Divergent impact of grazing on plant communities of created wetlands with varying hydrology and antecedent land use

Abstract

Destruction of natural wetlands has warranted the creation of wetlands to mitigate the reduction of valuable ecosystem functions and services. However, the complex interactions among key drivers of wetland community structure–hydrology, nutrient availability and herbivory-makes creation of functional wetland replacements challenging. We examined interactions among these drivers, and their impacts on plant communities and soil characteristics in two created wetlands with different hydrology and land use histories: a shallow emergent marsh on a previous gravel depository and a seasonally flooded marsh on a former cattle pasture. In paired plots open to or protected from large wetland grazers we observed seasonal variation in grazing impacts on plant communities and an increase in effect size over time. At the permanently flooded marsh with high waterfowl densities and low nutrients, open plots had significantly reduced plant growth and diversity, and an increase in submerged vegetation. In contrast, grazer density was lower and nutrients were higher in the seasonally flooded marsh, where grazer access enhanced plant diversity and reduced invasive plant cover. These results suggest the possibility of long-term grazer-induced shifts in community composition and delivery of key ecosystem services in young, vulnerable created wetlands. To improve created wetland design and function, we suggest that in addition to hydrologic conditions, the impact of prior land use on present nutrient availability be considered. Further, enhanced heterogeneity of spatial and bathymetric structure can provide conditions for diverse plant communities and balanced habitat use by wetland grazers.