HABITAT SELECTION BY MULE DEER: FORAGE BENEFITS OR RISK OF PREDATION?

Abstract

Risk of predation may affect individuals in prey populations by limiting their use of high-quality habitat. Predation risk, however, cannot be implicated as a factor in habitat selection by prey without data comparing quality of selected and avoided habitats, along with the predation risk associated with those habitats. If forage benefits and predation risk are not positively correlated among habitat types, then predation risk may have little influence on the habitat selected by prey. We evaluated habitat selection by mountain lions (Puma concolor) and mule deer (Odocoileus hemionus) in the eastern Sierra Nevada, California, USA, from 1994 to 1997, to determine how forage benefit or risk of predation by mountain lions affects habitat selection by mule deer. Mountain lions were the primary predator of mule deer in our study area. Stands of bitterbrush (Purshia tridentata) in the Great Basin provided more cover for mule deer than surrounding patches of rabbitbrush (Chrysothamnus nauseosum) or desert peach (Prunus andersonii). Bitterbrush also was important forage for mule deer during winter. We hypothesized that mountain lions would be more successful at stalking and killing mule deer in habitats with more concealment cover than in habitats with less cover, and therefore mule deer would choose between foraging on bitterbush and avoiding predation by mountain lions. We collected data on habitat characteristics in 3 types of locations: random locations (n = 180), deer foraging locations (n = 179), and locations where mountain lions killed deer (n = 41). Mule deer selected habitat at greater elevations (P < 0.001) with more bitterbrush (P < 0.001) and less rabbitbrush (P = 0.033) when compared with random locations. Logistic regression indicated that mountain lions killed deer in relatively open areas with more desert peach (P < 0.001) than at locations in which deer foraged. Therefore, deer were not confronted with a trade-off when selecting habitat on winter range, and they minimized the ratio of predation risk to forage benefit by selecting habitat with more bitterbrush. Changes in diet among seasons, which occur for herds of migratory deer, lead to individuals experiencing changing predation risk to forage benefit ratios throughout the year. Hence, migratory populations of mule deer likely adopt different strategies of habitat selection among seasons.