Ships and Aquaculture Blamed For Spreading Species
Invasive marine organisms now inhabit nearly all coastal areas. A comprehensive compilation of data finds that just 16% of marine ecoregions have no reports yet of exotic species. And data trace the extensive spread of species primarily to shipping and aquaculture.
While the database compiled by The Nature Conservancy doesn't contain every invasive species in the planet's oceans, researchers found enough information to include 329 species. Among these are 59 crustacean, 54 mollusks, 38 fish and 19 plants.
Well over half, 187 species, in the database have left their mark on ecosystems by interfering with numerous native organisms, including threatened ones, and upsetting ecosystem processes. Most of the species are nearly impossible to eradicate from their non-native habitat.
Northern temperate oceans have experienced the greatest degree of invasion, with 240 species recently moving into the North Atlantic and 123 into the North Pacific. The most highly invaded ecoregion is northern California, including San Francisco Bay, where 85 new species have established, of which 56 are harmful. The Hawaiian Islands, North Sea and Levantine Sea in the eastern Mediterranean are other invasive marine hotspots, with each having over 70 exotic species.
Hitching a Ride
Shipping is the biggest culprit in scattering species worldwide. Ships have carried 228 of the 329 species assessed for this project to new destinations. Invasive life is equally as likely to hitchhike in ballast water as on fouled hulls. In light of the link of species spread with global shipping patterns, researchers suspect that east and southeast Asia are areas more fraught with non-native marine organisms than the literature currently documents.
The aquaculture industry has also been a potent pathway, enabling 134 species to disperse, nearly two-thirds of these environmentally damaging. The Pacific coast of Oregon and Washington states and southern British Columbia stand out in having more species introduced by aquaculture than shipping. As one of the most invaded ecoregions in the world, this area now hosts 63 exotic marine species, including 41 harmful ones.
The main offender here is farming of Pacific oysters (Crassostrea gigas), originating from east Asia. Along with the oysters, 33 species have travelled to new locales by riding along on oyster shells or equipment. At least 26 of these species are nearly impossible to control once they've entered new habitat. As well, 18 organisms associated with the oysters dominate native species or destroy habitat.
Pacific oysters are not only a problem on the west coast of Canada and United States. The shellfish have been released into at least 45 ecoregions around the world.
While ships and aquaculture account for the majority of introduced species, other mechanisms have played a role. Canal construction has resulted in spreading 56 species. Notably, the Suez canal opened the way for marine life moving into the eastern Mediterranean. The aquarium trade and distribution of live seafood have also promoted the spread of several harmful species.
Jennifer L. Molnar, Rebecca L. Gamboa, Carmen Revenga and Mark D. Spalding. 2008. Assessing the global threat of invasive species to marine biodiversity. Frontiers in Ecology and the Environment. In press.