July 7, 1997 ATHENS, Ga. -- A new three-year study by ecologists at the University of Georgia and Virginia Polytechnic Institute and State University has discovered that falling leaves are much more crucial to the health of streams than was previously realized.
The research, which was funded by the National Science Foundation, was published today in the journal Science.
"We believe this is important new information for the overall health of streams," said Dr. Bruce Wallace, a UGA entomologist and ecologist. "Whether the idea is to restore streams or simply to reduce environmental impact, we need to be looking more closely at the streamside vegetation."
The three-year study, which was performed at the Coweeta Hydrologic Laboratory near Otto, N.C., is the first to exclude all leaf litter from a 180-meter-long stream flowing from a spring. Using an overhead canopy to exclude leaves from the entire length of the stream, scientists found startling results, with dramatic decreases in numerous species of animals that live in and around the water.
Also involved with the project from the University of Georgia were Dr. Judy Meyer, an ecologist, and research coordinator Susan Eggert. From VPI, Dr. Jack Webster was involved in the research.
Scientists have known for years that leaf litter is important in the overall scheme of life in free-flowing streams, especially in the southern Appalachians where the Coweeta site is located. But just how crucial falling leaves might be to productivity in the food chain had been unclear.
"I first started thinking about this research when I saw an article that said there is no clear relationship between resources and invertebrate abundance in streams," said Wallace. "And I just didn't believe it. I suggested we test the assumption to find out."
The mechanical requirements of the study were daunting. A canopy constructed of metal fence posts, a wooden frame and gill netting had to be installed along the 180-meter length of the stream, and workers had to make sure that leaves were kept off the netting when they fell. The project twice survived near-disasters, once during an ice storm and again when a limb fell during Hurricane Opal.
The area studied is in a dense oak-hickory forest, with many rhododendrons up to the edge of the stream. This meant that the canopy had to exclude leaf-fall from overhead and a lateral fence was installed to prevent the intrusion of materials from the sides. Susan Eggert and a crew of 12 people spent more than a week just putting the netting on the frame.
As a result of litter exclusion during the project, the ecologists "observed major changes in abundance, biomass and production of invertebrate fauna in the treated stream." A nearby stream that had no canopy served as a comparison or reference. Testing at the litter-exclusion site was done at sites determined by random sampling.
What Wallace and his colleagues discovered surprised them in its breadth and depth. Seventeen of 29 major taxa showed significant reductions in either abundance or biomass or both. This indicates that the leaves play a crucial role in maintaining the food chain in the rocky stream.
"Interestingly, we did find some invertebrates that didn't show decreases as significant as most did," said Wallace. "But these were mostly creatures that feed on woody debris and other organic matter that was present throughout the three-year exclusion period." Invertebrates collected during sampling were classified as scrapers, shredders, gatherers, filterers, primary consumers, invertebrate predators and salamanders.
The ecologists also found that the physical nature of the stream bottom dramatically affected the availability of food resources for the different invertebrates. For instance, the moss- covered, steep-gradient bedrock in the stream showed little decrease in the animal populations, suggesting they were less directly dependent on leaf-litter inputs.
Litter exclusion shows a strong bottom-up effect of food resources on stream communities, according to Wallace. Studies have been done in the past to determine bottom-up effects by adding nutrients to lakes and streams, but "ecosystem studies examining the effects of resource reduction on communities are rare," the Science paper noted.
The study could have important implications for the management of both streams and rivers, said Wallace.
"During the history of the country, we have destroyed many miles of the riparian zone [the area next to streams or rivers] and have decoupled the streams from the landscape," he said. "What we show is that leaf litter, among other things, is fuel for these streams. And there are so many threats, from grazing and agriculture to clear-cutting. The problem is the destruction of that linkage."
Wallace noted that many large rivers sequester detritus from the flood plain during floods, and yet flood plains were ignored for decades when planners or government agencies studied the health of these waterways. Even the Coweeta Forest was clear cut a century ago and now bears a second-generation regrowth, despite its pristine appearance.
The next part of the study is already underway. Last fall, members of the research team removed all the small wood in the litter-exclusion stream. This should give even more precise information about how terrestrial inputs affect stream productivity. Indeed, preliminary data shows that invertebrates populations are continuing to decline.
Results of the current study may become a vital part of future conservation efforts for streams and rivers, said Wallace.
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