Prevention strategies and management prioritization of nonnative plant species have increasingly been based on assessment of the risk of invasion and negative impacts. However, the literature on plant invasion has two possibly contradictory predictors: 1) weed risk assessments (WRA) can be used to identify harmful invaders with over 90% accuracy; and 2) invasion probability is directly dependent on minimum residence time (MRT) in the new habitat. Evidence of positive dependence of invasion risk on residence time suggests that all introduced plant species may become invasive given sufficient time. While WRA tools often include some element of MRT, they also include multiple ecological, environmental, and management variables. We hypothesized that MRT is a primary predictor of establishment, but that species predicted to be at high risk for invasion via WRA will 1) become invasive at lower MRT and 2) have higher probability of becoming harmful invaders than species predicted to be of low risk. We used a dataset on MRT, invasion status, and WRA score for 565 non-native plant species in Florida to evaluate these hypotheses. The WRA system used was the Australian WRA modified for the U.S., for which scores above 6 indicate species at high risk for invasion.
Results/Conclusions
Across all growth forms, the MRT of naturalized and invasive species was 40% longer than non-invaders (74 vs 52 years). The proportion of species becoming invasive linearly increased over time (r=0.96). We did not have data on the year in which species became invasive; however, species that were identified as invaders early in their introduction history (MRT ≤ 25 years) had significantly higher WRA scores (mean= 17 ± 3.42) than those that had naturalized (mean= 3.0 ± 7.27) or not become invasive (mean= 2.0 ± 5.30). Thus, WRA tools may less accurately predict species establishment probability than predict which species will become harmful invaders soon after introduction. As a result, WRA is valuable for prioritizing which species should be precluded from introduction pre-border, and for rapid management investment if they have been introduced.