Joint effects of density dependence and rainfall on abundance of San Joaquin kit fox

We analyzed time-series abundances of San Joaquin kit fox estimated during 1983-95 on the Naval Petroleum Reserves in California (NPRC). For the analysis, we modified a model of density-dependent, stochastic population growth to include the lagged effects of a weather covariate (vegetation growing season rainfall). Without the covariate in the model, a statistical test failed to detect significant density dependence in fluctuating kit fox abundances. However, when the covariate was added, strong density dependence was detected. According to an information-theoretic model-selection index, the model with both density dependence and rainfall is far superior to the models that result from deleting one or more of these factors. The 2- year time lag in the response of kit fos abundances to changes in rainfall is consistent with biological expectations of how rainfall affects habitat carrying capacity for kit fox. An additional covariate, a coyote abundance index failed to improve the model. A population viability analysis (PVA) performed with the combined density dependence-rainfall model suggests that the San Joaquin kit fox on NPRC could face a risk of up to 52% of falling to low levels within 20 years.