How does environment affect biodiversity? Did Humboldt get it right?

In the early nineteenth century, Alexander von Humboldt laid the foundations of a lot of ecology. One of his observations was that climate should help predict what plants you find in a location and the local biodiversity. Over two hundred years later, Jorge Antonio Gómez-Díaz and colleagues examined some of the places Humboldt had explored. In their paper, published in the open-access journal AoB PLANTS, they argue that while Humboldt was broadly correct, there are more factors to consider.

Alexander von Humboldt was a German naturalist and geologist who made important contributions to the study of elevational gradients through his journeys through the Neotropics. He proposed that climate regulates plant species composition and diversity, which has been the basis of numerous studies. Ecologists consider elevational gradients as natural laboratories to study environmental variables, as they contain many climatic variations in a small area. But what is it about elevation that matters? Botanists have proposed many hypotheses to explain species richness along elevational gradients, such as productivity, precipitation, and temperature.

Humboldt‘s influence on biogeography is still strong, especially in explaining species distribution due to its relationship with climate in elevational gradients, but is he out-of-date? Since his time, biologists have developed new ideas, including phylogenetics, that examines the diversity of species and the relationships between them. Furthermore, considering biodiversity facets beyond species richness, such as phylogenetic diversity, can help to reveal evolutionary and ecological processes determining elevational gradients. The current availability of phylogenetic information for large plant clades allows including such information to study species richness patterns. Finally, only a few studies have analyzed the impact of anthropogenic disturbances on biodiversity along elevational gradients, which is vital to consider during the current climate change and biodiversity crisis.

Gómez-Díaz and colleagues visited Cofre de Perote mountain, an extinct volcano of 4282 meters in elevation in Veracruz, Mexico. The study set up eight sites along a straight line of 81 kilometers, along an elevational gradient of 30 to 3500 meters, covering six vegetation types and a wide variety of climates, including warm-dry, temperate-humid, and cold-dry environments.

The botanists considered two gradients: the actual elevational gradient and a disturbance gradient with three habitats subjected to three different levels of forest-use intensity. The team set up five 20 m × 20 m plots at each level of forest-use intensity and sampled 120 plots in total. The authors obtained climatic variables, mean annual temperature, mean annual precipitation, and potential evapotranspiration from the MODIS/Terra Net Evapotranspiration 8-Day L4 Global 500 m Version 6. 

Species diversity was estimated as species richness, and the phylogenetic structure of species was obtained from a species-level mega-phylogeny of seed plants. The authors calculated the abundance-weighted net relatedness index (NRI) to measure the phylogenetic structure of each plot.

Gómez-Díaz and colleagues used a structural equation modelling approach to explicitly consider the direct and indirect effects of multiple environmental factors on species richness and NRI. They used a hypothetical pathway that considered climate (temperature and precipitation), forest-use intensity, potential evapotranspiration (PET), and its quadratic form PET². 

The team fitted piecewise structural equation models (SEM) separately for each diversity measure (species richness and NRI) and each plant life form. The models showed that while climate and forest-use intensity had direct effects on species richness and NRI, potential evapotranspiration had only a direct effect on species richness. Their results suggest that the impact of climate and forest-use intensity on the diversity of angiosperms is more or less direct. At the same time, the effect of potential evapotranspiration on species richness is more complex, with a direct and an indirect effect.

Overall, Gómez-Díaz and colleagues’ results support the hypothesis that climate strongly determines plant species richness along elevational gradients. Temperature had the strongest influence over species richness and phylogenetic structure, with higher temperatures associated with increased species richness and phylogenetic clustering. Precipitation and PET were also important for most plant life forms (except for shrubs and lianas), and anthropogenic disturbance only influenced trees. 

Gómez-Díaz and colleagues conclude:

Alexander von Humboldt noted that the types of habitats and the number of species varied predictably with changes in latitude and elevation. In his explorations of tropical mountains, Humboldt suggested that the climate is a crucial factor in determining plant species richness along elevational gradients. Honouring the work of Humboldt and continuing his legacy demands more research to understand the causes behind elevational diversity gradients. Finally, it is necessary to integrate species richness as well as the phylogenetic structure and ‘deconstruct’ the pattern by looking at life forms to have a better insight into the processes shaping biodiversity along elevational gradients.

Gómez-Díaz et al. 2022

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Gómez-Díaz, J.A., Carvajal-Hernández, C.I., Bautista-Bello, A.P., Monge-González, M.L., Guzmán-Jacob, V., Kreft, H., Krömer, T. and Villalobos, F. (2023) “Humboldt’s legacy: explaining the influence of environmental factors on the taxonomic and phylogenetic diversity of angiosperms along a Neotropical elevational gradient,” AoB PLANTS, 15(1), p. lac056. Available at: https://doi.org/10.1093/aobpla/plac056.