Oecologia (1992) 92:301-304 Oecologia 9 Springer-Verlag 1992 Extirpation of alternative prey during a small rodent crash Charles A. Drost I and Reed C. McCluskey 2 i Division of Environmental Studies, Universityof California, Davis, CA 95616, USA 2 7405 Santa Fe Trail NW, Albuquerque, NM 87120, USA Received May 11, 1992 / Accepted in revised form August 10, t992 Summary. We document two episodes, in different years, of Barn Owls (Tyto alba) preying on a winter population of Burrowing Owls (Athene cunicularia) on a southern California island. The predation in each case followed a marked shift in the diet of the Barn Owls, due to the cyclic decline of their normal small mammal prey. Heavy predation in the first year resulted in the extirpation of the Burrowing Owls on the island. Such heavy predation on alternative prey species is commonly reported in cyclic predator-prey systems, however this is the first docu- mented case of extirpation of the alternative prey. Com- plete elimination of any prey species by terrestrial preda- tors is, in fact, very rare. Key words: Owls - Predation - Population cycles Alter- native prey - Extirpation 1954; Lindstrom et al. 1987) but have also been noted for a variety of other species, ranging from shrews (Hansson 1984; Henttonen 1985) to shorebirds and waterfowl (Dhont 1987; Pehrsson 1986; Pienkowski 1983). Smaller predators may also become alternative prey of larger predators during these acute food shortages, which may hasten the decline of the small predators (Elton 1942, p. 458; Hagen, in Errington 1967, Keith 1963, p. 114). In this paper, we describe heavy predation by Barn Owls (Tyto alba), on another, smaller predator, the Burrowing Owl (Athene eunicularia), on a small island off of southern California, USA. Episodes of predation followed cyclic declines in the deer mouse (Peromyscus manieulatus) population on the island, and heavy Barn Owl predation following one of the declines resulted in the complete extirpation of Burrowing Owls from the island. The dramatic fluctuations in many microtine and other small mammal populations strongly affect the ecological communities in which they occur. As Lack (1954, p. 204) noted, there are three different groups of animals whose populations are intertwined in the cycle: the cyclic small mammal species itself; the predators of the cyclic species; and other ("alternative") prey species to which the preda- tors shift their attention during declines in the cyclic species. The cyclic small mammal population is generally seen as keying the complex of events which occur during the course of the cycle. Predator populations, in turn, increase to high numbers following the upswing and peak of the small mammal population, then are left to either starve or disperse when the small mammal population crashes. (Elton 1942; Hagen 1952; Lack 1954). During and after the small mammal decline, the large predator population may switch to preying on a variety of other species. The effects of this shifting predation have received particular attention for northern grouse populations (e.g. Angelstam et al. 1984, 1985; Lack Correspondence to: C.A. Drost Study area Santa Barbara Island is a 2.6 sq km island located 62 km from the southern California mainland, west-southwest of Los Angeles. The island is covered by large expanses of annual grassland, interspersed with patches of coastal bluff scrub (Philbrick 1972). Cliffs 60-150 m in height bound the island on three sides. Native deer mice are the only land mammal on the island. They are very abundant at times, but their num- bers fluctuate widely in a three- to four-year cycle (Drost and Fellers 1991). Several species of hawks and owls prey on the mice: Common Barn Owls and American Kestrels (Faleo sparverius) nest on the island and are present year-round, while Short-eared Owls (Asio flammeus) and Burrowing Owls are fall and winter residents. Methods The observations reported here were made during the course of a long-term studyof the deermiceand their predators. From Novem- ber 1982through April 1990, we censusedthe hawks, owls and deer