Going to the source: role of the invasion pathway in determining potential invaders

Biological invasions are an increasing agent of change in aquatic systems, and ballast-water transfer in ships is a leading pathway of these invasions. During sequential stages of ballast transfer (uptake, transport, and release), the density and diversity of the plankton assemblage is selectively filtered, determining the pool of potential invaders. Understanding taxon-specific patterns of mortality along the invasion pathway is key to understanding and predicting successful invasions. We quantified taxon-specific trends in plankton mortality during a 16 d trans-Atlantic ballast water voyage. In the ballast tanks, we collected a miminum of 50 live taxa. Over 50% of taxa and >98% of organisms collected in initial samples were not detected at the end of the voyage. No ballasted organisms survived experimental transfer to coastal harbor water. We suggest that the invasion success of a particular taxon may be predicted both by high density at the end of a voyage (which is comparatively easy to measure) and by low mortality during a voyage (which may indicate good body condition, but is harder to measure). These 2 predictors were not, however, correlated across taxa. Mid-ocean exchange, the most widely used method for reducing ballast-mediated invasion risk, alters the pool of potential invaders. In an experimental test of mid-ocean exchange, 93 to 100% of the coastal water and 80 to 100% of the coastal organisms were removed. However, the total density and diversity of plankton in the ballast tanks increased significantly following exchange and in laboratory experiments ocean water was not lethal to coastal organisms.