Among 473 of the alien plant species that have invaded from Europe and become naturalized in the United States as noxious weeds, the "most successful" traveled light – carrying fewer plant diseases from their native habitats – and were more immune to New World plant diseases.

That is the conclusion of Cornell University ecologists after examining plant-health records on both sides of the Atlantic. The study, reported in the latest issue of the journal Nature (Feb. 6, 2003) by Charles E. Mitchell and Alison G. Power as "Release of invasive plants from fungal and viral pathogens," is particularly significant in that it reconciles two theories, dating back to Charles Darwin in 1859, about successful naturalization of invading species.

Their findings, the ecologists say, should encourage biological-control strategists to look for weed-control pathogens both in the invading weeds' native and adopted habitats. However, they warn that biological control can negatively impact native species and is no panacea. What most surprised Mitchell and Power was the finding that pathogens can help keep invasive plants in check.

"We're coming to realize we should be grateful for our native plant pathogens," adds Mitchell, a postdoctoral researcher in Cornell's Department of Ecology and Evolutionary Biology. "Invasive plants cost an estimated $33 billion a year to the U.S. economy, but the damage would be worse were it not for our native fungi and viruses that control invasive plants to some degree."

A parallel study of invasive animals, ranging from mollusks to mammals, reported in the same issue of Nature as "Introduced species and their missing parasites" by ecologists at the University of California-Santa Barbara and Princeton University, reached similar conclusions about aliens' success in new lands.

Both the plant study and the animal study sought data for two long-standing and much-debated theories, explains Power, a professor 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 plants' impacts are a function of both release from and accumulation of natural enemies, including pathogens," the Cornell ecologists reported in Nature .

A second phase of the study – supported by the National Center for Ecological Analysis and Synthesis, the National Science Foundation, the U.S. Department of Agriculture and Cornell University – is being planned to consider the role of herbivorous insects and soil-borne plant diseases in controlling invasive plant species. In the meantime, this confirmation of the enemy-release and biotic-resistance hypotheses should inform efforts to control invasive plants, Mitchell says.

"This is also an answer for those who wonder, what has Mother Nature done for us lately? We continue to receive free service from our natural ecosystem in the form of pest control," Mitchell says. "The natural process of pathogen infection is helping to prevent invasive plants from becoming worse pests to humanity – without any subsidy from humans."