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Prey Specialization by Individual Cougars in Multiprey Systems

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Prey Specialization by Individual Cougars in Multiprey Systems Prey Specialization by Individual Cougars in Multiprey Systems Kyle H. Knopff University of Alberta Edmonton, Alberta Mark S. Boyce University of Alberta Edmonton, Alberta Introduction The cougar (Puma concolor) has made a remarkable recovery in North America over the last half century. This can be attributed primarily to the termination of predator-bounty programs and to a change in management status for cougar from vermin to big-game species. Most western states and provinces now boast healthy, harvestable populations of the big cats (Beausoleil and Martorello 2005), and the animals are even reoccupying parts of their historic range east of the Rocky Mountains (Neilsen et al. 2005). This is a rare success story in a world where most of our large carnivores are threatened, and many are even critically endangered through habitat loss and overharvest. A plethora of recent scientific work on predators (Ray et al. 2005) suggests that recovering and maintaining large carnivores in ecosystems can have benefits that go beyond their intrinsic value as wilderness icons, their recreational and economic value as big-game species and furbearers, or the inherent fascination that they hold for most people. Growing populations of wolves in North America, for example, have been shown to decrease populations of ungulate prey and, through what is known as a trophic cascade, to increase the biomass of plants that benefit numerous other species, from songbirds to beaver (Hebblewhite et al. 2005). Predators also may influence population dynamics and community structure by changing the behavior of their ungulate prey (Brown et al. 1999). Predation risk imposed by healthy populations of predators can cause ungulates to avoid certain areas, resulting in an alteration of ecosystem structure and increased biodiversity (Ripple and Beschta 2004). The predatory behavior of cougars, therefore, can have a substantial influence on ecosystems. 194 v Predator-Prey Workshop: Prey Specialization by Individual Cougars. This same predatory behavior, however, can result in unwanted declines in populations of prey that are locally or regionally endangered or that have recreational and economic importance for hunters and other wildlife enthusiasts. While cougar predation may have compensatory effects on ungulate populations in some cases (Hornocker 1970, Laundre 2005), although detailed experimental evidence is often required to clearly implicate predation as a major limiting or regulatory factor for prey (Boutin 1992), there have been several documented cases where cougar predation is the primary cause of population decline (Wehausen 1996, Festa-Bianchet et al. 2006). Indeed in some cases, cougars have taken prey to the brink of local extinction (Sweitzer et al. 1997, Logan and Sweanor 2001), leaving little doubt about the potential for predation by cougars to negatively affect the population viability of prey. Cougar predation, therefore, can be considered to have both positive and negative effects, depending on perspective and context. To optimize these effects through management, a firm understanding of cougar predatory behavior is required. This paper examines one important aspect of this behavior—prey specialization by individuals—which can have important implications for the extent that cougars influence populations of their prey. We begin by discussing cougar predation in multiprey systems because it is in these systems that the effects of cougar predation are most pronounced and because this is where strong individual preferences for a particular species of prey (specialization) is possible. Next, we review the literature as it pertains to cougar prey specialization and also provide some new data from the first year of an ongoing study of cougar predatory behavior along the eastern slopes of Alberta’s Rocky Mountains. Finally, we discuss the management implications of prey specialization by cougars and the gaps in our knowledge that still need to be addressed by future research to improve the performance of management actions. Cougar Impacts on Prey and the Importance of Multiprey Systems The effect that large carnivores have on populations of their ungulate prey has been studied and hotly debated in North American wildlife management circles for decades. It is a topic that has predominantly centered on wolf-ungulate systems and has focused chiefly on interactions between wolves and their primary prey (Bergerud et al. 1983, Boutin 1992, Messier 1994, White and Garrott 2005). Until recently, other predators and types of systems had not Transactions of the 72nd North American Wildlife and Natural Resources Conference v 195 received the same attention. In the case of cougars, this was no doubt due to preliminary data suggesting that their predatory behavior did not restrict the growth of prey populations (Hornocker 1970), perhaps because cougar populations were constrained by social factors to a level below that set by food supply (Seidensticker et al. 1973). In the 1990s, however, reports of cougars depressing populations of their prey began to emerge (Turner et al. 1992, Wehausen 1996, Ross et al. 1997, Sweitzer et al. 1997). These case studies, combined with research dispelling the idea that social factors kept cougar populations below what food availability would predict (Pierce et al. 2000, Logan and Sweanor 2001), catapulted cougars to the foreground of predator-prey debates. In direct opposition to Hornocker’s (1970) early belief, with respect to bighorn sheep (Ovis canadensis), that, “the numbers taken by lions are insignificant,” (23), several of the reported examples of cougars depressing populations of prey involve bighorn. For small populations of these sheep, even a single cougar is capable of causing substantial mortality. In Alberta, for example, a lone female cougar was responsible for killing 9 percent of a sheep population, including 26 percent of the lambs, over the course of a few months during winter (Ross et al. 1997). In the peninsular ranges of California, cougars reportedly killed 26 percent of the sheep in one population and are thought to be capable of impeding the recovery of endangered populations. In the Sierra Nevada, cougar predation has been identified as the single most important factor in the precipitous decline of what had previously been a successful reintroduction of bighorn (Wehausen 1996), and cougar predation has recently been identified as a primary cause of four major declines in three populations of sheep in Alberta and Montana (Festa-Bianchet et al. 2006). Bighorn are not the only species to be affected, however. Populations of mule deer (Odocoileus hemionus), feral horses (Equus caballus), mountain caribou (Rangifer tarandus) and even porcupines (Erethizon dorsatum) have suffered declines as a direct result of cougar predation (Turner et al. 1992, Sweitzer et al. 1997, Kinley and Apps 2001, Robinson et al. 2002). In nearly every case of prey depression reported for cougars, the species suffering a decline is a secondary prey item in a multiprey system. The critical feature of multiprey systems that exacerbates the ability of cougars to negatively affect prey is the ability of the cougar population to sustain itself on alternate prey. The impact this has on ungulate prey can take two forms. The first is 196 v Predator-Prey Workshop: Prey Specialization by Individual Cougars. known as apparent competition, which occurs when two or more prey species collectively contribute to the maintenance of a larger predator population than could be sustained on any one prey type alone, to the detriment of all types of prey (Holt 1977). The second is known as indirect amensalism, which occurs when the presence of one prey species negatively affects a second prey species, but the presence of the second has little or no effect on the first. Asymmetrical apparent competition (i.e., approaching indirect amensalism) may be common in vertebrate predator-prey systems (Chaneton and Bonsall 2000), and, if the smaller population of alternate prey is the more negatively affected, predation may rapidly become depensatory. This is especially true if the predator numerical response to population reductions of alternate prey is delayed or nonexistent because of a strong association of predator populations to those of their primary prey. In many predator-prey systems, predation on small populations of alternate prey is rare and may be incidental to the search for primary prey (Schmidt et al. 2001). In such cases, small populations are less likely to be adversely affected by predation, especially if they are able to occupy habitats that are rarely frequented by the predator in its search for primary prey species (Schmidt 2004). In systems where selection of prey by predators occurs, however, the negative effects of asymmetrical apparent competition on populations of secondary prey are greatly exaggerated if the secondary prey species also happens to be the preferred prey (i.e., taken at a rate greater than available). Cougars often exhibit prey selection in multiprey systems (Hornocker 1970, Kunkel et al. 1999) and, thus, are capable of such exaggerated impacts. For example, in a cougar, white-tailed deer (Odocoileus virginianus) and mule deer system in southern British Columbia, white-tailed deer were the primary prey of cougars and, consequently, a primary determinant of cougar carrying capacity (Robinson et al. 2002). Cougars in this system, however, selected heavily for mule deer, which suffered a predation rate of nearly double that
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