A 19-Year Study of Microtine Population Fluctuations Using Time-Series Analysis

This report involves time-series statistical analysis (including concurrent physical and community variables) of the population dynamics of 4737 voles (Microtus californicus) trapped over 19 yr while emigrating from two study enclosures on a Northern California grassland. Population fluctuations of voles, as documented in the literature as well as in this study, generally cannot be described by periodic or regularly cyclic equations, but rather are either random or occasionally pseudoperiodic where autoregressive correlation explains irregular cycles on the basis of the popu- lation's previous demographic history. For these two California vole populations, an autoregressive component accounted for approximately a third of the total variability in the population, while random extrinsic environmental variation explained almost all of the remaining variation. Weather played a key determinant role, influencing microtine populations both directly, and indirectly through an effect on vegetation. The distinction between periodic, pseudoperiodic, and random fluctuations in vole populations, and indeed in wildlife populations in general, cannot be dismissed as merely a question of semantics, because each entails a specific ecologic interpretation. The demographic characteristics of microtine rodents, although fluctuating more dramatically, were similar to those of other rodents.

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