An alien war is taking place on the beaches of New England. Two foreign crab species have invaded the ecosystem and are wreaking havoc. But instead of causing twice the damage--as many ecologists had expected--the aliens appear to be canceling each other out.
Like so many invasive-species stories, this one begins with stowaways. The European crab (Carcinus maenas) traveled from Europe to New England aboard a boat during the 1800s. Once there, the crustacean ate its way through the American soft-shell clam industry and spread like wildfire into Canada and the southeastern United States. In the 1980s, another stowaway crab arrived, this one from Asia. The Asian shore crab (Hemigrapsus sanguineus) favored the same habitat as the European crab and even hid in the same rocky holes, but it ate primarily algae.
Most studies indicate that two invasive species spell double trouble for an ecosystem. For example, multiple foreign bird species in Hawaii have triggered an invasive meltdown by destroying native plants and driving into extinction the birds that rely on these plants. But when ecologist Blaine Griffen and colleagues at the University of New Hampshire, Durham, began monitoring populations of the New England crab invaders, they noticed that in areas where the two interacted the most, the crabs were doing more harm to each other than to native species. For instance, the team found that the Asian shore crab often kicked the European crab out of its rocky crevice home. What's more, in the presence of the Asian shore crab, the European crab ate less shellfish and less food overall. The European crabs thus grew more slowly, leading to higher mortality from predation. For its part, the European crab occasionally feasted on the Asian shore crab, the team reports in an upcoming issue of The Journal of Animal Ecology.
"The most valuable thing we may learn from this is that when both invaders are similar, like these crabs, they mutually disrupt one another," says ecologist Edwin "Ted" Grosholz of the University of California, Davis. Griffen believes that the findings may help conservationists better control invasive species. "We have learned that by attacking invasive species in multiple ways simultaneously, we can more effectively reduce their ability to survive," he says. Past control measures have taken the one-dimensional approach of introducing a species meant to kill the invader, Griffen notes. The crab example shows that combining this approach with a focus on targeting an invasive species' diet and habitat may be more effective, he says.
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