Interactions among plants can have powerful impacts on species performance, community structure, and ecosystem properties. Some of the most dramatic examples of these ecologically important interactions involve invasive species. For example, invasive species can change from relatively poor performers with low abundance in their native communities to striking dominants that competitively exclude other species in their non-native ranges. This dramatic biogeographic shift in the way invasive species interact suggests there is much to learn about how evolutionary history and coevolved traits can affect how plants interact, and in turn how these interactions affect ecological processes. The last 30 years of invasive species research has led to new theories on plant competition and a better understanding of mechanisms of plant competition, such as allelopathy (biochemically mediated interactions). Here, we explore how invasive species research has furthered our understanding of the fundamental ways that plants interact and how future research can contribute to advances in plant theory.
In general, we find that invasive species are stronger competitors against species in new ranges when compared to their native range. In contrast to native plant communities which can maintain high levels of diversity and coexistence via complex suites of interactions, invasive species appear to form mono-dominant stands through competitive exclusion. Some species, such as in the genus Centaurea, use chemically mediated interference competition to dominate in their new ranges; a mechanism that can exhibit a strong biogeographic pattern in effect. Future research needs to progress from the lab and greenhouse to complex experimental communities in order to link mechanisms, such as allelopathy, to interaction outcomes that drive community patterns and processes.