The purple marsh crabs need tidal creek edge habitat to thrive, and do not venture into the inner heart of the marsh, where a shorter cordgrass species (the closely related, but squattier Spartina patens) and other high marsh plants dominate. The old WPA mosquito ditches also fulfill the crabs' habitat requirements. Once benign, the ditches nucleated dramatic reconstruction of the landscape with the loss of blue crab, striped bass, and smooth dogfish, and the subsequent boom of purple marsh crabs.
One of the remarkable features of the cordgrass die-off is its tight locality. Some areas of undeveloped marsh as close as a kilometer to the denuded banks around private residences and public docks appear healthy and unaffected. Mosquito ditches that can only be reached by a hard slog through undeveloped marshland do not display the striking die-off and bank erosion. The pattern cued the researchers to the possibility that recreational fishing was the trigger, Coverdale says. Few people wade into the swamp to fish.
Marshes are excellent model systems for observing the intersection of human impacts that can trigger environmental degradation, the authors say, because they have been exploited by humans for centuries, if not thousands of years, and are easily studied from aerial and satellite images.
"Marshes are one of the most heavily utilized resources worldwide," said Coverdale. "They are easily accessible, and provide shellfish, fuel, baitfish and opportunities for recreational anglers. A lot of those harvests are probably sustainable."But he is interested in the tipping points at which use of the marsh becomes unsustainable. The revelation of the slumbering menace of the mosquito ditches raises the prospect of other submerged impacts that may surface under the influence of new, contemporary pressures.
In the early
|Contact: Liza Lester|
Ecological Society of America