A neutral-metabolic theory of latitudinal biodiversity

Latitudinal gradients of species richness represent Earth's first-order biodiversity pattern. Most species groups display a near-monotonic decline in richness from the equator to the poles, yet there exists little mechanistic theory to derive such patterns from first principles. Here we integrate two key advances – neutral theory and the metabolic theory of ecology – to reconstruct global species richness gradients and test underlying causes.

Location
Simulated global meta-community.

Methods
We constructed a spatially explicit global meta-community with constant per capita rates of disturbance, speciation and dispersal. No gradient emerged in this neutral base model. Focusing on the oceans as a model system, we added a water temperature gradient of 0–30°C that independently affected rates of community turnover and speciation based on established metabolic scaling laws. We also added a gradient in habitat area that roughly parallels the observed decrease in ocean area from tropical to polar waters.

Results
Thermal effects on the community turnover rate caused a transient latitudinal gradient that ultimately disappeared. Thermal effects on speciation produced a dynamically stable but relatively weak gradient. Increasing habitat area towards the equator in combination with thermal effects on speciation rate produced a more realistic gradient that emerged from the combined effects of species–area and species–energy theory.

Main conclusions
This reasonably simple model provides a platform to explore support for processes underpinning large-scale biodiversity gradients, and the ability of prominent ecological theories, independently or in combination, to capture them.