Mammalian carnivores, mesopredator release, and avifaunal extinctions in a fragmented system (California)

Habitat fragmentation has been identified as a primary threat to biodiversity, and mammalian carnivores are thought to be particularly vulnerable to extinction in fragmented habitat. Further, the disappearance of dominant predators from fragmented systems may result in the ecological release of smaller carnivores and increased predation on small prey species, a process termed “mesopredator release.” I first examine the effects of habitat fragmentation on the distribution and abundance of mammalian carnivores in scrub habitats in coastal southern California. My results suggest a combination of landscape biogeographic descriptors (fragment area and isolation), local habitat variables (percent cover and distance to urban edge), and species characteristics (body size) help determine patterns of occupancy and relative abundance of a suite of mammalian carnivores. Second, I exploit an ecological experiment—spatial and temporal variation in the distribution and abundance of the coyote among urban habitat fragments in San Diego—to assess the direct and indirect effects of this top predator on community structure. My results, providing support for the mesopredator release hypothesis, indicate that the disappearance of coyotes result in elevated numbers and activity of smaller carnivores that exert strong predation pressure on scrub-breeding birds and other native prey. Third, I calculate local extinction and recolonization rates of scrub-breeding bird species in the San Diego urban habitat fragments by comparing recent surveys with those conducted in this system a decade ago. The results suggest ongoing local extinction and recolonization of native birds, dynamics that are a function of both extrinsic forces (fragment area, age) and intrinsic factors (density, body size). Finally, in a general review of my research, I use differential fragmentation sensitivities of mammalian carnivores to evaluate their roles as targets for conservation and management plans in the region. I argue that the choice of target species depends on the scale or intensity of fragmentation in an area, and the corresponding responses of carnivore populations.