Microtine population dynamics in a predictable environment

A noncycling population of Microtus montanus was studied for 8 years in a marsh community in Utah. Seasonal climatic and vegetational regimes were very similar each year. The food resource for voles in this habitat was almost solely salt grass (Distichlis stricta). Each year, salt grass began sprouting in late February and, several weeks later, most of the females were pregnant with their first litters. By August of each year, the salt grass had flowered, fruited, and was senescing; likewise, breeding had ceased by the end of August. Each year, four cohorts of young were produced between April and late August. Analysis of cohort life histories revealed that cohorts I and II matured rapidly and reproduced later in the same season. Cohort III animals grew very slowly, overwintered as subadults, and formed the primary breeding stock the following spring. Overwintering survival of cohort III animals was much higher than that of cohorts I and II. By the end of the breeding season each year, densities ranged between 150 and 225 animals/acre (1 acre = 0.405 ha). Densities at the end of winter varied between 50 and 120 animals/acre. The rate of population increase from April to August each year was virtually identical. Thus, the August densities were very strongly correlated with each preceding April density. The predictable seasonal phenology and uniform timing of cohort production each year seems to explain the yearly attainment of high density. Comparison of these data with studies of fluctuating populations of M. montanus in montane environments leads to the conclusion that the unique life history of each microtine cohort coupled with unpredictable environmental regimes may generate large-scale density oscillations. If this concept is valid, then microtine population dynamics are not cyclic at all, but rather pseudoperiodic.