Removing protected populations to save endangered species.

Conservation of species threatened with imminent extinction may require drastic measures that can be emotionally charged, politically unsavory, and legally challenging. A complex example relates to the decline of the island fox (Urocyon littoralis), endemic to the California Channel Islands, off the coast of the United States. The island fox exists as six subspecies, each found on a separate island. Serious population crashes recently occurred on three islands after colonization by golden eagles (Aquila chrysaetos). Preying mainly on abundant feral pigs (Sus scrofa), eagles reached numbers sufficient to drive the unwary fox toward extinction (1). Two subspecies are now extinct in the wild, and a third, endemic to Santa Cruz Island, dropped from approximately 1500 to fewer than 100 in less than a decade (2). Efforts to protect foxes by translocating golden eagles have been successful (31 eagles have been translocated since 1999, with an associated increase in fox survival), but efforts to remove the last 7 to 10 eagles have failed, preventing fox recovery (3). In the next few months, a plan to eradicate feral pigs will begin (4). This action will help restore the island’s natural vegetation, and might also help fox recovery by removing the eagles’ prey base. To assess this strategy, we investigated the effect of different levels of pig and eagle control on fox persistence (Fig. 1). Modifying a preexisting model of interacting dynamics (1, 5), we simulated fox population sizes after 6 years, the projected time to pig eradication (3, 4). Unexpectedly, pig control leads to decline or extinction of the fox population if eagle control is insufficient. This counterintuitive result occurs because eagle predation on foxes increases as pig availability declines. Congruent with this, fox recovery is directly proportional to the intensity of eagle removal. Our model suggests that, had the eagle translocations not occurred, pig eradication could trigger deterministic fox extinction (Fig. 1C). After the translocations, pig eradication might ultimately facilitate fox recov-