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
Being “invasive” might not always be so terrible, after all. Despite the negative connotation associated with its title, an invasive species of crab in Cape Cod marshes is disproving its infamous reputation and benefiting the salt marsh ecosystem.
Due to overfishing near Cape Cod and the subsequent lack of predatory animals, the herbivorous purple marsh crab (Seserma reticulatum) population has proliferated in the salt marsh and over-consumed their food source, the marsh’s cordgrass, and this has led to an overall decline in the health of the salt marsh ecosystem.
Mark Bertness and Tyler Coverdale, researchers in the Department of Ecology and Evolutionary Biology at Brown University, have found that the European green crab (Carcinus maenas) promotes the regrowth of marshland grass in and a subsequent improvement of Cape Cod’s marshes.
In order to better understand this phenomenon, the researchers collected data from sixteen marsh sites, eleven recovering sites and five healthy sites, and conducted controlled interaction experiments between individuals of both species of crab. This multifaceted approach allowed them to better determine the specific mechanisms of the ecological correlation they had observed.
According to the results of their marsh site observations, the abundance of the European green crab in recovering marsh sites positively correlates with the amount of regrowth of cordgrass at these sites. They also observed that the European green crabs are uncommon at healthy marsh sites.
For the first interaction experiment, the researchers placed green crabs in an enclosure with purple marsh crabs overnight and noted that the European green crabs had evicted the purple marsh crabs from their burrows by the next morning.
Furthermore, fewer than 15% of purple marsh crabs survived when enclosed with the European green crab, and in all cases of death, the researchers found evidence of green crab predation.
In addition to decreasing overall consumption of cordgrass by eating the purple marsh crabs, the European green crabs influence the consumption of cordgrass through intimidation. When the researchers placed an individual of the European green crab species in an isolated enclosure within a larger cage of purple marsh crabs, the mere presence of the European green crab intimidated the purple marsh crabs and subsequently, the purple crabs significantly reduced their grazing of the cordgrass.
These two effects, the “consumptive” and “non-consumptive” effects, lead to an overall promotion of cordgrass growth and salt marsh ecosystem recovery.
Some may worry that this invasive species will dominate the ecosystem, but the researchers cite that the European green crab’s dependence on the purple marsh crabs’ burrows prevent this predicament.
Additionally, while the European green crab dominates native species in other habitats such as cobble beaches and subtidal mudflats, they were not found in healthy marsh sites and generally only found in areas of marsh with increased purple marsh crab populations.
Through both direct and indirect interactions with the purple marsh crabs, the European green crab, an invasive species, has largely benefited Rhode Island’s marsh ecosystem by facilitating recuperation of the cordgrass. The European green crab challenges its seemingly negative reputation by bettering the salt marsh ecosystem in which it resides.
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