Plant Invasions Across the United States: Patterns and Clues

Forest plant invasions are driven by multiple factors that vary depending on the geographical area, the length of time invasive plants have been present, and other factors. Photo by Chris Evans, Illinois Wildlife Action Plan, courtesy of Bugwood.org.
Forest plant invasions are driven by multiple factors that vary depending on the geographical area, the length of time invasive plants have been present, and other factors. Photo by Chris Evans, Illinois Wildlife Action Plan, courtesy of Bugwood.org.

Garlic mustard, Japanese stiltgrass, Oriental bittersweet, and other non-native invasive plants are creeping across backyards, parks, forests, and roadsides throughout the southeastern U.S.

Scientists are still trying to understand what drives their relentless spread. Invasions are often assessed by measuring species richness, or the number of non-native species known to grow in a certain area. However, other measurements of plant invasions could offer more insights. “We can make stronger inferences about invasions when we account for multiple invasion measures, as well as the diversity of ecosystems across large geographic areas,” says U.S. Forest Service Southern Research Station (SRS) scientist Christopher Oswalt.

Oswalt, research forester at the SRS Forest Inventory and Analysis (FIA) unit, co-authored a recent study that evaluates the patterns and drivers of forest plant invasions over large scales. The study was led by Basil Iannone, a postdoctoral research associate at Purdue University, and published in the journal Diversity and Distributions. Multiple scientists from Purdue University, as well as the Forest Service Southern and Northern Research Stations collaborated on the study.

The scientists used data from FIA to compile and map the number of invasive species, as well as the percentage of plots invaded, and compared forest plant invasions in two ecologically distinct regions of the U.S. – eastern and western forests. Data from 2,524 counties were analyzed.

Two measures of invasion were modeled – richness and prevalence. Richness describes the number of invasive species present, while prevalence describes how common invasive plants are, and was defined as the percentage of FIA plots in a county that have at least one invasive plant species present. Invasion was also considered in relationship to the quality of an area’s habitat and its vulnerability to invasion, as well as propagule pressure – the number of viable seeds, fruits, root fragments, and other propagules that non-native plants produced. “We modeled invasion richness and prevalence as functions of 22 factors that describe propagule pressure and habitat invasibility,” says Oswalt. Eastern and western forests were analyzed separately.

On average, eastern forests were more heavily invaded, both in terms of the number of invasive species and how common they were. However, the geographical impact of plant invasions was not evenly spread through the region – forests with the highest number of invasive species were in the Southeast and East, while forests in the North, the Great Plains, and along the Mississippi had the lowest. Similarly, the prevalence of invasion differed throughout each region, and was highest in the Midwest, metropolitan areas, and the Southeast. In western forests, patterns of richness were varied, although prevalence was low throughout the region.

“Our results suggest that both propagule pressure and habitat invasibility contribute to large-scale patterns of forest plant invasions,” says Oswalt. “However, the extent to which they contribute varies considerably.” In both eastern and western forests, higher propagule pressure tended to be associated with more intense plant invasions, but study suggests that as forests become more heavily invaded, propagule pressure might become less important. Forest plant invasions may proceed in stages, and the high invasion richness and prevalence in the East suggests that these forests are in a later invasion stage than western forests. The findings underscore the need for considering the distribution and concentration of invasive species when developing management plans.

Read the full text of the article.

For more information, email Christopher Oswalt at coswalt@fs.fed.us

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