Intraspecific diversity poorly predicts invasibility and invasiveness: a meta-analysis of experimental invasions

Identifying the biotic factors underlying invasion success is critical to understand the dynamics of biological invasions; and while some studies suggest that intraspecific diversity may play an important role, there has not been a systematic evaluation of the evidence. We performed a meta-analysis on 34 experimental studies and 106 effect sizes to test two hypotheses: (1) higher intraspecific diversity in a resident population decreases the likelihood of it being successfully invaded by other species, and (2) higher intraspecific diversity in an invading population increases its ability to invade another species or community. We found small effects in the direction of our hypotheses, but these were not significant and were influenced by taxon and study duration. The lack of significant findings are somewhat unexpected, given the well-understood mechanisms analogous to those offered for interspecific diversity. These include sampling effects and complementarity, in which diversity is expected to maximize resource use and increase density, thereby reducing niche availability and subsequent invasibility. Additionally, we found evidence of publication bias for studies of invader intraspecific diversity. We recommend that future research focus on a wider diversity of organisms, include longer term experiments, and measure genetic dissimilarity to better understand the role of intraspecific diversity.