New research shows that two key causes of plant invasion — escape from natural enemies, and increases in plant resources — act in concert. This result helps to explain the dramatic invasions by exotic plants occurring worldwide. It also indicates that global change is likely to exacerbate invasion by exotic plants.

Agricultural Research Service (ARS) ecologist Dana Blumenthal reached these conclusions after studying 243 European plant species and their fungal and viral pests, both in Europe and in the United States.

The study was published this week in the Proceedings of the National Academy of Sciences.

Blumenthal, based at the ARS Rangeland Resources Research Unit in Fort Collins, Colo., and colleagues at the University of North Carolina and in the Czech Republic showed that fast-growing plant species adapted to moist, nitrogen-rich soils had many fungal and viral pathogens in the areas where the weedy species evolved. Once these species arrived here, they escaped many of their long-time enemies.

Such an escape from numerous enemies is thought to provide exotic species with an advantage over native species still burdened by their enemies. This is the first study, however, to show that whether a plant escapes from a few or an unusually larger number of enemies can be predicted from the type of plant: Exotic species that are fast-growing and weedy are likely to have more enemies to escape from.

Unfortunately, these are the same species most favored by global change. Fast-growing weedy species thrive in environments with ample plant resources. And global change increases key plant resources, such as carbon dioxide and soil nitrogen, through increases in the greenhouse gases carbon dioxide and nitrous oxide, respectively.

Fast-growing, weedy exotic species therefore have a double advantage in today's world. Increases in resources enable them to outcompete slow-growing plants. An escape from an unusually large number of enemies enables them to outcompete even fast-growing native plants. As global change proceeds, continuing increases in resource availability are likely to exacerbate such plant invasions.

The National Science Foundation, the European Union, and the Czech Republic supported the study.

ARS is the principal intramural scientific research agency in the U.S. Department of Agriculture.