Did Fur Trappers Trigger Landscape Changes?
Seeds, pollen in sediment yield clues to human impact on ecosystems
When 18th-century trappers trekked through the forests of what is now Baltimore, their minds were on money. They collected beaver pelts because the fur fetched high prices from clothing makers in Europe. At the same time, however, their hunting habits may have dramatically changed the landscape, altering the flow of local streams and the mix of vegetation nearby, new research at The Johns Hopkins University suggests.
Today, more than two centuries after the decimation of the beaver population, Hopkins scientist Grace Brush is digging into the mud beside Baltimore-area streams to find signs of how humans changed the ecology of a region once dominated by dense forests and meandering waterways. 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 little to interfere with the natural cycles of life. For the first time, two new research efforts, launched last year, are focusing on highly populated urban sites, Baltimore and Phoenix, where the impact of people on the environment will not be ignored..
"The idea is revolutionary in ecology," says Brush, a professor in Hopkins' Department of Geography and Environmental Engineering. "But the time has come to consider humans as part of ecosystems."
She is part of the Baltimore research team, led by the Institute of Ecosystem Studies in Millbrook, N.Y. A widely-respected paleoecologist, Brush will look for vegetation and chemical changes that have occurred within the Gwynns Falls Watershed, a network of streams and wetlands stretching from a semi-rural area northwest of the city to heavily populated inner city neighborhoods. Her findings could help public officials plan future developments that minimize ecological damage. "The purpose of this study is to help people become aware of and understand the environment within urban areas," she says. "We are beginning to think of interrelations between people and nature, and how each influences the other. Hopefully, what we learn here will be applicable elsewhere." Although the first six-year segment of the project just got under way, Brush and her students have already uncovered interesting signs that fur traders in the early 1700s may have inadvertently altered the terrain.
"The beaver population here was very high at the time of European settlement," the researcher says. "But within about 50 years, the beaver became pretty much extinct because of the fur trade. We know that from historical records. When they were thriving, beavers built dams that caused streams to flood into the surrounding lower areas. This activity would have changed the flow 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 a different vegetation," Brush says. "That's our hypothesis. Because there were no longer dams and stagnant water, marshy wetland areas near the streams disappeared. Sediment in the water would be 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 of some of the first sediment cores her team has extracted from the Gwynns Falls Watershed. Sedge plants grow in marshes, indicating the land beside the streams was once much wetter, probably because beaver dams were diverting water over the banks. Further research is needed, but Brush says, "This whole landscape could have been quite different under the influence of the beaver population."
To explore this and other hypotheses, Brush and her students are collecting sediment samples by driving an auger into the earth in places where moisture is likely to have preserved seeds, 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 is bagged, 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 200 years. The time when land was cleared and used for agriculture is identified in the cores by the depth where an increase in ragweed pollen is found. (Ragweed flourishes on plowed land.) The ragweed horizon can be dated from historical records of land use and agriculture. Sediment samples in the core at depths below the ragweed horizon are sent to a laboratory for carbon-14 dating. The oldest core collected in this region so far is 15,000 years old. It came from Indian Creek, a tributary of the Anacostia River near Washington, D.C.
When she studies the pollen, seeds, insects and chemicals in the sediment, Brush looks for signs of change in the natural environment. For example, pollen records may show that some species of trees once thrived in the watershed, even though they are no longer reproducing there today, perhaps because the soil is now too dry. She often looks at how such changes are related to human activity. Cutting down forests for agriculture, for example, can affect the level of nutrients carried by adjacent streams.
In the new Baltimore study, Brush hopes her sediment cores will reveal how land-use has changed the environment throughout the city's history.
Color slide available; contact Phil Sneiderman
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