DENSITY CYCLES IN AN ISLAND POPULATION OF DEER MICE, PEROMYSCUS-MANICULATUS

We report on eight years of data for a population of deer mice (Peromyscus maniculatus) on Santa Barbara Island, California which reaches exceptionally high densities and fluctuates markedly in an apparent three- to four-year cycle. The cyclic increase follows winters with high rainfall, and the decline may be similarly associated with low rainfall winters. The peak and early decline is marked by nearly complete cessation of breeding, along with heavy predation by barn owls (Tyto alba), whose numbers track those of the deer mice. This island population of Peromyscus differs from the well-established pattern for the species on the mainland, where populations occur in low to moderate numbers and are relatively stable from year to year. The pattern seen on the island is instead similar to that of cyclic microtines. We compare Microtus and the Santa Barbara Island deer mice and discuss parallels in the possible causes of the respective cycles.