Weeds Flourish in Diverse Habitats

ALBUQUERQUE, NEW MEXICO--Hot spots of plant biodiversity may also be magnets for invading weeds that thrive in prime growing conditions, according to a study described here yesterday at the annual meeting of the Ecological Society of America. The findings, which run counter what many ecologists have believed, could complicate efforts by land managers to fight off weedy invaders, the study's lead author says.

Natural resource managers and biodiversity experts are becoming increasingly concerned about invasions of weeds such as leafy spurge, which has swept across Western landscapes, and the melaleuca tree, a major threat to ecosystems in Florida. There has been one reason for optimism, however: ecologists have long believed that landscapes with lots of different species should be more resistant to exotic invaders, since their panoply of species types maximizes the use of resources like nitrogen and sunlight, and limits the availability of these resources to weeds.

While some experiments with plots of plants have supported this idea, others have not. A team led by Thomas Stohlgren of the U.S. Geological Survey's Biological Resources Division decided to revisit the problem by comparing two biomes--temperate grasslands and mountains--at several spatial scales. The researchers counted plant types and amount of cover in 180 one-meter-square plots in four prairie types in the Western United States and also in 200 plots in forests and meadows in Rocky Mountain National Park, Colorado.

When the researchers examined the results across many plots, at a 1000 square meter scale, they found that the mountain biome had more weeds in diverse areas. In the prairie, weeds were more numerous in areas with denser coverage of native plants. For both biomes, the weedy areas also had higher levels of nutrients such as nitrogen and carbon, which Stohlgren believes is part of the good conditions that favors the invading weeds' success. "Native plants and exotic plants like the same things--light, soil, water--the good life," he says.

Stanford ecologist Peter Vitousek, calls the finding "original and useful." The bottom line, says Stohlgren, is a wake-up call for conservation biologists and managers of national parks, who "want to know where alien species are likely to invade." But the finding may also complicate efforts to counter the weeds, he adds, because it may not be desirable to use herbicides to kill weeds in a highly diverse, nutrient rich area.