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Explanations for latitudinal diversity gradients must invoke rate variation.

¹Department of Earth Sciences, University of Oxford, Oxford OX1 3AN, United Kingdom.

Proceedings of the National Academy of Sciences of the United States of America

|August 3, 2023

Summary

The latitudinal diversity gradient (LDG) shows more species near the equator. Explaining this requires understanding varying speciation, extinction, and dispersal rates across regions and time.

Keywords:

age and area hypothesis biodiversity gradient climate change tropics as older tropics as stable

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Area of Science:

Ecology
Macroecology
Biogeography

Background:

The latitudinal diversity gradient (LDG) is a fundamental pattern in macroecology, describing increased species richness from polar to equatorial regions.
Despite being recognized for over two centuries, the underlying mechanisms driving the LDG remain a subject of active debate.
Existing hypotheses often implicitly assume differential rates of evolutionary and ecological processes without explicitly focusing on them.

Purpose of the Study:

To argue that any comprehensive explanation for the latitudinal diversity gradient must incorporate differential rates of speciation, extinction, extirpation, and dispersal.
To highlight the necessity of understanding regional variations in these rates throughout Earth's history.
To emphasize that clarifying rate variations is crucial for identifying the ultimate drivers of the LDG.

Main Methods:

Conceptual synthesis and theoretical argumentation.
Critique of existing hypotheses for the latitudinal diversity gradient.
Identification of core processes (speciation, extinction, dispersal) as essential explanatory components.

Main Results:

Proposed that differential rates of speciation, extinction, extirpation, and dispersal are indispensable components of any valid LDG explanation.
Argued that hypotheses like 'age and area' or 'time for diversification' presuppose rate variation rather than explaining it.
Identified significant uncertainty regarding the historical and regional variation of these core ecological and evolutionary rates.

Conclusions:

Understanding the latitudinal diversity gradient necessitates a focus on the varying rates of speciation, extinction, and dispersal across geographic regions and geological time.
Further research is required to constrain the historical variation of these rates.
Clarifying rate variations will illuminate the abiotic and biotic factors that drive them, leading to a clearer understanding of the LDG.