Impending extinctions of North American freshwater mussels "Unionoida# following the zebra mussel "Dreissena polymorpha# invasion

0[ Freshwater mussels "Order Unionoida# are the most imperiled faunal group in North America^ 59) of described species are considered endangered or threatened\ and 01) are presumed extinct[ Widespread habitat degradation "including pollution\ siltation\ river channelization and impoundment# has been the primary cause of extinction during this century\ but a new stress was added in the last decade by the introduction of the Eurasian zebra mussel\ Dreissena polymorpha\ a biofouling organism that smothers the shells of other molluscs and competes with other sus! pension feeders for food[ Since the early 0889s\ it has been spreading throughout the Mississippi River basin\ which contains the largest number of endemic freshwater mussels in the world[ In this report\ we use an exponential decay model based on data from other invaded habitats to predict the long!term impact of D[ polymorpha on mussel species richness in the basin[ 1[ In North American lakes and rivers that support high densities "×2999m# of D[ polymorpha\ native mussel populations are extirpated within 3Ð7 years following invasion[ Signi_cant local declines in native mussel populations in the Illinois and Ohio rivers\ concomitant with the establishment of dense populations of D[ poly! morpha\ suggest that induced mortality is occurring in the Mississippi River basin[ 2[ A comparison of species loss at various sites before and after invasion indicates that D[ polymorpha has accelerated regional extinction rates of North American freshwater mussels by 09!fold[ If this trend persists\ the regional extinction rate for Mississippi basin species will be 01) per decade[ Over 59 endemic mussels in the Mississippi River basin are threatened with global extinction by the combined impacts of the D[ polymorpha invasion and environmental degradation[