Top predators still prowled the seas after the biggest mass extinction

The worst known mass extinction wiped out over 80 per cent of marine species. But despite these huge losses, many ecosystems did not collapse, with a variety of animals and even top predators managing to survive the cataclysm.

The findings suggest that each ecosystem’s fate was determined, in part, by its own unique mix of species. The same may be true of modern marine ecosystems, which are also facing major threats from climate change.

The end-Permian extinction struck about 252 million years ago. It seems to have been caused by massive volcanic eruptions in what is now Siberia, which led to drastic global warming, low oxygen levels in the oceans and a host of other threats. Some animal groups, such as trilobites and eurypterids (sea scorpions) were entirely wiped out; others suffered huge losses. In the aftermath, many new groups arose, including dinosaurs and ichthyosaurs.

Given that so many species died out, researchers have assumed that ecosystems became much simpler in the wake of the extinctions. A fully functioning ecosystem has a range of species that depend on each other: plants that produce sugar using energy from sunlight, herbivores that eat the plants, predators that eat the herbivores, and possibly top predators that eat smaller predators. However, animals at higher “trophic levels”, like top predators, might be more vulnerable to extinction because they can’t survive without prey to eat. So a mass extinction like the end-Permian would remove trophic levels, leaving simpler ecosystems.

To find out if this really happened, Baran Karapunar at the University of Leeds in the UK and his colleagues studied the preserved remains of seven marine ecosystems from around the world, from just before and just after the extinction. Based on the species that were present, they inferred the structure of each ecosystem. Karapunar declined to be interviewed because the study is not yet peer-reviewed.

Despite species losses of up to 96 per cent, five of the seven ecosystems retained at least four trophic levels throughout.

In most regions and especially towards the poles, the worst losses were among herbivores, which were often slow-moving and lived on the seabed. In contrast, organisms that could freely swim in open water, such as fish, were less affected.

In the aftermath, ecosystems recovered differently depending on how close they were to the equator. Tropical ecosystems were dominated by low-trophic-level animals such as herbivores, often living on the seabed. In contrast, ecosystems closer to the poles acquired additional trophic levels as predatory animals like fish moved away from the equator to escape the worst of the heat.

The findings suggest that today’s marine ecosystems will also respond in diverse ways to climate change and other hazards caused by human activities.

“I am not aware of any other study that’s pulled so many regions together,” says Peter Roopnarine at the California Academy of Sciences in San Francisco. He agrees with the finding that many ecosystems did retain their trophic levels despite the extinctions, something smaller-scale studies had already suggested.

However, Roopnarine says we cannot place too much faith in the specifics of the researchers’ ecosystem models. For instance, they had to lump all the photosynthesising organisms together, because the fossil record does not reveal which ones survived and which didn’t – so they couldn’t simulate the consequences of such organisms going extinct. “They are ground-truthed by the fossil record, but the fossil record is incomplete,” he says.

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