fessor of ecology and evolutionary biology who also serves as dean of the Cornell University Graduate School. "The enemy-release hypothesis argues that invaders' success results from reduced attacks by natural enemies from their native habitat, while the biotic-resistance hypothesis says invaders' impacts are limited by interactions with native species, including natural enemies, in their new habitat," she notes. " Our study found that both factors -- enemy release and biotic resistance -- are important in determining whether an invading plant species thrives to become a noxious weed or struggles to survive."

The Cornell plant study began with 4,100 naturalized plant species, regarded by the U.S. Department of Agriculture as invaders surviving in wild populations in the United States without human intervention, and focused on 1,165 randomly chosen species. A further focus on plants from Europe, particularly the Mediterranean region, narrowed the study to 473 species, such as leafy spurge, sulphur knapweed and Russian thistle.

The Cornell ecologists then compiled information on viral and fungal infections of the 473 species in their native and adoptive habitats. Mitchell credits Cornell undergraduates Jennifer Gardell and Brian Youn for assistance in what he calls a "mammoth data-mining task. For the first time, we were able to bring real numbers to the theories."

The results: Invasive plants in the United States, on average, have 77 percent fewer diseases (84 percent fewer fungal diseases and 24 percent fewer viral diseases) compared with the same species in their native European habitats. (Viral diseases are harder for plants to escape because the viruses can travel, systemically, in the plants or in their seeds, the ecologists note.) And invading plants that had acquired the most pathogens in their naturalized ranges were less likely to become widespread, noxious weeds that are costly to agriculture. "These results suggest that invasive plan
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Contact: Roger Segelken
hrs2@cornell.edu
607-255-9736
Cornell University News Service
5-Feb-2003

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