Geographic spread of exotic species: Ecological lessons and opportunities from the invasion of the zebra musel Dreissena polymorpha

The spatial and temporal dynamics of the recent invasion of North American fresh waters by the zebra mussel Dreissena polymorpha are reviewed in terms of the mechanistic bases behind the dispersal and colonization processes. The planktonic phase of the life cycle (the veliger), the ability of the benthic stage to attach to submerged objects, and the prominence of human activities as vectors for dispersal has promoted rapid spread of this aquatic pest to 18 states in the USA and two provinces in Canada within the first seven years of its introduction into the Laurentian Great Lakes. So far, the majority of range expansion has occurred within commercially navigable waters, and thus commercial shipping appears to be the most important vector of spread within connected bodies of water, especially to areas upstream of established populations. In contrast, overland spread to isolated inland waters appears to occur more slowly; and by early 1994 adult mussels had only been found in eight inland lakes. Although there are many potential vectors of overland spread, transient recreational boating activity is suspected of being the primary means of overland dispersal, and several mechanisms associated with boating have been shown to be capable of transporting mussels in large numbers. Studies on waterfowl indicate that although ducks are capable of transporting zebra mussels, the rate of transport is quite small relative to boating activity. Other methods of inferring the relative importance of dispersal vectors are outlined, and an example of predicting the spread on the basis of regional patterns of recreational boating traffic is given. Finally, studies on the demographic conditions necessary for the establishment of new populations are suggested as a rewarding area of further research.