Marine invaders: a worldwide threat

Posted: 25 February 2008

Some 84 per cent of the world's marine ecosystems have been infected by invasive species - and this number could be even higher due to under-reporting, according to a new study from the Nature Conservancy.

Comb jellyfish
Comb jellyfish
Comb jellies introduced to the Black Sea from the United States in shipping ballast water destroyed commercial fishing, costing thousands of jobs. © L. Madin, Woods Hole, Oceanographic Institute
The report quotes the case of the population of comb jellyfish sucked into the ballast of a US tanker and shipped halfway around the world in 1993, where it was unceremoniously dumped into the Black Sea when the tanker discharged its ballast water.

This event caused one of the most alarming species invasions in European history. At their peak in the mid-1990s, the comb jelly invaders made up 90 per cent of living organisms in the Black Sea - the sheer weight of the invasive population exceeded the weight of the world's entire fish catch. The jellyfish destroyed the Black Sea's commercial fishing industry and cost thousands of jobs.

Chinese mitten crab
Chinese mitten crab
The Chinese mitten crab (Eriocheir sinensis) is a migrating crab which has invaded Europe and, more recently, North America. It contributes to the local extinction of native invertebrates and causes erosion by its intensive burrowing activity. Photo: Stephan Gollasch, GoConsult
The Chinese mitten crab is one of more than 200 exotic species which have invaded North American waters. These small crabs, which were brought to the San Francisco Bay via ships' ballast water, reproduce rapidly and have spread throughout the Delta. They may imperil the state's threatened and endangered salmon populations due to the crabs' appetite for juvenile salmon.

"The scale of this problem is vast," said Jennifer Molnar, conservation scientist at The Nature Conservancy and lead author of the study, Assessing the Global Threat of Invasive Species to Marine Biodiversity.

Chinese mitten crab infestation
Chinese mitten crab infestation
Chinese mitten crab infestation. Photo: Stephan Gollasch, GoConsult
"Every day, thousands of vessels cross our oceans with invasive species hitchhiking on their hulls," Molnar said. "Because of this, as many as 10,000 species are estimated to be in transit at any one time."

Difficult to remove

The study also found that most of the invasive species that have taken hold around the world - from San Francisco Bay to the eastern Mediterranean Sea - are difficult if not impossible to remove. Other major findings of the study include:

  • Many marine invasives harm other species and disrupt entire natural systems.
  • 57 per cent of marine invasive species in our study can be classified as harmful to the native ecosystems.
  • Most invasive species are unintentionally transported to new habitats through shipping.
  • Many species are introduced through the farming of non-native fish and shellfish.

Although only a small fraction of the many marine species introduced outside of their native area are able to thrive and invade new habitats, their impact can be dramatic and have transformed marine habitats around the world. The most harmful of these invaders displace native species, and change natural structures and food webs.

Caulerpa is a tropical seaweed that has wreaked havoc in the Mediterranean and in Australia. Once established, it is quickly transported on the anchors of fishing and recreational boats. It overgrows native seagrass and is toxic to many fish. © Alexander Meinesz,
Caulerpa, a tropical seaweed that has invaded the Mediterranean and in Australia, is transported on the anchors of fishing and recreational boats. It is toxic to many fish and blankets large areas of the sea floor where it has invaded the Mediterranean, smothering native species and getting tangled in boat propellers.

Alien invasive species can also significantly alter human lives, and have damaged economies by diminishing fisheries, fouling ships' hulls, and clogging intake pipes. Some can even directly impact human health by causing disease.

Stopping invasions before they happen makes economic as well as environmental sense, as the catastrophe in the Black Sea demonstrates.

But dealing with invasions when they occur is very expensive and rarely effective. For example, the United States spends $120 billion annually on the control and mitigation of impacts of more than 800 invasive species infestations.

Invasives typically hitch rides to new habitats either through the discharge of ballast water which ships carry for stability or through the "bio-fouling" of ships hulls, when aquatic species hitchhike to new places on the bottom of ships. Coordinated action from governments is needed to prevent further introductions through these pathways.

The new report provides evidence to support ongoing efforts to improve ballast water management practices and to tackle the problem of the fouling of ships' hulls and lines. However, the major impacts of ship-fouling species suggest that ballast water agreements alone may be insufficient.

The study also confirms the role of aquaculture operations such as non-native fish and shellfish farming in marine invasions. Stricter, industry-wide control measures should be developed and enforcement strengthened to restrict intentional and accidental introductions of harmful invasive species.

The study could also inform biosecurity decisions by helping identify species that have not yet invaded an ecoregion but have had considerable impacts on similar habitats elsewhere. Government action needed

It is vital that governments begin working now to make strong decisions on invasive species that will halt new introductions around the world before this hidden threat becomes more costly to people and nature, says the report.

Map of invasive hotspots
Map of invasive hotspots
Invasive species in the marine environment - problem regions. The locations of major problem areas for invasive species infestations or occurrence of exotic species in the marine environment. Credit: UNEP. Click to enlarge
To counter the devastating effects of invasive species around the world, the Nature Conservancy is encouraging governments to make a strong decision on invasive species that will:

  • Halt new introductions through priority pathways;
  • Support screening of new species before they are introduced; and
  • Ensure countries have the technical and financial resources required to combat invasions.
And governments this year have an unprecedented opportunity to address these issues: Marine invasives will be a top agenda item at a meeting in May of the UN-sponsored Convention on Biological Diversity in Bonn, Germany.