Effects of Wolf Predation

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Effects of Wolf Predation

Abstract: This paper discusses four hypotheses to explain the effects

of wolf predation on prey populations of large ungulates. The four

proposed hypotheses examined are the predation limiting hypothesis,

the predation regulating hypothesis, the predator pit hypothesis, and

the stable limit cycle hypothesis. There is much research literature

that discusses how these hypotheses can be used to interpret various

data sets obtained from field studies. It was concluded that the

predation limiting hypothesis fit most study cases, but that more

research is necessary to account for multiple predator - multiple prey


The effects of predation can have an enormous impact on the

ecological organization and structure of communities. The processes of

predation affect virtually every species to some degree or another.

Predation can be defined as when members of one species eat (and/or

kill) those of another species. The specific type of predation between

wolves and large ungulates involves carnivores preying on herbivores.

Predation can have many possible effects on the interrelations of

populations. To draw any correlations between the effects of these

predator-prey interactions requires studies of a long duration, and

statistical analysis of large data sets representative of the

populations as a whole. Predation could limit the prey distribution

and decrease abundance. Such limitation may be desirable in the case

of pest species, or undesirable to some individuals as with game

animals or endangered species. Predation may also act as a major

selective force. The effects of predator prey coevolution can explain

many evolutionary adaptations in both predator and prey species.

The effects of wolf predation on species of large ungulates have

proven to be controversial and elusive. There have been many different

models proposed to describe the processes operating on populations

influenced by wolf predation. Some of the proposed mechanisms include

the predation limiting hypothesis, the predation regulating

hypothesis, the predator pit hypothesis, and the stable limit cycle

hypothesis (Boutin 1992). The purpose of this paper is to assess the

empirical data on population dynamics and attempt to determine if one

of the four hypotheses is a better model of the effects of wolf

predation on ungulate population densities.

The predation limiting hypothesis proposes that predation is the

primary factor that limits prey density. In this non- equilibrium

model recurrent fluctuations occur in the prey population. This

implies that the prey population does not return to some particular

equilibrium after deviation. The predation limiting hypothesis

involves a density independent mechanism. The mechanism might apply to

one prey - one predator systems (Boutin 1992). This hypothesis

predicts that losses of prey due to predation will be large enough to

halt prey population increase.

Many studies support the hypothesis that predation limits prey

density. Bergerud et al. (1983) concluded from their study of the

interrelations of wolves and moose in the Pukaskwa National Park that

wolf predation limited, and may have caused a decline in, the moose

population, and that if wolves were eliminated, the moose population

would increase until limited by some other regulatory factor, such as

food availability. However, they go on to point out that this upper

limit will not be sustainable, but will eventually lead to resource

depletion and population decline. Seip (1992) found that high wolf

predation on caribou in the Quesnel Lake area resulted in a decline in

the population, while low wolf predation in the Wells Gray Provincial

Park resulted in a slowly increasing population. Wolf predation at the

Quesnel Lake area remained high despite a fifty percent decline in the

caribou population, indicating that mortality due to predation was not

density-dependent within this range of population densities. Dale et

al. (1994), in their study of wolves and caribou in Gates National

Park and Preserve, showed that wolf predation can be an important

limiting factor at low caribou population densities, and may have an

anti-regulatory effect. They also state that wolf predation may affect

the distribution and abundance of caribou populations. Bergerud and

Ballard (1988), in their interpretation of the Nelchina caribou herd

case history, said that during and immediately following a reduction

in the wolf population, calf recruitment increased, which should

result in a future caribou population increase. Gasaway et al. (1983)

also indicated that wolf predation can sufficiently increase the rate

of mortality in a prey population to prevent the population's

increase. Even though there has been much support of this hypothesis,

Boutin (1992) suggests that "there is little doubt that predation is a

limiting factor, but in cases where its magnitude has been measured,

it is no greater than other factors such as hunting."

A second hypothesis about the effects of wolf predation is the

predation regulating hypothesis, which proposes that predation

regulates prey densities around a low-density equilibrium. This

hypothesis fits an equilibrium model, and assumes that following

deviation, prey populations return to their pre-existing equilibrium

levels. This predator regulating hypothesis proposes that predation is

a density-dependent mechanism affecting low to intermediate prey

densities, and a density-independent mechanism at high prey densities.

Some research supports predation as a regulating mechanism.

Messier (1985), in a study of moose near Quebec, Canada, draws the

conclusion that wolf-ungulate systems, if regulated naturally,

stabilize at low prey and low predator population densities. In

Messier's (1994) later analysis, based on twenty-seven studies where

moose were the dominant prey species of wolves, he determined that

wolf predation can be density-dependent at the lower range of moose

densities. This result demonstrates that predation is capable of

regulating ungulate populations. Even so, according to Boutin (1992)

more studies are necessary, particularly at high moose densities, to

determine if predation is regulatory.

A third proposal to model the effects of wolf predation on prey

populations is the predator pit hypothesis. This hypothesis is a

multiple equilibria model. It proposes that predation regulates prey

densities around a low-density equilibrium. The prey population can

then escape this regulation once prey densities pass a certain

threshold. Once this takes place, the population reaches an upper

equilibrium. At this upper equilibrium, the prey population densities

are regulated by competition for (and or availability of) food. This

predator pit hypothesis assumes that predator losses are

density-dependent at low prey densities, but inversely

density-dependent at high prey densities. Van Ballenberghe (1985)

states that wolf population regulation is needed when a caribou herd

population declines and becomes trapped in a predator pit, wherein

predators are able to prevent caribou populations from increasing.

The final model that attempts to describe the effects of

predation on prey populations is the stable limit cycle hypothesis.

This hypothesis proposes that vulnerability of prey to predation

depends on past environmental conditions. According to this theory,

individuals of a prey population born under unfavorable conditions are

more vulnerable to predation throughout their adult lives than those

born under favorable conditions. This model would produce time lags

between the proliferation of the predator and the prey populations, in

effect generating recurring cycles. Boutin (1992) states that if this

hypothesis is correct, the effects of food availability (or the lack

of) should be more subtle than outright starvation. Relatively severe

winters could have long- term effects by altering growth, production,

and vulnerability. Thompson and Peterson (1988) reported that there

are no documented cases of wolf predation imposing a long-term limit

on ungulate populations independent of environmental influences. They

also point out that summer moose calf mortality was high whether

predators were present or not, and that snow conditions during the

winter affected the vulnerability of calves to predation. Messier

(1994) asserts that snow accumulation during consecutive winters does

not create a cumulative impact on the nutritional status of deer and


All of the four proposed theories mentioned above could describe

the interrelationships between the predation of wolves and their usual

north american prey of large ungulate species. There has been ample

evidence presented in the primary research literature to support any

one of the four potential models. The predation limiting hypothesis

seems to enjoy wide popular support, and seems to most accurately

describe most of the trends observed in predator-prey populations.

Most researchers seem to think that more specific studies need to be

conducted to find an ideal model of the effects of predation. Bergerud

and Ballard (1988) stated "A simple numbers argument regarding

prey:predator ratios overlooks the complexities in multi-predator-prey

systems that can involve surplus killing, additive predation between

predators, enhancement and interference between predator species,

switch over between prey species, and a three-fold variation in food

consumption rates by wolves." Dale et al. (1994) stated that further

knowledge of the factors affecting prey switching, such as

density-dependent changes in vulnerability within and between prey

species, and further knowledge of wolf population response is needed

to draw any firm conclusions. Boutin (1992) also proposed that the

full impact of predation has seldom been measured because researchers

have concentrated on measuring losses of prey to wolves only.

Recently, bear predation on moose calves has been found to be

substantial, but there are few studies which examine this phenomenon

(Boutin 1992). Messier (1994) also pointed out that grizzly and black

bears may be important predators of moose calves during the summer.

Seip (1992), too, states that bear predation was a significant cause

of adult caribou mortality. These points emphasize that

multiple-predator and multiple-prey systems are probably at work in

the natural environment, and we must not over generalize a one

predator - one prey hypothesis in the attempt to interpret the overall

trends of the effects of predation of wolves on large ungulate


Literature Cited

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predation in limiting a moose population. Journal of

Wildlife Management. 47(4): 977-988.

Bergerud, A. T., and W. B. Ballard. 1988. Wolf predation on caribou:

the Nelchina herd case history, a different interpretation. Journal of

Wildlife Management. 52(2): 344- 357.

Boutin, S.. 1992. Predation and moose population dynamics: a critique.

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Dale, B. W., L. G. Adams, and R. T. Bowyer. 1994. Functional response

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