The Impacts of Water Lily Invasion and Removal on Wetland Ecosystem Function

Colonization by native and non-native invasive plants is considered a primary reason for the failure of wetland restoration and creation projects. We investigated the impact of invasion and subsequent removal of Nymphaea odorata (white water lily) in two permanently flooded wetlands in Western New York State. Long-term grazer exclusion experiments at these sites demonstrated that selective grazing by herbivores, such as waterfowl, reduced emergent vegetation and overall plant diversity, simultaneously facilitating N. odorata expansion. This interaction ultimately promoted a negative feedback to waterfowl use of the wetlands because of the lack of open water space. To evaluate potential remediation options, we experimentally removed N. odorata in both small and large-scale plots and assessed impacts on methane emission, plant diversity, soil characteristics, potential denitrification, and waterbird use. In small plots, N. odorata removal was crossed with grazer exclusion to evaluate interactive effects. In smaller plots, removal resulted in a marginal decrease in N. odorata cover, but only where grazers were excluded. There were no persistent effects among years. However, plant diversity increased in grazed plots with N. odorata removal, trending towards diversity measured in exclusion plots. Soil characteristics, methane flux, and potential denitrification were not impacted by removal efforts. In large zones, bird use increased significantly with removal in spite of the lack of reduction in N. odorata cover. These results highlight the importance of considering multiple drivers of ecosystem functions, including invasive plants and herbivory, during efforts to improve wetland restoration outcomes.