Invasive species threaten healthy ecosystems
Spotted Knapweed is an aggressive competitor. First spotted in Montana in 1920, it has spread widely on rangelands. One plant can produce 1,000 seeds, and seeds can lay dormant for eight years
May 17, 1999
Web posted at: 4:00 PM EDT
Even healthy ecosystems are vulnerable to invasions by exotic species, according to scientists with the U.S. Geological Survey.
The conventional wisdom on invasive species is that they leap into a void, quickly colonize and then crowd out native species. The assumption has been that habitats rich in native species are relatively immune to such invasions because the native species are using all the available resources, leaving no room for wandering weeds.
A team of ecologists is challenging that position, saying that in a study across a broad array of central grassland and Rocky Mountain habitats, from Colorado to Minnesota, areas with the most native species are exactly where invasives tend to take up residence.
Their findings suggest that invasibility depends more on the presence of basic resources, such as sunlight, water and soil nutrients, than on an absence of competitors. Conversely, in less favorable habitats, a small number of native plants may monopolize existing resources, making the community more resistant to invasion.
Previous studies, suggesting that low-diversity communities are more invasion-prone, may have simply not cast a wide enough net, says USGS ecologist Thomas Stohlgren, primary author and an ecologist at the USGS Midcontinent Ecological Science Center. At his central grasslands study sites, for example, data from small, one-meter-square plots did support the classical view that high native species richness deters invasives. But this changed as the scale of vegetation sampling expanded to 10-, 100- and 1,000-meter-square plots.
"The pattern reversed itself at larger scales because resources are patchily distributed in the landscape," says Stohlgren. "Nitrogen, light and water are present in some areas but not in others. At large spatial scales, it's those hot spots of resources that are being invaded."
From a conservation perspective, the results are disturbing. "It's an alarming pattern," says Stohlgren. The invasions may threaten some of the last strongholds of certain biologically rich habitats, such as tallgrass prairie, aspen woodlands and moist riparian zones.
The researchers also found that rivers and streams act as havens for invasive species as well as networks for their spread from one pristine area to another. Invasions of streamside corridors are particularly worrying because they may also disrupt the relationships of plants and pollinators that evolved together. "This is really troubling, because it means that all along their migratory routes, birds and butterflies, for instance, are running into more and more non-native plants that they may not benefit from as much," says Stohlgren.
One consistent message from these studies, Stohlgren says, is that ecologists should avoid making sweeping generalizations based on work done at a small spatial scale or a single study site. Conservationists and land managers should be aware of the potential for invasives to move into biologically rich areas, but conservation strategies need to be dictated by the particular characteristics of the habitats and species involved.
The study has been published in the current issue of the journal Ecological Monographs. Stohlgren worked with researchers at the Natural Resource Ecology Laboratory at Colorado State University.
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Midcontinent Ecological Science Center
Natural Resource Ecology Laboratory, Colorado State University
Invasive species - Ecological processes and human impact
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