Recovery from last mass extinction

Marine fossils reveal that it took millions of years for ecosystems to recover from the extinction caused by the asteroid impact widely believed to have wiped out the dinosaurs.

In addition to killing the dinosaurs, the impact caused many other species to go extinct, including 90 percent of “calcifying plankton” – a type of phytoplankton that produce calcium carbonate shells, a team led by Sarah Alvarez of the University of Gibraltar reports in the journal Nature.

Such phytoplankton are important to the ocean food web but are also useful to palaeontologists because their shells leave easy-to-study fossils. 

Using 54 metres of core sample from the Shatsky Rise, 1,500 kilometres east of Japan, Alvarez’s team painstakingly teased out 700,000 plankton fossils from a 13-million-year time span, beginning shortly before the Cretaceous/Paleogene (K/Pg) extinction, as the dino-killing event is known to geologists.

This allowed them to obtain a “remarkable glimpse” into how a marine ecosystem “reboots” following a mass extinction. “We wanted to find out how long the ocean ecosystems took to recover and how this happened,” Alvarez says.

They also wanted to see how the recovery related to the restoration of the carbon cycle, in which phytoplankton grow in surface waters, then die and sink to the depths, carrying their stored carbon with them – a major part of the Earth’s carbon cycle in which they help remove excess carbon dioxide from the atmosphere.

One thing the scientists found was that while most species of phytoplankton went extinct, others rebounded rapidly. But for the first 1.8 million years, their numbers and types varied wildly. Those that were present were also unusually tiny.

At the same time, the amounts of carbon reaching the seafloor (measured by prior researchers) were low, meaning that the carbon cycle wasn’t functioning all that well, says Alvarez’s co-author, Andy Ridgwell, a climate scientist at the University of California, Riverside.

When the carbon cycle did recover, it corresponded to stabilisation of the ecosystem, as well as a shift to larger (and presumably healthier) cells. But interestingly, this occurred millions of years before plankton had recovered to anything close to their pre-extinction levels of biodiversity. 

The result is a mix of good news and bad news for those worried about an extinction caused by such human activities as habitat destruction and anthropogenic climate change.

The good news is that an ecosystem can lose a lot of diversity and still be functional, as was the case 1.8 million years after the K/Pg mass extinction.

From that point of view, Ridgwell says, “the climate trolls would be right in saying that high biodiversity doesn’t matter.”

But, he says, to function, an ecosystem has to have the species needed to do the job, and it’s not obvious which ones are lynchpins until it’s too late.

“You could look at biodiversity as a backup,” he says. “The key to preserving biodiversity is that the system could collapse if you remove the right [or wrong] piece, and if you don’t know the right piece you can’t risk losing things.”

And if a collapse does occur, recovery could take a long time – 1.8 million years in the case of the K/Pg extinction.

Not to mention that utilitarian values aren’t the only ones. “There’s also all those things about beauty,” Ridgwell says. “Everything you can’t quantify in terms of dollars or petagrams of carbon.”

Other scientists are impressed.

“Beyond the importance for understanding the K/Pg event, studies like this one I find to be sobering cautionary tales that warn how very long earth systems can take to recover from perturbations,” says Ken MacLeod of the University of Missouri, Columbia, Missouri.

“This paper represents an enormous amount of work,” adds Sean Gulick, a geophysicist at the University of Texas, Austin.

Related reading: Fossil hints at rapid mass extinction recovery