Experiments reveal that environmental heterogeneity increases species richness, but they are rarely designed to detect the underlying mechanisms.

Environmental heterogeneity (EH) plays a central role in hypotheses used to explain distributions of species diversity. Here, we conducted a meta-analysis of species richness-EH (S-EH) relationships reported for experimental or quasi-experimental studies. We controlled for the lack of independence among effect sizes and additionally performed a cumulative meta-analysis. We found that EH has a consistent positive effect on species richness. This pattern was consistent across spatial scales, methodological issues and among taxonomic groups and realms. Furthermore, our cumulative meta-analysis highlighted that a positive S-EH relationship has been evident for nearly two decades. However, experiments often did not deeply explore the mechanisms underlying the positive relationship between EH and species richness. Taken together, our results present a robust pattern for the S-EH relationship, but our understanding of the main mechanisms is still in its infancy.