In a mass extinction, why only the smallest survive

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When times are good, it pays to be the big fish in the sea. In the aftermath of disaster, however, smaller is better.

According to new research a mass extinction 359 million years ago known as the Hangenberg event triggered a drastic and lasting transformation of Earth’s vertebrate community.

Beforehand, large creatures were the norm, but, for at least 40 million years following the die-off, the oceans were dominated by markedly smaller fish.

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Ancient Mass Extinction Led to Dominance of Tiny Fish. An imagined post-extinction scene, when the ocean was filled with tiny fish. Illustration: Bob Nicholls

“Rather than having this thriving ecosystem of large things, you may have one gigantic relict, but otherwise everything is the size of a sardine,” says Sallan, an assistant professor in Penn’s Department of Earth and Environmental Science.

The finding, which suggests that small, fast-reproducing fish possessed an evolutionary advantage over larger animals in the disturbed, post-extinction environment, may have implications for trends we see in modern species today, such as in fish populations, many of which are crashing due to overfishing.

Paleontologists and evolutionary biologists have long debated the reasons behind changes in animal body sizes. One of the main theories is known as Cope’s rule, which states that the body size of a particular group of species tends to increase over time because of the evolutionary advantages of being larger, which including avoiding predation and being better able to catch prey.

Other theories suggest that animals tend to be larger in the presence of increased oxygen, or in colder climates.

Still another idea, known as the Lilliput Effect, holds that after mass extinctions, there is a temporary trend toward small body size. But this theory has only been supported with a limited number of species and is highly debated. It’s named after a fictional island in the book “Gulliver’s Travels” that’s inhabited by tiny people.

Many scientists believe that we are on the brink of – if not in the midst of – a sixth mass extinction. This summer, scientists released a report indicating that humans are chiefly to blame for the mass extinction that is already underway.

But these same scientists say that aggressive conservation efforts may yet stave off a true mass extinction. Humpback whales, for example, were recently recommended for removal from the endangered species list.

“This will require rapid, greatly intensified efforts to conserve already threatened species and to alleviate pressures on their populations – notably habitat loss, overexploitation for economic gain, and climate change,” wrote the research team, including scientists from Stanford, Princeton, and Berkeley, in their report.

If the present extinction does eliminate the planet’s largest animals, the new study suggests they will not be replaced any time soon.

“It doesn’t matter what is eliminating the large fish or what is making ecosystems unstable,” Sallan said. “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.”

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