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When It Comes to Surviving Mass Extinctions, Smaller Is Better

Extinction events literally have bigger fish to fry.

by Becky Ferreira
Nov 15 2015, 9:47pm

Concept drawing of period following Devonian extinction. Image: Bob Nicholls

Over the last 500 million years, life on Earth has been hit with five mass extinction events. A sixth, human-driven die-off is currently in the works. While each of these upheavals is unique in its own apocalyptic way, there is increasing evidence that common patterns tend to arise in the evolutionary aftermath.

To that point, researchers, led by University of Pennsylvania paleobiologist Lauren Sallan, have presented evidence that smaller animals have an advantage in newly razed ecosystems.

Sallan and her colleagues compared the body sizes of 1,120 fish fossils that lived before and after the late Devonian extinction event, a glacial period that wiped out about 96 percent of vertebrate life some 359 million years ago.

The team found that the late Devonian waters were filled with enormous sea monsters, such as the ten-meter-long Dunkleosteus terrelli, as well as plentiful smaller species. In the wake of the extinction event, however, fish measuring under one meter in length exploded in biodiversity, while larger vertebrates either shrank in size, or, in most cases, unceremoniously died off.

"[V]ertebrates experienced persistent reductions in body size for at least 36 million years [after the Devonian extinction]," the team said in the new research, published in the journal Science. "Large-bodied, slow-breeding survivors failed to diversify, facing extinction despite earlier evolutionary success."

Larger animals are simply not able to outbreed their smaller counterparts, with makes them less adaptable to the punishing conditions imposed by mass extinctions

This evolutionary trend is known as the Lilliput effect, and it has been observed in the ashes of many other global ecological collapses. The extinction of the dinosaurs, for instance, ushered in the age of birds and mammals, clades that emerged from very small creatures that lived in the shadows of the tyrannosaurs or titanosaurs. Many of these birds and mammals evolved into enormous creatures that were, in turn, edged out by the recent Ice Age megafauna extinction which killed off mammoths, cave lions, monster birds, and dire wolves some 10,000 years ago.

Even today, the ongoing ecological collapse of Earth's oceans is placing immense pressures on larger vertebrates like whales and sharks, while favoring smaller vertebrates and invertebrates like jellyfish. As with the late Devonian period, larger animals are simply not able to outbreed their smaller counterparts, with makes them less adaptable to the punishing conditions imposed by mass extinctions.

Sallan noted that once some measure of ecological equilibrium is reached, many species tend to invest their energies in growing larger once more, a phenomenon known as Cope's law. But this process can take tens of millions of years, which is a timescale that humans have certainly not taken into account when projecting the ecological losses of Earth's larger creatures.

"It doesn't matter what is eliminating the large fish or what is making ecosystems unstable," Sallan said in a statement. "These disturbances are shifting natural selection so that smaller, faster-reproducing fish are more likely to keep going, and it could take a really long time to get those bigger fish back in any sizable way."

This is bad news for humans, no matter how you slice it. If we continue to lose these larger vertebrates—the lynchpins of their ecosystems—we can expect to inherit a much more unstable foodweb. We're talking "gross cockroach paste in Snowpiercer" unstable. Sallan's team has provided yet another example of how studying the extinction events of the past can, hopefully, help us mitigate the one we are currently entering.