An Invasive Crab Is Helping Cape Cod’s Marshes To Recover

crabInvasive species are generally not good for an ecosystem. Zebra mussels, snakehead fish, feral hogs, nutria, domestic cats, purple loosestrife — I could probably fill inches of your screen with the names of species that have caused havoc when moved into a new landscape. But not all species prove troublesome when transported out of where they are traditionally found, and now there’s a case where an invasive crab is actually helping an ecosystem to get healthier.

The case comes from Cape Cod, where overfishing has removed most of the predators from the salt marshes. Without those predator species to keep it in check, the crab Sesarma reticulatum has grown in numbers. The species denudes the marshes of the cordgrass that holds the land together, and also burrows into the ground, making that soil more susceptible to erosion.

In recent years, though, some of those marshlands have started to recover, and the cordgrass began to return. That recovery can be tied to the arrival of another crab, the European green crab (Carcinus maenas), say researchers from Brown University in a study in Ecology.

“When we started seeing the marshes recover, we were baffled,” study coauthor Mark Bertness said in a statement. “To see very quickly the marshes start to come back, at least this veneer of cordgrass, it seemed pretty impressive. When we started seeing this recovery we started seeing loads of green crabs at the marshes that were recovering. We went out and quantified that.”

The researchers looked for crabs in healthy and recovering marshes. There weren’t many green crabs in the healthy marshes, and there also weren’t many¬†Sesarma burrows in those areas. But in recovering marshes, there were lots of burrows and lots of green grabs using them.

They then tried an experiment, putting a Sesarma crab and a green crab in a cage near a burrow. The green crab not only took over the burrow, but it often killed the other crustacean. In another test, they caged the crabs in a plot of grass. When a Sesarma crab was on its own, it would eat a lot of grass, but when it had a companion of the other species, it was put off its feast, even when the green crab was restrained from attack.

What’s happening in these recovering marshes, the researchers say, is that the green crabs are finding refuge from predators and the sun in the burrows of Sesarma crabs, and then feeding on the Sesarma crabs. Those crabs are not only getting reduced in numbers through this competition and predation, but they’re also reducing their cordgrass destruction because they’re afraid of the green crabs.

“Non-consumptive effects can be much more powerful because whereas a consumptive effect is one crab eats another crab, a non-consumptive effect is one crab scares dozens of crabs,” Bertness said. “The ecological effect can be much greater much quicker.”

The result is that the cordgrass in those recovering areas is getting the chance to grow back. And when the grass recovers, other species may follow.

The researchers — and I — called the green crab an “invasive species” but that’s probably not right in this context. The definition of an invasive species is “an alien species whose introduction does or is likely to cause economic or environmental harm or harm to human health.” The green crab may be an alien, but we probably need another term for one that causes an environmental benefit.

Image (a European green crab) credit: Catherine Matassa/Northeastern University