Distribution of Freshwater Mussels: Coastal Rivers as Biogeographic Islands

Distribution of freshwater mussels: coastal rivers as biogeographic islands. Syst. Zool. 23:165–188.—Coastal rivers are isolated habitats that can be studied as biogeographic islands. Forty-nine rivers of the North American Atlantic and Eastern Gulf of Mexico Coastal Plains and their freshwater mussel (Unionidae) faunas are analyzed as a discrete island biogeographic system. Cluster analyses group the rivers on the basis of faunal similarity into five intergrading zoogeographic provinces. Distributional patterns in these provinces are strongly influenced by the proximity of species source rivers. Independent variables measuring stepping-stone distances (number of intervening rivers) from species source rivers, area of river drainage basins, and water quality parameters were tested in multiple regression models for their ability to predict numbers of species. Area of drainage basins is the best predictor of numbers of species. The speciesarea effect is greatest for provinces containing important source rivers and is lower for provinces in which rivers were colonized primarily by stepping-stone dispersal. The influence of stepping-stones on dispersal was assessed by constructing a stochastic model that uses immigration and extinction probabilities on serially arranged islands. Simulations of the model demonstiate that numbers of species on the islands should decrease approximately geometrically with distance from the species source. Numbers of freshwater mussels in the coastal rivers display a similssar geometrical decline. The species-area and stepping-stone dispersal effects are combined in a descriptive model that stresses an interaction between area and distance from source rivers in determining species numbers. When combined with measures of environmental quality (hydronium concentration, amount of dissolved solids, and nitrate concentration), this model explains up to 80 percent of the variance in unionid species numbers among the coastal rivers.