Every other Friday on Morning Edition, Outside/In Host Sam Evans-Brown answers listeners' questions about the mysteries and quirks of the outside world.
Laurie from California asks: “In all of the five big extinction events, how did plants fare versus animals? Are trees going to take over after we’re gone? Are trees with flowers still going to be around?”
Sam responds: Wow, Laurie. We’re headed straight to the dark place, huh? Though I guess it’s not terribly surprising that in this particular moment one might start thinking about the end of the humanity.
For the less catastrophically minded: Paleontologists say there have been five big mass extinction events, or five moments in the history of the planet when geologic records from the bottoms of ancient oceans show a huge drop in the diversity of life.
The idea that we are entering a sixth great extinction can be traced back to a paleo-archaeologist from Kenya, Richard Leakey. More recently, this idea has entered the popular psyche because of a Pulitzer prize-winning book, The Sixth Extinction, by Elizabeth Kolbert.
I talked to two people for this question. While there are many, many, many caveats about the limited evidence available on this subject — it's based on sedimentary deposits, which often don’t last terribly long before being smooshed into metamorphic or melted into igneous rock, for instance — both were uncharacteristically willing to put forth their answers.
“Generally, plants did fare better, we think they fared better, during these mass extinction events,” says Emmy Smith, a geologist with Johns Hopkins University.
Peter Brannen, a science journalist and author of The Ends of the World which is about the big five, echoed that takeaway.
“Well, I think it’s actually fairly straightforward," Brannen said. "It seems like plants make it through mass extinctions, they have an easier time making it through mass extinctions than animals do."
Why is that? One of the easiest explanations that plants might be the cause of the extinctions. The so-called “kill mechanism.”
The first of the big five — the still-mysterious Late Ordovician Extinction, about 450 million years ago — is thought to be caused by a big uptick in the growth of plant life in the oceans, which caused oxygen levels in the oceans to plummet, suffocating as much as 85 percent of all species out of existence.
But when you look closely at the kill mechanisms associated with these events, a disturbing number of them relate to levels of atmospheric CO2 and global temperature, which animals may not be as well-equipped to handle.
“A rise in CO2 is, to me, the easiest way to wipe out a lot of animals, particularly marine animals, and that wouldn’t have a big effect on plant communities,” Smith says. “Given what’s happening now, to answer the listener’s original question, I would say yeah, trees are probably going to fare better than a lot of animals in oceans.”
Now, when she says “big effect” she’s speaking relatively. Because, it’s actually a little hard to comprehend how drastic events like the big five actually were. There have actually been many more than five instances in the history of the earth when biodiversity seems to have plunged, though these smaller die-offs were not as severe.
Which brings us to an important nuance: While it is almost certainly the case that, thanks to human beings, the earth is experiencing many more extinctions than the “background rate” (or normal extinction rate), our current species loss is nowhere near the same scale as one of the big five.
“In historical times, humans have driven close to zero percent of species in the ocean extinct, and in the worst mass extinction of all time it’s close to 96 percent of species extinct,” says Peter Brannen. “And I think there’s a risk of when people read that we’re already in one of the biggest mass extinctions of all time, to get fatalistic, to think ‘oh it’s too late to do anything.’ and actually this is our last chance to avoid that.”
A grim subject, for Ask Sam, but as a congenitally optimistic person, I find this takeaway encouraging. Our fate is in our hands.
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