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