Digging A Deeper Hole

Drainage ditches intended to fight mosquitoes during the Great Depression have taken a toll on salt marshes at Cape Cod, even decades after they were created, scientists report.

Cape Cod, Massachusetts has been overrun by humans, whose local population increased by more than seven times between 1930 and 2000. But even before the boom, people were changing the landscape by digging ditches. These ditches were meant to drain areas that mosquitoes used for breeding.

In recent years, Cape Cod fishers also have changed the ecosystem by catching animals such as striped bass. Freed from predators, the purple marsh crab has flourished and munched cordgrass along the banks of creeks and ditches, leading to erosion.

The study authors wanted to find out if the mosquito ditches had made the problem worse. They analyzed aerial photos of 12 Cape Cod salt marshes from 1939 to 2005, looking for patterns of development, ditching, and cordgrass death.

Purple marsh crabs began to damage marshland after about 1976, and some areas lost more than 90 percent of their low marsh, the team reports in Frontiers in Ecology and the Environment. The cordgrass die-off was worse in developed patches that had mosquito ditches, probably because these changes had created better habitat for the crabs and more favorable conditions for fishers. The study suggests that even though mosquito ditching “was a relatively benign disturbance for decades,” it has aggravated a problem triggered by fishing and development, the team writes. Roberta Kwok | 29 January 2013

Source: Coverdale, T.C. et al. 2013. Latent impacts: the role of historical human activity in coastal habitat loss. Frontiers in Ecology and the Environment doi: 10.1890/120130.

Image © DenisNata | Shutterstock.com