Identifying the biogeographic traits of invasive plants’ native habitats

William G. Pfadenhauer, Organismic and Evolutionary Biology, University of Massachusetts Amherst, Amherst, MA, Michael F. Nelson, Environmental Conservation, University of Massachusetts Amherst, Amherst, MA, Brittany B. Laginhas, Organismic & Evolutionary Biology, University of Massachusetts, Amherst, MA and Bethany A. Bradley, Environmental Conservation, University of Massachusetts, Amherst, Amherst, MA

Background/Question/Methods
Reducing ecological threats from invasive plants is most effective if we proactively identify invasive species and stop their introduction and spread. Risk assessments, which identify potentially invasive plants, typically focus on biological traits (e.g., growth rates, dispersal capability). However, the environmental characteristics of invasive plants’ native habitats (termed “biogeographic traits”) may also provide important clues about invasion risk. Biogeographic traits may reflect species environmental tolerance and competitive ability - traits that are difficult to measure based solely on biology. Here, we compiled biogeographic variables associated with the native habitats of 5,500 vascular plant species native to the continental United States (CONUS). We categorized each species’ invasion status outside of CONUS as “native”, “established”, or “invasive” based on information from existing databases (USDA Plants, CABI, GRIIS, GloNAF, and Global Plant Invaders). We hypothesized that species with broader abiotic niches in the native habitat and species more accessible to human discovery (e.g., proximal to human footprint) would be more likely to establish and invade outside of the U.S. To test these hypotheses, we used logistic regressions and support vector machines to assess the significance and accuracy of biogeographic traits for predicting invasion status.
Results/Conclusions
Our results suggest that species with broader abiotic niches are more likely to become established and invasive. We found that the range of temperature, precipitation, soil pH, and soil texture in a species’ native habitat had the strongest predictive power for explaining the likelihood of both establishment and invasion. Models that incorporated these variables correctly predicted the invasion status for ~72% of plant species and explained more variation than models that used combinations of other biogeographic traits. While we found some support for our second hypothesis - human footprint variables were positively correlated with invasiveness - these variables did not offer as much predictive power as variables describing niche breadth. The support for our hypotheses suggests that invasive plants have higher physiological tolerances than both native and established species, supporting the inclusion of niche breadth variables in invasive plant risk assessments. Our results also suggest that accessibility to humans increases the likelihood that a species will be established elsewhere. However, variables representing human footprint did not help to distinguish established species from invasive species, suggesting these traits may not be useful for risk assessments.