Marsh Crabs Show Ecological Benefits Of Invasive Species
April 4, 2013

Marsh Crabs Demonstrate Ecological Benefits Of Invasive Species

Alan McStravick for - Your Universe Online

The marsh systems around Cape Cod have been eroding at a rapid rate in the past few decades. As redOrbit reported in January, researchers from Brown University were able to find a cause and effect relationship between human interaction with the ecosystem and the rapid increase in the native purple crab population.

As their research detailed, human manipulation of the region via recreational overfishing and the creation of drainage ditches that zig-zag across the marsh systems led to the depletion of the purple crab´s natural predators. Their previous results urged conservation efforts yet offered few concrete suggestions as to how to stem the tide of the deleterious die-off of the marsh grasses that the purple crabs subsist on exclusively.

While scientists were searching for a solution, Mother Nature silently offered an answer to the problem with the introduction of a non-native invasive species. This has led to a measurable regrowth of the marsh grasses responsible for protecting the wetlands from further erosion.

“Humans have had far-reaching impacts on ecosystems,” said author Tyler Coverdale, a researcher in the lab of lead author Mark Bertness, chair of the Department of Ecology and Evolutionary Biology.

“Some of those impacts, like overfishing, cause species to decline in their native ranges. Others, like shipping and trade, cause species to become more common outside of their native ranges. Most of the time these opposing types of impacts have negative results. In this case, an invasive species is potentially restoring a lost ecological function.”

In a new study published online in the journal Ecology, the team details how the invasive green crab species has stepped in to fill the void left by the decline of the native predators of the grass-eating purple crabs. The increase in the presence of green crabs has effectively driven the purple crab out of the distressed ecosystem.

The Bertness lab has been observing and conducting research on the marshes for years. Their focus has predominantly been on trying to determine the cause of the rapid erosion in the region. Only recently, they noted that specific areas where the soil had not yet disappeared were enjoying a resurgence of marsh grass growth, though still far short of what could be termed a full recovery.

“When we started seeing the marshes recover, we were baffled,” Bertness said. “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.”

In their study results, the team states they learned the invasive green crab existed in greater abundance in the marsh areas that were in the most distress. As the previous research showed the distressed areas were overrun by the purple crabs, this new information offered a possible predator-prey correlation. This assertion was bolstered by the observation that in seemingly healthy marsh areas where there were significantly fewer purple crab burrows, it was also difficult to locate a green crab.

Bertness and Coverdale´s measurements of cordgrass regrowth also showed that locations with high green crab density correlated positively with locations of grass regrowth.

With the above information in hand, the researchers wanted to determine if their observations were the result of coincidence of coexistence or if there was a relationship between the two crabs that could explain the presence of the new growths of cordgrass.

To test their theory, Bertness and Coverdale placed a green crab and a purple crab into a wire cage at a burrow. This experiment was conducted multiple times and always presented the team with the same result: When they returned to the cage, they would find that the green crab had won the struggle for the burrow. Throughout these experiments, the grass-eating purple crab only survived 15 percent of the time. A control to their experiment involved placing two of the purple crabs into one cage together. With the control experiment, the purple crabs survived every time.

The researchers devised one further experiment to determine whether the invasive green crab had to physically engage the purple crab to protect the grass or if their presence in the area alone was enough of a deterrent to drive them off. This experiment involved fencing the purple crabs off so that they could not directly interact with the green crabs. Each of the fenced areas was populated with purple crabs. Into some, a free roaming green crab was introduced. In others, the green crab was caged.

In the fenced areas where the purple crab had neither a free-roaming or caged green crab, grass consumption was notably higher. However, both the free-roaming and caged green crabs presented enough of a threat to the purple crab that in those fenced areas grass consumption was significantly lower.

In the ecological world, these results represent what is known as a ℠non-consumptive effect´. To the lay person, a ℠non-consumptive effect´ basically means ℠scaring things away.´

“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 team´s study, according to Bertness, provides evidence for two newer views within the field of ecology. The first is the idea that an invasive species is not always a bad thing. The second is that ecologists should perhaps broaden their understanding of the predator-prey relationship to include the threat the predator presents, not just its direct attacks.

While this latest study is interesting news for ecologists, Bertness says that the introduction of the green crab to the marsh system alone is not going to be enough to restore the blighted areas.

“The marshes are slowly coming back but they were destroyed much faster than they are going to be able to rebuild,” he said.

Funding for this study was provided by a grant from the National Science Foundation (NSF).