Wolf Predation Essay, Research Paper

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 ir 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.

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 ir 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.