Biodiversity and elevation gradients: Insights on sampling biases across worldwide mountains

Abstract

Identifying macroecological patterns and biases in species distribution is a challenging but essential task in biodiversity-oriented studies. Despite extensive attempts to find consistent species richness elevation (SRE) patterns, the topic remains controversial owing to widespread conflicting, idiosyncratic and non-generalizable underlying mechanisms. We used a meta-analytical review to answer why patterns of species-richness in elevation gradients remain elusive, a long-standing, central but contentious macroecological and biogeographical question. Location: Global elevation gradients. Taxon: Major terrestrial taxa (invertebrates, vertebrates and plants). Methods: We tested the effect of elevation on species richness using multilevel mixed-effects meta-analytical models. Data from 127 studies spawning almost one century of research were integrated to test the effect of elevation across distinct (1) SRE models, (2) quality of primary data (e.g. mountain sampling coverage), (3) biogeographic realms, (4) studied taxa and (5) organism mobility. Results: The linear negative pattern showed the strongest model fit followed by the hump-shaped and the linear positive models. Studies with higher sampling sizes showed a consistent decrease in the strength of SRE patterns. Further, the larger the mountain coverage and sampled range, the stronger the detection of some SRE patterns. Overall, the elevational effect on species richness was consistent across biogeographical realms, taxonomic groups and organism mobility. Main Conclusions: This study indicates a bias in the detection of SRE patterns, driven mostly by mountain comprehensiveness, namely the number of sampling units, sampled range and mountain sampling coverage. These results call attention to the evidence that undersampled elevation gradients may bias our understanding on the complex relationships between elevation and biodiversity, thus impairing a broad understanding on the ecology, evolution, biogeography and conservation of mountain biota.