Florian Hartig

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A Minimal Model for the Latitudinal Diversity Gradient Suggests a Dominant Role for Ecological Limits

Rampal S. Etienne, Juliano Sarmento Cabral, Oskar Hagen, Florian Hartig, Allen H. Hurlbert, Loïc Pellissier, Mikael Pontarp & David Storch

The American Naturalist, 194(5), E122–E133 (2019)
Cite this
@article{etienne2019minimal,
  author = {Rampal S. Etienne and Juliano Sarmento Cabral and Oskar Hagen and Florian Hartig and Allen H. Hurlbert and Loïc Pellissier and Mikael Pontarp and David Storch},
  title = {A Minimal Model for the Latitudinal Diversity Gradient Suggests a Dominant Role for Ecological Limits},
  journal = {The American Naturalist},
  volume = {194},
  number = {5},
  pages = {E122--E133},
  year = {2019},
  doi = {10.1086/705243},
}

DOI: 10.1086/705243
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Funding

This study was supported through funding by sDiv, the Synthesis Centre of iDiv (DFG FZT 118); Netherlands Organization for Scientific Research (VICI grant); Czech Science Foundation (16-26369S).

View article (DOI)

Abstract

The latitudinal diversity gradient (LDG) is one of Earth’s most iconic biodiversity patterns and still one of the most debated. Explanations for the LDG are often categorized into three broad pathways in which the diversity gradient is created by (1) differential diversification rates, (2) differential carrying capacities (ecological limits), or (3) differential time to accumulate species across latitude. Support for these pathways has, however, been mostly verbally expressed. Here, we present a minimal model to clarify the essential assumptions of the three pathways and explore the sensitivity of diversity dynamics to these pathways. We find that an LDG arises most easily from a gradient in ecological limits compared with a gradient in the time for species accumulation or diversification rate in most modeled scenarios. Differential diversification rates create a stronger LDG than ecological limits only when speciation and dispersal rates are low, but then the predicted LDG seems weaker than the observed LDG. Moreover, range dynamics may reduce an LDG created by a gradient in diversification rates or time for species accumulation, but they cannot reduce an LDG induced by differential ecological limits. We conclude that our simple model provides a null prediction for the effectiveness of the three LDG pathways and can thus aid discussions about the causal mechanisms underlying the LDG or motivate more complex models to confirm or falsify our findings.

What the paper shows and why it matters

(AI-generated)

Explanations of the latitudinal diversity gradient (LDG), the rise in species richness from the poles towards the tropics, are often grouped into three broad pathways. The tropics may gain species faster (through higher speciation or lower extinction), may support more species in total (higher “ecological limits”, a ceiling on how many species can coexist), or may have had more time to accumulate species. These ideas have mostly been argued in words. According to the paper, simulation models show that each pathway can produce a gradient, and one study explored how several pathways act together (Pontarp and Wiens 2017), but such models require a multitude of modelling decisions. No consensus on the dominant process has been reached. How effective each pathway is at creating a gradient when taken on its own was an open question, and it is the one the paper addresses.

The paper argues that, in a deliberately minimal model of the latitudinal diversity gradient, the gradient arises most easily from a gradient in ecological limits. The model has only two zones, tropical and temperate, with speciation, extinction, dispersal between the zones and a limit on diversity in each. Each pathway is represented by a gradient in one factor, and the resulting richness differences are compared. Over a much wider range of conditions than for the other pathways, the gradient in limits produces the largest difference. A gradient in diversification rates produces a stronger one only when speciation and dispersal rates are low; the paper reads this as a system still far from equilibrium, meaning richness has not yet levelled off, where the predicted gradient seems weaker than the observed one. Range dynamics, meaning species shifting or spreading between zones, can reduce a gradient created by diversification rates or by time to accumulate species, but not one created by limits.

The paper treats this as suggestive, not as proof. Together with general parsimony arguments, it shifts the evidence towards ecological limits playing an important role, possibly together with other mechanisms. The paper offers the model as a null prediction, a reference against which more complex models can test and challenge these findings.

References

  • Pontarp & Wiens (2017). The origin of species richness patterns along environmental gradients: uniting explanations based on time, diversification rate and carrying capacity. Journal of Biogeography 44, 722–735. doi:10.1111/jbi.12896