Northern Prairie Wildlife Research Center
We reviewed data from field studies of kit foxes (Vulpes macrotis) and swift foxes (Vulpes velox) to gain insights into factors that may strongly influence the population dynamics of these foxes. The functional responses of kit foxes appear to be relatively constant across all but the lowest prey densities, suggesting that these foxes may not shift to alternate prey until their staple prey is scarce. As a result, reduced prey resources caused by extrinsic events such as low precipitation could result in rapid numerical responses such as lower reproductive rates and survivorship. Furthermore, unpredictable, short-term fluctuations in precipitation and prey abundance could generate longer term, aperiodic fluctuations in fox density. Hence, long periods of increase or decrease in a fox population may be intrinsic to the system due to fluctuating weather patterns, and need not reflect special or persistent causes such as predation or disease. Our analyses also indicated that the proportion of juvenile foxes killed by coyotes increased linearly over low to moderate fox densities, whereas cause-specific adult mortality and other forms of juvenile mortality were density independent. Density-dependent juvenile mortality may provide a feedback mechanism that reduces the amplitude of fox population dynamics; thereby limiting fox populations to lower densities than they might otherwise attain. Diseases such as canine distemper virus, canine parvo virus, and rabies virus may have contributed to substantial declines in some fox populations.