WHY ARE SOME MICROTINE POPULATIONS CYCLIC WHILE OTHERS ARE NOT

Mark-recapture, radiotelemetry, and behavioural tests were used to determine whether mechanisms suggested to cause cyclic patterns in microtine rodents are present in a noncyclic Microtus agrestis (Linnaeus, 1761) population studied in southern Sweden. Several of the factors suggested to cause cyclicity occur also in our noncyclic population, e.g. between-year differences in individual quality and variations in food conditions. The population was found to have a high potential for fluctuating in numbers, and peak densities varied greatly between years. Intense predation, especially during winter, by various generalist predators resulted in low densities in spring and early summer every year. This probably prevented between-year cyclic patterns. Predation by specialist predators whose numbers are closely related to the abundance of their prey, might promote cyclicity in some microtine populations.