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Cheatgrass (Bromus tectorum, right) on the march in western Nevada.

Cheatgrass (Bromus tectorum, right) on the march in western Nevada.

Bethany Bradley

Invasive plants taking over the U.S.

Invasive plants are conquering the United States. The first comprehensive assessment for the continental United States has found that nonnative plants are more widely distributed than native plants are. And humans are largely to blame.

Invasive plants have been a problem in the United States for years. They range from the fast-growing Japanese kudzu, which has smothered more than 3 million hectares, mostly in the South, to the bristly cheatgrass, native to Eurasia and the Mediterranean, which has wreaked havoc on western lands, crowding out other grasses and fueling bigger fires in places like Nevada. Yet scientists have long assumed that native species are more widely distributed than these newcomers, because the natives have had more time to fill in their potential ranges.

The new study calls that assumption into question. Researchers led by biogeographer Bethany Bradley of the University of Massachusetts, Amherst, created lists of 13,575 plants that are native (9402), endemic (2397), alien (1201), or invasive (755) in the continental United States using the U.S. Department of Agriculture (USDA) PLANTS database. (Endemic plants are natives with limited ranges, whereas aliens are nonnative but noninvasive. Invasive species in the study are nonnatives identified as highly aggressive, noxious weeds by the federal government and many states or listed in the Invasive Plant Atlas of the United States.) Then, they divided the country into squares the size of an average county, about 2500 square kilometers, and determined which plants were present in each grid cell based on distribution data from the Global Biodiversity Information Facility, an international organization that provides open-access biodiversity data.

For each plant species, the researchers documented the total number of grid cells occupied, determined the extent of its range, and modeled its potential range based on the climate of the grid cells that it occupied.

The team found that invasive and alien plants are more widely distributed than natives across the continental United States. Even invasive species that are still relatively rare in the United States are already widely introduced. For example, tropical spiderwort, an aggressive weed in agriculture, was first identified as a problem in Florida in the 1990s and has already made it to California. Invasives and aliens also had larger potential ranges than natives, the team reports online ahead of print in Global Ecology and Biogeography.

These relative newcomers have more footholds in more diverse ecosystems than was previously thought, but that’s not all. By comparing the plants’ actual and potential distributions, the scientists also calculated each plant’s range infilling, or the fraction of its potential range that it was currently actually occupying. They found that the average invasive plant now inhabits only about 50% of its expected range. That means there’s still plenty of space for these plants to invade.  

So why are the nonnative plants doing so well? Bradley says that even if invasives enjoy intrinsic advantages—like broader climate tolerances and stronger competitive abilities—over natives, biological factors do not fully account for the invasives’ success. The key may be human activity. People assist nonnatives intentionally—planting ornamentals like English ivy—and by accident—sowing grass seed contaminated with nonnative seeds—but the result is the same: a takeover by nonnative plants.

“The research shows that the problem isn’t just the initial introduction of an invasive,” says Dana Blumenthal, a USDA plant ecologist in Fort Collins, Colorado, who was not involved in the study. “It’s also the repeated introduction of invasives by people.”

There is a morsel of good news, according to the researchers. The widespread distribution of nonnative, noninvasive alien plants suggests that native plants could survive in climates and geographical areas that they now don’t inhabit. They also may be able to survive longer in their current habitats, even with a changed climate.

“That’s a potential silver lining for near-term conservation efforts,” Bradley concludes. “We might have a little more time before climate change makes the current ranges of many species uninhabitable.”