OOS 20-2
Dynamics of black-tailed deer in a single ungulate prey, multiple predator system in northern California
Ecological theory predicts that single prey systems with one predator species will persist over time. Persistence is a consequence of numerical and functional responses of specialist predators to temporal fluctuations in prey abundances. In contrast, the dynamics of prey populations that are affected by multiple predator species are less clear. How prey are affected by predation in such systems can depend on a variety of factors including hunting strategies of predators, individual variability in predator diet, specialization on body size and age of prey, and existence of spatial and temporal prey refugia. Here we describe the dynamics of black-tailed deer (Odocoileus hemionus columbianus) in northern California, the sole ungulate prey species in a system containing 4 predators. Our analyses are based on age-specific survival probabilities of individually identifiable deer (80 female deer >1 year old captured between 2004 and 2013; 136 fawns captured between 2009 and 2013). We quantified cause specific mortality of deer through investigations of kill sites (21 adults, 93 fawns), and estimated kill rates of pumas (Puma concolor), a specialist predator feeding primarily on adult deer, using investigations of GPS clusters from satellite-enabled collars for 7 individuals monitored between 2010 and 2012. Finally, we used remote cameras to estimate occupancy and abundances of predators on deer summer range and used vegetation surveys to quantify cover as well as biomass of important food species for deer.
Results/Conclusions
Observed kill rates of pumas were among the highest reported in the literature, and may have been a result of competition from black bears (Ursus americanus) usurping their kills. Survival of female deer > 1 year old was consistently low and primarily a consequence of high predation from pumas over the summer. Predation was also the primary cause for low observed annual survival of fawns. Most fawn mortalities occurred at < 3 months of age and were predominantly attributed to black bears and coyotes (Canis latrans). Fawn mortality risk including from coyotes, however, was significantly reduced in areas with more oak forage indicating potential interactions between predation and cover. Resulting estimates of population growth rates were indicative of a strongly declining deer population (λ = 0.81). Our results suggest that population dynamics of ungulates in systems containing multiple predators are complex, that predation can result in significant short-term population declines, and that ungulates may struggle to adapt to opposing selection pressure from predators targeting different age classes.