By Leah Topper
Mature adult European green crabs
Image by [Ramon F Velasquez] via [Wikimedia] http://commons.wikimedia.org/wiki/File:Liliw,Lagunajf5731_29.JPG?uselang=en-gb
Witches, vampires, dragons, ogres, and the monsters hiding under your bed – all these creatures have terrifying reputations that precede them. But have you ever had tea with the witch next door, played fetch outside with the dragon, or asked the monster to help you make your bed? Sometimes, even the scariest creatures are not so bad, after all.
Invasive species, like these ogres and vampires, have an overwhelmingly negative reputation. But are all invasive species really so terrible? As researchers from Brown University discovered, a particular invasive species of crab contradicts the reputation of its title and proves that it is beneficial to its ecosystem, the marshes of Cape Cod.
We’ll get back to that part of the story in a minute, but first, you’ll need some background information. Our story, similar to many recent conservation biology narratives, begins with humans. Overfishing in New England marshes has greatly decreased the number of predators in these marshes, and this has allowed the herbivorous purple marsh crab (Seserma reticulatum) to explode in population size and chow down (excessively, mind you) on the marsh’s cordgrass.
At this point in our narrative, our overly-criticized monster makes his first appearance. The European green crab (Carcinus maenas) first arrived on North American soil in the early 19th century. Since then, this crab has assisted in the recovery of the marshland grass, and Mark Bertness and Tyler Coverdale of Brown University’s Department of Ecology and Evolutionary Biology wanted to determine exactly how this invasive species was improving the marshland.
First, they ventured into the swampy marshes, surveyed eleven recovering and five healthy marsh sites, and collected totals for both species of crabs and cordgrass growth at each site. The researchers found that the number of European green crabs correlated with cordgrass growth at these particular marsh sites.
Approaching the research from an experiment-based avenue, the researchers then completed parts two and three of their research through controlled cage experiments to observe any interactions between European green crabs and purple marsh crabs When placed in the same cage, the green crab evicted the purple marsh crab from its burrow like a landlord who hasn’t received his rent payments, and even more unfortunately for the herbivorous crab, fewer than 15% survived this close-quarters experience.
Clear blue skies were also not forecasted for the purple marsh crabs interacting indirectly with isolated European green crabs placed in the center of their enclosures. The mere presence of the green crab lowered the appetite of the purple marsh crabs, and this caused them to eat less cordgrass.
As the Rhode Island researchers discovered, the marshland monster of this ecosystem, the European green crab, constructively uses its scary reputation and carnivorous appetite to improve the overall health of the marshland ecosystem.
“So, that’s pretty neat!” you might think to yourself, “But if the European green crab is an invasive species, how will we know that they won’t try to take over the whole world and wreak havoc on other marshes where they are not benefitting the ecosystem?”
European green crabs were not found in marsh sites deemed “healthy” by the researchers, and furthermore, they are typically only found in marsh areas with high numbers of purple marsh crabs. As inadequately-skilled construction workers and distasteful interior designers, the European green crabs require burrows already constructed, designed, and furnished by purple marsh crabs. This necessary arrangement keeps the European green crab’s population in Cape Cod’s marshes dependent on the purple marsh crab population.
So, next time you come across an invasive species in your backyard and immediately judge it because of its title, take a second and learn more about it. Play a round of cards with it; read it a book. Perhaps, like the European green crab, it benefits its ecosystem in a manner by which we are not yet aware.
Mark D. Bertness and Tyler C. Coverdale 2013. An invasive species facilitates the recovery of salt marsh ecosystems on Cape Cod. Ecology 94:1937–1943. http://dx.doi.org/10.1890/12-2150.1