By
Cat Viglienzoni - July 6, 2010
Small nocturnal crabs are putting a clamp on the Cape Cod
environment. Researchers working over several years say the two-inch, purple-tinged
crustaceans are eating the cordgrass in Atlantic marshes en masse and turning
the wetlands into mudflats. However, humans may be at least partially
responsible for the population boom.
Researchers hypothesize that the Sesarma reticulatum crabs
were able to cause such damage because of a decrease in predation, which led to
an increase in the crab population. They point to humans, though, as a possible
cause for the decrease in predators. Predators of the crab - including other
crabs, some fish and birds - are less prevalent in marshes that have been disturbed
by human activities like fishing.
In one 2008 study to test if the crab was the culprit in
grass decline, researchers from Brown University monitored 12 Cape Cod marshes to
see if the cordgrass die-off on creek banks was related to the amount of crabs
in the area. Using archived aerial images, they analyzed the die-off over the
past two decades to test the predation hypothesis.
When they studied the areas, researchers found the areas
where the die-off occurred had higher levels of crab grazing than those that
were healthy. In fact, they attributed more than 80 percent of the variation in
vegetation to the crab's voracious appetite. The study said surveys concluded nearly
half of the creek banks in the Cape marshes had been denuded as a result of
crab appetites. They also said the rates of die-off expansion and area of marsh
affected have more than doubled since 2000.
To test predation theories, researchers conducted crab-tethering
experiments. In healthy marshes, crabs were eaten at a faster rate than in
marshes with a die-off. However, an effect of the booming crab population could
be an increase in predators that would otherwise rarely visit those marshes.
Currently, researchers are setting traps and nets to try to assess the number
of blue crabs and fish that typically prey on the Sesarma crab.
Researchers are also growing the cordgrass in biodegradable
protective nets to see if that will help stop the crabs from eating the grass. By
allowing the grass to grow, researchers hope to allow the marshes to recover. The
netting may also help to control the erosion that has developed as a result of
decreased grass.
To get a more definitive picture of the problem,
researchers must now study every level of the marsh ecosystem food chain,
measuring everything from predators to crab levels and grass die-off. Once they
have determined the scope of the problem, they can recommend better ways to
help the marshes.
