It doesn't take a very long time to irreversibly change the planet. In just a few dozen millennia — a geologic blink of an eye — three quarters of Earth's living things went extinct 66 million years ago. The cause was probably a massive asteroid impact, possibly accompanied by colossal volcanic eruptions. The consequences of what scientists call the Cretaceous-Tertiary (K-T) mass extinction were catastrophic: All the dinosaurs died (except for birds). Millions of microscopic organisms were killed off. Invertebrates vanished from land and sea. In North America, close to the site of the Chicxulub asteroid impact crater, more than half of all plant species were wiped away.
Recovering from the damage, on the other hand, is a very lengthy process.
Scientists working to understand how life rebounded after the K-T extinction found that it took another 4 million years before biodiversity returned to healthy levels in South America — making the recovery period 125 times as long as the actual extinction.
And that's short, by global standards, said Penn State paleontologist Michael Donovan, the lead author of a study in Nature this week: North America took 9 million years to recover. It appears that ecosystems are a lot like trust: They take a moment to break, and forever to rebuild.
Donovan and his colleagues draw their conclusions from tiny holes found in thousands of fossilized leaves — evidence of insect bites taken long ago. By cataloging and quantifying damage from leaves taken from four moments in geologic time — one right before the extinction event, three in the millions of years after — the scientists could track how plants and their insect predators recovered in the wake of the catastrophe.
“What we do in these studies is we take up a fossil leaf, look at it under the microscope to see if it has any damage, and if it does, we categorize it by numbers,” Donovan said. “From there we can quantify the damage and compare how it changes through time.”
Plants and insects are the most diverse multicellular organisms on Earth, and they provide the foundation for most terrestrial food webs. The relationship between the two groups is often a bellwether for how other organisms will fare. Studies in modern rain forests suggest that diversity of insect damage on leaves is a good proxy for overall insect diversity, which in turn can be used to understand the overall health of the ecosystem.
At the end of the Cretaceous period, roughly 67 million years ago, bugs were nibbling away with abandon. But in his sample from right after the extinction event, which included leaves that were about 65 million years old, the diversity of bites fell by more than 20 percent.
“It suggests that those ecosystems are less healthy than the more diverse ecosystems that existed before this major catastrophe,” Donovan said.
Samples from 64 million years ago showed gradual improvement, and by 62 million years ago the insect damage rates were back up to their normal levels. Donovan can't say for sure whether this means that insect species that had died off were replaced, or that species that had been depleted returned to their former numbers. But it does indicate that overall diversity was strong once more.
Most of our understanding of the K-T extinction (which is technically called the K-Pg extinction now, because even though the rocks don't change, geologists' jargon often does) comes from the badlands in the American West, because that is where evidence of the catastrophe was best preserved. But that region was also one of the hardest hit, so it skewed how scientists viewed the world's overall recovery.
“I expected that the recovery [in South America] would be faster, but I didn't necessarily think it would be this fast,” Donovan said.
This work is more than just an intellectual exercise, he added. “Everything that's alive today is descended from organisms that survived this extinction event” — it's effectively the last time life hit the “reset” button. That means that the overall patterns of biodiversity we recognize today were shaped in the wake of this disaster.
“If you look at many modern forests in South America, there’s really highly diverse insect feeding,” Donovan said. “They might be part of the legacy of these insects that survived the extinction or diversified during this early recovery period.”