The process of invasion, whereby individuals establish into an existing community, is central to community assembly and has important pragmatic implications. As such, understanding the controls over invasion success is a central goal in ecology, but these controls remain unresolved. Broadly, invasion success is determined by the interplay of competition with neighbors (limiting similarity) and suitability to the local environment (environmental filtering). Functional trait-based approaches are a promising avenue for understanding the relative importance of these two processes. Species that are functionally more dissimilar from the local community should more readily invade a locality due to lower niche overlap and weaker competitive interactions with the resident community members. Alternatively, invaders may be the most successful when their traits match those of the community they are invading because they are suited to local environmental conditions. This sets up two hypotheses: successful invaders may either match (environmental filtering) or differ (liming similarity) from resident community in their traits. Moreover, invasion success may also be dependent on the local environment where certain environments favor invasion broadly, by increasing rates of establishment, independent of the resident community. We tested these ideas by experimentally invading 10 restored tallgrass prairies that differed in their environmental conditions and resident species composition. We invaded each site with 10 prairie species that were never seeded into these sites and do not occur at these sites or in the surrounding landscape. We focused on two main questions. (1) Does functional trait similarity between invader and the standing community predict invasion success? (2) What role do environmental conditions play in determining invasion success?
We found support for the environmental filtering hypothesis, but not the limiting similarity hypothesis. Species that were more functionally distinct from a resident community were less likely to successfully invade that community (r=-0.32, p<0.001). We found no effect of environmental conditions on invasion. Species invaded equally well regardless of sand content, organic matter, phosphorous, or whether or not the site was burned prior to the invasion (p>0.05). This suggests a greater role of trait matching to local conditions, relative to competitive interactions, for invasion during community assembly across large spatial scales. This work has important pragmatic implications from understanding the spread of invasive species and for inter-seeding to increase diversity during restoration.