Concerning Fluctuations in Populations of the Prolific and Widely Distributed Muskrat

Abstract

The data for this paper on Ondatra zibethicus were obtained from approx. 25,000 hrs. of field study, largely on about 30 square miles of Iowa habitats kept under observation from year to year since 1934. A strong tendency for rates of population gain to conform to density-determined patterns in relation to specific environments implies a great deal of automatic adjustment or intercompensation between fecundity and mortality. The indications are that dominant patterns may be well maintained despite many substantial variations in a population''s breeding success and losses through different agencies[long dash]in other words, that many variations in impacts of so-called limiting factors may have slight if any actual effect on the course of populations. Observed departures from patterns have resulted for the most part from profound environmental changes, climatic emergencies (notably those brought on by droughts), sweeping epizootics, and phenomena attributed to changes in "cyclic" phases. The Iowa studies of manifestations of the "10-year cycle" in muskrat populations have been concerned far less with numerical fluctuations of the species (which, indeed, can be very misleading if overemphasized as "cyclic" criteria) than with other population symptoms. Of the latter, changes in average size of letters conceived, in tolerance to crowding, in adaptability during crises, and in syndromes of a lethal disease'' have been particularly suggestive of certain ways in which the depressed phase of the "10-year cycle" may operate, whether accompanied by evident population declines or not.