Island fox populations on San Miguel, Santa Rosa, and Santa Cruz Islands underwent severe and unprecedented declines in the 1990s. Island fox populations on all three islands were naturally small and had historically fluctuated, but as far as was known had never been as low as they were during this period and had never come close to extinction. The source (or sources) of the mortality that was causing the declines was at first elusive; a species-wide investigation revealed no evidence of disease, and no significant changes in food supply or habitat condition were detected. Radiotelemetry studies of foxes on Santa Cruz and San Miguel ultimately revealed evidence of predation by golden eagles, a species which had not previously bred on the islands and was, until this time, rarely observed. Fox mortality rates due to predation were so high that by 1999 the San Miguel and Santa Rosa fox subspecies were nearing extinction; on each of those islands total abundance had declined from approximately 450 and 1,500, respectively, to 15. The remarkable influence of golden eagle predation on island fox populations was due to the success of golden eagles in colonizing the northern Channel Islands (supported by introduced prey species), and the vulnerability of island foxes to predators with which they had not evolved.

Detecting change

Prior to the 1990s, the island fox populations on San Miguel, Santa Rosa, and Santa Cruz were thought to have fluctuated over time, perhaps markedly, but never to near-extinction levels.