More Than Just Seagulls Munch On The Seashore

journal.pone.0068221.g005Walk along pretty much any beach and at the high tide mark will be a line of debris. There may be seaweed or shells, bits of driftwood or plastic debris. You probably won’t see any fish, though. And that’s a little odd, because fish do die, and their bodies have to go somewhere. Surely some would wash ashore.

A group of researchers in Australia think that they’ve figured out where the fish go – the fish quickly get scavenged by the critters that live along the shore. But there’s more than just seagulls finding their meals here, the team reports in PLOS One.

The scientists set up a series of experimental plots along a sandy beach on North Stradbroke Island on the east coast of Australia. They picked a spot far from humans, where dogs and beachgoers would be scarce. Then they set out 20 plots, three meters by 10 meters each, and for eight days added about five kilograms of flathead mullet fish to half the plots about two hours before sunset.

The beach was nearly picked clean. Over the eight days, 720 fish were set out and 97 percent were completely eaten. Gulls (silver gulls in this case) ate some of the mullet, but there were several other birds species as well: Torresian crows, whistling kites (b in image above), brahminy kites (a), and white-bellied sea eagles (c and d)

These avian scavengers scoured the beach most often at sunrise and in the first few hours of morning. On three occasions at night, however, red foxes (f) visited the plots, snapping up the easy meal. And on one day, a lace monitor (e) – a large, carnivorous lizard – was spotted on the beach snacking away. That was a surprise because there have been few reports of terrestrial reptiles scavenging on a beach.

One invertebrate also got in on the action. The researchers measured the diameter of ghost crab burrows and, using burrow size as a proxy for crab size, discovered that when they added fish to the experimental plots, bigger ghost crabs moved in to take advantage of the free food.

But birds were the dominant consumers of the carrion fish. And they probably play an important role in this seashore ecosystem, the researchers say, helping to transfer nutrients from the sea onto land and providing a vital link between water and soil.

Image used under Creative Commons license, Schlacher TA, Strydom S, Connolly RM, Schoeman D (2013) Donor-Control of Scavenging Food Webs at the Land-Ocean Interface. PLoS ONE 8(6): e68221. doi:10.1371/journal.pone.0068221

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