Stochastic predation events and population persistence in bighorn sheep

Abstract

Many studies have reported temporal changes in the relative importance of density-dependence and environmental stochasticity in affecting population growth rates, but they typically assume that the predominant factor limiting growth remains constant over long periods of time. Stochastic switches in limiting factors that persist for multiple time-steps have received little attention, but most wild populations may periodically experience such switches. Here, we consider the dynamics of three populations of individually marked bighorn sheep (Ovis canadensis) monitored for 24-28 years. Each population experienced one or two distinct cougar (Puma concolor) predation events leading to population declines. The onset and duration of predation events were stochastic and consistent with predation by specialist individuals. A realistic Markov chain model confirms that predation by specialist cougars can cause extinction of isolated populations. We suggest that such processes may be common. In such cases, predator-prey equilibria may only occur at large geographical and temporal scales, and are unlikely with increasing habitat fragmentation.

Demography, not inheritance, drives phenotypic change in hunted bighorn sheep

“…1A and 2A and Table S1). Initial mixed models accounted for density-dependent effects, using population size as a fixed effect, but we found these to be misleading; the Ram Mountain population has increased dramatically and then declined over the past 40 y, and the low-density decline phase has shown possible time-lag effects and major effects of sudden changes in cougar (Puma concolor) predation (37). Because our models detected and predicted unrealistic positive density dependence, we assumed density independence over the entire time series.…”

Estimating genome-wide heterozygosity: effects of demographic history and marker type

“…Between 1988 and 2010, census size fluctuated between 38 and 210 sheep, declining recently because of low recruitment (Jorgenson et al, 1997) and cougar (Puma concolor) predation (Festa-Bianchet et al, 2006).…”

Declines of Bighorn Sheep, <em>Ovis canadensis</em>, on Deteriorating Winter Range in Jasper National Park, Alberta, 1981-2010

“…To explain the decline, by way of hypothesis, the various proximate factors that can negatively affect sheep dynamics, apart from disease, are (1) population density, (2) fecundity, (3) competition for forage from other grazers, (4) predation, and (5) poor habitat (Cowan 1947;Stelfox 1971;Geist 1971;Jorgenson et al 1997;Festa-Bianchet et al 2006).…”

“…At two localities, telemetry studies revealed that individual Cougars were having a major impact on small, isolated populations of sheep (Festa-Bianchet et al 2006). The declines were halted after the death of these individual Cougars.…”