Diverging Effects of Elevation on Species Richness of Freshwater Organisms: A Global Meta‐Analysis

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Abstract
A primary ecological challenge is to disentangle how abiotic factors affect species richness using measured environmental variables in addition to broad proxies such as elevation. Considering direct ecogeographical gradients, such as climate harshness, it is therefore essential to understand whether these underlying abiotic factors might consistently drive the distribution of different groups of organisms along elevational gradients. To address how elevation shapes species richness patterns, we performed a systematic review and a subsequent meta‐analysis to answer the following questions: (i) Does elevation influence species richness in global freshwater zooplankton and macrophyte communities? (ii) Can climatic harshness, such as temperature and precipitation variability, geography and different freshwater ecosystem types explain relationships between elevation and species richness? (iii) Are these ecogeographical patterns consistent for both organismal groups? Our meta‐analysis included 84 effect sizes spanning 48 countries and both hemispheres from −54.9 S to 84.7 N, with an elevational range up to 4200 m above sea level within six distinct freshwater ecosystems. We found that the relationship between elevation and species richness in zooplankton and macrophyte communities is context‐dependent. These responses varied among different taxonomic groups and broad habitat categories (i.e., flowing vs. standing waters). We found a general negative relationship between zooplankton species richness and elevation. Zooplankton species richness decreased along the elevational gradient, but this pattern was primarily associated with a concomitant decrease in cladoceran species numbers. The negative correlation between elevation and species richness was observed solely in flowing waters for macrophytes. Additionally, we found that negative elevation–richness relationships were more pronounced at high latitudes in the global north for both zooplankton and macrophyte communities. This research challenges the prevailing assumption that elevational diversity gradients are universal, with species richness decreasing with increasing elevation in freshwater ecosystems. We also emphasize that the conventional ecogeographical measures employed in terrestrial studies, such as air temperature, may be insufficient on their own to explain richness patterns in freshwater organisms. These findings underscore the necessity for future freshwater assessments to build on well‐established biogeographical traditions rooted in terrestrial systems while recognizing the unique features of inland waters.
Funding Information
  • Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (001)

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