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Population Changes During the 1957‐1958 Vole (Microtus) Outbreak in California

Murray, Keith F.
Ecology 1965 v.46 no.1-2 pp. 163-171
Microtus, Salmonella, Siphonaptera, Streptococcus, basins, breeding, food availability, habitats, lakes, spring, tularemia, viability, voles, watersheds, winter, California
A high population peak in Microtus montanus during 1957—1958 was synchronous throughout at least the California part of its range. Densities reached drastic outbreak levels only in relatively large cultivated basins. In the Tule Lake basin of northern California, populations survived the winter in sharply reduced numbers but did not suffer a drastic crash. From April 1958 onward, in spite of the resumption of breeding and renewed food and cover, voles in most fields continued to decline to the point of disappearance. Although breeding was fairly normal, relatively few young were successfully recruited into the population. A temporary resurgence of numbers occurred in some peripheral fields. Voles in favorable natural habitats showed less evidence of reaching extreme numbers and declined more slowly during spring than did those in cultivated fields. Voles in Bridgeport Valley, Mono County, had nearly disappeared by January 20, and were thought to have crashed in the wake of a severe precipitation—freeze episode. Tularemia was found at Tule Lake in December and early January, but subsequent testing was negative and there was no further evidence of die—offs. There were positive findings of Salmonella enteridis and Group A Streptococcus. Flea densities on Microtus were low. Neither the effects of disease nor destruction of food supply were thought to explain adequately the termination of the outbreak. Concepts of reduced viability due to crowding were compatible with observed events.