Tracking marine alien species by ship movements

Under human influence, plants, animals, and microbes are spreading beyond their native ranges faster and farther than ever before. Rates of invasion are increasing worldwide, especially in large aquatic systems (1). Many of these alien species appear to be innocuous, whereas others cause substantive impacts on biodiversity, ecosystem services, and human and animal health (2, 3). Effective management requires an ability to anticipate and prioritize significant invasion threats from among the enormous numbers and diversity of organisms being introduced by transportation mechanisms that facilitate human travel and trade, such as shipping. Global ship traffic has increased by fourfold since the early 1990s (4). Shipping is the dominant vector of unintentional species introduction in estuarine and coastal marine systems worldwide (2, 3, 5), and connects distant regions using ports as stepping stones (6). In PNAS, Seebens et al. (7) predict the probability of invasion by marine alien species, using a modeling approach that considers global ship movements, habitat suitability, and patterns of species occurrence. Cargo ships carry large volumes of ballast water, which they take up and release at various times to regulate stability. This water typically contains an abundant and diverse assemblage of phytoplankton, zooplankton, bottom-dwelling invertebrates, and fish that were taken up at multiple ports of call along the shipping route (8). At any given moment, several thousands of species are being moved in the ballast tanks across distances they could …