Evolution’s Death Row: Groups surviving mass extinction still go bust

Sets of species may persist through major extinction events only to die off in the aftermath, new research suggests.

Paleontologists recognize five cataclysmic episodes in Earth’s history, times when 50 to 95 percent of existing species abruptly vanished. Scientists have long studied the causes and casualties of these mass-extinction events (SN: 2/24/01, p. 116: Extinctions Tied to Impact from Space). Recently, focus has shifted to survivors, but only those that went on to “fame and fortune,” not those that later failed, says paleontologist David Jablonski of the University of Chicago.

While studying the aftermath of the Cretaceous-Tertiary, or K-T, extinction, which wiped out the dinosaurs 65 million years ago, Jablonski noticed that many surviving lineages of plants and animals lingered a paltry few million years and then petered out. Intrigued by this anecdotal evidence, Jablonski set out to test the hypothesis that a disproportionate number of survivors wind up dead in the recovery phase of a mass extinction. His results, presented in the June 11 Proceedings of the National Academy of Sciences, may have implications for the current biodiversity crisis.

Using a database of marine fossils, Jablonski considered two types of groups of related organisms, or clades–in this case, genera and orders. Genera are the smallest basic groups of related species; higher up on the taxonomic ladder, orders encompass hundreds of genera. Jablonski compared percent extinctions of these groups in the geologic stages immediately before and after each mass extinction. Geologic stages last 5 to 10 million years, the average life span of a single species.

Jablonski found elevated genera-extinction rates in four of the five postextinction stages and elevated ordinal losses in three. He named his doomed lineages “Dead Clade Walking,” alluding to the film Dead Man Walking, about a death-row inmate.

Groups that survive a mass extinction with few intact species could fizzle out just by chance, and Jablonski wondered if this could account for the belated die-offs. But he found that successful survivors were no less squeezed by the mass-extinction bottleneck than their condemned counterparts were. Alternative explanations of the phenomenon include biological competition or environmental stresses, such as changing climates and sea levels or colliding continents.

“This is a paradigm-changing paper,” says paleontologist Mary L. Droser of the University of California, Riverside. Scientists can no longer assume that a species that safely crosses an extinction boundary has made it, she says.

Not everyone is convinced by Jablonski’s data, however. “I think the results are likely to be entirely a statistical artifact and can be dismissed,” says paleontologist John Alroy of the University of California, Santa Barbara. Extinction rates vary widely between geological stages, he notes.

Jablonski maintains that the study is a first step, intended to stimulate further research. His work certainly piqued the interest of conservation biologists, such as Stuart L. Pimm of Columbia University. “I think it’s really exciting. Nobody’s looked at this in any kind of quantitative way before,” Pimm says. Unfortunately, the results are “more bad news” for the plight of endangered species, Pimm adds. “It tells us the estimates we have of how many extinctions are going to happen are probably underestimates.” Jablonski concurs: “It’s very sobering.”

From the Nature Index

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