Muskrat (Ondatra zibethicus) disturbance to vegetation and potential net nitrogen mineralization and nitrification rates in a freshwater tidal marsh

The muskrat (Ondatra zibethicus) is a wetland mammal whose disturbance acti vities include grazing, burrowing and lodge construction. We evaluated the effects of these disturbances on plant biomass, species richness and divers ity, stem density and potential net nitrogen mineralization and nitrificati on rates in a freshwater tidal marsh on the Hudson River in New York. We hy pothesized that muskrats increase floristic richness and diversity by decre asing the biomass of narrowleaf cattail (Typha angustifolia) and that muskr ats increase potential net nitrogen mineralization and nitrification rates through aeration and reduced plant uptake because of herbivory. Because mus krats commonly build lodges on or close to creek banks, we separated the di sturbance effects of muskrats from the disturbance effects of the creek ban k by sampling quadrats along transects placed perpendicular to creek banks at lodge sites. Muskrats decreased biomass, particularly of cattail, but ha d no measurable effect on stem count, species richness or species diversity . Muskrats increased potential net nitrogen mineralization and nitrificatio n rates; however, this effect was limited to active sites. Creek bank distu rbance increased stem count but had no effect on the other variables. Altho ugh muskrats did not significantly affect floristic diversity in this study , their disturbance activities did influence soil nitrogen dynamics, which is an important component of wetland function.