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Wolf Predation

     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.

Bibliography

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.