Top down or bottom up: which process drives a population of an endangered mesocarnivore?

Context The San Joaquin kit fox (Vulpes macrotis mutica) is listed as endangered in the United States and threatened in California. Primary processes influencing populations of this mesocarnivore are not well quantified. Aims The study aim was to determine whether bottom up or top down processes most influenced the demographic and ecological characteristics of kit foxes. Methods During 2014–19, we assessed prey abundance on 60 0.5-km transects. We then assessed food item use by kit foxes and coyotes (Canis latrans) by examining scats, abundance of both canids using camera stations, and kit fox survival, reproduction, and space use during low and high prey periods using radiotelemetry. Coyotes are the main kit fox predator. Key results Due to higher precipitation, prey abundance increased markedly during the study period. Dietary overlap between kit foxes and coyotes was high in all years (0.73–0.92). Abundance of both kit foxes and coyotes increased as did overlap at camera stations. Between low and high prey years, survival (0.69 vs 0.87) and reproductive success (44.4% vs 72.7%) of kit foxes increased and mean home range size decreased significantly (4.3 km2 vs 1.3 km2). Conclusions As prey increased, kit fox abundance, survival, and reproduction all responded positively despite a concomitant increase in the abundance of their main competitor, coyotes, and a high potential for interference and exploitative competition. We concluded that bottom up processes are more influential on kit foxes than top down processes. Implications Management strategies that enhance prey abundance, such as grazing in wet years, should benefit kit foxes. Coyote control is unlikely to benefit kit foxes.