Plant Invasions Across the United States: Patterns and Clues

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.

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.

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.

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