Predicting invasiveness and range size in wetland plants using biological traits: a multivariate experimental approach

Summary Predicting invasion potential across a wide range of plant species using only their biological traits is often challenging. We present a novel approach, aligning species along a multivariate axis of putative invasiveness traits. The traits were quantified in a common garden in which we manipulated two critical variables thought to strongly influence invasiveness: herbivory and resource availability. We used principal component analysis ( PCA ) to characterize 20 species based on seven key traits (percentage germination, specific leaf area, seed mass, above‐ground biomass, below‐ground biomass, response to fertilization and response to herbivory). We then assessed whether the linear combination of traits identified by axis 1 of the PCA could predict invasiveness. Axis 1 of the trait‐based PCA explained nearly 40% of variation in two literature‐based invasiveness scores; thus, we considered it to be useful as a trait‐based invasiveness index. Literature‐based invasiveness was greatest in large species (both above‐ground and below‐ground) with high percentage germination, small positive responses to fertilization and small negative responses to herbivory. Contrary to expectations from the enemy release hypothesis, species that were native to our region and highly invasive elsewhere were associated with only weak responses to insect herbivory. This suggests that invasion potential may be highest in species that are insensitive to herbivore pressure, rather than species benefitting the most from enemy release. Native range size is often an effective predictor of invasiveness, although the underlying mechanisms are unclear; we therefore compared the predictive ability of range size versus biological traits. Our trait‐based index was a better predictor of invasiveness than range size, while also explaining nearly 20% of the variation in longitudinal range size. Synthesis . We present a novel method for using traits to quantify invasiveness that appears to have broad predictive utility. Our results explicitly link biological traits to range size and global invasiveness, perhaps reflecting a general ecological strategy in plants. Following additional testing, this simple method could be utilized in a pre‐introduction screening protocol, and we provide practical recommendations for doing so.