A deep-time perspective on the latitudinal diversity gradient

Abstract

Today, species richness is highest in the tropics and declines toward the poles. Although there are exceptions, this pattern is pervasive within both the terrestrial and marine realm and across taxonomic groups (1). This latitudinal diversity gradient (LDG) was first recognized by Alexander von Humboldt over two centuries ago. Despite this, understanding the mechanisms that underlie the LDG remains one of the great challenges of biodiversity science (1). Whether this is hyperbolic or not, this gradient is the first-order macroecological pattern that needs to be explained if we are to understand the broader question of what determines the distribution of biodiversity. In turn, this is critical to determining which geographical regions of Earth’s biosphere are most vulnerable to the ongoing climate emergency. Dozens of hypotheses have been proposed to explain the LDG, pertaining to a broad suite of climatic, environmental, geographical, and historical variables (2), but the answer remains elusive, in part because the proposed drivers covary in space today. However, the distribution patterns we see among living organisms provide only a snapshot of life: Earth’s geological record presents a unique window into the past, during which time these variables fluctuated substantially. Critically, this record also reveals the response of species to these changes. In PNAS, Song et al. (3) evaluated the evolution of the LDG in the marine realm from 254 to 201 Ma. Notably, this time span includes the most devastating of all mass extinctions, at the Permian/Triassic boundary, 252 Ma. Although Song et al. (3) recovered an LDG that is similar to the present-day pattern for much of their study interval, the distribution of biodiversity in the 5 My after the mass extinction event was characterized by a flat gradient, with no tropical peak. Song et al. (3) attributed this pattern to higher extinction rates in the … [↵][1]1Email: philipdmannion{at}gmail.com. [1]: #xref-corresp-1-1

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