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Manuscript - Vole population fluctuations: factors affecting peak densities and intervals between peaks
Lowell L. Getz, Joyce E. Hofmann, Betty McGuire, Madan K. Oli
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Abstract
Factors associated with initiation of population cycles of Microtus ochrogaster and M. pennsylvanicus were studied in alfalfa, bluegrass \nand tallgrass habitats for 25 years. For both species, increased survival appeared to be the most important factor associated with initiation of a population cycle during a given year. There was no difference in reproduction the previous winter or during spring (both species) and autumn (M. ochrogaster) of cycle and non-cycle years. \nWeather differences, including episodes of extreme conditions, were not associated with cycle and non-cycle years. There was no indication \nthat cyclic phenomena were a result of habitat degradation owing to high densities during peak phases. We found no relationship between \npeak densities and rate of decline, length and extent of the decline, population density during the subsequent trough, or the interval until \nthe next cycle. Population cycles appeared to be initiated by relaxation of predation pressure which occurred erratically across years.