There have been five mass-extinction events in Earth’s history. In the worst one, 250 million years ago, 96 per cent of marine species and 70 per cent of land species died off. It took millions of years to recover.
Nowadays, scientists are predicting that the planet is on pace for a sixth mass extinction. The world’s species are vanishing at an unnaturally rapid rate. And humans are altering the Earth’s landscape in far-reaching ways: We have hunted animals like the great auk to extinction. We have cleared broad swaths of rainforest. We have transported species from their natural habitats to new continents. We have pumped billions of tonnes of carbon dioxide into the atmosphere and oceans, transforming the climate. Those changes are pushing more species to the brink.
What happens if the extinction rate keeps speeding up? That is one question New Yorker science writer Elizabeth Kolbert explores in her book, “The Sixth Extinction”, an in-depth look at the science of extinction and the ways we are altering life on the planet. Excerpts:
Let’s start by walking through the history of science here.
There is an interesting history there. Until the early 1800s, the concept of extinction didn’t really exist. Even early in the 19th century, you had Thomas Jefferson hoping that when he sent Lewis and Clark to the Northwest, they would find mastodons roaming around. Mastodon bones had been unearthed — a very famous one was unearthed in New York and displayed in Philadelphia — and people thought they must still exist somewhere.
But right around that time, a French naturalist named Georges Cuvier came to the realisation that if these animals were out there, we would have seen them by now. They are not there. And that made sense of many things. There were these bones that were very, very hard to explain. And more and more of them as Europeans colonised the New World — they were getting these bones shipped to them. It made sense of these weird nautical creatures that had been found.
So extinction actually predated the concept of evolution by about half a century — people knew that things went extinct, even though they didn’t really understand how species came into being. But there was still some debate. Cuvier thought that when extinctions happened, it must be because the Earth changed quickly and catastrophically. Why else would an animal that was perfectly suited to life on this planet go extinct? His theory became known as “catastrophism”. And Charles Lyell and Charles Darwin came along and said, “That’s ridiculous, the Earth changes slowly, we’ve never seen a catastrophe, that’s because they don’t exist.”
That paradigm persisted until the 1980s and 1990s. That was when Walter Alvarez and his father, Luis Alvarez, came up with the theory that an asteroid impact had done in the dinosaurs. And that idea was actually resisted for the same reasons — the dominant view was that the Earth does not change quickly. But then it was proven.
And so now the prevailing view of change on planet Earth, as one palaeontologist put it, is that the history of life consists of long periods of boredom interrupted occasionally by panic. It usually changes slowly, but sometimes it changes fast, and when it does, it is very hard for organisms to keep up.
Nowadays, scientists are aware of five mass-extinction events in the past, starting with the end-Ordovician extinction 450 million years ago and up to the end-Cretaceous extinction that killed off the dinosaurs 66 million years ago. Is there a lot we still don’t know about what caused these events?
It depends. So I think with the dinosaurs, the asteroid theory is quite widely accepted at this point. Although there are still a couple of holdouts.
The worst mass extinction of all time came about 250 million years ago. There is a pretty good consensus there that this was caused by a huge volcanic event that went on for a long time and released a lot of carbon-dioxide into the atmosphere. That is pretty ominous, considering that we are releasing a lot of CO2 into the atmosphere and people increasingly are drawing parallels between the two events.
The very first extinction event seems to have been caused by some kind of sudden cold snap, but no one’s exactly sure how that happened.
At some point scientists realised that modern-day extinction rates seem to be elevated — that species are now going extinct faster than the normal “background” rate. How did they realise this?
I think a point that’s important to make is that, normally, you shouldn’t be able to see anything go extinct in the course of a human lifetime. The normal background rate of extinction is very slow, and speciation and extinction should more or less equal out. But that’s clearly not what is happening now. Any naturalist out in the field has watched something go extinct or come perilously close. Even children can name things that have gone extinct.
So as soon as this concept of background versus mass extinction came into being in the 1980s, people realised that what we are seeing today is not just background extinction. If you are looking at this in a rigorous way, you can see that something unusual is going on.
One thing your book explores is that there’s no one factor causing modern-day extinctions. There’s hunting. There’s deforestation. There are changes in land use. There’s climate change and the acidification of the oceans. Which of these stands out as most significant?
I’m relying on what scientists tell me. And I think many scientists would say that what we’re doing to the chemistry of the oceans is the most significant. One third of the carbon dioxide that we pump into the air ends up in the oceans almost right away, and when CO2 dissolves in water, it forms an acid. That’s just an unfortunate fact.
The chemistry of the oceans tends to be very stable, and to overwhelm those forces is really hard. And we are managing to do it. When people try to reconstruct the history of the ocean, the best estimate is that what we’re doing to the oceans or have the potential to do is a magnitude of change that hasn’t been seen in 300 million years. And changes of ocean chemistry are associated with some of the worst crises in history.
Are there lessons we can learn from past extinctions that provide clues for what the changes today hold?
A lot of people are trying to tease out what survived previous extinctions and ask what are the characteristics of those that survived. It’s called the selectivity of extinction events. Why did some groups survive and others didn’t? It turns out to be, 65 million years after the fact, very, very difficult.
But speaking very broadly, the species that tend to survive mass-extinction events often tend to be very widely distributed, or groups that have a lot of species.
You discuss global warming in your book. And the big concern here seems to be that a lot of species are adapted to particular climate ranges, and if those heat up, some species may not be able to move or relocate fast enough to more suitable climates. How much do we really know about these dynamics?
What the scientists that I was with in the Peruvian cloud forest are finding is that things move at different rates. People have calculated how fast species would have to move to keep up with rising temperatures, whether it’s moving up a mountain or moving to higher latitudes.
And some organisms can keep up with that fantastically high pace — for example, in Peru, there was this one genus of tree called Schefflera, which is sometimes used as a house plant, and that genus is moving really fast up the mountain. But some of the other plants weren’t moving at all, and others were moving but not nearly fast enough.
So the lesson is that all those pretty complicated relationships, which in the tropics have been pretty stable for a long time, are going to break up.
So you end up with pretty wide estimates for how many species could go extinct if the planet heats up this much. Some studies suggest that 20 per cent to 30 per cent of species could go extinct if the planet warms 2C. Other scientists think those estimates are flawed.
You often hear that what we’re doing is a planetary experiment, but we have only one planet, and we can run this experiment only once. So some of these modelling efforts get pretty complicated. Just because a species lives in a certain climate under a certain set of conditions, could it live under different conditions? Or is this just where it’s maximally competitive? What happens if some of your competitors are disadvantaged? We just don’t know. Life turns out to be incredibly complicated.
Most of the people in your book who study these extinction trends tend to think they’re horrible news. Did you come across researchers who had a more optimistic view about extinction? Who say that we might be surprised by the resilience involved?
I guess one point to make: Even in moments of extremes, certain organisms thrive. They’re sometimes called “disaster taxa”, and they do very well. After the end-Permian extinction, which was the worst mass extinction of all time, there was an animal called Lystrosaurus that did phenomenally well. It was the biggest animal on the planet; you find fossils everywhere. And the question of why did it do so well? We just don’t know.
But some things will thrive. Some things will thrive in an acidified ocean because all of their competitors will drop out. So some things will do well, and undoubtedly there will be surprises. But I have not met anyone who hasn’t said, “We’re going to be vastly simplifying the web of life.” A lot of things are going to drop out. It’s hard to make predictions of what they are.
The spread of people across continents has transported all sorts of species to new habitats — and sometimes that’s had catastrophic results, like when the brown tree snake was introduced into Guam and wiped out the native birds. Is this sort of exchange speeding up, or are there efforts to slow it?
There are certain moments of time where you see a huge exchange of species. After Columbus arrived in the New World, there was this huge exchange. And as global travel becomes very rapid, that speeds up exchanges. Organisms that couldn’t survive on the Mayflower could survive in a modern supertanker or aeroplane and get transported from one continent to another. So we’ve ratcheted things up a notch.
We don’t do as much purposeful moving of species as we used to — where we’ve decided we’d like to have this bird in a new place. We’ve done a lot to prevent that. You’re not supposed to just take a bird from South America and release it in Australia. But the unconscious transport of species, I think there’s no doubt that is increasing very dramatically as the sheer amount of cargo increases.
And it can still have devastating effects. Look at the Asian carp, working their way towards the Great Lakes. The disease that’s killing off bats in the Northeast and in the DC area, that’s an invasive pathogen that was brought in, it’s a fungus. And I’m sure if we have this conversation a year from now there will be new ones.
What about attempts to save species from extinction?
A lot of them involve zoos or conservation organisations. So there are these fascinating and pretty ugly animals called hellbenders. They’re these big salamanders that could be featured in a horror movie. They are endangered, and what people are trying to do is raise them to a certain size at the Bronx Zoo, and then repopulate streams in Upstate New York.
What’s the big thing you took away after writing this book?
It’s a very sobering thought: Many of our best qualities as humans — our creativity, our cleverness, our cooperation, the fact that we can work in these huge societies, and pass knowledge on from generation to generation — those things can turn out to be damaging. It’s not just that we go out and poach things, although that’s a problem. We can change the planet by doing things that have no evil intent. For example, going on vacation and bringing a bat fungus from Europe to the United States unintentionally. So it’s not always clear how you would separate out what we do just by being human from what we do that has all of these unfortunate side-effects.
