How can cougar predation have such dramatic impacts on bighorn populations? Cervids are the main prey of cougars in North America (Ross et al. 1997
), and to prey on bighorn sheep, cougars would require very different hunting techniques. Predation on bighorn sheep is risky: at Sheep River one cougar fell to its death while attacking a lamb (Ross et al. 1997
). Cougars that learn the very predictable space-use patterns of bighorn sheep, however, become highly successful at preying on this species (Wehausen 1996
; Ernest et al. 2002
; Rosas-Rosas et al. 2003
; Rominger et al. 2004
). We could not determine if each predation episode was due to a single individual specialist, but clearly cougar predation on sheep did not simply involve opportunistic killings while searching for other prey. Once they started preying on bighorns, cougars concentrated on this species. Cougars and bighorns coexisted for decades in all study areas without any evidence that cougar predation affected sheep population dynamics. Yet, during 12% of years at Bison Range, 16% at Ram Mountain and 36% at Sheep River, cougar predation was the overriding factor affecting bighorn numbers. Because of the availability of alternative prey, predation on bighorn sheep could continue until the last individual is eaten, as reported with other species that are not typical cougar prey (Sweitzer et al. 1997
; Kinley & Apps 2001
). In addition, high predation over consecutive years generates a positive autocorrelation in mortality rates, leading to a more negative impact on population growth rate than if years of high predation were not consecutive (Pike et al. 2004
The impact of specialist predators on small populations of prey is important, yet unpredictable (Bonsall & Hastings 2004
). An opportunistic predator may begin the process, or changes in the availability of other prey species may be involved. No previous study has accurately quantified the demographic impacts of stochastic predation. We were able to fill this gap because we monitored for a minimum of 25 years three populations of bighorn sheep where we knew the sex and exact age of each individual.
The apparent unsustainability of cougar predation on bighorn sheep raises questions about the ecological relationship between these two species and underlines the importance of very long time-scales to understand population dynamics and evolution of life histories in large mammals. There is no clear evidence that cougar numbers have increased over the past 20 years. On the other hand, mule deer have generally declined while white-tailed deer have increased in much of western North America (Robinson et al. 2002
). Other ecological changes could affect the relationship between bighorns and cougars, such as increased forest cover (providing visual cover for ambush) and changes in the abundance of wolves (Canis lupus
), whose ecological relationships with cougars are unclear (Husseman et al. 2003
). Owing to the very patchy distribution of bighorn sheep, very few cougars have sheep within their home range, compared to those that overlap with deer. Probably, most cougars learn to hunt deer rather than sheep, possibly explaining why in most years we found no evidence of cougar predation. Bighorn sheep antipredator behaviour appears effective against coursing predators such as wolves, but may not be as effective against ambush predators such as cougars (Festa-Bianchet 1991
Because it is impossible to accurately predict when a population will go extinct, the use of population viability analyses as a management tool has received criticism (Coulson et al. 2001b
). Population viability analysis, however, can be useful to compare the performance of contrasting management strategies, rather than the exact consequences of one strategy. Our simulation shows that the cougar predation we quantified in three studies can lead to extinction of small bighorn populations. An increase in the frequency and length of cougar predation events will increase the probability of extinction via the combined effect of heightened mortality and demographic stochasticity in small populations.
If cougar predation regularly led to the extirpation of local populations of bighorn sheep, then metapopulation dynamics involving local extinctions and recolonizations may become important, but because bighorn ewes are extremely philopatric (Festa-Bianchet 1991
), those processes would require a very long time-scale. Regardless of the ecological or behavioural processes that lead to them, stochastic episodes of elevated predation can have severe negative effects on small populations and may be unsustainable over the long term (Williams et al. 2004
). We documented that switches to predation as the limiting factor occur surprisingly frequently in bighorn sheep populations. Multiple processes are associated with declines and extinction of small populations including human-induced and genetic effects (Coltman et al. 2003
; Spielman et al. 2004
). We identified a previously unsuspected process that can further threaten the persistence of isolated populations.