Modern coexistence theory as a framework for invasion ecology

In the 60-year development of invasion ecology, many hypotheses have been proposed to explain the mechanisms behind biological invasion. However, it remains difficult to integrate these hypotheses into a unified framework. In particular, whether exotics ecologically differ from natives, and how ecological differences between exotic and native species could determine invasion outcome, have been hotly debated. By categorizing exotic-native ecological differences into niche differences and fitness differences, modern coexistence theory provides a framework to place different invasion hypotheses and mechanisms into a common context. This framework emphasizes that invasion success depends on either a niche difference between exotics and natives, or that exotics have fitness advantage over natives. By reviewing the leading invasion hypotheses, we found that most invasion hypotheses can be incorporated into this framework, as they considered different aspects of exotic-native niche and fitness differences. This framework can well explain why exotic-native phylogenetic distance and trait difference have complex influences on invasion, and therefore may help to reconcile the long-standing Darwin’s naturalization conundrum and the debate regarding the value of native versus exotic trait comparisons. Together, this framework provides a new opportunity to better understand the mechanisms of ecological invasion.