Integrating hypotheses of invasion ecology into a single theoretical framework

Background/Question/Methods

To understand how and why alien species can dominate and change the communities and ecosystems they invade, ecologists need to first understand the processes that enable and drive successful invasion. Invasion ecology includes myriad hypotheses. Evidence suggests that most of these can explain the success of some invaders to some degree in some circumstances, but what is the relative importance of these hypotheses and the invasion mechanisms they invoke?

We review and synthesize 29 leading hypotheses in plant invasion ecology. Structured around propagule pressure (P), abiotic characteristics (A) and biotic characteristics (B), with the additional influence of humans on P, A and B (hereon PAB), we show how these hypotheses fit into one paradigm. P is based on the size and frequency of introductions, A incorporates ecosystem invasibility based on physical conditions, and B includes the characteristics of invading species (invasiveness), the recipient community and their interactions. Having justified the PAB framework, we propose some ways in which invasion research could progress.

Results/Conclusions

By highlighting the common ground among hypotheses, we show that invasion ecology is encumbered by theoretical redundancy that can be removed through integration. Through a range of approaches, the PAB framework can be used to quantify the relative importance of underlying invasion mechanisms at different spatial and temporal scales. If the prime aim is to identify the main cause of invasion success, we contend that a top-down approach that initially focuses on PAB maximises research efficiency; it identifies the most influential factors first and narrows the number of potential causal mechanisms. An overarching framework helps to organize research and foster links among subfields of invasion ecology and ecology more generally. By viewing invasion as a multifaceted process that can be partitioned into major drivers and broken down into a series of sequential steps, invasion theory can be rigorously tested, understanding improved, and effective weed management techniques identified.