New studies show that not all ships are created equal when it comes to the supply of potentially invasive organisms to new environments. Scientists at the Smithsonian Environmental Research Center (SERC) found that the type of ship, the hailing port, and the length, frequency and route of a ship’s voyage all influence the total propagule supply, that is, the number of new plants and animals that could become established in a new port.
For coastal ecosystems, one of the main sources of biological invasions—the establishment of nonnative species in new environments—is the dispersal of freshwater and marine organisms through commercial shipping. Plants and animals are transported from one area to another in ships' ballast water or on the outer surfaces of these vessels. What determines the likelihood of invasion in a bay or estuary not only depends on the characteristics of the environment, but also on the type, quantity, and quality of the organisms released into the new ecosystem.
Scientists at SERC’s Marine Invasions Research Laboratory studied the factors that influence propagule supply such as frequency of ship arrivals from overseas, ship types, amount and frequency of ballast discharge, and source regions for the ballast. They found that each port examined has a unique makeup of ship types, foreign arrivals, and ballast water discharge levels—all factors influencing the risk of invasion from non-native species. Ports visited by container ships for example, which discharge relatively little ballast in U.S. waters, may not have the same invasions risk as a port mainly visited by bulk carriers, which discharge the most ballast into U.S. waters. Arrivals to the Los Angeles/Long Beach port are dominated by container ships that mostly come from the Pacific Basin while arrivals to Houston are mainly tanker ships and are mostly from the northwestern Atlantic. Furthermore, the authors found that though survivorship of zooplankton—tiny aquatic animals—declines in ballast water as voyage length increases, the magnitude of the decline varies depending upon the where the ballast water originated and the length of the voyage.
Previous assessment models assumed that all ship arrivals present the same degree of risk. This research however, reveals that the real story on the supply of nonnative species by shipping is much more nuanced. Future models may have to consider the complex interactions that take place between ship types, voyage lengths, and individual source and discharge ports when estimating invasions risk and predicting where and when coastal invasions might occur. Moreover, SERC scientists say, using this multi-faceted approach to evaluate transfer mechanisms of non-native species is broadly applicable to other methods of transport outside the shipping industry.
This study appeared in Volume 272 of the journal Proceedings of the Royal Society and was funded by Maryland and National Sea Grant Programs, Prince Williams Sound Regional Citizen Advisory Council, Smithsonian Scholarly Studies Program, US Coast Guard and US Fish and Wildlife Service.
Verling E, GM Ruiz, LD. Smith, B Galil, AW Miller and K Murphy. 2005. Supply-side invasion ecology: characterizing propagule pressure in coastal ecosystems. Proc. R. Soc. B 272: 1249-1256.