Intra- and Interannual Changes in Habitat Selection by the Muskrat

Abstract

Habitat selection by the muskrat (Ondatra zibethica), as reflected by house location, was studied over a 7-yr period (1970-1976) in a shallow lake, rich in emergent aquatic plants (belts of sedges, water horsetail (Equisetumfluviatile), and bulrush (Schoenoplectus lacustris)), in northern Sweden. The houses were built in or very close to the water, and were constructed with the plant material available near the house site. Most houses were found in about 0.3 m of water and they were often situated on locally elevated points. The spatial distribution of inhabited houses during the peak year was random in late June, but contagious in late October. The shallow sedge belt was most utilized in May and June, but was to a large extent abandoned later on, when the deeper water horsetail belt became well stocked. In late October the outer bulrush belt was also exploited, although only to a minor extent, as during other parts of the year. Utilization of bulrush was determined by the seasonally-prevailing water depth rather than population size. The water horsetail belt was utilized increasingly in late October with the increase in the muskrat population, but only up to a certain level, after which the sedge belt was utilized instead. J. WILDL. MANAGE. 42(3):540--49 The muskrat, previously confined to North America, was introduced into Europe in 1905. Its present range in Europe and Asia covers an area greater than the former, natural distribution area in USA and Canada (Hoffmann, in litt.). Around 1950 the species entered northern Sweden and soon occupied vast areas (Marcstrom 1964, Danell 1977a), where, as in Finland (Artimo 1960), it is most abundant in well-vegetated localities, usually building its houses in lakes and digging burrows in riverbanks (Danell 1978b). For studies concerning the impact of muskrats on plant (Danell 1977b) and animal life in Swedish lakes a detailed knowledge of the factors influencing its habitat selection was needed. Because muskrats build their houses throughout the ice-free season, then abandon some of them and later perhaps reoccupy them, it was assumed that the distribution of inhabited houses at any given moment reflected the habitat selection habits of the population as a whole. The individual houses are short-lived, as they are destroyed by the spring flood, while burrows can persist in use for many years. H wever, it is more difficult to check whether burrows are inhabited or not, so it was therefore desirable to choose a lake at which only houses were present. This requirement is fulfilled in lakes with very shallow shores, where muskrats only construct houses (Danell 1978b). The aim of this investigation was to study habitat selection by the muskrat in a lake, paying special attention to any seaso al differences shown during the ice-free season, as well as to any differences due to population size, i.e., between years with low and high muskrat densities. The study was carried out during a 7yr period (1970-76) in a shallow lake in northern Sweden, where the muskrat population had not been disturbed by trapping. This study was sponsored by the Research Committee of the Swedish National Environment Protection Board (contract 6-3/73-76). I am further indebted to L.-O. Eriksson, A. Gothberg, V. Marcstrom, K. Sjoberg, and T. Sorlin for 540 J. Wildl. Manage. 42(3):1978 This content downloaded from 157.55.39.223 on Wed, 24 Aug 2016 04:43:10 UTC All use subject to http://about.jstor.org/terms MUSKRAT HABITAT SELECTION Danell 541 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..... ..... .. .. . . . .. .. .. . . . .. .. . . .... .... ... ... ... ... . . . . . . . . . . . . . . I) i~:: ~ Ii:?I; Y;~~q~q b9 t :czsjup:HER F-' ~ ~ b~~5HHE 40 H+E FE FE......... iX 65 W. I~ . ... . .... .... ... ... .... .... .... ... ... . . . . .