Feral pigs facilitate hyperpredation by golden eagles and indirectly cause the decline of the island fox

Abstract

Introduced species can compete with, prey upon or transmit disease to native forms, resulting in devastation of indigenous communities. A more subtle but equally severe effect of exotic species is as a supplemental food source for predators that allows them to increase in abundance and then overexploit native prey species. Here we show that the introduction of feral pigs (Sus scrofa) to the California Channel Islands has sustained an unnaturally large breeding population of golden eagles (Aquila chrysaetos), a native predator. The resulting increase in predation on the island fox (Urocyon littoralis) has caused the near extirpation of three subspecies of this endemic carnivore. Foxes evolved on the islands over the past 20,000 years, pigs were introduced in the 1850s and golden eagles, historically, were only transient visitors. Although these three species have been sympatric for the past 150 years, this predator-prey interaction is a recent phenomenon, occurring within the last decade. We hypothesize that this interaction ultimately stems from human-induced perturbations to the island, mainland and surrounding marine environments.

Pleistocene to historic shifts in bald eagle diets on the Channel Islands, California

“…Recent golden eagle (Aquila chrysaetos) predation of island foxes on the NCI resulted in swift and costly action to capture both predator and prey to avoid probable extinction of the fox (40,(45)(46)(47). It is hypothesized that golden eagles naturally colonized the NCI to take advantage of the high abundance of feral ungulates and switched to preying on foxes after eradication programs reduced ungulate densities (40,45,48).…”

“…It is hypothesized that golden eagles naturally colonized the NCI to take advantage of the high abundance of feral ungulates and switched to preying on foxes after eradication programs reduced ungulate densities (40,45,48). Both golden eagles and feral ungulates now have been removed from the islands, but a monitoring program recently has discovered evidence of island fox predation by eagles on SCI and SRI that occurred after golden eagle removal (49).…”

Burrowing Owl Diet at a Migratory Stopover Site and Wintering Ground on Southeast Farallon Island, California

“…Eagles then began preying upon the rare, endemic island fox (Urocyon littoralis), 13 resulting in a steep catastrophic decline of the island fox population (Roemer et al 2001). After 14 removal of feral pigs and eagles from Santa Cruz Island combined with fox reintroduction efforts 15 the fox population recovered quickly to near historic population levels (Coonan 2011).…”

Home Range and Habitat Use of Santa Rosa Island Foxes (Urocyon littoralis santarosae)

“…Of 28 fox carcasses recovered on Santa Cruz and San Miguel Islands, 24 (86%) showed signs of raptor predation. This correlates with a high number of golden eagle sightings during this period (Roemer et al 2001b, Latta et al 2005. The declines in the northern islands were due to a novel predation event by non-native golden eagles.…”

“…Roemer et al (1994) (Roemer 1999). Activity pattern results showed a significant correlation between mortality and levels of diurnal activity, specifically with higher predation rates at the open grasslands versus in chaparral and oak woodlands (Roemer et al 2001b). …”