Effects of Wolf Predation

Effects of Wolf Predation Effects of Wolf PredationAbstract: This paper discusses four hypotheses to explain the effectsof wolf predation on prey populations of large ungulates. The fourproposed hypotheses examined are the predation limiting hypothesis,the predation regulating hypothesis, the predator pit hypothesis, andthe stable limit cycle hypothesis. There is much research literaturethat discusses how these hypotheses can be used to interpret variousdata sets obtained from field studies. It was concluded that thepredation limiting hypothesis fit most study cases, but that moreresearch is necessary to account for multiple predator - multiple preyrelationships.The effects of predation can have an enormous impact on theecological organization and structure of communities. The processes ofpredation affect virtually every species to some degree or another.Predation can be defined as when members of one species eat (and/orkill) those of another species. The specific type of predation betweenwolves and large ung ulates involves carnivores preying on herbivores.English: A camouflaged Menemerus sp Jumping spider...Predation can have many possible effects on the interrelations ofpopulations. To draw any correlations between the effects of thesepredator-prey interactions requires studies of a long duration, andstatistical analysis of large data sets representative of thepopulations as a whole. Predation could limit the prey distributionand decrease abundance. Such limitation may be desirable in the caseof pest species, or undesirable to some individuals as with gameanimals or endangered species. Predation may also act as a majorselective force. The effects of predator prey coevolution can explainmany evolutionary adaptations in both predator and prey species.The effects of wolf predation on species of large ungulates haveproven to be controversial and elusive. There have been many differentmodels proposed to describe the processes operating on populationsinfluenced by wolf predation. Some of th e proposed mechanisms includethe predation limiting hypothesis, the predation regulatinghypothesis, the predator pit hypothesis, and the stable limit cyclehypothesis (Boutin 1992). The purpose of this paper is to assess theempirical data on population dynamics and attempt to determine if oneof the four hypotheses is a better model of the effects of wolfpredation on ungulate population densities.The predation limiting hypothesis proposes that predation is theprimary factor that limits prey density. In this non- equilibriummodel recurrent fluctuations occur in the prey population. Thisimplies that the prey population does not return to some particularequilibrium after deviation. The predation limiting hypothesisinvolves a density independent mechanism. The mechanism might apply toone prey - one predator systems (Boutin 1992). This hypothesispredicts that losses of prey due to predation will be large enough tohalt prey population increase.Many studies support the hypothesis that predat ion limits preydensity. Bergerud et al. (1983) concluded from their study of theinterrelations of wolves and moose in the Pukaskwa National Park thatwolf predation limited, and may have caused a decline in, the moosepopulation, and that if wolves were eliminated, the moose populationwould increase until limited by some other regulatory factor, such asfood availability. However, they go on to point out that this upperlimit will not be sustainable, but will eventually lead to resourcedepletion and population decline. Seip (1992) found that high wolfpredation on caribou in the Quesnel Lake area resulted in a decline inthe population, while low wolf predation in the Wells Gray ProvincialPark resulted in a slowly increasing population. Wolf predation at theQuesnel Lake area remained high despite a fifty percent decline in thecaribou population, indicating that mortality due to predation was notdensity-dependent within this range of population densities. Dale etal. (1994), in their study of wolves and caribou in Gates NationalPark and Preserve, showed that wolf predation can be an importantlimiting factor at low caribou population densities, and may have ananti-regulatory effect. They also state that wolf predation may affectthe distribution and abundance of caribou populations. Bergerud andBallard (1988), in their interpretation of the Nelchina caribou herdcase history, said that during and immediately following a reductionin the wolf population, calf recruitment increased, which shouldresult in a future caribou population increase. Gasaway et al. (1983)also indicated that wolf predation can sufficiently increase the rateof mortality in a prey population to prevent the population'sincrease. Even though there has been much support of this hypothesis,Boutin (1992) suggests that "there is little doubt that predation is alimiting 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 thepredation regulating hypothesis, which proposes that predationregulates prey densities around a low-density equilibrium. Thishypothesis fits an equilibrium model, and assumes that followingdeviation, prey populations return to their pre-existing equilibriumlevels. This predator regulating hypothesis proposes that predation isa density-dependent mechanism affecting low to intermediate preydensities, 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 theconclusion that wolf-ungulate systems, if regulated naturally,stabilize at low prey and low predator population densities. InMessier's (1994) later analysis, based on twenty-seven studies wheremoose were the dominant prey species of wolves, he determined thatwolf predation can be density-dependent at the lower range of moosedensities. This result demonstrates that predation i s capable ofregulating ungulate populations. Even so, according to Boutin (1992)more studies are necessary, particularly at high moose densities, todetermine if predation is regulatory.A third proposal to model the effects of wolf predation on preypopulations is the predator pit hypothesis. This hypothesis is amultiple equilibria model. It proposes that predation regulates preydensities around a low-density equilibrium. The prey population canthen escape this regulation once prey densities pass a certainthreshold. Once this takes place, the population reaches an upperequilibrium. At this upper equilibrium, the prey population densitiesare regulated by competition for (and or availability of) food. Thispredator pit hypothesis assumes that predator losses aredensity-dependent at low prey densities, but inverselydensity-dependent at high prey densities. Van Ballenberghe (1985)states that wolf population regulation is needed when a caribou herdpopulation declines and becomes trapped in a predator pit, whereinpredators are able to prevent caribou populations from increasing.The final model that attempts to describe the effects ofpredation on prey populations is the stable limit cycle hypothesis.This hypothesis proposes that vulnerability of prey to predationdepends on past environmental conditions. According to this theory,individuals of a prey population born under unfavorable conditions aremore vulnerable to predation throughout their adult lives than thoseborn under favorable conditions. This model would produce time lagsbetween the proliferation of the predator and the prey populations, ineffect generating recurring cycles. Boutin (1992) states that if thishypothesis is correct, the effects of food availability (or the lackof) should be more subtle than outright starvation. Relatively severewinters could have long- term effects by altering growth, production,and vulnerability. Thompson and Peterson (1988) reported that thereare no documented cases of wolf preda tion imposing a long-term limiton ungulate populations independent of environmental influences. Theyalso point out that summer moose calf mortality was high whetherpredators were present or not, and that snow conditions during thewinter affected the vulnerability of calves to predation. Messier(1994) asserts that snow accumulation during consecutive winters doesnot create a cumulative impact on the nutritional status of deer andmoose.All of the four proposed theories mentioned above could describethe interrelationships between the predation of wolves and their usualnorth american prey of large ungulate species. There has been ampleevidence presented in the primary research literature to support anyone of the four potential models. The predation limiting hypothesisseems to enjoy wide popular support, and seems to most accuratelydescribe most of the trends observed in predator-prey populations.Most researchers seem to think that more specific studies need to beconducted to find an ide al model of the effects of predation. Bergerudand Ballard (1988) stated "A simple numbers argument regardingprey:predator ratios overlooks the complexities in multi-predator-preysystems that can involve surplus killing, additive predation betweenpredators, enhancement and interference between predator species,switch over between prey species, and a three-fold variation in foodconsumption rates by wolves." Dale et al. (1994) stated that furtherknowledge of the factors affecting prey switching, such asdensity-dependent changes in vulnerability within and between preyspecies, and further knowledge of wolf population response is neededto draw any firm conclusions. Boutin (1992) also proposed that thefull impact of predation has seldom been measured because researchershave concentrated on measuring losses of prey to wolves only.Recently, bear predation on moose calves has been found to besubstantial, but there are few studies which examine this phenomenon(Boutin 1992). Messier (1994) als o pointed out that grizzly and blackbears may be important predators of moose calves during the summer.Seip (1992), too, states that bear predation was a significant causeof adult caribou mortality. These points emphasize thatmultiple-predator and multiple-prey systems are probably at work inthe natural environment, and we must not over generalize a onepredator - one prey hypothesis in the attempt to interpret the overalltrends of the effects of predation of wolves on large ungulatepopulations.Literature CitedBergerud, A. T., W. Wyett, and B. Snider. 1983. The role of wolfpredation in limiting a moose population. 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