PS 38-208 - Invasive hybrid cattail Typha x glauca distribution and relative abundance does not impact muskrat (Ondatra zibethicus) population density

Background/Question/Methods

Recent studies have confirmed suspicions of long-term muskrat (Ondatra zibethicus) population declines in North America. Water level management is known to affect muskrat populations, but the spread of invasive cattails may also play a role. An invasive hybrid cattail, Typha x glauca, has spread throughout North American wetlands in recent decades. T. x glauca produces tall, dense stands that outcompete native vegetation and may reduce cattail-water interspersion, an essential component of muskrat habitat. We compared the distribution of cattail genotypes to an index of muskrat density in wetlands across south-central Ontario to determine whether the observed muskrat population declines may be related to invading T. x glauca. We selected thirty-eight cattail-dominated marshes in south-central Ontario, including both inland marshes and coastal Lake Ontario marshes. At each site, we sampled cattails along representative transects and identified genotypes present based on pollen and bracteole characteristics outlined in a key to Typhaceae. We then estimated muskrat density by conducting muskrat house counts using recent, high-resolution aerial imagery. We predicted a negative correlation between muskrat house densities and relative abundance of T. x glauca.

Results/Conclusions

Invasive hybrid T. x glauca was the dominant cattail genotype at 18 of 19 coastal sites and 17 of 19 inland sites. T. x glauca occurred at all sites, while native T. latifolia only occurred at two coastal and six inland sites. The slightly higher occurrence of T. latifolia at inland marshes may owe to these sites being higher in elevation than Lake Ontario and thus may have had less exposure to the invasive cattail (T. angustifolia) and resulting hybrid from aquatic dispersal. We did not find a correlation between relative abundance of T. x glauca and muskrat house density. Preliminary muskrat house counts show mean house densities of 0.3 houses/ha at coastal sites and 0.7 houses/ha at inland sites. These findings suggest that other factors, such as water level regulation, may be more important predictors of muskrat population declines. Water levels at both coastal and inland sites used in our study are regularly managed for flood prevention. However, the decades-long seasonal management regime for Lake Ontario, Plan 1958, which ended in 2017, has been considered detrimental for coastal muskrat populations, likely explaining the lower densities at coastal sites. We recommend continued monitoring of muskrat densities at these sites.