High abundance of an invasive species gives it an outsized ecological role.
Many communities are shifting composition, with losses of native species and increases of non-indigenous species (NIS). At its extreme, such alteration of ecological guilds can result in simplification with a single NIS performing an ecological role once carried out by a suite of natives. This alteration has occurred in many rivers of the south-eastern U.S.A., where the invasive filter-feeding freshwater clam Corbicula fluminea has proliferated following the nearly complete extirpation of native mussels. We investigated the factors controlling the distribution and abundance of Corbicula, as well as estimated the ecological service it provides via water filtration. With a nested design, we surveyed multiple transects within four to six sites within each of four rivers that spanned three large catchments in the Georgia piedmont, collecting data on Corbicula density and physical habitat characteristics associated with its presence. We found Corbicula present in over half of the 1,536 sampled 0.044 m2 sampled plots, 90 of the 93 transects that spanned the width of the river, and all 1-2 km sample sites, underscoring the clam's ubiquity in the study region. At the river scale, Corbicula densities ranged from 50-212 Corbicula m-2, although individual sites ranged from 7-483 Corbicula m-2. Corbicula was more abundant in areas with higher proportions of gravel, and less abundant with higher proportions of bedrock. A hierarchical model with river, site, and these two substrate variables explained 32% of the variation in Corbicula density. Using observed densities and published per capita feeding rates, we calculated system-wide collective filtration rates provided by Corbicula. In the four rivers surveyed and based on estimated residence times for median flows for the summer of 2012, Corbicula is estimated to filter water as many as seven times during median flows and 18 times during minimum flows before water flows out of a 10-km reach. Due to high abundances and per biomass filtration rates, Corbicula plays an important role in these rivers. Invasive species, biotic homogenisation, and the loss of functional redundancy may mean that many more rivers are similar to our studied rivers, with a single, often invasive, species dominating ecosystem function. Understanding the influence of biotic homogenisation on ecosystem function is of foremost importance to evaluate the resilience of natural systems.