Wolf Predations Essay, Research Paper

Hypotheses of the Effects of

Wolf Predation

John Feldersnatch

December 1st, 1995

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 relationships.

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 moose.

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 populations.

Literature Cited

Bergerud, A. T., W. Wyett, and B. Snider. 1983. The role of wolf 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. Journal of Wildlife Management. 56(1): 116-

127.

Dale, B. W., L. G. Adams, and R. T. Bowyer. 1994. Functional response of wolves preying on barren-ground caribou

in a multiple prey ecosystem. Journal of Animal Ecology. 63: 644- 652.

Gasaway, W. C., R. O. Stephenson, J. L. Davis, P. E. K. Shepherd, and O. E. Burris. 1983. Interrelationships of

wolves, prey, and man in interior Alaska. Wildlife Monographs. 84: 1- 50.

Messier, F.. 1985. Social organization, spatial distribution, and population density of wolves in relation to moose

density. Canadian Journal of Zoology. 63: 1068-1077.

Messier, F.. 1994. Ungulate population models with predation: a case study with the North American moose. Ecology.

75(2): 478-488.

Seip, D.. 1992. Factors limiting woodland caribou populations and their interrelationships with wolves and moose in

southeastern British Colombia. Canadian Journal of Zoology. 70: 1494-1503.

Thompson, I. D., and R. O. Peterson. 1988. Does wolf predation alone limit the moose population in Pukaskwa Park?:

a comment. Journal of Wildlife Management. 52(3): 556-559.

Van Ballenberghe, V.. 1985. Wolf predation on caribou: the Nelchina herd case history. Journal of Wildlife

Management. 49(3): 711-720.