American Society of Naturalists

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“A minimal model for the latitudinal diversity gradient suggests a dominant role for ecological limits”

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Rampal S. Etienne, Juliano Sarmento Cabral, Oskar Hagen, Florian Hartig, Allen H. Hurlbert, Loïc Pellissier, Mikael Pontarp, and David Storch (Nov 2019)

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A model for the latitudinal diversity gradient predicts that it is most affected by a gradient in ecological limits

Why are there so many species in the tropics?

Regions of maximum climatic stability.<br />(After figure 1 of <a href="https://www.journals.uchicago.edu/doi/abs/10.1086/282179">Klopfer&nbsp;&&nbsp;MacArthur&nbsp;(1961)</a>, ©&nbsp;The&nbsp;University of Chicago Press)
Regions of maximum climatic stability.
(After figure 1 of Klopfer & MacArthur (1961), © The University of Chicago Press)

Why are there more species in the tropics than at higher latitudes? This question has been boggling the minds of biologists for decades. Is it because the formation of species in the tropics is higher, or species extinction is lower? Or is it because species have had more time to accumulate in the tropics? Or because the tropics can host more species than temperate areas? Each of these explanations is feasible, but which explanation creates the strongest gradient in species richness across latitudes? Eight researchers from many different countries got together at the Centre of Integrative Biodiversity Research in Leipzig several times over the past few years to discuss this question, and they built a mathematical model to compare the effect of all the possible explanations on the distribution of species across different latitudes. They found that the last answer, the tropics can host more species, creates the strongest patterns. They made their model available online for scientists and other interested people to explore: https://seldig.shinyapps.io/RBM-ODE/.


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 than a gradient in the time for species accumulation or diversification rate under 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.