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Diverse Plant Communities Resist Invading Species

Invasions by exotic species have bedeviled ecosystems from lawns and farms to prairies and oceans. Recent invaders of North America include zebra mussels, purple loosestrife and Eurasian watermilfoil. In 1958 British ecologist Charles Elton hypothesized that communities with diverse arrays of organisms were better equipped to resist invaders. Now, a study at the University of Minnesota has confirmed the connection between biodiversity and resistance to ecological invasion in a prairie ecosystem.

The work suggests that plant communities, whether intact areas of native prairie or plots of land undergoing restoration, will suffer less from exotic species invasions if a diverse mix of species is present.

Elton, who has been called the father of ecology, published a 1958 book on invasions of exotic species in which he proposed his idea, which is known as the Diversity-Resistance Hypothesis. Since then, many scientists have explored the hypothesis, said Theodore Kennedy, a graduate student in ecology and lead author of the Nature paper.

"Researchers studying sedentary marine invertebrates have found that diverse communities make more complete use of the limited space available to them, and this leaves invaders with no place to settle," he said. "But the mechanism by which diverse plant communities keep out newcomers was previously unknown."

Kennedy and his colleagues asked whether land supporting relatively large numbers of prairie plant species would do a better job of excluding plants of Eurasian origin. They studied 147 square plots of land, each three meters on a side, at the university's Cedar Creek Natural History Area near Minneapolis St. Paul.

Each plot had been planted with either 1, 2, 4, 6, 8, 12 or 24 grassland plant species, and the researchers had already noticed more weedy biomass in the less diverse plots. For two years they ceased weeding and examined relations between Eurasian weeds and "resident" species (those that had been purposely planted).

The team found that the total number of invading weeds, the number of extremely large individual weeds, and the total amount of ground covered by weeds all decreased as the number of resident plant species increased.

The results suggest a mechanism of invader repulsion similar to what goes on with marine invertebrates, Kennedy said.

"Here, it appears we also see a more complete use of space," he said. "We found the neighborhoods of weeds were more crowded in the plots with high species diversity. We think that in addition to a lack of space, there are fewer other resources available to invaders in crowded neighborhoods. That is, when you pack more plants into a small area, you'll have more complete utilization of nitrogen, phosphorus, water, light, and other resources that will thereby limit the success of invaders."

The findings imply that losses of biodiversity could also mean loss of local resistance to invasion by species that, thanks to human transport, are no longer kept out by geographical or other barriers, the researchers said. Therefore, restoration and revegetation efforts would likely benefit from establishing as diverse a plant community as possible. Diverse communities should require less maintenance and monitoring because "they are generally very effective at excluding undesirable invaders."