FUNCTIONAL AND NUMERICAL RESPONSES OF KIT FOXES TO A SHORT-TERM DECLINE IN MAMMALIAN PREY

Diet and abundance of San Joaquin kit foxes (Vulpes macrotis mutica) w ere monitored over 45 months (1989-1992) at a semi-arid site in southc entral California. From 1990 to early 1991, the mammalian prey of thes e endangered foxes declined markedly to densities 2-5X lower than at p eak. During late 1991 and 1992, however, the populations of at least t hree species of mammalian prey irrupted. Regardless of these fluctuati ons, foxes maintained consistent preferences among small mammals, and did not shift their diets to other prey when mammalian prey was scarce . Although we did not detect a clear functional response, foxes did re spond numerically because their abundance decreased after the decline in mammalian prey. This decline in abundance was due to proportionatel y fewer females successfully rearing young and high coyote-induced mor tality, rather than foxes abandoning their territories. Because kit fo xes continue to prey on their staple species during times of prey scar city, declines in rates of consumption could exert a strong influence on the population dynamics of this endangered fox. We suggest that pop ulation dynamics of kit foxes may be similar to those of obligate pred ators due to their apparent unwillingness or inability to switch to ab undant, alternate prey during declines in density of their preferred p rey.