Impoundments and the Decline of Freshwater Mussels: a Case Study of an Extinction Gradient

Abstract: One major factor leading to the imperilment of freshwater mussels ( Bivalvia, Unionidae) has been the large‐scale impoundment of rivers. We examined the distribution and abundance of mussels at 37 sites along a 240‐km length of the Little River in southeastern Oklahoma, U.S.A., which is affected by both mainstem and tributary reservoirs. We observed a mussel extinction gradient downstream from impoundments in this river: with increasing distance from the mainstem reservoir there was a gradual, linear increase in mussel species richness and abundance. Mussel species distributions were significantly nested, with only sites furthest from the impoundment containing relatively rare species. Below the confluence with the inflow from the second reservoir these same trends were apparent but much weaker, and overall mussel abundance was greatly reduced. Our results suggest that considerable stream lengths are necessary to overcome the effects of impoundment on mussel populations, and such information should be considered in conservation and management plans.