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PORTLAND - Native prairies, probably the single most endangered ecosystem in the Pacific Northwest, were historically shaped by fire, one expert said today, but in the future they may be brought back to health by one of the more noisy, pedestrian inventions of the modern age - the lawnmower.

This is one of the conclusions emerging from years of research on how best to recover the native prairies that once dominated such regions as the Willamette Valley of western Oregon, which have shrunk to less than half of 1 percent of their former abundance, along with the multitude of plants and animal species which depended upon them.

"It's no secret that fire was a major factor in shaping these historic prairie ecosystems, and that suppression of fire, along with agriculture and urbanization, led to their demise," said Mark Wilson, an associate professor of botany and plant pathology at Oregon State University.

"However," Wilson said, "the rules have all changed now. We have huge fuel loads, hotter burns, invasive weeds and fragmented parcels of prairie to deal with, and we can't really expect fire to have the same effects that it did 200 to 300 years ago. Fire is not a silver bullet for prairie management."

Wilson spoke today at the annual meeting of the Ecological Society of America, and outlined future management regimes for prairie recovery in which fire may be one of the tools used, but by no means the only one and sometimes not even the most important one.

In recent research, scientists have found that fire was particularly effective at removing tall shrubs and trees that have replaced some native prairies - that's a good start, you can't very well have prairie unless you get rid of most of the trees first. But fire also releases nutrients and other resources that can favor non-native or exotic growth at the expense of native prairie plants. In one study in a wetland prairie, native herbaceous cover remained steady after seven years of prescribed fire, but non-native herbaceous cover almost doubled.

Some of these non-native herbaceous plants are aggressive pests, Wilson said, and can lead to an eventual decline in native plant abundance. Fire can also harm some federally protected plant species, and its use is a particular concern when used in some of the drier sites.

What's the alternative? Well, in some cases, Wilson said, it makes more sense to use a mower.

"We're not suggesting that we take native prairie and try to turn it into a lawn," he said. "But sometimes you can mow at precise times and heights, such as 4-6 inches, and kill upper-level biomass that you don't want while preserving low-growing, native plants that you are trying to encourage."

There may also be a role for selective, careful use of herbicides, Wilson said, especially on lands that have had no native plants on them for a very long time, or in spot applications on good prairie.

"In too many places, when our primary management tool was fire, we just haven't been getting the recovery of native prairie that we had hoped for," Wilson said. "Fire is non-selective, it's expensive and dangerous. There are still places and times when it will make sense to use fire, but it's not an open and shut case."

The advent of invasive species will mean that lands which are being restored as native prairie will need active management forever, Wilson said. But the good news, he said, is that many areas of native prairie are now getting pretty good stewardship, with good results.

Fender's blue butterfly, for instance, was an insect species once believed to be extinct before OSU researchers 15 years ago re-discovered some small populations, living on a particular native plant - Kincaid's lupine - that was essential to their survival. Now officially listed as endangered, the population numbers of this butterfly have exploded in recent years along with the recovery of native prairies at various sites.

According to scientists, Oregon's native prairies are an ecosystem that dates to the last Ice Age and were nurtured by frequent fire, often set by Kalapuya Indians. Prior to European settlement, both wetland and native upland prairie were the dominant land form in the Willamette Valley. They featured a range of native grasses, flowers, shrubs, insects and other plant and animal species that were rare or nonexistent elsewhere in the world.

At this point native prairies are even more at risk than the region's old growth forests. And a variety of groups - university researchers, student volunteers, the Army Corps of Engineers, U.S. Fish and Wildlife Service, the Nature Conservancy - have collaborated to help bring these ecosystems back from the brink of extinction.

The arrival of settlers, plowing of farmland, growth of roads and cities, and suppression of fire all contributed to the destruction of these ecosystems, researchers say.