Population dynamics of kit foxes

Abstract

We evaluated data from field studies of kit foxes to identify factors that strongly influence or regulate their population dynamics. Two density-dependent regulating mechanisms were detected. First, the rate of juvenile recruitment was inversely related to the density of adult foxes because a higher proportion of juveniles were killed by coyotes at higher fox densities. The mortality rates of adult foxes were independent of population density. Second, populations of kit foxes are bounded by their territorial spacing behavior, which limits recruitment at high densities. These regulatory factors may operate in concert to curtail population growth at high densities, whereas decreased juvenile mortality by coyotes can operate independently to increase population growth at low densities. Despite the presence of these strong regulating factors, density-independent variations in reproductive rates often result in population crashes or irruptions that are the fundamental causes of instability in populations of kit foxes. Desert systems are characterized by unpredictable fluctuations in precipitation, which contribute to high-frequency, high-amplitude fluctuations in the abundance of mammalian prey (leporids, rodents) for kit foxes. These fluctuations subsequently contribute to density-independent variations in fox reproductive rates, which are strongly influenced by food supply. Projection models of a kit fox population showed that unpredictable, short-term fluctuations in precipitation and, in turn, prey abundance could generate longer term, aperiodic fluctuations in the density of foxes. Consequently, high amplitude fluctuations in the abundance of kit foxes may be intrinsic to the desert systems that they inhabit and need not reflect special or persistent causes such as predation or disease.