IMPENDING EXTINCTIONS OF NORTH-AMERICAN FRESH-WATER MUSSELS (UNIONOIDA) FOLLOWING THE ZEBRA MUSSEL (DREISSENA-POLYMORPHA) INVASION

1. Freshwater mussels (Order Unionoida) are the most imperiled faunal group in North America; 60% of described species are considered endang ered or threatened, and 12% are presumed extinct. Widespread habitat d egradation (including pollution, siltation, river channelization and i mpoundment) has been the primary cause of extinction during this centu ry, but a new stress was added in the last decade by the introduction of the Eurasian zebra mussel, Dreissena polymorpha, a biofouling organ ism that smothers the shells of other molluscs and competes with other suspension feeders for food. Since the early 1990s, it has been sprea ding throughout the Mississippi River basin, which contains the larges t number of endemic freshwater mussels in the world. In this report, w e use an exponential decay model based on data from other invaded habi tats to predict the long-term impact of D. polymorpha on mussel specie s richness in the basin. 2. In North American lakes and rivers that su pport high densities (> 3000 m(-2)) of D. polymorpha, native mussel po pulations are extirpated within 4-8 years following invasion. Signific ant local declines in native mussel populations in the Illinois and Oh io rivers, concomitant with the establishment of dense populations of D. polymorpha, suggest that induced mortality is occurring in the Miss issippi River basin. 3. A comparison of species loss: at various sites before and after invasion indicates that D. polymorpha has accelerate d regional extinction rates of North American freshwater mussels by 10 -fold. If this trend persists, the regional extinction rate for Missis sippi basin species will be 12% per decade. Over 60 endemic mussels in the Mississippi River basin are threatened with global extinction by the combined impacts of the D. polymorpha invasion and environmental d egradation.