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European green crab

The European green crab (Carcinus maenas) is a predator and an invasive species to the rocky intertidal shores of New England.

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The National Science Foundation (NSF) supports research on invasive species such as the European green crab (C. maenas). For example, for her dissertation research, Catherine M. Matassa, a Ph.D. candidate at Northeastern University's Marine Science Center, is studying the "ecology of fear," that is, how predators, by scaring their prey, can have a large and sometimes surprising effect on natural communities.

Fear of predators, or "predation risk," can cause prey to increase their use of refuge habitats and decrease how often they feed, thus minimizing the possibility that they will become a meal. These anti-predator behaviors have direct consequences on prey fitness and indirectly affect the abundance and distribution of other species in a community (most notably the prey's food) and the nature and efficiency of ecological processes--the transfer of energy and nutrients among trophic levels.

Although there has been a growing appreciation for the magnitude, ubiquity and importance of these "nonconsumptive effects" of predators, so-called because predators do not need to consume their prey in order to have these effects, further study is needed to identify the ecological factors and biological mechanisms that determine how prey make behavioral decisions when exposed to predation risk. There is an abundance of theory and meta-analyses, but more empirical work is needed, especially in the field.

Matassa's dissertation research is providing some of the first empirical tests of these theories. She has developed a model system for investigating the effects of predation risk using a simple rocky intertidal food chain: The green crab, an invasive predator; the Atlantic dog whelk (Nucella lapillus), an intermediate consumer; and barnacles (Semibalanus balanoides) and mussels (Mytilus edulis), basal resources.

Matassa has already completed two large-scale field experiments and one lab experiment that tested predictions about the effects of temperature, habitat quality and prey state (i.e., starvation level) on the response of prey to predation risk.

Another example is research by a team at Brown University who found that, although an invasive species, these green crabs have become an asset to the ecosystem in the distressed salt marshes of Cape Cod. A decline of native predators of the Sesarma crab (Sesarma reticulatum), primarily brought on by human activity, has allowed them to run amok and deplete the marsh grasses.

The Brown team, led by Mark Bertness, chair of the Department of Ecology and Evolutionary Biology, has been studying the extent and causes of the marsh damage for many years. But beginning a few years ago, they began to see signs of the marshes recovering. In some places where there was still soil, some of the grasses were growing back to some degree. In these distressed but healing marsh areas, Bertness' group noticed an abundance of green crabs (as many as 2.8 green crabs per square meter), where they could take over Sesarma burrows. On-site tests found that the mere presence of a green crab, even one caged but out of reach, of the Sesarmas was enough of a deterrent that the Sesarma crabs ate far less grass.

"Humans have had far-reaching impacts on ecosystems," said Tyler Coverdale, a researcher in Bertness' lab. "Some of those impacts, like overfishing, cause species to decline in their native ranges. Others, like shipping and trade, cause species to become more common outside of their native ranges. Most of the time these opposing types of impacts have negative results. In this case, an invasive species is potentially restoring a lost ecological function."

To learn more about Matassa's research (supported by NSF grant IOS 11-10675), visit her website Here. To read more about the Brown University study (supported by NSF grant OCE 09-27090), see the Brown news story Invasive crabs help Cape Cod marshes. (Date of Image: September 2010)

Credit: Catherine M. Matassa, Marine Science Center, Northeastern University

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