Traveling light

Natural History, May, 2003 by Stephan Reebs

Why do some plant immigrants spread so widely and destructively in their adopted lands, yet remain relatively innocuous back home? Presumably the new host country lacks some of the disease-causing fungi and viruses that afflict the plant in its native land. Hence, as long as the plant can resist the new pathogens it encounters in its adoptive home, it will become ... a weed.

Charles E. Mitchell and Alison G. Power, both biologists at Cornell University, delved deep into the databases of the U.S. Department of Agriculture and identified 473 plant species introduced (whether by accident or on purpose) into the United States from Europe. They found that, on average, 84 percent fewer fungal species and 24 percent fewer viral species infected the plants in the U.S. than in Europe. And for individual species, the lighter the burden of pathogens, the more states officially listed the plant as a "noxious weed." The few pathogens still afflicting the expatriate plants were an even mix of introduced and indigenous ones. Thus, both their escape from old pathogens and their resistance to new ones contributed to the invaders' success. ("Release of invasive plants from fungal and viral pathogens," Nature 421:625-27, February 6, 2003)

Stephan Reebs is a professor of biology at the University of Moncton in New Brunswick, Canada, and the author of Fish Behavior in the Aquarium and in the Wild (Cornell University Press).