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
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
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
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.
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