Poff said biotic exchange is a much greater threat to freshwater systems, particularly lakes, than to terrestrial biomes. Aquatic species have typically evolved in relatively isolated habitats, and introduction of a new species often produces stress on the food chain or some other aspect of native species' existence. Freshwater ecosystems are very sensitive to poor land-use practices, Poff said, because as low points on the landscape, they accumulate damaging silt and excess nutrients from runoff.

The earth's surface wasn't the only focus of the study. Wall, a professor of rangeland ecosystems and a specialist in soil biodiversity and microscopic invertebrates, said belowground species, from bacteria to worms, currently are most harshly effected by land-use change.

"That's because soil is usually torn up," she said. "It's taken perhaps a hundred years to form an inch of soil, and whether we're digging a ditch or cultivating a garden, we're disturbing that ecosystem, the habitat for thousands of species."

Of the global change drivers, carbon dioxide levels are probably the hardest to assess in terms of their direct influence on soil biodiversity, Wall said. Nonetheless, the indirect effects through plants of increased carbon dioxide concentrations may be significant, she said.

On land, the researchers found, Mediterranean-climate and grassland ecosystems will probably experience the greatest proportional change in biodiversity because all five factors affect them. Northern temperate ecosystems will likely experience the least biodiversity change, primarily because they've already been so extensively affected by major land-use change. The most land-use change will occur in tropical forests and the temperate
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Contact: David Weymiller
dweymiller@ur.colostate.edu
970-491-6851
Colorado State University
9-Mar-2000