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
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