Seeds, pollen in sediment yield clues to human impact on ecosystems
When 18th-century trappers trekked through the forests of what is now Baltimore, theirminds were on money. They collected beaver pelts because the fur fetched high prices fromclothing makers in Europe. At the same time, however, their hunting habits may have dramaticallychanged the landscape, altering the flow of local streams and the mix of vegetation nearby, newresearch at The Johns Hopkins University suggests.
Today, more than two centuries after the decimation of the beaver population, Hopkinsscientist Grace Brush is digging into the mud beside Baltimore-area streams to find signs of howhumans changed the ecology of a region once dominated by dense forests and meanderingwaterways. By studying the pollen, seeds, tiny animals and chemicals preserved in sediment,Brush hopes to learn how natural resources were affected by the people who hunted animals,farmed the land and finally turned Baltimore into a bustling metropolis.
Her work is a key facet of a new long-term ecological research effort funded by the National Science Foundation. The project is unusual because these in-depth environmental studies have traditionally been done in "pristine" areas such as Antarctica, where humans have done littleto interfere with the natural cycles of life. For the first time, two new research efforts, launchedlast year, are focusing on highly populated urban sites, Baltimore and Phoenix, where the impactof people on the environment will not be ignored..
"The idea is revolutionary in ecology," says Brush, a professor in Hopkins' Department ofGeography and Environmental Engineering. "But the time has come to consider humans as part ofecosystems."
She is part of the Baltimore research team, led by the Institute of Ecosystem Studies inMillbrook, N.Y. A widely-respected paleoecologist, Brush will look for vegetation and chemicalchanges that have occurred within the Gwynns Falls Watershed, a network of streams andwetlands stretching from a semi-rural area northwest of the city to heavily populated inner cityneighborhoods. Her findings could help public officials plan future developments that minimizeecological damage. "The purpose of this study is to help people become aware of and understandthe environment within urban areas," she says. "We are beginning to think of interrelationsbetween people and nature, and how each influences the other. Hopefully, what we learn here willbe applicable elsewhere." Although the first six-year segment of the project just got under way, Brush and herstudents have already uncovered interesting signs that fur traders in the early 1700s may haveinadvertently altered the terrain.
"The beaver population here was very high at the time of European settlement," theresearcher says. "But within about 50 years, the beaver became pretty much extinct because of thefur trade. We know that from historical records. When they were thriving, beavers built dams thatcaused streams to flood into the surrounding lower areas. This activity would have changed theflow and cycling of nutrients through the landscape."
As the beavers disappeared, however, so did their dams. "As a result, streams would have been more free-flowing, resulting in less flooding, drier land adjacent to the streams and adifferent vegetation," Brush says. "That's our hypothesis. Because there were no longer dams andstagnant water, marshy wetland areas near the streams disappeared. Sediment in the water wouldbe transported farther downstream, perhaps as far as the Chesapeake Bay."
Brush's theory is supported by her discovery of sedge pollen in the oldest portions ofsome of the first sediment cores her team has extracted from the Gwynns Falls Watershed. Sedgeplants grow in marshes, indicating the land beside the streams was once much wetter, probablybecause beaver dams were diverting water over the banks. Further research is needed, but Brushsays, "This whole landscape could have been quite different under the influence of the beaverpopulation."
To explore this and other hypotheses, Brush and her students are collecting sedimentsamples by driving an auger into the earth in places where moisture is likely to have preservedseeds, pollen and other fossilized material. With the tool, the researchers can extract a cylinder-shaped sediment core, about 2 inches across and up to 4 feet long.
Inside her Hopkins lab, Brush cuts each core into 1-centimeter-wide slices. Each isbagged, labeled and placed in cool storage to deter bacterial growth while it awaits analysis.Depending on how quickly sediment has accumulated, each slice can represent one year or 200years. The time when land was cleared and used for agriculture is identified in the cores by thedepth where an increase in ragweed pollen is found. (Ragweed flourishes on plowed land.) Theragweed horizon can be dated from historical records of land use and agriculture. Sedimentsamples in the core at depths below the ragweed horizon are sent to a laboratory for carbon-14dating. The oldest core collected in this region so far is 15,000 years old. It came from IndianCreek, a tributary of the Anacostia River near Washington, D.C.
When she studies the pollen, seeds, insects and chemicals in the sediment, Brush looks forsigns of change in the natural environment. For example, pollen records may show that somespecies of trees once thrived in the watershed, even though they are no longer reproducing theretoday, perhaps because the soil is now too dry. She often looks at how such changes are relatedto human activity. Cutting down forests for agriculture, for example, can affect the level ofnutrients carried by adjacent streams.
In the new Baltimore study, Brush hopes her sediment cores will reveal how land-use haschanged the environment throughout the city's history. Reporters: Color slide available; contact Phil Sneiderman (see above).
Related Web Sites:
Baltimore Ecosystem Study: http://baltimore.umbc.edu/lter/
Grace Brush's Home Page: http://www.jhu.edu/~dogee/brush.html
Hopkins Department of Geography and Environmental Engineering: http://www.jhu.edu:80/~dogee/
Materials provided by Johns Hopkins University. Note: Content may be edited for style and length.
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