[Damaged Salt Marsh]. Bertness Lab, Brown University. (2012). [Online Image]. http://news.brown.edu/pressreleases/2012/06/saltmarsh
We’re all familiar with the unfortunate tale of the invasive species. Human action introduces a non-native species into a new habitat, where it thrives and wreaks havoc on the native populations. We lament the onslaught, shake our fists at the pests, and write articles about how invasive species are a top cause of biodiversity loss. So we should work to get rid of them, right?
Not so fast. A recent study of New England salt marshes reveals an unlikely hero in the restoration of the degraded ecosystem. An outsider has adopted a crucial ecological role left empty by extensive overfishing, helping to restore balance and revitalize the area.
Carcinus maenas, or the European green crab, is an invasive species that has taken hold in the damaged, die-off salt marshes of Cape Cod. Human impacts – specifically overfishing – have depleted the native predator populations, allowing the voracious marsh crab Sesarma reticulatum to proliferate. The herbivorous and burgeoning Sesarma population has devastated much of the salt marsh ecosystem, chowing down on the cordgrass that makes up the marshes.
Carcinus did what invasive species do best – interfere with local populations. This time, however, the effects were welcome.
The presence of Carcinus has both directly and indirectly reduced the ability of Sesarma to feed on cordgrass, allowing damaged regions to begin restoring healthy cordgrass populations. Predation by Carcinus is a major factor, as the crab adopted the ecological function of the overfished and depleted local predators. Researchers also observed that Carcinus induce behavioral changes in the local populations they interact with – you could consider it the playground bully.
In addition to directly preying on Sesarma, Carcinus evicts the marsh crabs from their burrows, leaving them vulnerable to desiccation and predation. In fact, Sesarma find the invasive species so intimidating that they lose their appetite when the predators are around. One of the experiments of the study placed Sesarma in enclosures with either a free Carcinus, or one in a cage within the enclosure. In both cases, Sesarma grazing was greatly reduced. In enclosures with a free Carcinus, Sesarma survival rates were around 50%, which reflect predation. In the enclosures with a caged Carcinus, the Sesarma survival rates were approximately 75%, whereas survival rates in Carcinus-free environments were near 90%. This difference reflects the fear factor that Carcinus exerts on the Sesarma populations.
Carcinus may be the big, bad wolf to the ever-hungry Sesarma, but how involved are they in bringing the cordgrass marshes back? Researchers studied the correlation between Carcinus density and cordgrass regrowth, observing that Carcinus populations were rare in healthy marshes, but were common in formerly degraded areas that were recovering. Areas containing the largest Carcinus populations were noted to be experiencing the greatest recovery. Healthy sites have stable native predator populations, which can keep the ravenous Sesarma in check. The lack of local predators in damaged marshes allows Carcinus to live predation-free, while leaving a niche for the invasive species to fill – and to restore balance.
Recovery of the salt marshes is attributed to not only the consumptive and nonconsumptive effects of Carcinus – its diet of Sesarma and its intimidating tendencies – but also to the ability of a few stands of cordgrass to spread and facilitate ecosystem restoration. Together, these processes have spurred the rebuilding of the salt marsh ecosystems.
Carcinus maenas encourages us to at least revisit the invasive species paradigm. Instead of contributing to our environmental woes, it is solving some of the man-made problems that were leading to biodiversity loss. This is one invasive species that’s managed to cast off its villainous cloak for a more flattering fit – a hero’s cape.
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