Document: GAR-3-99-135

Exotic herbivores facilitate the near extinction of an insular mesopredator.

ROEMER, G.W.* ¹, D.K.GARCELON ² and T.J.COONAN ³

University of California, Los Angeles, CA 90095 USA ¹
Institute for Wildlife Studies, Arcata, CA 95518 USA ²
Channel Islands National Park, Ventura, CA 93001 USA ³

Abstract:
Top-down processes influence community dynamics and structure, but their influence is often dampened by the vast array of interactions that occur among trophic levels. Trophic cascades are more common in simple systems where perturbations, like the introduction of a novel predator, have a more profound impact. Here we show that anthropogenic disturbance to the California Channel Islands caused a shift in the apex predator, from island fox to golden eagle, which changed the trophic hierarchy from a 3-level to a 4-level system. Golden eagles have subsequently caused the near extirpation of three subspecies of the island fox, a rare island endemic. Fox populations have declined by 90% and have an estimated 50% chance of persistence over the next decade. Of 28 fox carcasses discovered on two islands, 24 (86%) showed signs of being eaten by a large raptor, 18 (64%) had talon holes in the head or body, and 8 (29%) had golden eagle feathers nearby. The discovery of the first golden eagle nest ever recorded for the Channel Islands confirms recent colonization. A predator-prey logistic model supports the premise that mainland golden eagles were attracted to the archipelago and sustained by the presence of a novel prey, feral pigs. Regression analyses suggest that an increase in the abundance of the island spotted skunk, a competitor of the island fox, is also related to the decline in foxes.

Keywords: predator-prey, fox

This abstract is being presented at: 2:15 PM in session:
Oral Session #14: Disturbance Effects on Bird Populations.