Synergistic impact of water level fluctuation and invasion of Glyceria on Typha in a freshwater marsh of Lake Ontario

The effects of multiple stressors on the native Typha marsh community (mainly Typha latifolia) were examined using historical records of water levels, human census population, and field vegetation maps. Percent cover of the major plant species was estimated in a GIS, and the percent cover of Typha was related to changes in water level, human population growth, and percent cover of exotic Glyceria maxima and invasive Phragmites australis. Water level fluctuation was the major natural disturbance and it alone accounted for 88% of the variation in Typha. After partitioning out the effect of water level, both human population growth and the presence of exotic species were still significantly related to the decline of native Typha. We suggest that multiple stressors interact with each other to influence changes in native Typha community and cause greater detrimental impact. An important implication of our results is that projected water level decline due to climate change may not necessarily favor the restoration of a desirable native marsh because of the presence of other disturbances such as exotic and invasive species and altered nutrient regime.