Freshwater mussel decline linked to ancient corn growing

OXFORD, Mississippi, June 10, 2005 (ENS): A decline in the abundance of freshwater mussels about 1,000 years ago across what is now the southeastern United States may have been caused by the cultivation of maize, or corn, by Native Americans, according to new research by scientists with the U.S. Forest Service.

In the April 2005 issue of "Conservation Biology," Wendell Haag and Mel Warren, who work at the Forest Service's Southern Research Station unit in Oxford, report results from a study of archaeological data from 27 prehistoric sites in the southeastern United States.

Of 297 freshwater mussel species found in the United States, 269 species are found in the Southeast.

“We can tie declines of specific mussel populations to the construction of dams, stream channelization, or pollution from a specific source,” says Haag, “but the worldwide patterns of decline in these animals implies that larger-scale disturbances such as sedimentation and nonpoint-source pollution may have an equal impact.”

Worldwide, freshwater mussels have proven to be susceptible to human disturbance, and they are the most endangered group of organisms in North America, say Haag and Warren.

Among freshwater mussels, members of the genus Epioblasma, known as riffleshells, are the most endangered. Epioblasma consists of 20 species and eight subspecies; at least 13 of these species and four subspecies are presumed extinct. Of those remaining, the snuffbox mussel, Epioblasma triquetra, is the only species not listed on the federal endangered list.

“Human population in the Southeast began to increase steadily about 5,000 years ago,” says Warren. “With increasing population came land disturbance from agriculture. This intensified about 1,000 years ago, with the beginning of large-scale maize cultivation. No one has really tried to look at how this change in land use impacted water quality and aquatic organisms such as freshwater mussels.”

Working with Evan Peacock from the Cobb Institute of Archaeology at Mississippi State University , Warren and Haag used survey data from prehistoric shell middens - refuse heaps of shells discarded after eating - to examine differences in the abundance of Epioblasma species before and after maize cultivation started in the Southeast.

They compiled data from both published and unpublished archaeological reports from 27 different sites along 12 rivers in the Southeast.

“As far as we can tell, Native Americans harvested mussels without preference for species,” says Haag. “Shell middens provide us with a way to establish the range of freshwater mussel species before human impacts, and to chart changes in relative abundance as impacts increased.”

The researchers found that the relative abundance of riffleshell mussels in the rivers they studied declined gradually during the period between 5,000 and 1,000 years ago and the decline accelerated during the period between 1,000 and 500 years ago, when thousands of acres of land were cleared for farming.

“We know that freshwater mussels are very sensitive to stream alterations,” says Warren. “Although we cannot entirely rule out the influence of long-term changes in climate, the dramatic changes in land use in this period provide a compelling explanation for the changes in mussel abundance we found.”

Today, none of the riffleshell species the researchers found in ancient middens survive at the study sites, where they were gathered by Native Americans over the millennia before European settlement. Most are extinct as a result of modern land disturbances.