Doomsayers may predict melting ice caps and drowned cities, but more subtle fallout from global warming has already struck the American West. Milder temperatures on the Colorado prairie have led to a decline of a native grass species, the prime feed for local livestock. Experts say the finding, reported in today's Science , is an ominous warning of how warmer temperatures may shift entire ecosystems.
Climate records over the last century point to a rise in average global temperature by about half a degree Celsius. Most ecological studies have tried to tie biological changes, such as the loss of wildflower biodiversity in the Alaskan Tundra, to either this average or to changes in the maximum annual temperatures. But this average hides a more complicated story: Days haven't been getting much warmer over the years, but nights now cool down significantly less than before, i.e., the diurnal temperature range is waning. That's why ecologist Richard Alward of the Colorado State University in Fort Collins and his colleagues decided to examine the daily temperature minima, collected over more than 20 years in the dry short-grass steppe of northeastern Colorado.
The team has combined the temperature data with vegetation data, such as counts of individual grasses and flowers and bioproductivity, for various species at a site near Fort Collins, Colorado. They have found that rising minimum temperatures correlate strongly with a shift in plant diversity, although it's not clear why. The biomass of blue grama, the predominant grass species and main forage for cattle, has declined by more than 50%, while weedy flowers and other non-grassy plants have invaded and established a strong beachhead. Alward suspects that these invaders probably won't resist grazing and drought as well as the blue grama does, possibly leading to soil erosion. "It's possible that the long-term sustainability of this ecosystem is jeopardized," he says.
For Jerry Melillo, an ecologist at the Marine Biology Laboratory in Woods Hole, the study offers an early grasp on the consequences of climate changes. "It's an interesting example of how subtle changes in climate can have a significant effect on the vegetation of an area people care about." In the long run, entire vegetation zones may shift toward the polar regions. The bottom line, Melillo says, is to "be prepared for changes in ecosystems that we're taking advantage of to support our lifestyle. This is the kind of things we'll be seeing in the future."