Functional and Numerical Responses of Kit Foxes to a Short-Term Decline in Mammalian Prey

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Abstract
Diet and abundance of San Joaquin kit foxes (Vulpes macrotis mutica) were monitored over 45 months (1989–1992) at a semi-arid site in southcentral California. From 1990 to early 1991, the mammalian prey of these endangered foxes declined markedly to densities 2–5 × lower than at peak. During late 1991 and 1992, however, the populations of at least three species of mammalian prey irrupted. Regardless of these fluctuations, 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 respond numerically because their abundance decreased after the decline in mammalian prey. This decline in abundance was due to proportionately fewer females successfully rearing young and high coyote-induced mortality, rather than foxes abandoning their territories. Because kit foxes continue to prey on their staple species during times of prey scarcity, declines in rates of consumption could exert a strong influence on the population dynamics of this endangered fox. We suggest that population dynamics of kit foxes may be similar to those of obligate predators due to their apparent unwillingness or inability to switch to abundant, alternate prey during declines in density of their preferred prey.

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