amoenus

were subsequently retrapped in either arc�a. But since

the neighboring

areas

had

habitat

(forest) more suitable for E. clmoenus,

it is

l

ikely that E.

amoenus

moved into them,

a

possibility noted'

by

S

h

ep

pa

r

d

.

The results of these experiments have been quoted by Miller

(96)

as e

v

i

­

dence

for

interspecific co

mpe

ti

t

i

o

_n,

_{but t}

_h

_e

_{y might eq}

_u

_a

_l

_l

_y

_{well be e}

_xpl

_ai

_n

_ed

on the basis o

f d

iff

e

r

e

n

t

habi

t

a

t

pre

f

er

e

n

c

es and the availability or lack of more suitable habitat adjacent to the area into which the introductions were made. Nevertheless, the possibility of i

n

te

rs

pe

ci

fi

c competition in the wild,

with

E.

a

m

oe

n

u

s tending

to e

xclu

d

e

E.

minimus

from forested areas, is sup­ ported

by

the results of Sheppard's laboratory studies. These showed E.

amoenus

to be

generally

dominant to the smaller

E. minimus.

LABORATORY EXPERIMENTS

Some of the

_e

_v

_i

_de

_nc

_{e has been}

_{mentioned already, and most of}

_{it has} been d

i

s

c

u

s

s

ed in Part II

of

this se

ri

es

(64;

see also

8, 99, 103, 104, 108) , s

o

only

the essentials

will

be

stated here. Individuals of different species are fre­

quently aggressive to each other under the confined conditions of the

labora­

tory test arena. Aggressive interactions have been observed occasionally in

the wild

as

well (2, 80) ,

in

di

c

ati

ng that the laboratory studies reflect, how­ ever imperfectly, behavior exhibited in the wild. Many of

the species which

ap

p

ea

r

to interact competitively for space in the wild interact aggressively in the laboratory, the outcome being the establishment of a dominance relation­ ship. This

is

generally stable

(8).

In those instances where the field evidence indicates that one species m

i

gh

t co

nsi

st

e

n

tl

y

be

co

mpe

tit

iv

el

y

superior

to the

other, it is also

g

en

e

r

a

ll

y

the dominant o:ne in the laboratory

tests (29,45, 82,

106, 124, 144, 147 ) ; conflicting results

from laboratory

te

sts

(99, 104)

are

probably attributable to differences in t€�st procedures, and to different envi­

ronmental conditions from which the animals were drawn (92, 103, 137; see

also p. 85) .

It

h

as

also been s

ho

wn

that when

two

sp

e

c

i

es of different habitat

affinity are

pl

aced

to

gether in a

l

a

b

or

a

t

ory

test

arena which has

segments of

both habitats, the species disperse into their usual or "preferred"

habitats, apparently as a consequence

of

the

_{ago:nistic behavior exhibited under thos}

_e

conditions

(64, 137) .

Annu. Rev. Ecol. Syst. 1972.3:79-106. Downloaded from www.annualreviews.org

THE SERIES OF EXPERIMENTS WITH WOODLAND AND GRASSLAND SPECIES Choice of specie

s.

-T

he experiments were designed to demonstrate com­

petitive interaction between species under near-natural conditions. It was de­

cided to use species which might compete for space, because the results of

competition for space are easier to measure than are the results of competi­

tion for food. It was further decided to use two species of animals whose

activities are more or less restricted to the surface of the ground (two-dimen-­

sional space) for ease of recording their distributions.

A

final requirement

was small individual body size, hence potentially large population size, in an

area sufficiently small for suitable experimental manipulations.

The choice of species was made on the basis of indirect evidence of com­

petitive interaction in areas of sympatry. The species were rodents of the

family Microtinae:

Microtus pennsylvanicus pennsylvanicus

(Ord) from

grassland; and from woodland,

Clethrionomys gapperi gapperi

(Vigors) in

some experiments and Peromyscus maniculatus gracilis (Wagner) in others.

On the mainland of North America these species are sympatric over

large

areas, yet the grassland and woodland species are rarely found outside their

usual habitats. On islands the restriction to habitat is not so pronounced. On

islands in Notre Dame Bay off Newfoundland

(121) and Penobscot Bay in

Maine

(K. L.

Crowell personal communication), M. pennsylvanicus occur in

both grassland and woodland in the absence of the two woodland species.

Conversely, P. maniculatus have been recorded in grassland on islands

largely or entirely devoid of M. pennsylvanicus

(27,

51, 114,

K.

L.

Crowell

personal communication).

C.

gapperi

have not been reported in grassland on

islands without M. pennsylvanicus (28); but on islands lacking a Microtus

species in the British Isles

(40,44, 55) and in Japan (113) , species of

Cleth­ rionomys

similar to

C. gapperi

have been found to occupy the grassland to

a

greater extent than on the nearby mainland where Microtus species are pres­

ent. These facts indicate, but by no means prove, that the habitat restriction

in mainland areas is caused by other species.

A

competitive interaction be­

tween species, however rarely it might occur, would adequately account for

the restriction observed in areas of sympatry; and the lack of restriction on

islands

is

accounted for by a hypothesis of ecological release (28, 65, 66).

The requirements for a demonstration of interaction.-For

a demonstra­

tion of competitive interaction in a two-species two-habitat system, one of the

species (Species 1) should occur in the other's habitat more frequently in the

absence that in the presence of the other species (Species 2). This result can

be obtained more clearly with enclosed populations than with unrestrained

ones. Two approaches to the conduct of such an experiment in enclosures are

possible:

(a)

a contemporaneous approach, with control (one-species) and

experimental (two-species) conditions in separate enclosures at the same

time; and (b) a sequential approach, with control and experimental condi­

tions in the same enclosure either at different times of the year or, prefer

a

bly,

Annu. Rev. Ecol. Syst. 1972.3:79-106. Downloaded from www.annualreviews.org

at the same time in different years. Each approach has its advantages and disadvantages. For example, the contemporaneous approach is more costly to follow in terms of materials, but it does allow one to eliminate the (un­ known) effects of time (season, years) upon the results. However, if con­ ducted in this way the experiment should be repeated, with experimental and control conditions reversed, in order to eliminate the effects upon the results of small differences in habitat features between enclosures.

Conclusive evidence for interaction is then obtained if, following the in­ troduction of Species

2

into its usual habitat, where Species

1

has been occur­ ring frequently, Species

1

leaves this habitat and now stays more or less en­ tirely in its own usual habitat.

These steps in the demonstration are shown diagrammatically and hypo­ thetically in Figure

2.

The contemporane:ous approach is used. For the pur­ pose of illustration, steps

1

and

2

show the woodland species C.

gapperi,

as Species

1,

occurring in the grassland more frequently in the absence than in the presence of the grassland species

M. pennsylvanicus

as Species

2

(these steps could have been illustrated just as well with

M. pennsylvanicus

entering the woodland). Step

3

shows that, following the transposition of

M. pennsyl­

vanicus

from Enclosure I grassland to Enclosure II grassland, C.

gapperi

oc­ curs in Enclosure

II

_{grassland less frequc:ntIy than before, and also less fre­} quently than in Enclosure III grassland (control). In addition, C. gapperi now occur more frequently in Enclosure I grassland than when

M. pennsylvanicus

were there.

The most obvious biases are eliminated by this procedure. One set, which is difficult to control, comprises those population factors that cause members of a species to leave their usual habitat and enter another. These include the density. sex and age composition of the population and associated behavior, which may not be the same in two enclosures. Replicates of experimental and control conditions would help to minimize the effects upon the results of dif­ ferences

in

these factors. A further value of replicates lies

in

providing an assessment of the consistency, and hence reliability. of the results. However, for practical reasons replicates are not feasible in studies of this type and scale with vertebrates. Therefore, in their absence, it must be shown that dif­ fernces in the population features of a species between enclosures do not account for the difference between enclosures in the frequency of occurrence of that species in the habitat of the other species.

There are alternative outcomes of these experiments which lead to an op­ posite conclusion (providing that all biases have been eliminated). One spe­ cies might occur in the other's habitat as jfrequently. or even more frequently, in the presence as in the absence of the other species. The conclusion from both of these outcomes is that competitivl� interaction between the species did not occur. Therefore, the results allow a decision to be made as to whether or not the two species interact competitively.

The experimental evidence jor interaction in the grassland.-Experiments

were conducted

in

three enclosures in soJuthern Quebec in three successive

Annu. Rev. Ecol. Syst. 1972.3:79-106. Downloaded from www.annualreviews.org

1. Wood c -

i

--

i

--Groll 2. c

--t---M 3. c --

t

--

i

--Enclolures II III c

---i----M c

--t--i-c . ,

tt-ti

\_, '-' M c --

l-

--

l-

--c -

i

---

r

- e-c

tt

-FIGURE 2. The requirements for a demonstration of interaction between Species C (Clethrionomys gapperi) and M (Microtus pennsylvanicus) in grass­ land habitat. Arrows indicate degree of movement but do not represent move­ ment of all animals from one habitat to another. Steps

1

and 2 are shown as separate experiments. Step 3 is a second part of Experiment

2

and involves the transposition, by the experimenter, of all of Species M from Enclosure I to II. years and have been reported in detail

(63,65).

The disposition of the enclo­ sures and habitats is as shown in Figure

2.

Each contained

0.5

acre of grass­ land and the same area of deciduous woodland. The distribution of the ani­ mals was determined by weekly trapping with Longworth live-traps for a pe­ riod of

4

to

5

months. The significance of the difference in distribution of a species in woodland and grassland habitat in control and experimental enclo­ sures was assessed by X2.

Animals used in the experiments were collected at another locality in southern Quebec. C.

gapperi

was used in the first experiment. Unexpectedly it was almost completely absent from the source locality in the second and

Annu. Rev. Ecol. Syst. 1972.3:79-106. Downloaded from www.annualreviews.org

90

third years. Rather than use animals of this species from a different locality, and hence with different experience, in the second and third experiments, it was decided to use another woodland spl:cies, P.

maniculatus,

from the same locality.

M.

pennsylvanicus

was used in

all

three experiments.

The first requisite for these experiments was that one species enter the other's habitat. This happened in all three years; the woodland species en­ tered the grassland.

The results of the three experiments go most of the way towards fulfilling the requirements outlined in the previous section. In the first experiment the result was identical to the hypothetical f€�sult given in Figure

2

(

1)

for enclo­ sures I and II (Enclosure III had

M. pennsylvanicus

only). It needs no fur­ ther comment. In the second- and third experiments with P.

maniculatus,

Step

2

of the progression illustrated in Figure

2

was by-passed. Step

1

was re­ peated, and a modified Step

3

was performed, in the course of which part of Step

2

was demonstrated. The results of the second and third experiments are given diagrammatically in Figure

3,

which may be compared with the scheme in Figure 2.

The first part of the second experiment duplicated the findings with C.

gapperi,

which justified the tacit assumption that these two species are similar with respect to their response to

M. pennsylvanicus.

In both ex:periments

1

and

2

the numbers, sex, and age distributions of the woodland species in con­ trol and experimental enclosures varied in approximately the same way, so the results are not obviously biased by these population factors. There are two small differences between experiments

1

and

2.

Two control enclosures

(I

_{and III) were used in the second experiment, instead of just one as in the} first experiment, and

P. maniculatus

occurred frequently in the grassland of both (Figure

3,

1).

Although it was pronounced, the difference in occurrence of

P . maniculatus

in the grassland of c:ontrol and experimental enclosures was not statistically significant, due to small sample sizes.

In the second part of the second experiment

M. pennsylvanicus

was trans­ posed to the grassland of Enclosure III (Figure

3, 2) ,

with two results. There occurred an immediate and 'significant increase in the frequency of occur­ rence of

P. maniculatus

in the grassland of Enclosure

II,

from which

M.

pennsylvanicus

had been removed. Within

3

weeks the frequency of occur­ rence there was indistinguishable from that in the remaining control enclo­ sure

(I).

_{The other result was that in Ellclosure III, into which}

M.

_pennsyl­

vanicus

had been newly introduced, the frequency of occurrence of

P. mani­

culatus

in the grassland did not decline, but P.

maniculatus

were only captured in the area of grassland close to the habitat interface. A comparison of the distribution of

P. maniculatus

captures in the grassland before and after the introduction of

M. pennsylvanicus

shclwed that P.

maniculatus

captures were significantly restricted within the grassland after the introduction of

M.

pennsylvanicus.

Complete exclusion of P.

maniculatus

from the grassland was demon­ strated in the third experiment. M.

pennrylvanicus,

at a much higher density

Annu. Rev. Ecol. Syst. 1972.3:79-106. Downloaded from www.annualreviews.org

1. Wood Grass 2.

-91

II III 3. p p p

-tt- --t--

-tt-M 4. p p p -

t

--

r

--

-t-t-

--

t

--

t

:

;

M p �--

t

---p

---t---p

--t�r--p .. ..

t-t-t-+

1_,

)

M p

---i---M p ----

t

----FIGURE 3. The demonstration of interaction between Species P

(Peromyscus

maniculatus)

and M

(Microtus pennsylvanicus)

in grassland habitat. The mean­ ing of the arrows is the same as in Figure 2. Steps

1

and 2 were performed in the second experiment, steps 3 and

4 in

the third experiment.

than in the second experiment, were introduced into the grassland of Enclo­ sure II, in which P.

m

an

i

c

ulat

us had been occurring frequently (Figure

3, 3).

The frequency of occurrence of P.

maniculatus

declined to zero in the next

3

weeks (Figure

3, 4),

despite recruitment and generally higher numbers of P.

maniculatus

i

n Enclosure II than in the preceding

4

weeks. It remained at �ro for

the

following

5

weeks. Contemporaneous controls cannot be used to assess the significance of this result, because P.

maniculatus

rarely occurred in the grassland of enclosures I and III, despite the absence of M.

pennsyl­

vanicus

(Figure

3, 4).

Therefore, the distribution,of P.

maniculatus

in Enclo­ sure II

when

M.

pennsy[vanicus

was

present

must

be compared

with sequen­

tial controls in the same enclosure, (a) earlier

in

the same year, and (b) at

the same time of year in the previous year (Figure

3, 2).

The results of the two comparisons are the same. Following the introduction of M.

pennsylvani­

cus

into the grassland of Enclosure II, the frequency of occurrence of P.

maniculatus

there was significantly lower than when M.

pennsylvanicus

was absent earlier in the year· ana "lower than at the same time of year in the previous year.

The

purpose of Step

2

in the progression illustrated in Figure

2

was to show that differences in habitat features between enclosures I and II were not

responsible

for the

result

in Step

1. Th

at

a woodland species could occur

Annu. Rev. Ecol. Syst. 1972.3:79-106. Downloaded from www.annualreviews.org

frequently in the grassland of Enclosure II, in the absence of

M. pennsylvani­

cus,

was shown in the second and third experiments (Figure 3, 2 and 3), us­

ing P. maniculatus

as

the woodland species. It is still

po

ssible, however, that

C.

gapperi,

unlike

P. m

a

ni

c

u

l

a

t

u

s

,

would not occur frequently in the grass­

land of Enclosure II for reasons independent of the presence of

M. pennsyl­

vanicus. This is extremely unlikely becam:e the Woodlands in enclosures I and

II

w

e

r

e

so

similar, and the grasslands were similar

too

(63). Furt

he

rm

o

re

,

the population sizes of

C.

gapperi in the two enclosures were nearly the same

throughout the experiment.

The other aspect of Step

2

is the demonstration that the woodland species occurs rarely in the grassland in the presence of

M. pennsylvanicus i

n one year,

and frequently in the grassland

of

the same enclosure in the absence

of

M.

p

e

n

n

sy

lva

n

i

cus

in

another year. In other words the experimental

result, differing from the control result, is demonstrated in more than one enclosure.

This

was

not attempted in these e

x

p

eri

me

nts

due

to shortage of time, but it is

not essential for the

demonstration of comp

et

itiv

e interaction.

Thus, the chief features of the results which demonstrate an interaction

between the woodland and grassland

spe

cies in the

grassland

habitat are: (a)

a

woodland species occurs more frequently

in the grassland without

than

with a grassland species, and (b) a wotJdland species no longer occurs in

grassland, or

occurs there rarely,

after

a

gras

s

l

an

d species

is

introduced there. The ecological release hypothesis is tested. and supported by the results.

The one piece of conflicting evidencl� is that in the third experiment P.

maniculatus did

not

occur frequently

in th

e

grassland of

enclosures I and III,

even

in t

he

absence of M.

pennsylvanicus

(Figure

3, 3

and

4).

However, this conflict is more apparent than real, because on all occasions

P. maniculatus

occurred frequently in the grassland only following recruitment, and recruit­

ment

did

n

ot

take place

in

enclosures I and III

in the third experiment (i.e.

conditions necessary for frequent movement into the grassland did not

occur

in enclosures I and III) .

The animals can be observed in

the

fiJ�ld only

with considerable difficulty,

so the mechanism of the interaction is

Do

t

known.

Nevertheless, lab

or

a

to

r

y

experiments with these species give a strong indication that the mechanism is

aggression and avoidance. When

to

ge

t

her

, the

grassland and woodland

spe­ cies were

more

restricted to

their

usual habitats

(r

ecreate

d

in a

laboratory

test

arena)

than when alone, and apparently this resulted from the agonistic

behavior exhibited (and observed) under these conditions (64).

The experimental evidence for interaction in the woodland.-M. pennsyl­

vanicus

did not enter the woodland in

th��

above experiments, even when the

w

oodl

a

n

d was without

a woodland speci��s. A

subsequent

exp

e

rim

ent

in

En­

closure II (66)

showed that the

p

roba

b

l

e

reason for this

was

that the densi

ty

in the gr

a

s

s

l

an

d was

insufficient to induce movement into the woodland.

In

this subsequent e

xper

i

m

e

nt

M. pennsylvllnicus

did e

n

t

er the

w

o

od

lan

d

,

but

Annu. Rev. Ecol. Syst. 1972.3:79-106. Downloaded from www.annualreviews.org

only when the density

in

the grassland was considerably higher than the den­

sities prevailing in the previous three experiments. The single-species experi­

ment was not designed to investigate the possibility that

M.

pennsylvanicus

interacts competitively with woodland species in the woodland, but

R. D.

Morris has conducted such investigations in an enclosure in Saskatchewan

(102).

T

h

e si

ngl

e

e

nc

los

u

re

c

o

nt

a

in

e

d a continuous piece

of g

ras

s

l

and,

1.4

acres in area, and three pieces of aspen woodland with grasses and sedges in the herb layer,

0.6

acres in combined area. Differing from the Quebec experi­ ments'in some details, but similar in plan, three experiments with

C. gapperi

a

nd

M.

pennsylvanicus

were conducted in successive years.

The requirements for a demonstration of interaction in this system are shown dia

gr

am

m

ati

c

a

lly

i

n

Fi

gu

r

e 4.

In the presence of

C. gapperi

in

the

woodland,

M.

pennsylvanicus

are shown to occur infrequently

in the wood­ land (Step

1).

Following the removal of

C.

gapperi from the

woodland,

the

frequency of occurrence there of

M.

pennsylvanicus

increases (Step 2).

Following the re-introduction of C.

gapperi

to the woodland, the frequency

of occurrence there of

M.

pennsylvanicus decreases (Step 3).

The three experiments were conducted as follows. In the first experiment

M.

pennsylvanicus

were introduced alone, and their distribution was recorded in six trapping

p

eri

o

ds in

4

months. In

the

second year M.

pennsylvanicus

and

C. gapperi

were introduced into their appropriate habitats and their distribu­

tions were recorded in two subsequent trapping period's. C. gapperi were then

removed and the distribution of M.

pennsylvanicus

was recorded. C.

gapperi

were re-introduced, and distributions were determined once more. Since the

C.

gapperi

population declined drastica

ll

y following re-introduction, a third ex­ periment was performed in the

next year

to

repeat

the

re

-i

n

t

ro

du

c

ti

on phase.

M.

pennsylvanicus

which had survived the winter were allowed to remain,

1. 2. 3. Wood C C ,-,

---t---

-

---

�

--

i-

- - ---

l

---

H

-, Grass M M M

FIGURE

4.

The demonstration of interaction between Specie

s

C (C.

gapperi)

an

d

M (M.

pennsylvanicus)

in woodland habitat. The meaning of the arrows is the same as in Figure 2. The

s

hape of the enclosure and habitat is not

as

shown; the enclosure is octagonal and the three pieces of woodland are peripheral. Steps 1 and 2 were performed in one experiment. Step 3 in the next one.

Annu. Rev. Ecol. Syst. 1972.3:79-106. Downloaded from www.annualreviews.org

alone, in the enclousre for a 3-month period, during which trapping was per­

formed weekly.

C.

gapperi

were then introduced and distributions were re­

corded in the following 3 months.

For a study with only one enclosure the sequential approach to compar­

ing experimental and control results must be used. Unfortunately, the density

of traps varied from year to year, and only in the third experiment was it the

same in woodland and grassland. Since frequency of capture is a function of

trap density over a large range of animal densities, statistical comparisons

cannot be �de between years, but only within years.

Steps

1

and

2

were demonstrated in the second experiment: following the

removal of C. gapperi, the frequency of ,[)ccurrence of

M.

pennsylvanicus in

the woodland increased significantly. No Buch increase was found at that time

of year in the first experiment, in which

M.

pennsylvanicus

were alone

throughout. Step 3 was attempted in the second experiment, but it failed due

to the disappearance and presumed dea1h of most of the re-introduced C.

gapped. Step 3 was then demonstrated in the third experiment. Although not

immediately influenced by the introduced C. gapperi,

M.

pennsylvanicus oc­

curred in the woodland less frequently, overall, after the introduction of

C.

gapperi

than before. Furthermore, the relationship between frequency of oc­

currence of

M.

pennsylvanicus in the woodland and population size was dif­

ferent before and after the introduction of

C.

gapperi.

For a given population

size of

M.

pennsylvanicus there were fewer captures of

M.

pennsylvanicus

in

the woodland in the presence of

C.

gappeti

than

in

their absence.

Although the Quebec and Saskatchewan experiments were all single repli­

cate ones, they provide a demonstration of competitive interaction between a

woodland and a grassland species in both habitats. Competitive exclusion

( 69) of some or all individuals of an invading species is the result of inter­

specific interaction. The only other experiment with any two of these three

species, which was performed by Clough (36) with unconstrained populations

of

M.

pennsylvanicus

and

C.

gapperi,

lacked a control but yielded somewhat

similar results.

C.

gapperi were added to two areas which had only

M.

penn­

sylvanicus but appeared to be favorable,

m

terms of habitat, for both species.

Later it was found that

C.

gapperi had completely disappeared from one area

and that

M.

pennsylvanicus

had completely disappeared from the other.

Clough ( 36) concluded that competitive: interaction had occurred between

the species, and he suggested that the mechanism might be interspecific ag­

gression.

Conclusions.-Thus, several studies have provided evidence of varying

degrees of completeness that competitiv(: interaction occurs between rodent

species. Owing to the difficulties of obtaining critical evidence, it is striking

that so much has been obtained with 50 many 5pe<;ies. Therefore. it is likely

that competitive interaction for space

is

a general phenomenon among rodent

species and is not restricted to the few slPecies that have been studied in

de-Annu. Rev. Ecol. Syst. 1972.3:79-106. Downloaded from www.annualreviews.org

tail. We may inquire why this is so, and ask under what conditions it occurs and what are the selective forces upon the interactants.

SOME GENERALIZ:ATIONS AND THEORETICAL CONSIDERATIONS To understand why interspecific competition for space is apparently wide­ spread among rodents, let us first consider a simple situation, with Species

1

in Habitat A and Species

2

in Habitat

B.

They can only compete when indi­ viduals of one species enter the other habitat. There is an abundance of evi­ dence to suggest that at the time of reproduction animals seek space first in the habitat to which they are most adapted. They are forced to seek space else­ where if, as a result of interactions with other animals of the same species in that habitat, they are unable to acquire the sought-after space

(33, 37,

63-67,

145) .

They enter another habitat ( let us call them members of Species

1

which enter Habitat

B) .

Whether or not they secure space and remain there depends upon two classes of factors:

(a)

the suitability of Habitat

B

in terms of food and cover, nest-holes, microclimate, etc, and

(b)

the frequency of interaction with Species 2. _{Having once left Habitat A, an individual of Spe­} cies

1

will eventually settle in Habitat A or

B

according to which side of the following equation is the larger

SA

-

111

�

SB

-

112

where S represents the suitability of a habitat and I represents a product of interaction frequency and intensity. Of course, S and I are measured in dif­ ferent (although unspecified) units, but they could be converted' to a com­ mon scale of energy units to make them comparable.

With this as background we now ask why Species

1

and

2

are not mutu­ ally tolerant in Habitat

B.

A possible reason is that they are not able to dis­ criminate between members of their own and of the other species. In this case, interspecific interaction would be an inadvertent consequence of similar­ ity in body size, behavior, etc. However, many interacting species are not particularly similar in appearance or behavior; For example, M.

pennsyl­

vanicus,

C. gapperi,.

and

P. maniculatus

differ conspicuously in size, color, pattern, behavior, vocalizations, and possibly odor (unpublished data) . There­ fore, the species can probably distinguish each other fairly readily in the wild. An alternative view is that the interactive behavior is advantageous. If it were not, it would be selected against, because interactions between individuals are energy-consuming, and they may also render the animals more con­ spicuous to a searching predator.

There could be an advantage to competitive interaction for space if the diets of the two species overlapped, i.e. pre"emptive competition for food. Put another way, in the event of mutual tolerance in Habitat

B,

Species

1

might appreciably reduce part of the food supply of Species

2,.

and this could have' serious consequences for Species

2

in terms of reproduction,. survival, etc. There could also be an advantage if the probability of succumbing' to a p

red-Annu. Rev. Ecol. Syst. 1972.3:79-106. Downloaded from www.annualreviews.org

ator increased as the overall density of the two species increased.

A

predator species which preyed upon both Species

1

and

2

in Habitat B might be ex­ pected to exhibit a numerical and functional response to an increase in the overall density of the two species combiIled (cf.

116).

For the reasons of food and predation, there should be a selective advantage to those individuals of Species

2

which respond to the presence of individuals of Species

1

by attempting to drive them away.

The same considerations apply to situations more complex than this sim­ ple two-species two-habitat situation, such as two or more species within a single habitat. They may achieve complete spatial separation within the same habitat. The occupation of overlapping areas is more likely to occur, but still there will be a tendency to minimize the overlap by competitive interaction. Since use of the same space by two species is of more frequent and regular occurrence in this situation than in the two-habitat situation described above, selection will also tend to minimize the overlap in diet, spatial and temporal patterns of activity, etc. At the extreme, this would result in ecological differ­ ences between two species being so large that, providing the predator factor (above) was unimportant, mutually interactive behavior might confer no benefits and would in fact be selected again:;t.

Diets are not well-enough known to enable one to say whether the species which compete for space have generally overlapping diets. However, there is substantial overlap in diets of at least three pairs of inferred competitors:

Mi­

crotus ochrogaster

and

M. pennsyZvanicus

( 1 48), M. ochrogaster and Sigmo­

don hispidus

(54),

and

Peromyscus Zeucopus

and

P. manicuZatus bairdii

(47 ) .

The foregoing reasoning is consistent with the generally accepted view of the significance of competition for space in birds [for a different view see

( 105)].

Much more is known about birds than rodents because they are eas­ ier to study and have been studied more extensively. Numerous studies have shown that where systematically related spi�cies of birds use the same volume of space, they forage in it in different ways" thereby harvesting different foods (e.g.

12, 59, 72, 89) .

Conversely, where rdated species use different and ad­ jacent volumes of space, they forage in their separate spaces in a similar manner and harvest similar foods

(62, 71, 97, 111, 112).

The latter spatial separation is achieved by means of interspecific territorialism. Such studies have led to the conclusion that, under conditions where the variety of food resources is too small to permit division between species, selection favors aggressive behavior which results in mutual dispersion of the species. Inter­ specific territoriality is exhibited most frequently in structurally simple habi­ tats or portions of habitats, where the foraging space is restricted in the verti­ cal plane

( 1 12).

The activity of many rodents is more or less restricted to a single (hori­ zontal) plane of the environment. In general, they have not been able to achieve the spatial niche separation in the vertical plane, unlike birds and the shrew species

Sorex araneus

and

S. minutus (94 ) .

Thus, they are unable to

Annu. Rev. Ecol. Syst. 1972.3:79-106. Downloaded from www.annualreviews.org

avoid each other and forage in different

ways to the extent

that birds can

( 82 ) ;

_this

_is

_{particularly true of the principally subterranean species (95,}

1 15 ) . The absence of strict territoriality among many rodent species (but see

1 30), in contrast to its prevalence among birds, does not undermine the argu­

ment since spacing-out of conspecifics is achieved to some degree by the be­

havior associated with the establishment and maintenance of home ranges.

From this c

o

mparison

with

birds, it may be pre

di

c

t

ed

that i

n

te

rs

pe

c

i

fic compe

tit

ion for space

i

s relati

v

e

l

y more widespread among rodents than

among birds. It probably also occurs widely among other grou

n

d

-

livi

ng

mam­

mals, such as shrews (42, 94) , and ground-living forms of other vertebrate

classes, such as lizards ( 100), salamanders (48, 68, 77, 78) , and frogs (76).

The conditions under which interaction occurs are not uniform.

This

has

been alluded

to

already.

Our

knowledge

of

the interspecific interactions

in

nature

is

too fragmentary to offer a comprehensive theory of when and under

what conditions interaction occurs. There are, however, three identifiable

states of coexistence between rodent species, and these are illustrated in Fig­

ure 5. They represent coexistence with or without competitive interaction,

and, in the former, with or without predictable outcomes. Predictable, in this

Habitats A C 0) 1 (2

1·<, I

I) 2 �

I

·

I

1 =2

FIGURE 5. Three stages of coexistence of Species

1

and 2. A, B, and C refer

to

habitats (the precise definition of habitat is unimportant prOViding that each

s

p

a

ce

so

designated has its own characteristic plant and animal composition ) . The symbols indicate dominance or lack of it (equal sign). The consistent domi­

nance

of Species

1

in Hab

i

ta

t B(2)

tends to lead to the exclusion of Species 2

and

therefore to minimal coexistence, unless Species

2

posses

s

e

s

some compen­ sating ad

v

an

t

age (see text, p. 98).

Annu. Rev. Ecol. Syst. 1972.3:79-106. Downloaded from www.annualreviews.org

sense, means on the average. Since the animals are not at maximum size at .the age of first maturity

(

-

50%

in the case of M. pennsylvanicus) , their interactive behavior is likely to change wilth age

(64)

and reproductive status

(34),

which introduces a varying component into the determination of an outcome.

Type l' coexistence represents coexistence over a small area. The two spe­ cies generally select different habitats, in which they are dominant to the other species should the other species ·enter. In a small area of overlap

(B)

the habitat characteristics are intermediatf:. Coexistence is permanent, or near permanent, and the outcome of .competiHve interaction is indeterminate. It may be determined by a "beachhead" pheJilomenon

(39, 101 ) ;

whichever ani­ mal arrives first at a piece of ground holds sway. Habitats

A

and C may be adjacent at some localities or else may be separated by such a thin strip of intermediate habitat,

B,

that there is insufficient space in it for the home range requirements of any animal. An ex.ample of this type of coexistence is M. pennsylvanicus with C. gapperi or

P.

maniculatus, which was described in detail in an earlier section (see also

1 02 ) .

Type

2

_{coexistence represents coexistence in a moderately large area of} overlap. Habitat characteristics are intermediate here, and competitive inter­ action yields a predictable outcome since Species

1

is consistently dominant to the other. However, Habitat C does not offer the requirements of Species I, and so Species

1

will rarely enter it and will not remain there long when it does. There are really two variants of this type of coexistence, which may be designated

a

and b. As a result of competitive interaction, the subordinate Species 2 tends to be excluded from Habitat B and is never abundant there in the presence of Species

1 ;

_{this is Variant a.}

_An

_{example of this is given by} Wirtz & Pearson

( 1 47 ) .

M. pennsylvaniclls occurs in grassland (Habitat

A) ;

P.

leucopus occurs in shrubbery and gOlden-rod plant association (Habitat

C) .

In an intermediate habitat of broomsedge (Habitat

B) ,

M. pennsylvani­

CIlS

is

apparently dominant to and tends to exclude

P.

leucopus. Alterna­

tively, Species

1

_{does not exclude Species 2 from Habitat}

_B

_{because Species}

₂

enjoys some compensating advantage; this might reside in the foods

( 1 3 1 )

_or structural aspects of the habitat

( 1 8, 74 ) ,

_{which it is better able to exploit, or} in its superior ability to avoid predators

( 1 22 ) .

_{This is Variant b. It might be} widespread if many of the rodent species which coexist over large areas ex­ hibit a regular dominance-subordinate relationship

( 1 , 2 5 ) .

_{Coexistence types}

1

and

2

may be expected to give rise to the numerical and spatial dynamics depicted in Model

I

of Figure

1 .

Type

3

coexistence represents largely coextensive ranges and either no interaction between the species, as is shown, or else infrequent and unpredict­ able interaction. According to Catlett & Shellhammer

(30),

Reithrodontomys megalotis and Mus musculus do not inteJC3ct aggressively; and because they coexist in the wild, they may constitute an example of Type

3

coexistence.

Obviously, the three types of coexistence do not exhaust all possibilities. They are presented in this way in order to suggest a possible sequence of

Annu. Rev. Ecol. Syst. 1972.3:79-106. Downloaded from www.annualreviews.org