Removing protected populations to save endangered species.

Abstract

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 ][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 ][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 ][3]). In the next few months, a plan to eradicate feral pigs will begin ([ 4 ][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][5]). Modifying a preexisting model of interacting dynamics ([ 1 ][1], [ 5 ][6]), we simulated fox population sizes after 6 years, the projected time to pig eradication ([ 3 ][3], [ 4 ][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. ![ Fig. 1. ][7] Fig. 1. Three-dimensional plot showing fox population size after 6 years of controlling pigs and/or eagles. ( A ) Initial conditions (solid circle) taken from ([ 1 ][1]): 25 eagles, 154 foxes, and 7561 pigs. ( B ) Conservative 2003 levels: 10 eagles, 100 foxes, and 1000 pigs. Controlling pigs only (black line) can result in fox extinction [gray area in (A)]. ( C and D ) Change in fox population size over 100 years following eradication of pigs only (red line), of pigs and eagles (blue line), and no control of either species (green line). Our model suggests that, had the eagle translocations not occurred, pig eradication could trigger deterministic fox extinction ([Fig. 1C][5]). After the translocations, pig eradication might ultimately facilitate fox recovery through eagle disappearance ([Fig. 1D][5]). However, fox numbers may first decline to <30 individuals, at which point population viability analysis predicts a high extinction probability ([ 5 ][6]): Demographic and environmental stochastity, additional sources of food, Allee effects, and disease epizootics all are expected to increase the probability of fox extinction. Our parsimonious model predicts that foxes are highly sensitive to predation by eagles and warns of the potentially disastrous effect of removing pigs while eagles remain. Because eagle eradication is doubtful by translocation alone, other means should be pursued, including lethal removal, if the fox is to be saved. The advocated removal of a magnificent bird of prey has significant legal challenges. In the United States, the Migratory Bird Treaty Act and the Bald Eagle and Golden Eagle Protection Act both prevent take of golden eagles except under special circumstances granted by the Secretary of the Interior. This exemplifies how solving conservation problems is often more complex than redressing its primary cause (here, pigs). As often, biodiversity managers are now faced with a difficult dilemma. This time, it is associated with a paradox: the protection of the island fox, an endangered species, depends upon complete removal, by any and all means, of a small population of golden eagles, a protected species. Supporting Online Material [www.sciencemag.org/cgi/content/full/302/5650/1532/DC1][8] Materials and Methods