Effects of Alien Species on Freshwater Mollusks in North America

Alien species (those carried outside their original ranges by human activities) have strongly affected the distribution and abundance of mollusks in many North American fresh waters. The best known of these aliens in North America is the zebra mussel (Dreissena polymorpha), which has nearly extirpated native unionid clams from infested lakes and rivers by fouling their shells and outcompeting them for food. Zebra mussels also have reduced populations of native sphaeriid clams, and both increased and reduced populations of snails. The effects of the other well-known alien bivalve in North America, Corbicula fluminea, are surprisingly poorly known. Corbicula probably caused some populations of native bivalves to decline, but other native populations seem to coexist with Corbicula. Several plausible mechanisms of interaction between Corbicula and the native biota have been proposed, but not demonstrated. Other aliens, including the recently arrived snail Potamopyrgus antipodarum, probably compete strongly with native freshwater mollusks under some circumstances. Several alien species, such as round goby and some sunfishes and crayfishes introduced outside their native ranges in North America, are effective predators on native mollusks and have strong effects on their distribution and abundance. Other aliens (particularly aquatic plants) affect mollusks by altering the food base or the physicochemical environment. Alien species can affect water quality, cycling of contaminants, and performance of biological indices of water quality. Because of ineffective control of aliens in North America, they may be an increasingly important factor in molluscan distribution as new species arrive from other continents and established species spread throughout the continent.