Minimizing invasion risk by reducing propagule pressure: a model for ballast- water exchange

© The Ecological Society of America www.frontiersinecology.org B invasions, a major and increasing agent of global biodiversity change, are often the result of inadvertent releases from trade and travel pathways (Levine et al. 2003; Ruiz and Carlton 2003; Drake and Lodge 2004). Empirical and theoretical evidence indicate that invasion risk can be decreased by reducing propagule pressure, specifically, the quantity, quality, and frequency of introduced individuals (Grevstad 1999; Rouget and Richardson 2003; Drake and Lodge 2004; Verling et al. 2005). In marine and estuarine systems, the dominant invasion pathway worldwide is the ballast water of commercial ships (Carlton and Geller 1993; Carlton 1998; Ruiz and Carlton 2003; Drake and Lodge 2004; Holeck et al. 2004). Current estimates suggest that a global fleet of approximately 35 000 commercial vessels transports an annual volume of about 3.5 x 10 metric tons of ballast water, containing some 7000–10 000 species – mostly marine – at any one time (Carlton 1999; Endresen et al. 2004; Figure 1) This invasion pathway is currently managed primarily by open-ocean ballast-water exchange (IMO 2004; Minton et al. 2005). Under this practice, a ship’s ballast tanks are loaded as usual at the start of a voyage, emptied and refilled in mid-ocean, and subsequently emptied in or near the destination port (Figure 2). Exchange is based on three assumptions: (1) that most initial organisms are flushed out; (2) that remaining organisms survive poorly, if at all, in the newly ballasted ocean water; and (3) that oceanic organisms released in the destination port pose a minimal invasion risk. We focus here on the interaction between the first and second assumptions in determining exchange effectiveness. Ballast-water exchange was originally developed in the context of ships sailing from freshthrough saltwater back to freshwater, so that any freshwater organisms remaining after exchange would be expected to die in the newly loaded oceanic water. Exchange has since been recommended or required by a number of coastal ports and nations, and a recently adopted International Maritime Organization convention now requires vessels arriving in all 164 member states to conduct open-ocean exchange or equivalent management (IMO 2004; Minton et al. 2005). However, it is not clear if exchange would be as effective for saltwater organisms, where post-exchange survival in oceanic water could be equal to or greater than that in the initial water. Here we develop a simple theoretical framework for evaluating and maximizing the effectiveness of ballastwater exchange. Using this framework, we show when exchange is predicted to reduce propagule pressure, and when it can, counterintuitively, increase propagule pressure relative to a nonexchanged tank. We then apply the model to evaluate exchange effectiveness for a series of RESEARCH COMMUNICATIONS RESEARCH COMMUNICATIONS