Mapping the travel of invasive marine species
From nearly invisible plants to the slimy, toothy vampires of the deep (lamprey eels, image below), species that are not indigenous to a specific region are hitchhiking in on the global transport network. It only takes a couple specimens of an invasive species with a significant competitive advantage over native flora or fauna for an ecological disaster to ensue.
The costs of invasive species, both in terms of loss of indigenous species, imbalance of ecosystems, and devastation of commercial interests like fisheries or infrastructures, are estimated in the billions of dollars per year. The cost of fighting an almost inevitably losing battle take tens of millions of dollars out of taxpayer pockets.
These invaders most often arrive in the ballast water of large freighters. The ballast water, necessary to stabilize the ship especially on empty legs, is pumped into holding tanks at the ship's point of origin and dumped again as the ship is loaded at its destination port. The video below describes this silent invasion:
Preventing the next invasion, especially considering the costs involved, can be enabled by knowing where the risks are highest. That was the endeavor undertaken by a team of scientists funded by the Volkswagen foundation. Scientists from the Universities of Bristol, UK, and Oldenberg, Germany collected the logs of 3 million voyages from 2007-2008.
The risk of a successful invasion by foreign species depends not only on the frequency of ship traffic. According to U. Oldenburg professor Bernd Blasius: “Our model combines information such as shipping routes, ship sizes, temperatures and biogeography to come up with local forecasts of invasion probabilities.” Field testing of invasive species that are successfully establishing new populations were used to validate the predictions of the models.
The results show that in spite of intensively trafficked ports, the North Sea's cold temperatures protects it from many invasive species. Treatment of ballast water from ships that originate in colder waters, such as those from the Northeast Atlantic coast of the US, deserve priority in prevention efforts.
Large ports in more temperate climates, such as Singapore and Hong Kong, face the greatest risks. In the USA, ports like New York and Long Beach are "among the sites of highest invasion probability."
The study provides statistics that support the value of ballast water treatment:
By successfully removing a species from 25 per cent of the ballast tanks arriving at each port (eg with filters, chemicals or radiation), the overall invasion probability decreases by 56 per cent. The reduction is so disproportionately large because the effect of ballast water treatment multiplies at successive stopovers.
Given the challenges of stopping the spread of an invasive species once established, a focus on treatment to prevent the spread of invasive species must have priority. It is not just the potentially invasive species, but also the "polluting" freighters that enjoy a free ride -- they benefit from the profits of transporting goods while the ecosystem (and ultimately the taxpayers) pay the costs of the invasive species discharged in untreated ballast water.
The study, The risk of marine bioinvasion caused by global shipping, is published in April Ecology Letters.