American Society of Naturalists

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Synthesis: “Shaping the latitudinal diversity gradient: New perspectives from a synthesis of paleobiology and biogeography”

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David Jablonski, Shan Huang, Kaustuv Roy, and James W. Valentine

The perfect storms that make or break biodiversity

Bivalve morphological diversity is much greater in the tropics than in the high latitudes. Some tropical species from Ernst Haeckel’s 1904 <i>Kunstformen der Natur</i> [<i>Art Forms in Nature</i>]
Bivalve morphological diversity is much greater in the tropics than in the high latitudes. Some tropical species from Ernst Haeckel’s 1904 Kunstformen der Natur [Art Forms in Nature]

The factors shaping the world’s latitudinal diversity gradient, the dramatic rise in diversity from poles to equator, have been debated for 150 years. A synthesis of both fossil and present-day data on a major component of the marine biota shows that the diversity gradient must involve mutually reinforcing causes – a perfect storm – rather than a single mechanism.

By analyzing the past and present distribution of species and evolutionary lineages of marine bivalves – scallops, mussels, cockles, and their kin, known for their exceptionally rich fossil record – the researchers find that the strong diversity trend of this group cannot be explained solely by the factors like climate or nutrients within each specific region, or by the movement of species into and out of those regions. Both effects, often cast as alternatives, must be operating.

A strong role for local effects is seen in the patterns of diversity loss over the past 3 million years, with once-rich fossil faunas declining in direct proportion to the size of the temperature drop experienced by each region as ice ages set in. On the other hand, the fossil record shows that bivalve lineages tend to start in the tropics and spread to higher latitudes while retaining their tropical presence, expanding across temperature lines and so contradicting an exclusive role for local conditions. With both the youngest and oldest lineages found in the tropics, this finding challenges another pair of supposed alternatives, often phrased as the question, are the tropics a cradle or museum of biodiversity?

Comparing diversity patterns among, rather than along, coastlines also shows that the expansion of lineages helps shape latitudinal diversity trends. In their global database of 62,000 records of living marine bivalve species, the researchers find that temperate southeast Australia and southeast Japan each contains more species than the entire tropical Caribbean region. This result was unexpected, because the Caribbean is 10 times larger, much warmer and less seasonal, and contains a much wider array of habitats including coral reefs. Poleward “spillover” of species from the massively diverse tropical west Pacific into the adjacent temperate zones evidently explains their remarkably high diversities, an out-of-the-tropics dynamic that overwhelms expectations drawn from a single coastline.

Taken together, the evidence shows that the global latitudinal gradient is shaped both by local effects and by the long-term expansions of geographic ranges – neither explanation alone is sufficient. Many of the biggest biological patterns today and in the history of life may be such “perfect storms,” where mutually reinforcing processes generate the most extreme outcomes. Other candidates for perfect storms include the “Coral Triangle” diversity peak in the today’s oceans, the Cambrian Explosion of multicellular life 530 million years ago, the greatest mass extinction in the history of life 250 million years ago, and the explosive diversifications of flowering plants and insects over the past 100 million years.

The research team cuts across academic generations: lead author David Jablonski, now at the University of Chicago, was a postdoctoral fellow with James Valentine, now at the University of California, Berkeley; Kaustuv Roy, now at the University of California, San Diego, was Jablonski’s PhD student and then a postdoc with Jablonski and Valentine; and Shan Huang, now at the Senckenberg Biodiversity & Climate Research Center in Frankfurt, Germany, was a postdoc with Jablonski, Valentine, and Roy. Read the Article