Muskrats (Ondatra zibethicus) in treatment wetlands

Muskrat grazing can change treatment wetlands from being densely vegetated to a patchwork of open and emergent areas. Muskrats consume a portion of the annual net primary productivity, primarily rhizomes, but their mounds represent a greater share of this production. Densities of 20 or more animals per ha have been found, which can destroy the majority of the macrophyte standing crop in a given year. At such an exacerbated scale, muskrat herbivory may be termed as an “eatout,” and is evidenced by the removal of essentially all emergent plant parts. Destruction of the wetland vegetative infrastructure may create an attendant loss of some water quality functions, but may not harm others. The integrity of berms may be threatened by burrowing. Impacts on wetland hydraulics are also possible. In all cases, loss of the emergent vegetation has been viewed with dismay by owners, wetland practitioners, regulators and the general public. Several case histories are reviewed to illustrate the breadth and severity of muskrat damage. Muskrat control is given scant attention in existing treatment wetland literature, which provides very limited information on potential muskrat problems, or on the means to control them. Controls include trapping, shooting, poisoning, hazing, and exclusion in order to protect the wetland from excessive vegetation destruction by these rodents. This paper summarizes available muskrat controls, as well as their effectiveness. While many of these approaches have had a limited effect on deterring these industrious creatures, there are some methods that have proven to be effective over the long run, and should be considered in wetland design.