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Mass Species Loss Stunts Evolution for Millions of Years

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Mass Species Loss Stunts Evolution for Millions of Years

When searching for causes of Earth’s mass extinctions, scientists instinctively turn to geophysical calamities: erupting volcanoes, methane bursts, asteroid strikes and other obvious dooms.

But in the most massive extinction of all, when most of everything that lived died out some 250 million years ago, a more subtle form of destruction has been suggested. Following an initial volcanic upset, the loss of life itself may have fueled further extinctions, then slowed life’s recovery.

That possibility, suggested by massive analyses of fossils and chemical signals left during the Permian-Triassic extinction event, represents more than an interesting wrinkle to a notable period in history. By this reading of the fossil record, biological diversity — something that’s now imperiled by human appetites — may be a sustaining, stabilizing force on planetary scales, and its disruption self-perpetuating.

“Earlier interpretations have looked at the Permian-Triassic extinction as purely the result of external physical processes,” said paleobiologist Jessica Whiteside of Brown University. “But low diversity itself can be an external feedback.”

Whiteside’s specialty is mass extinctions and their geophysical consequences, cycles of energy and nutrients that play out over millions of years.

'Normally when people think of extinctions, it's of single species. This is a systems approach.'

Her latest study, co-authored with University of Utah paleobiologist Randall Irmis and published Oct. 26 in Proceedings of the Royal Society B, follows on earlier findings that the taxonomic richness of ammonoids, a once-dominant class of marine invertebrates, rose and fell with in tandem with fluctuations in Earth’s carbon cycle for 10 million years after the Permian-Triassic extinction.

During that time, the carbon cycle — the flow of life’s essential element through all Earth’s systems — oscillated wildly, a period known as a “chaotic carbon interval.” And rather than rebounding and steadily filling suddenly open niches, as might be expected, life appears to have entered a boom-and-bust cycle. Species flourished and collapsed, over and over, a planet-level version of the jellyfish bloom-and-bust cycles now seen in overfished oceans.

One seemingly plausible explanation is ongoing Permian-Triassic volcanic activity, which could have decimated new species as they arose. However, carbon chaos continued for millions of years after volcanoes cooled. A newer explanation, favored by Whiteside, draws from the work of ecological theorists who say that, at planetary scales as well as local, complexity generates resilience.

Applied to mass extinctions, this idea is somewhat radical — but in a coral reef or rainforest, or even a computer network, it’s an accepted notion. Just as distributed systems are more secure than a handful of mainframes, ecosystems composed of many interlocking and sometimes redundant species are especially sturdy. Because they’re stable, they in turn nourish life’s diversification over evolutionary time. It’s a biological catch-22: A richness of life requires stability to develop, but stability requires a richness of life.

Mass Species Loss Stunts Evolution for Millions of Years

Species abundance (black bar) before, during and after the Permian-Triassic extinction event. Species "evenness," a measure of food web stability, is in blue. Fossil records are from Africa (above box) and Russia (below box), with carbon cycle measurements in the middle. Image: Whiteside & Irmis/Proceedings of the Royal Society B

Through this lens, “chaotic carbon intervals record the instability of the ecosystem itself,” wrote Whiteside and Irmis. It doesn’t simply reflect the lingering aftereffects of the Permian-Triassic extinction’s initial causes, but life’s new post-extinction rules.

Whiteside’s earlier study joined marine fossil records with carbon analyses taken from ancient seafloor sediments. The new study ventures onto land: Irmis painstakingly classified some 8,600 terrestrial vertebrate fossils collected in southern Africa and the Ural region of Russia, spanning the period from 260 to 240 million years ago. Carbon records came from several studies of Permian-Triassic sediments.

When Whiteside joined the terrestrial fossil record to carbon cycling, the same basic patterns emerged as she’d seen at sea. Over several million years, roughly 80 percent of all terrestrial animal species vanished. A few tough groups — so-called “disaster taxa,” such as a German shepherd-sized lizard called Lystrosaurus, pictured above — survived and prevailed in post-extinction ecosystems, which were dramatically simplified compared to what existed before.

Ecological chaos reigned, with slight natural perturbations of climate and circumstance repeatedly sending these simple new ecosystems into drastic decline. Another eight million years passed before species abundance and ecological richness recovered to pre-extinction levels. Cause and effect are difficult to establish, especially at the distance of geological time, but the patterns fit with evolutionary simulations that show biodiversity loss limiting post-mass extinction recovery in the early Triassic.

“We are very interested to see that this specimen-oriented study supports our conclusion,” said those simulations’ authors, paleobiologists Peter Roopnarine of the California Academy of Sciences and the Field Museum’s Kenneth Angielczyk, in a joint email. “It really is a pioneering approach.”

Whiteside and Irmis plan to conduct similar studies of other mass extinctions. If reduced biological diversity really did make it harder for life to rebound, then the most pressing questions aren’t historical, but immediate.

Scientists say that Earth may now be entering another period of mass extinction, with species dying at a pace seen only five times in life’s history, including the Permian-Triassic. Exactly how current extinction rates compare to those episodes is an open question, all the more pressing if modern extinctions represent not just the loss of a lineage but a constraint on evolution for the foreseeable future, if not millions of years to come.

“We’re showing that low-diversity systems take a long time to recover,” said Whiteside. “When you destroy links in the food web, effects exist that are difficult to see. Normally when people think of extinctions, it’s of single species. This is a systems approach.”

According to Roopnarine and Angielczyk, their analyses differ subtly from Whiteside’s in more precisely tracing post-extinction instabilities to losses of specific animal groups, especially large predators and herbivores. Those equilibrium-maintaining animals are the ones now dying off fastest. Species that survive are often so rare as to be, in ecological parlance, functionally extinct.

“My own personal feeling is that many modern communities are probably well on their way to the type of instability inferred for the Early Triassic,” said Roopnarine. “The more we understand of the workings of these systems, the better equipped we are to make good choices.”

Image: Artist’s rendering of Lystrosaurus, one of the “disaster taxa” to survive the Perminan period, as did the now-extinct spore-tree Pleuromeia, which flourished in the aftermath. (Victor Leshyk)

Citation: “Delayed recovery of non-marine tetrapods after the end-Permian mass extinction tracks global carbon cycle.” By Randall B. Irmis and Jessica H. Whiteside. Proceedings of the Royal Society B, Vol. 278 No. 1723, Oct. 26, 2011.

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