On the heels of yesterday’s post about how to measure human impact on the oceans comes a New York Times follow-up article on exactly that subject. After mentioning the Science mapping effort, the author goes on to discuss the declining alkalinity of the oceans, and efforts by teams of scientists to map the number of invasive species worldwide, and to complete a Census of Marine Life, which I will address in a later post.
This study, funded by The Nature Conservancy, represent the first international survey of the number of invasive marine species. Classified by eco-region, the paper analyzed the threat posed by invasive marine species according to their ecological impact, geographical extent, invasive potential, and management difficulty. This paper provides both an extendable framework with which to analyze the ecological impact of invasive species, and the knowledge of their means of introduction necessary to prevent their spread.
Map of Invasive Species. You can download the full text of the article here.
The numbers are striking: 84% of the world’s 232 marine eco-regions have documented invasive species. 57% of the invasive species are harmful to their new ecosystems, including 84% of plants. The average harmfulness invasive species fell in between “disrupts single species with little or wider ecosystem impact” and “disrupts multiple species, some wider ecosystem function.”
The most heavily impacted areas were around shipping ports, including the North Sea, Northern California, the Hawaiian Islands, and the eastern Mediterranean. This is particularly significant as shipping introduced the greatest number of alien species, 69% of the 329 invasive species analyzed, compared to 41% introduced by aquaculture and 17% by canal construction. However, of those species introduced by aquaculture, 64% were harmful, compared to 57% of species introduced by shipping.
This study, in both quantifying the number and harmfulness of invasive marine species, also points towards the potentially most effective means to limit their spread. For example, of the 205 species introduced by shipping where more specific information is known, 39% were introduced by ship fouling, i.e. on the sides or hulls of ships, while 31% were transported in the ballast water taken up in the departure port and discharged at the arrival port. 31% of shipping-introduced species were transported either by ship fouling or by ballast water.
This strongly suggests that while global measures to manage the discharge of ballast water will have a significant effect, a means of preventing the introduction of invasive species by ship fouling is also necessary to slow the introduction of alien marine species via sea transport. Furthermore, the harmfulness of species introduced by aquaculture must not be overlooked, especially considering the high concentration of aquaculture ventures in specific eco-regions, for example oyster farming off of the coast of the Pacific Northwest.
Jennifer L Molnar, Rebecca L Gamboa, Carmen Revenga, Mark D Spalding. 2008. Assessing the global threat of invasive species to marine biodiversity. Frontiers in Ecology and the Environment. 6, doi:10.1890/070064.