One of the most invasive exotics in the western United States, the yellow starthistle, is successful both at "invasion" in non-native areas and "colonization" in native ones. However, new research from an international team of researchers finds that a disturbance such as fire or grazing actually increased the success of yellow starthistle far more in non-native than in its home regions. Furthermore, yellow starthistle was able to establish virtual monocultures in disturbed plots only where it is exotic.

"Our results are novel," says Jose Hierro (University of Montana and Universidad Nacional de La Pampa). "No one else has ever shown that ruderals, that is, plants that are generally adapted to disturbance, respond differently to disturbance in native versus non-native regions."

The researchers conducted their research over three years in southern Turkey, where the weed is native, and in California and central Argentina, two regions where the weed is non-native and remarkably abundant. Their findings, published in the August issue of The American Naturalist, question the assumption that disturbance alone is sufficient to explain the remarkable success of invasive plant species in non-native ranges. Instead, the researchers argue, the common and powerful effects of disturbance must act in concert with other factors to allow certain species to dominate plant communities only where they occur as exotics.

The researchers suggest that soil pathogens suppress the growth of certain species and may contribute to the disproportionately powerful effect of disturbance in introduced regions.

"The potential for disturbance to have much stronger effects in invaded systems than in native systems is not trivial," says Ragan Callaway (University of Montana). "If disturbance in non-native regions is no different than in native regions, then clearly the management response is to limit disturbance and thus to limit invasions. However, if dis
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Contact: Suzanne Wu
swu@press.uchicago.edu
773-834-0386
University of Chicago Press Journals
26-Jul-2006