When protected areas produce source populations of overabundant species

Abstract

While protected areas are a cornerstone of biodiversity conservation, human activities in these areas can negatively affect native species in direct and indirect ways. However, the potential effects of food subsidies provided by visitors on the local- and landscape-scale population dynamics of overabundant species, and how these effects may be augmented by human development outside of parks, are largely unexplored. Here, we investigated how human foods at heavily-visited sites within California parks benefited populations of Steller's jays (Cyanocitta stelleri), an overabundant predator of a threatened seabird that nests in remnant old-growth forests. Our study population was heavily subsidized with both juveniles and adults consuming substantial amounts of human foods by biomass (46% and 57%, respectively), as indicated by stable isotope analyses. Juvenile survival, adult survival, and fecundity—estimated from radio-telemetry data—were high and the population was projected to grow rapidly (25% annually). Most juveniles dispersed from parks following seasonal declines in human visitation to areas with high housing densities and more stable resources—with almost half of dispersers returning to old-growth forests the following breeding season. Thus, we found that local food subsidies at heavily-visited sites promoted source populations of jays that may bolster populations in old-growth forests at broader spatial scales, and that this effect was likely augmented by more temporally stable resources in developed areas outside of parks during the nonbreeding season. These novel results indicate that curbing populations of overabundant predators may require reductions in food subsidies both within protected areas and surrounding landscapes.