Multiple mechanisms in woodland plant species invasion.
Aims: A plethora of theories explain species invasion, yet when tested in isolation, support or falsification becomes contingent on study species, system and approach. Our objective was to examine community-level species invasion as a function of multiple competing hypotheses. Methods: We used data from >3500 woodland plant species in 2750 plots in 49 national parks in eastern US deciduous forests to test multiple competing theories of species invasion: competition, empty niche, propagule pressure and latitude matching. We also tested interactions with residence time to account for non-native species naturalization and spread since arrival. Important Findings: The non-native herbs generally thrived at latitudes similar to those from which they originated, but not necessarily where they were originally introduced to the eastern US. Overall, we found that each hypothesis explained at least some aspect of woodland plant species invasion, but examining them simultaneously allowed assessment of their relative strengths and interactions. Our results suggested that residence time is a strong predictor of non-native woodland plant success, particularly as it interacts with other mechanisms of invasion, such as competition (abundance of native woodland plants), climate matching (similar invaded latitude as home range), propagule pressure (distance to putative seed sources) and empty niche (relatedness to native plants). We found that initial barriers, such as distance from propagule source or suboptimal habitat, were overcome, as was resistance from native relatives. However, the biggest challenge for the non-native woodland plants appeared to be time, as they declined after ∼1 to 2 centuries.