Sudden Increase in a Rare Endemic Carnivore: Ecology of the Island Spotted Skunk

Abstract

The island spotted skunk (Spilogale gracilis amphiala), an insular endemic carnivore, recently increased unexpectedly from rarity to abundance on Santa Cruz Island, California. Two explanations have been proposed for this striking increase: competitive release due to decline of the island fox (Urocyon littoralis santacruzae) and vegetative recovery due to removal of feral livestock. To examine the causes and consequences of the increase, we assessed abundance, body mass, home-range size, spatial resource use (den sites and habitat use), temporal resource use, and diet of island spotted skunks during abundance in 2003–2004 and compared it with similar measures during rarity in 1992. Capture success of skunks increased exponentially from 1992 to 2004 (r ¼ 0.38), leading to extraordinarily high densities (9–19 individuals/km 2 ). Both body mass and home-range size remained unchanged, suggesting that per capita resource abundance was not higher in 2003–2004 compared to 1992. We found modest shifts in habitat use, diet, and possibly diurnal activity, providing some support for release from exploitative competition as an explanation for the increase. However, there was a marked shift in den selection, toward unprotected dens and away from multiple use, that is not likely attributable to either release from exploitative competition or to island recovery. Thus, our results suggest that, although both of these processes have likely played a role, release from interference competition also may be a factor behind the dramatic increase in number of skunks. Skunks on Santa Cruz Island may be nearing or even exceeding carrying capacity, and the future of the population and its effects on the endangered island fox remain unclear.