The notion that invasive species are always a bad thing for the ecosystem has been challenged by research published in a recent issue of the journal Ecology.
Andrew Altieri and colleagues from the US and the Netherlands examined the effects of the invasive Asian shore crab (Hemigrapsus sanguineus) on beach ecosystems along the Atlantic coast of the United States.
The Asian shore crab was probably introduced into American waters via ballast in commercial shipping more than 20 years ago, and has since proliferated along the Atlantic coast northwards to Maine and southwards to North Carolina.
The authors counted the crabs at four sites on Narragansett and Mount Hope bays in Rhode Island and found that the crabs were found at their highest densities where Cordgrass (Spartina alterniflora) and Ribbed mussels (Geukensia demissa) proliferated, with more than a 100-fold difference observed between areas with cordgrass and mussels and areas without one or the other.
Data from the field and from experimental manipulation of the environment carried out by the authors suggested that the crabs were taking advantage of the moist, shaded environment that the Cordgrass and the mussels were providing.
Known as a "facilitation cascade" in ecological terms, the Cordgrass attracts the Ribbed mussels by providing shade and shelter, as well as a suitable substrate for the mussels to anchor themselves to.
The presence of the mussels in turn creates crevices, which are attractive to the crabs for the protection from predators and exposure to the sun that they afford (the Cordgrass cements this relationship by also providing shade to the crabs).
Although one would logically assume that the high densities of the crabs in these areas would come at the expense of the native species, the authors found that such was not the case. In fact, the survey results indicated the opposite case, as the diversity of the native species (such as littorine snails and barnacles) was actually higher in areas where the invasive crabs were found in higher densities.
According to lead author Andre Altieri, the cordgrass-ribbed mussel association "…may be promoting co-existence, allowing for this ecosystem to absorb a new species."
The Asian shore crab has also invaded the shores of northern France, the Netherlands and the Channel Islands, where its effects may be considerably less benign.
For more information, see the paper: Altieri, AH, BK van Wesenbeeck, MD Bertness and BR Silliman (2010) Facilitation cascade drives positive relationship between native biodiversity and invasion success. Ecology 91, pp. 1269–1275.