Northern Prairie Wildlife Research Center
Iowa State University, Department of Animal Ecology, 124 Science II, Ames, IA 50011
Muskrat (Ondatra zibethicus) population dynamics and habitat relationships were studied during 1985-89 as part of the Marsh Ecology Research Program at Delta, Manitoba. After draining, the experimental wetland complex was flooded to one of three different levels: normal (long-term average elevation), medium (30 cm above normal), and high (60 cm above normal). Trapping in October and May was used to estimate population parameters including density and survival. I used locations where muskrats were trapped or built houses, superimposed on vegetation types and water depth in a geographic information system, to determine habitat selection. Populations reached densities >30/ha in October 1986, two growing seasons after flooding. Muskrats were captured in all vegetation types, but significantly avoided areas with water <1 cm dominated by squirrel-tail barley (Hordeum spp.). However, >78% of all individual captures were made in bulrush (Scirpus sp.), whitetop (Scolochloa festucacea), or cattail (Typha sp.). Muskrats preferred locations within cattail and bulrush stands for houses, although some were built in whitetop and reed (Phragmites communis). Water depth at houses averaged 38 cm and >90% were in water ≥10 cm. Juvenile muskrats gained an average 319 g body mass between October and May, while adults gained 142 g body mass. During the first two winters, only 3% of the animals had lost body mass, whereas 25% lost body mass during the winter of 1987. Based on approximations of energetic requirements, I estimated that peak densities of muskrats could have consumed <3% of the estimated emergent biomass, although they destroyed >70% of the shoots within a 3-m radius of a house. The estimated consumption was insufficient to cause the vegetation decline, which had already begun in August 1987 before peak densities of muskrats were reached. Populations were reduced to about 3/ha in all treatments in May 1988 due to a significant reduction in winter survival from 0.31 in 1986 to 0.09 in 1987. The hypothesized underlying causes of changes in body mass and survival were reduced access to food and suitable shelter due to the increasing area of open water and a corresponding decreased area of emergent vegetation resulting from the flooding treatments. Emergents such as cattail and bulrush are good building material and provide quality above- and below-ground food resources. Furthermore, they catch snow, which insulates the house and prevents water from freezing to the bottom. The winter mass gain of most muskrats is evidence that food quality is not an energetic constraint for muskrats. Emergent vegetation in adequate water not only provides access to rhizomes and tubers, but facilitates escape from predators such as mink (Mustela vison) and reduces the need to desert lodges over ice. As vegetation succession proceeded, driven by flooding the prairie marsh complex, relative use of less-suitable habitat first increased, accounting for the increase in density. But as prime emergent habitat was reduced, a smaller proportion of the population used habitat where survival and reproduction were greatest, and total survival, recruitment, and density declined.