[Damaged Salt Marsh]. Bertness Lab, Brown University. (2012). [Online Image]. http://news.brown.edu/pressreleases/2012/06/saltmarsh
The introduction of invasive species is often lamented as a major threat to local biodiversity. A recent study challenges the universality of this view, demonstrating that the invasive species Carcinus maenas is contributing to the restoration of degraded salt marsh ecosystems in New England.
Carcinus maenas, the European green crab, is facilitating the regrowth of cordgrass in Cape Cod’s damaged marshes, performing an ecological function that was previously lost due to overfishing. Human impacts in the area have depleted the native predator population, allowing the herbivorous marsh crab Sesarma reticulatum to strip vast areas of habitat.
Through both consumptive and nonconsumptive effects, Carcinus maenas has reduced the salt marsh die-off caused by Sesarma reticulatum and its ingestion of cordgrass.
The overfishing of local predators, which spurred the four-fold increase in Sesarma reticulatum population, also allowed the establishment of the invasive Carcinus maenas. Because Sesarma herbivory constitutes a major cause of marsh die-off, the increased predatory pressure from Carcinus has enabled the recovery of marsh cordgrass after years of degradation.
The study examined the correlation between Carcinus density and cordgrass regrowth, identifying recovery patches by the presence of tall-form cordgrass in once degraded plots of marsh. Researchers found that the crabs were rare at healthy sites but existed commonly in damaged and recovering marshes.
A burrow competition experiment was also conducted, the results of which indicate that Carcinus evict Sesarma from their burrows, leaving the Sesarma vulnerable to desiccation and predation.
To examine the nonconsumptive effects of Carcinus, researchers placed the crabs either directly with Sesarma in enclosures, or in cages within the enclosures. In both cases, the presence of a Carcinus – either caged or free – significantly diminished the feeding of Sesarma on the cordgrass, leaving behind a greater amount of grass biomass.
Sesarma contained in enclosures with free Carcinus experienced a survival rate of approximately 50%, due to direct predation by Carcinus.
The Sesarma held in enclosures with caged Carcinus had survival rates near 75%, while those contained in areas lacking any Carcinus had survival rates of over 90%. The presence of the caged Carcinus, though the crab could not directly prey on the Sesarma, affected its behavior and survival.
These behavioral changes are accompanied by a substantial loss of fitness, as the Carcinus evict Sesarma from their burrows and cause them to lose vital protection from dehydration and predation.
Carcinus have been observed to induce similar behavioral variations in other species, such as littorine snails and American lobsters.
The study suggests that these nonconsumptive effects may in large part be fueling the salt marsh ecosystem recovery. Such effects have replaced direct predation as factors believed to lie at the heart of prey behavior and community dynamics, as they can influence multiple organisms at once.
Recovery of the cordgrass species is attributed to both the consumptive and nonconsumptive effects of Carcinus, as well as to the ability of cordgrass to expand from a single area to cover a wide swath of formerly degraded marsh, when given the opportunity. The interrelated process includes the protection of existing cordgrass stands by the predatory and deterrent effects of Carcinus, followed by the growth and expansion of the plant population stemming from those stands.
The level of human impact in the New England salt marshes makes the location suitable for determining the effects of the Carcinus invasive species. Human intervention introduced the species to the area, and was also responsible for the decline of native predators that allowed Carcinus to thrive.
The adoption of a lost ecological function by Carcinus enabled the degraded salt marshes to begin restoring their cordgrass. Contrary to a common view, this invasive species is contributing to – not threatening – the wellbeing of its new ecosystem.
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