Muskrat Abundance Responses to Water level Regulation Within Freshwater Coastal Wetlands

Abstract

Muskrats (Ondatra zibethicus) are aptly described as “ecosystem engineers” for their influences associated with herbivory, house building, and movement within freshwater wetlands. Sensitivity to flow regime alteration via hydrologic regulation, however, may limit their populations and ability to facilitate wetland diversity and heterogeneity. Muskrat house surveys in coastal wetlands in tributaries (reference wetlands) subject to regional scale water level regulation were compared to treatment sites where local scale water-control structures were installed to alter the regulation effect. Data were used to develop a two-step model to predict wetland occupancy then house density to assess effects associated with proposed water level regulation plans. Field surveys indicated low house densities for cattail-dominated reference wetlands, and nearly 85% of houses located were in treatment sites. House distribution at reference sites was limited to channel edges, whereas houses at treatment sites were found throughout the floodplain. Occupancy of wetlands by muskrats was estimated by winter water depth, and fall water depth, and winter air temperature were selected as predictors of house density. Model validation indicated complete agreement for wetland occupancy, but density tended to be underestimated. Simulations provided a tool to evaluate water management plans and indicated that muskrat populations are suppressed under the current water level regulation regime.