LB 27-232 - Intraspecific diversity poorly predicts invasibility and invasiveness: a meta-analysis of experimental invasions

Biodiversity loss, including both species diversity and intraspecific diversity, is a major global problem affecting the livelihood of natural ecosystems and agricultural productivity. Research suggests that intraspecific diversity significantly impacts a range of ecological properties, including community structure and competition. However, a critical knowledge gap exists in understanding the role of intraspecific diversity on the invasibility and invasiveness of a species. Knowing this has implications for conservation efforts, including protecting communities more resistant to invasion, and for cultivating agricultural crops more resistant to weeds. So far, there have been limited efforts to systematically evaluate evidence linking intraspecific diversity to invasibility or invasiveness. To address this research gap, we conducted a systematic meta-analysis using Web of Science. Our search yielded 7,109 published results, from which we identified 37 relevant studies that manipulated genetic or phenotypic variation of resident or invader species and measured invader biomass. We hypothesized that (1) higher intraspecific diversity in a resident decreases the likelihood of it being successfully invaded by other species and (2) higher intraspecific diversity in an invader increases its ability to invade another species or community. We extracted 111 effect sizes, comparing populations with no intraspecific diversity to those with intraspecific diversity. We recorded additional potential mediators, including taxa, study setting, and experiment duration. Lastly, we calculated publication bias using Egger's test, which showed that there is potentially an underreporting of studies finding a negative effect of invader diversity on invasion success. This meta-analysis revealed (1) a small negative effect of variation in the resident (Hedges’ g = -0.14, se = 0.12, p = 0.22) and (2) a small positive effect of variation in the invader on invasion success, but these effects were not significant (Hedges’ g = 0.11, se = 0.07, p = 0.09). We saw larger effects in bacterial studies, which were also the studies that ran for more generations. The lack of significant findings are somewhat unexpected, given that intraspecific diversity affects many other ecological processes and given the well-understood mechanisms, such as sampling effects and complementary, in which diversity is expected to maximize resource use and increase density. However, our conclusions are somewhat limited by the number and diversity of systems studied. Therefore, we recommend that additional experiments be conducted, focusing on organisms other than plants that are conducted for a longer period, and include measures of genetic dissimilarity to better understand the role of intraspecific diversity.