Interactions between propagule pressure, native diversity and local assembly processes in mediating invasion outcomes

Invasion of native communities by introduced species is known to depend on the introduced species' traits, biotic resistance and propagule pressure. Each of these factors has been studied individually, but the interactions between them need further attention. For example, propagule pressure is considered an important predictor of invader success, but its effect is likely mediated by native composition and invader traits. Similarly, biotic resistance arises through the effects of local community assembly processes on native-exotic interactions, but is generally studied using native diversity alone. Here we examined interactions between propagule pressure and native community assembly processes in determining exotic species abundance, using spatially-explicit community assembly simulations. Unlike most previous studies on propagule pressure, we focused on long-term abundance of the focal introduced species, rather than the probability of establishment, as the former is more strongly associated with invasive species' impacts than the latter. We found that the abundance of a generalist invader in simulated communities varied widely with the characteristics of the native community, particularly the degree of native niche specialization. Similarly, the sensitivity of invader abundance to propagule pressure strongly depended on the community context. But despite the importance of native community characteristics in general, native richness in particular was an unreliable predictor of invasibility. The frequently-invoked negative diversity–invasibility relationship was only seen when the native community showed strict limiting similarity without character displacement. This likely explains why empirical evidence for the negative diversity–invasibility relationship is mixed, as previous community assembly research suggests that natural communities do not always show limiting similarity. Thus, these results provide a fuller and more nuanced understanding of the role of propagule pressure and native diversity in biological invasions.