GLOBAL INVASIONS OF MARINE AND ESTUARINE HABITATS BY NON-INDIGENOUS SPECIES - MECHANISMS, EXTENT, AND CONSEQUENCES

Non-indigenous species (NIS) are increasingly conspicuous in marine an d estuarine habitats throughout the world, as the number, variety, and effects of these species continue to accrue. Most of these NIS invasi ons result from anthropogenic dispersal. Although the relative importa nce of different dispersal mechanisms varies both spatially and tempor ally, the global movement of ballast water by ships appears to be the largest single vector for NIS transfer today, and many recent invasion s have resulted from this transfer. The rate of new invasions may have increased in recent decades, perhaps due to changes in ballast water transport. Estuaries have been especially common sites of invasions, a ccumulating from tens to hundreds of NIS per estuary that include most major taxonomic and trophic groups. We now know of approximately 400 NIS along the Pacific, Atlantic and Gulf coasts of the U.S., and hundr eds of marine and estuarine NIS are reported from other regions of the world. Although available information about invasions is limited to a few regions and underestimates the actual number of NIS invasions, th ere are apparent differences in the frequency of NIS among sites. Mech anisms responsible for observed patterns among sites likely include va riation in supply of NIS, and perhaps variation in properties of recip ient or donor communities, but the role of these mechanisms has not be en tested. Although our present knowledge about the extent, patterns a nd mechanisms of marine invasions is still in its infancy, it is clear that NIS are a significant force of change in marine and especially e stuarine communities globally. Taxonomically diverse NIS are having si gnificant effects on many, if not most, estuaries that fundamentally a lter population, community, and ecosystems processes. The impacts of m ost NIS remain unknown, and the predictability of their direct and ind irect effects remains uncertain. Nonetheless, based upon the documente d extent of NIS invasions and scope of their effects, studies of marin e communities that do not include NIS are increasingly incomplete.