Classic Hypotheses of Area, Time, and Climatic Stability Fall Short in Explaining High Tropical Species Richness

Aim

Tropical biodiversity overshadows the number of species inhabiting other regions. Age, area, and stability constitute three classical ideas used to explain the higher richness in these warm and humid zones. In this study, we measured the global dynamics of tropical, arid, temperate, cold, and polar climate zones over the last 5 million years (Ma). We aimed to evaluate whether the age, area, and stability of these climate zones contribute to explain the observed differences in species richness.

Location

Global land.

Taxa

Amphibians, birds, and mammals.

Methods

We classified the paleoclimatic layers generated by the PALEO-PGEM climatic emulator—temperature and precipitation for the last 5 Ma at 1000-year intervals—into the main Köppen-Geiger climate zones: tropical, arid, temperate, cold, and polar. We then calculated three variables: age, area, and stability. Age represents the duration that each map cell has remained within its current climate zone since its last change (map cell-based measure). Area quantifies the total extent of each climate zone over time by summing all map cells corresponding to that climate zone (climate zone-based measure). Stability indicates the number of times a given map cell changed between climate zones over time (map cell-based measure). We implemented regression and correlation tests, Structural Equation Models, and decision trees to measure the relationship between these estimates and current global patterns of amphibian, bird, and mammal richness.

Results

Our results indicate that age, area, and stability do not account for the observed differences in species richness among the 5 climate zones.

Main Conclusions

None of these classical hypotheses alone can explain the high vertebrate tropical richness observed. Further investigation, incorporating additional taxa (e.g. invertebrates or plants), or integrating new perspectives (such as the influence of local variations in diversification processes) will provide a more comprehensive understanding of the factors shaping large-scale biodiversity patterns.