Productivity gradients cause positive diversity-invasibility relationships in microbial communities

Models predict that community invasibility generally declines with species diversity, a prediction confirmed by small-scale experiments. Large-scale observations and experiments, however, find that diverse communities tend to be more heavily invaded than simple communities. One hypothesis states that large-scale environmental heterogeneity, which similarly influences native and invasive species, can cause a positive correlation between diversity and invasibility, overriding the local negative effects of diversity on invasibility. We tested this hypothesis using aquatic microbial communities consisting of protists and rotifers consuming bacteria and nanoflagellates. We constructed a productivity gradient to simulate large-scale environmental heterogeneity, started communities with the same number of species along this gradient, and subjected equilibrial communities to invasion by non-resident consumer species. Both invaders and most resident species increased their abundances with resource enrichment, resulting in a positive correlation between diversity and invasibility. Intraspecific interference competition within resident species and the positive effect of enrichment on the number of available resources probably accounted for the higher invasibility with enrichment. Our results provide direct experimental evidence that environmental heterogeneity in productivity can cause a positive diversity-invasibility relationship.