The Port of Los Angeles, via Lance Cunningham/Flickr
Invasive species are taking over the world, and marine ecosystems aren't immune. And as worldwide shipping volume grows, the risk of spreading invasive species in the ballast tanks and on the hulls of ships grows. If we want to stop more species from piling onto the list of thousands that have successfully taken over new homes, we first have to figure out where they're likely to succeed.
Thanks to a comprehensive assessment of worldwide shipping and ecological factors, a team of researcher has quantified the probability of species invading marine ecosystems for thousands of shipping routes globally. The British and German team of researchers, who published their results in Ecology Letters (PDF mirror here), found that there's a Goldilocks zone for species hitching rides on ships.
If the trip is too long, mortality rate skyrockets; surviving a trip around the world in a ballast tank isn't easy for clams and grasses and any other invading species. At the same time, invasive species are successful because they can exploit ecosystems that have never seen them before, and thus that haven't evolved checks and balances against them. That means that short trips may not carry a species out of its home range.
That means that simply charting global shipping data isn't enough to figure out where invasive species may succeed. Instead, the team combined almost three million shipping records from 2007 and 2008 and combined it with "ballast water release protocols, port environmental conditions (i.e. temperature and salinity) and biogeography to develop a model of marine bioinvasion by worldwide shipping," according to their report.
The team pushed that data through a series of equations to produce their calculated probability of a species invading along a specific trade route, or P(inv) for short. As you can see in the above graphic, intermediate routes across continents (i.e., to novel habitats) had the highest P(inv). The super long and very short routes, meanwhile, had much lower probabilities of success.
The key point to note is that while higher ship traffic is generally correlated with more invasive species risk, it doesn't hold true completely. Compared to Southeast Asia, the Middle East, and the US, which all have high risk, ports in the North Sea, which have an enormous amount of traffic, have low invasion risk.
According to the authors, "These differences result from a combination of traffic volume, distance to the most connected regions and environmental similarity. For instance, with respect to maritime traffic, the Northern European Seas are most strongly connected to tropical and sub-tropical ecosystems. However, among the adjacent water bodies only the North-West Atlantic provides sufficiently similar climatic conditions and thus dominates as the major source region of invasions."
Those model results were tested with field data from four recent regions invaded by exotic species, and the "intermediate hypothesis," as the authors call it, held up well.Considering how many ships are sent around the world daily, figuring out where the highest-risk environments are goes a long ways towards directing mitigation efforts into worthwhile regions. With marine systems on the brink worldwide, that's welcome news.