Prehistoric decline of freshwater mussels tied to large-scale maize cultivation


USDA Forest Service (FS) research suggests that a decline in the abundance of freshwater mussels about 1000 years ago may have been caused the large-scale cultivation of maize by Native Americans.

In the April 2005 issue of Conservation Biology, Wendell Haag and Mel Warren, researchers with the FS Southern Research Station (SRS) unit in Oxford, MS, report results from a study of archaeological data from 27 prehistoric sites in the southeastern United States that shows how the erosion and sediment associated with agriculture affect freshwater mussel populations.

Worldwide, freshwater mussels have proven to be highly susceptible to human-caused disturbance, and represent the most endangered group of organisms in North America. Of 297 species found in the United States, 269 freshwater mussel 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."

Among freshwater mussels, members of the genus Epioblasma -- commonly called riffleshell -- 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 the 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 5000 years ago," says Warren. "With increasing population came land disturbance from agriculture. This intensified about 1000 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 They compiled data from both published and unpublished archaeological reports from 27 different sites along 12 rivers in the Southeast.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 5000 and 1000 years ago; however, the decline accelerated markedly during the period between 1000 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.

"Our results from prehistory support the notion that increases in human activities such as land clearing have measurable effects on freshwater mussel communities," says Haag, "and that prehistoric human activities put pressures on aquatic ecosystems that were similar to -- though certainly less acute than -- present-day activities."

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Last reviewed: By John M. Grohol, Psy.D. on 21 Feb 2009
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