Resolving the context-dependency of local heterogeneity-diversity relationships across rocky reefs worldwide.

Environmental heterogeneity is widely thought to promote biodiversity, yet its variable effects limit its predictive power. This variability can be better understood by considering key mediating factors: different forms of local heterogeneity, organismal groups and their associated traits, and the broader environmental context (e.g. latitude). To address these factors, we analysed 144 studies (24 412 data points and 2815 effects) from rocky reefs worldwide. Heterogeneity was defined as spatial variability, and heterogeneity metrics were grouped into facets such as three-dimensional (3D) structure (e.g. substrate rugosity), complexity (e.g. fractal dimension) and feature variability (e.g. mussel size classes). All facets promoted biodiversity, but effects were context-dependent, with 3D structures having the strongest impact, likely owing to increased niche provision and substrate area. Responses also varied across organismal groups, with small-bodied and mobile species benefiting the most. Additionally, heterogeneity reduced grazing and enhanced recruitment, helping promote biodiversity. Effects were strongest on biogenic substrates, at lower latitudes and in more stressful intertidal zones. Overall, the influence of local heterogeneity depends on how it is generated and measured, organisms' traits, and prevailing environmental conditions. These insights inform the development of a conceptual model predicting heterogeneity's context-dependent effects on biodiversity across systems.