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Essay Name       : 1228.txt 
Uploader         : John Feldersnatch
Email Address    : 
Language         : English
Subject          : Biology
Title            : Wolf Predations
Grade            : 96 %
School System    : University of Pittsburgh
Country          : USA
Author Comments  : Lots of Research.  I really outdid myself on this one!
Teacher Comments : She said that it put me up to an A for the semester..
Date             : December 1, 1995
Site found at    : Through a friend at CMU.
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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 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 their 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. 
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