The Decline of a Muskrat Population

Abstract

Responses of a high muskrat (Ondatra zibethicus) population to a deteriorating habitat were studied at Goose Lake, a 135-acre central Iowa marsh, during 1961. Apart from certain cyclic implications in the situation, the most remarkable findings from this particular study relate to the indifferent breeding performance of the muskrats from spring to midsummer, followed by a late-breeding spurt in August and September after ear corn in fields adjacent to the marsh became utilized by the muskrats on an extensive scale. To summarize the differences in seasonal distribution of 88 litters born to or conceived by adult females at Goose Lake in 1961 and samples totaling 3,209 litters from Iowa study areas collectively, 1935-57. There were about three times the proportion of early-season litters in the long-term Iowa series as for Goose Lake in 1961, similar proportions of mid-season litters, and over four times the proportion of late-born litters in the 1961 Goose Lake sample as for the Iowa series. The muskrat (Ondatra zibethicus) has been studied with respect to life history, ecology, and population dynamics not only in its native North America (O'Neil 1949, Sather 1958, Errington 1961, 1962) but also in many parts of its recently acquired Eurasian range (Hoffmann 1958). The resulting literature covers fairly well the muskrat's reactions to drought conditions, but there are few descriptions of what happens when a muskrat marsh goes from one environmental extreme to another in a relatively short period of years. One of the best examples of dramatic change of this sort is provided by Goose Lake, a privately-owned central Iowa marsh of about 135 acres, which passed through stages from complete drought exposure in 1956 to an open-water lake in 1961. Goose Lake is a shallow, glacial marsh having two islands near its center. The larger island supports a heavy growth of hardwood trees, with a mainly brushy understory. Surrounding the marsh are cultivated fields and former pastures that have been ungrazed by livestock for at least 15 years. The marsh itself is usually covered or partly covered by emergent vegetation consisting predominantly of sedges (Carex spp.), hardstem and river bulrushes (Scirpus validus and S. fluviatilis), and cattails (Typha latifolia, T. angustifolia, and hybrids). Other marshy emergents include 1 Journal Paper No. J-4378 of the Iowa Agricultural and Home Economics Experiment Station. Projects No. 1389 and No. 1504. A contribution from the Iowa Cooperative Wildlife Research Unit, the Bureau of Sport Fisheries and Wildlife (U. S. Department of the Interior), Iowa State University of Science and Technology, Iowa State Conservation Commission, and the Wildlife Management Institute, cooperating.