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Invasive Species: What We Do

Q&A: Marine Invaders

84 percent of the world's coastal ecoregions have been invaded by at least one species.

See Marine Invaders

Striking images of marine invasive species found around the world.

"The most effective way to reduce the threat of marine invasives is to prevent them from being introduced in the first place."

Jen Molnar, conservation scientist for The Nature Conservancy

Unwanted, invading species are causing economic and environmental havoc in coastal regions around the world, according to a new study from The Nature Conservancy.

The study—the first-ever global review of marine invasives — finds that 84 percent of the world’s coastal ecoregions have been invaded by at least one species. asked study lead author Jennifer Molnar what the results mean for the world’s oceans and coasts and what we can do to prevent further invasions. 

Jennifer Molnar is a conservation scientist at The Nature Conservancy's Center for Global Trends, where she leads global analyses of habitat condition and threats across multiple habitats, with a focus on marine and freshwater systems.  Her current work also includes a global assessment of freshwater invasive species and research on the impacts of nutrient pollution on aquatic habitats and the risks and opportunities posed by agriculture. 84 percent of all coastal ecoregions — that sounds like an awful lot.

Jennifer Molnar: Yes, we found that these species are very widespread. While we documented the greatest numbers of invaders in industrialized regions such as the United States, Europe, and Australia, species capable of causing significant ecological and economic damage are found in many more regions as well.

And it might even be worse. Our results likely under represent invasions in regions with little available data but large shipping industries, such as east and Southeast Asia
How did all these marine invasives become invasive?
Jennifer Molnar: Many species are being introduced outside of their native range by increasing trade and travel. Although a large number of these species don’t survive in new habitats, a small percentage are able to thrive and can end up dominating their new home.

There are three major pathways for invaders: 

  • Most marine invasive species are introduced by hitchhiking on ships (69 percent of the species in our study). These species are either accidentally taken up in ballast water, or they directly attach themselves to the bottom of a ship in port. It has been estimated that up to 10,000 species are being carried in ships around the world every day  many of which are being transported to new habitats.
  • Aquaculture activities were responsible for the introduction of 41 percent of species in our study.  A majority of those species were inadvertently moved with farmed species or equipment. 
  • Canals connect previously isolated waters and can act as a conduit for invasion (17 percent of species in our study).  For example, many species have migrated from the Red Sea to the eastern Mediterranean Sea since the Suez Canal was built in 1869. Why are marine invasive species such a big deal?

Jennifer Molnar: The most harmful invasive species can not only disrupt and transform coastal habitats — they can put native species at risk. 

Invasive species often find themselves with no local predators or competitors, so they can displace native plants and animals, disrupt food webs, and change natural environmental processes. They can also damage local economies by reducing productivity of fisheries, interfering with navigation, and clogging intake pipes.   

For example: A green seaweed called Caulerpa blankets large areas of the sea floor where it has invaded the Mediterranean Sea, smothering native species and getting tangled in boat propellers.

As part of our assessment, we developed a system for scoring each species’ threat to biodiversity. It allows us to determine which invasive species are most likely to damage a new habitat or impact native species. This information is key in prioritizing conservation actions. What about the intentional introduction of marine invaders — how big a problem is that?

Jennifer Molnar: Quite a few non-native species are introduced intentionally—usually for economic reasons. Some are introduced to be farmed or released into the wild to enhance or create new fisheries; others are used for biocontrol.

For example, the Pacific oyster – native to Japan – is being cultivated in the coastal waters of at least 5 continents.  After its introduction, this oyster can invade nearby natural habitats, squeezing out native oysters and other species.

But there's a long-term risk to ecosystems and economies in a lot of these situations. Farmed species can invade surrounding natural habitats or carry pests or pathogens with them.  For example, a kind of seaweed that has been accidentally introduced with farmed oysters is commonly known as the “oyster thief” because it can overgrow and smother both native and cultured oyster beds.           
And once a marine invader has established, it is nearly impossible to remove it. So what can we do?

Jennifer Molnar: The most effective way to reduce their threat is to prevent them from being introduced in the first place. And a global approach is needed.

Working as a member of the Global Invasive Species Programme, the Nature Conservancy is helping to guide the development of policies that will address the major pathways for invasive species. We also support regional initiatives such as the Pacific Invasives Learning Network. We are also making our database available so that the information we’ve collected about harmful species and their pathways can be a resource to others. 

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