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Invasive species are usually the bad guys in conservation. But an invasive crab is helping to restore salt marshes on Cape Cod by forcing out more destructive crabs, a new Ecology study suggests.

Along the New England coast, fishing has left many marshes bereft of predatory animals. As a result, marsh crabs that would otherwise have been eaten by the predators have multiplied. The marsh crabs have gobbled cordgrass along creek banks, making the land erode more easily.

Enter the European green crab, which invaded North America a couple hundred years ago and has settled on Cape Cod. Researchers studied 10 recovering marshes in the area last summer and discovered that the more green crabs they found at a site, the more the cordgrass had regrown.

The team wondered if green crabs were stealing the marsh crabs’ homes. So they performed experiments in which they put a green crab and a marsh crab in an enclosure with one burrow. In other experiments, they put a marsh crab alone in the enclosure.

Nearly all the lone marsh crabs staked out the burrow as their home. But when a green crab was present, none of the marsh crabs were able to settle in the burrow, and more than 85 percent of them were killed by the green crabs.

Finally, the team placed green crabs in enclosures with marsh crabs and cordgrass. After a month, the amount of cordgrass left was several times higher than in enclosures without green crabs. “These results suggest that invasive species can contribute to restoring degraded ecosystems,” the team concludes. Roberta Kwok | 3 April 2013

Source: Bertness, M.D. and T.C. Coverdale. 2013. An invasive species facilitates the recovery of salt marsh ecosystems on Cape Cod. Ecology doi: 10.1890/12-2150.1.

Image © Hans Hillewaert / CC-BY-SA-3.0



  • Carlo April 4, 2013 at 7:00 pm

    This is a seemingly fortunate scenario because the habitat was already compromised in the first place because of the lack of predatory animals due to fishing. But if the invasive overruns the native it will still be an ugly situation.


  • Edward Norkett April 10, 2013 at 1:59 am

    Balance is a delicate thing, if the preditory fish were alowed to recolonise they would probably control all the crabs enabling nature to rebalance itself. A close eye would need to be kept though!


  • Greg Hitchcock April 16, 2013 at 6:42 am

    “These results suggest that invasive species can contribute to restoring degraded ecosystems”.

    What utter tosh. If you lose another natural component of your ecosystem (i.e. the marsh crab) how is that restoring a degraded ecosystem?

    The team are confusing habitats and ecosystems.


  • Alex April 16, 2013 at 11:23 pm

    That’s right Carlo if the invasive overruns the native then it will be a serious issue.

    I don’t know if completely agree with this post as i’ve seen these tyepes of things happen all too frequently and it has had devastating effects to the environment.


  • Alex Ameen April 19, 2013 at 11:23 am

    While the destruction that can result from biological invasion is well documented, it frustrates me that the only acceptable viewpoint is to treat invasive species as an evil that must be eradicated.

    What’s interesting in this case is that the native crabs were behaving like an invasive (especially if you consider the enemy-release mechanism often associated with invasiveness). At this point the invasive presents competition which inhibits the disruptive effects (herbivory) of the native, although it does not say if the invasive defoliates the environment in a similar fashion.

    Nobody (I hope) is suggesting that green crabs (or any invasive) should be introduced into new systems based only on these findings. It does suggest that in systems that have already been irreparably disturbed, it’s worth looking into the functional roles or ecosystem services provided by all community members, native or otherwise.

    This brings up the related but separate debate relating to restoration, should we be trying to manipulate ecosystems back to a “pristine” state that may no longer be attainable or sustainable, or should management practices encourage any sustainable ecosystem, allowing for adaptation to human activities and their consequences?


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