What do We Mean by Environmental Heterogeneity? Toward A Mechanistically Explicit and Scale-Dependent Framework

Environmental heterogeneity (EH) is widely recognized as a key driver of biodiversity, yet its conceptual foundations remain insufficiently structured. Here we present a semi-systematic narrative synthesis of 360 studies retrieved from Web of Science and Scopus, spanning 2000 to 2025, to evaluate how EH has been defined, quantified, and analyzed across taxa, regions, and biological disciplines. Only 10.0% of studies (n = 36 of 360) provided an explicit definition of EH, and definitions were predominantly descriptive, lacking specification of scale, structural properties, or underlying mechanisms. Most studies (91.9%; n = 331 of 360) reported significant EH effects across taxa, regions, and disciplines. Building on our synthesis, we identify four operational interpretations of EH: environmental variability, habitat diversity, environmental differentiation, and structural heterogeneity, each linked to distinct quantification approaches and biological mechanisms. We propose an operational definition of EH as spatial and/or temporal variation in abiotic and/or biotic environmental conditions, across one or multiple scales, that influences resource distributions, environmental filtering, niche differentiation, demographic dynamics, or adaptive processes. Misalignment among EH definitions, measurement strategies, and hypothesized mechanisms emerges as the primary source of inconsistency in the literature. We provide a Research Roadmap linking EH metrics and spatiotemporal scales to detectable biological mechanisms, advancing EH from a descriptive concept toward a predictive, mechanistically explicit framework in biodiversity science, and formulate five broad mechanistic hypotheses linking environmental heterogeneity to biological responses across scales.