Florian Hartig

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Environmental heterogeneity predicts global species richness patterns better than area

Kristy Udy, Matthias Fritsch, Katrin M. Meyer, Ingo Grass, Sebastian Hanß, Florian Hartig, Thomas Kneib, Holger Kreft, Collins B. Kukunda, Guy Pe'er, Hannah Reininghaus, Britta Tietjen, Teja Tscharntke, Clara-Sophie van Waveren & Kerstin Wiegand

Global Ecology and Biogeography, 30(4), 842–851 (2021)
Cite this
@article{udy2021environmental,
  author = {Kristy Udy and Matthias Fritsch and Katrin M. Meyer and Ingo Grass and Sebastian Hanß and Florian Hartig and Thomas Kneib and Holger Kreft and Collins B. Kukunda and Guy Pe'er and Hannah Reininghaus and Britta Tietjen and Teja Tscharntke and Clara-Sophie van Waveren and Kerstin Wiegand},
  title = {Environmental heterogeneity predicts global species richness patterns better than area},
  journal = {Global Ecology and Biogeography},
  volume = {30},
  number = {4},
  pages = {842--851},
  year = {2021},
  doi = {10.1111/geb.13261},
}

DOI: 10.1111/geb.13261
Citation statistics: 101 (Google Scholar) · 86 (OpenAlex), as of 29 October 2026

Google Scholar citations per year, as of 22 October 2026 (yearly counts are fetched separately from the total)

Funding

This study was supported through funding by Deutsche Forschungsgemeinschaft (DFG), Research Training Group 1644 'Scaling problems in statistics' at the University of Göttingen.

View article (DOI)

Abstract

Aim: It is widely accepted that biodiversity is influenced by both niche‐related and spatial processes from local to global scales. Their relative importance, however, is still disputed, and empirical tests are surprisingly scarce at the global scale. Here, we compare the importance of area (as a proxy for pure spatial processes) and environmental heterogeneity (as a proxy for niche‐related processes) for predicting native mammal species richness world‐wide and within biogeographical regions. Location: Global. Time period: We analyse a spatial snapshot of richness data collated by the International Union for Conservation of Nature. Major taxa studied: All terrestrial mammal species, including possibly extinct species and species with uncertain presence. Methods: We applied a spreading dye algorithm to analyse how native mammal species richness changes with area and environmental heterogeneity. As measures for environmental heterogeneity, we used elevation ranges and precipitation ranges, which are well‐known correlates of species richness. Results: We found that environmental heterogeneity explained species richness relationships better than did area, suggesting that niche‐related processes are more prevalent than pure area effects at broad scales. Main conclusions: Our results imply that niche‐related processes are essential to understand broad‐scale species–area relationships and that habitat diversity is more important than area alone for the protection of global biodiversity.

What the paper shows and why it matters

(AI-generated)

Species richness rises with the area examined, and two kinds of process could explain this. Niche-related processes predict that a larger, more heterogeneous area offers more niches, so more species can coexist. Pure spatial processes need no niches: individuals placed at random, colonisation and extinction on islands, or larger populations that are less prone to chance extinction. Both ideas are plausible, and area is an excellent predictor of richness on its own. The difficulty is that environmental heterogeneity itself tends to increase with area, so species–area patterns, and analyses of one predictor alone, make the two difficult to separate. The paper therefore takes on their relative importance by analysing area and heterogeneity together, a combination that, to its authors’ knowledge, no earlier study has examined for the whole globe and within biogeographical regions.

The study finds that, for native terrestrial mammals worldwide (bats excluded), environmental heterogeneity explains richness better than area does. It used range maps for 4,954 species on a grid of cells of about 111 km × 111 km. A “spreading dye” algorithm grew random patches of 1 to 50 neighbouring cells, and the richness of the patches was regressed on their area and on two measures of heterogeneity, the range of elevation and the range of precipitation. Variance partitioning then separated how much of the variation each predictor explains alone and how much it shares with the others. Area alone explained less than the heterogeneity variables together for the globe and in every biogeographical region; globally, precipitation range alone accounted for 36.6% of the variation and area alone for 1.0%. Which variable, or combination of variables, mattered most differed between regions. The paper concludes that the increase of richness with area is primarily attributable to heterogeneity increasing with area, and only secondarily to area itself.

Figure 1. Maxima of elevation (in metres), precipitation (in millimetres) and number of mammalian species per grid cell across the globe. From Udy et al. (2021), Global Ecology and Biogeography, licensed under CC BY 4.0. Click the image to enlarge.

The paper reads the results as implying that niche-related processes are essential for understanding broad-scale species–area relationships, and that habitat diversity matters more than area alone for protecting global biodiversity. The paper notes limits: regional species-pool size, isolation, fragmentation history and human influence were not accounted for, and other predictors may matter. Malanson et al. (2023) cite the study as evidence that heterogeneity outweighs area, and test in simulations of alpine plants how that balance varies with extent.

References

  • Malanson et al. (2023). Area, environmental heterogeneity, scale and the conservation of alpine diversity. Journal of Biogeography 50, 743–754. doi:10.1111/jbi.14573