The first comprehensive study of native versus non-native plants in the United States has found that non-native species are considerably more widespread than natives. Researchers with the University of Massachusetts-Amherst, the University of Exeter, U.K. and the University of California-Irvine studied more than 13,000 plant species—both native and non-native selections—to determine differences in overall geographic spread.
“Ecologists typically think of invasive species as being introduced in one spot and gradually spreading out from there. But, we found that even species with only a handful of occurrences were distributed all across the U.S.,” lead author Bethany Bradley says. “The future may already be here. … For non-native, invasive species in particular, predicting invasion risk is difficult because those species that have recently arrived may not have yet spread into all the environments where they could get a toehold.”
The comparative analysis highlights the fact that native plants are strongly limited in their distributions compared to non-native plants, probably because they have a harder time dispersing into suitable climates.
The authors believe that this pattern is a result of widespread human introduction of non-native and invasive plants. Regionally, the ornamental plant trade and other human activities like planting of seeds with weed contaminants help non-natives overcome dispersal barriers that limit the distribution of native species.
“One silver lining for biological conservation is that native species are not so strongly limited by climate as once assumed,” she adds. In other words, native species’ distributions aren’t defined by their climate tolerances. Instead, other, non-climate-related dispersal barriers or interactions with other species can prevent native species from moving into environments where they could otherwise exist.
“With this study, we’re showing that inability to disperse, not climate tolerance, is likely stopping some species from inhabiting a broader range. This could mean that many species predicted to go extinct with climate change could persist for longer than previously anticipated under novel climates.”
But, Bradley warns, “Dispersal barriers aren’t going away, so even if native species can survive a little longer with climate change, most are clearly not going to be able to shift into newly suitable climate without our help.”
Bradley says although non-native and invasive species are much more widespread than natives, they have “filled in” much less of their potential range. Native species on average occupied about 50 percent more of their potential range than non-native species.
She adds, “We’re likely to see more problems from invasive species ahead as they continue to expand locally into suitable environments.”