John FeldersnatchDecember 1st, 1995Abstract: This paper discusses four hypotheses to explain the effects of wolfpredation on prey populations of large ungulates. The four proposed hypothesesexamined are the predation limiting hypothesis, the predation regulatinghypothesis, the predator pit hypothesis, and the stable limit cycle hypothesis.
There is much research literature that discusses how these hypotheses can beused to interpret various data sets obtained from field studies. It wasconcluded that the predation limiting hypothesis fit most study cases, but thatmore research is necessary to account for multiple predator – multiple preyrelationships. The effects of predation can have an enormous impact on the ecologicalorganization and structure of communities. The processes of predation affectvirtually every species to some degree or another. Predation can be defined aswhen members of one species eat (and/or kill) those of another species. Thespecific type of predation between wolves and large ungulates involvescarnivores preying on herbivores.
Predation can have many possible effects onthe interrelations of populations. To draw any correlations between the effectsof these predator-prey interactions requires studies of a long duration, andstatistical analysis of large data sets representative of the populations as awhole. Predation could limit the prey distribution and decrease abundance. Suchlimitation may be desirable in the case of pest species, or undesirable to someindividuals as with game animals or endangered species. Predation may also actas a major selective 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 have proven to becontroversial and elusive. There have been many different models proposed todescribe the processes operating on populations influenced by wolf predation. Some of the proposed mechanisms include the predation limiting hypothesis, thepredation regulating hypothesis, the predator pit hypothesis, and the stablelimit cycle hypothesis (Boutin 1992). The purpose of this paper is to assess theempirical data on population dynamics and attempt to determine if one of thefour hypotheses is a better model of the effects of wolf predation on ungulatepopulation densities. The predation limiting hypothesis proposes that predation is the primary factorthat limits prey density.
In this non- equilibrium model recurrent fluctuationsoccur in the prey population. This implies that the prey population does notreturn to some particular equilibrium after deviation. The predation limitinghypothesis involves a density independent mechanism. The mechanism might applyto one prey – one predator systems (Boutin 1992). This hypothesis predicts thatlosses of prey due to predation will be large enough to halt prey populationincrease. Many studies support the hypothesis that predation limits prey density.
Bergerudet al. (1983) concluded from their study of the interrelations of wolves andmoose in the Pukaskwa National Park that wolf predation limited, and may havecaused a decline in, the moose population, and that if wolves were eliminated,the moose population would increase until limited by some other regulatoryfactor, such as food availability. However, they go on to point out that thisupper limit will not be sustainable, but will eventually lead to resourcedepletion and population decline. Seip (1992) found that high wolf predation oncaribou in the Quesnel Lake area resulted in a decline in the population, whilelow wolf predation in the Wells Gray Provincial Park resulted in a slowlyincreasing population.
Wolf predation at the Quesnel Lake area remained highdespite a fifty percent decline in the caribou population, indicating thatmortality due to predation was not density-dependent within this range ofpopulation densities. Dale et al. (1994), in their study of wolves and caribouin Gates National Park and Preserve, showed that wolf predation can be animportant limiting factor at low caribou population densities, and may have ananti-regulatory effect. They also state that wolf predation may affect thedistribution and abundance of caribou populations.
Bergerud and Ballard (1988),in their interpretation of the Nelchina caribou herd case history, said thatduring and immediately following a reduction in the wolf population, calfrecruitment increased, which should result in a future caribou populationincrease. Gasaway et al. (1983) also indicated that wolf predation cansufficiently increase the rate of mortality in a prey population to prevent thepopulation’s increase. Even though there has been much support of thishypothesis, Boutin (1992) suggests that “there is little doubt that predation isa limiting factor, but in cases where its magnitude has been measured, it is nogreater than other factors such as hunting.
“A second hypothesis about the effects of wolf predation is the predationregulating hypothesis, which proposes that predation regulates prey densitiesaround a low-density equilibrium. This hypothesis fits an equilibrium model, andassumes that following deviation, prey populations return to their pre-existingequilibrium levels. This predator regulating hypothesis proposes that predationis 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 astudy of moose near Quebec, Canada, draws the conclusion that wolf-ungulatesystems, if regulated naturally, stabilize at low prey and low predatorpopulation densities. In Messier’s (1994) later analysis, based on twenty-sevenstudies where moose were the dominant prey species of wolves, he determined thatwolf predation can be density-dependent at the lower range of moose densities.
This result demonstrates that predation is capable of regulating ungulatepopulations. 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 isthe predator pit hypothesis. This hypothesis is a multiple equilibria model. Itproposes that predation regulates prey densities around a low-densityequilibrium.
The prey population can then escape this regulation once preydensities pass a certain threshold. Once this takes place, the populationreaches an upper equilibrium. At this upper equilibrium, the prey populationdensities are regulated by competition for (and or availability of) food. Thispredator pit hypothesis assumes that predator losses are density-dependent atlow prey densities, but inversely density-dependent at high prey densities. VanBallenberghe (1985) states that wolf population regulation is needed when acaribou herd population 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 of predation on preypopulations is the stable limit cycle hypothesis. This hypothesis proposes thatvulnerability of prey to predation depends on past environmental conditions. According to this theory, individuals of a prey population born underunfavorable conditions are more vulnerable to predation throughout their adultlives than those born under favorable conditions. This model would produce timelags between the proliferation of the predator and the prey populations, ineffect generating recurring cycles. Boutin (1992) states that if this hypothesisis correct, the effects of food availability (or the lack of) should be moresubtle than outright starvation. Relatively severe winters could have long- termeffects by altering growth, production, and vulnerability.
Thompson and Peterson(1988) reported that there are no documented cases of wolf predation imposing along-term limit on ungulate populations independent of environmental influences. They also point out that summer moose calf mortality was high whether predatorswere present or not, and that snow conditions during the winter affected thevulnerability of calves to predation. Messier (1994) asserts that snowaccumulation during consecutive winters does not create a cumulative impact onthe nutritional status of deer and moose. All of the four proposed theories mentioned above could describe theinterrelationships between the predation of wolves and their usual northamerican prey of large ungulate species. There has been ample evidence presentedin the primary research literature to support any one of the four potentialmodels.
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 needto be conducted to find an ideal model of the effects of predation. Bergerud andBallard (1988) stated “A simple numbers argument regarding prey:predator ratiosoverlooks the complexities in multi-predator-prey systems that can involvesurplus killing, additive predation between predators, enhancement andinterference between predator species, switch over between prey species, and athree-fold variation in food consumption rates by wolves. ” Dale et al. (1994)stated that further knowledge of the factors affecting prey switching, such asdensity-dependent changes in vulnerability within and between prey species, andfurther knowledge of wolf population response is needed to draw any firmconclusions.
Boutin (1992) also proposed that the full impact of predation hasseldom been measured because researchers have concentrated on measuring lossesof prey to wolves only. Recently, bear predation on moose calves has been foundto be substantial, but there are few studies which examine this phenomenon(Boutin 1992). Messier (1994) also pointed out that grizzly and black bears maybe 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 areprobably at work in the natural environment, and we must not over generalize aone predator – one prey hypothesis in the attempt to interpret the overalltrends of the effects of predation of wolves on large ungulate populations. Literature CitedBergerud, A. T. , W. Wyett, and B.
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