Photo by dbencek/ Getty Images.

After decades of researching the impact that humans are having on animal and plant species around the world, Chris Thomas has a simple message: Cheer up. Yes, we’ve wiped out woolly mammoths and ground sloths, and are finishing off black rhinos and Siberian tigers, but the doom is not all gloom. Myriad species, thanks in large part to humans who inadvertently transport them around the world, have blossomed in new regions, mated with like species and formed new hybrids that have themselves gone forth and prospered. We’re talking mammals, birds, trees, insects, microbes—all your flora and fauna. “Virtually all countries and islands in the world have experienced substantial increases in the numbers of species that can be found in and on them,” writes Thomas in his new book, Inheritors of the Earth: How Nature Is Thriving in an Age of Extinction.

Thomas is a professor of conservation biology at the University of York in England. He is not easily pigeonholed. He has been a go-to scientist for the media and lawmakers on how climate change is scorching the life out of animals and plants. At the same time he can turn around and write, “Wild geese, swans, storks, herons and cranes are returning as well, and the great whales, the largest animals ever to have lived on Earth, are once more plying their way across our seaways in numbers after centuries of unsustainable butchery.” Glass half empty, meet Chris Thomas.

Why He Writes: “I’m concerned a lot of resources being spent on conservation are focused on trying to keep things exactly as they are, or revert to some imagined past,” says Chris Thomas (above). “As great as those aspirations might be, in the long run, they’re doomed.”

Inheritors of the Earth collects years of Thomas’ field research, illuminating plant and animal species—notably one of his specialties, butterflies—flourishing all over the Earth. Thomas also puts big ideas on display. Humans are just another animal on the planet, he wants us to know. Our actions are not outside the engine of evolution, even though we have the most horsepower. Environmentalists need to stop fencing off nature from humans, he argues, understand the mechanics of evolution better, including our role in it, and quit being such nattering nabobs of negativity. Once they do all those things, real conservation has a chance. The Sixth Great Extinction, he tells us, is premature.



There may be a bit too much of Dr. Pangloss in Inheritors of the Earth, and despite its ample footnotes, I would have liked to have learned more about how Thomas quantifies some of his general assertions about increases in species. I did ask Thomas to cite the research he has drawn from to support his views, and he responded with the names of five scientists who have influenced him. You can read his answer in a footnote at the end of the interview.1 (I didn’t include it because it seemed kind of wonky.) In any event, Thomas was a pleasure to talk to. What is it about a wry Englishman that so enchants an American interviewer? Thomas and I had a jolly conversation, even if it got contentious at times, as I reminded him of the environmental wreckage that hung like a dark cloud over his thesis.

You write, “It is entirely possible that the long-term consequence of the evolution of Homo sapiens will be to increase the number of species on the Earth’s land surface.” That sure goes against the grain of what we have been hearing for generations.

Yes, it does.

What first caused you to you come to that conclusion?

I knew there was a new hybrid plant living in my hometown of York, England, and nowhere else in the world, and I had also heard about a new kind of fly evolving on introduced apple trees in North America. I started to reflect on the fact that so many of our crop plants started out as hybrids between different species. So I privately asked myself: How many new species might come into existence because of humans? All I needed was a pencil and the back of an envelope for my first calculation. I was gobsmacked, and eventually pleased with my preliminary answer. I reckoned that we might, very roughly, double the number of species on Earth over the next million years. Brought up on stories of extinction and environmental doom, it took me several years to believe my own answer.

Give us a convincing example of how humans boost the number of species.

The Italian sparrow is a really good example of a rapid evolution of a new species. It began when the house sparrow colonized out of Asia, following the development of agriculture. In the Mediterranean Basin, it met the Spanish sparrow. At some point, probably about 6,000 years ago, the house sparrow and the Spanish sparrow hybridized, and their offspring became sufficiently genetically distinct. Although they can interbreed with both of their parents, they basically don’t. So a new species came into existence by hybridization. I really like this example because the Italian sparrow has probably already survived for several thousand years. It’s not one of these species that come into existence and suddenly disappear.

Yes, there are monsters. Biodiversity is not a one-way street to a beautiful, happy world.

Most of the new hybrids exist because we humans have either deliberately or accidentally brought the parents—which used to live in different parts of the world—into contact with one another. This is an extraordinary feature of the modern world. There has been no time in the history of life when species have been mixed up within and between continents at the rate that’s going on at the moment. The consequence of this human-caused transport is that hybrids must be coming into existence faster than ever before.



Is there a mammal that fits your scenario? Mammals have not fared well with humans.

You are quite right. The heavyweight, large body-sized mammals are where we have most systematically exterminated other species over the last 60,000 years. There’s no doubt about that whatsoever. But hybridization is taking place in mammals too, when they’ve been introduced to new locations. In Britain, the native red deer has been mating with the sika deer. There’s some hybridization among the wapiti, or elk, and red deer. Presumably, these new populations will start to diverge with a new mixture of genes that they didn’t have previously.

In 1997, the biologist E.O. Wilson wrote, “Extinction is now proceeding thousands of times faster than the production of new species.” Is that just wrong?

At the time he wrote that, nobody had made any estimate whatsoever of the current speciation rate. It was based on a presumption that the present-day speciation rate is the same that it appears to have been in the long-term historical past. For animals, particularly vertebrates, it’s pretty clear that over the last few thousand years, the extinction rate has indeed been a lot higher than the speciation rate. However, it’s not so obvious for plants. If you take the mainland North America north of Mexican border, for which there’s good data, and mainland Europe, we know of more hybrid plant species in both of these regions that have come into existence over the last 300 years than we know of plant species that have become completely extinct.

I’m not disputing that what Wilson and others are talking about is real. What I’m saying is that, simultaneously, large numbers of biological gains are also going on, and that they are at least as worthy of scientific study as the losses. As environmental managers and conservationists, we should start incorporating these gains into our thinking of how we manage the planet, rather than taking a stance of simply trying to fight the losses.

How do we benefit by taking those gains into account?

It depends on what sort of gain you are talking about. A number of scientists have suggested the benefits of ecosystems to humans may increase with the number of species in an ecosystem. Most ecologists accept that various plants can stabilize the soil, purify water, or fix carbon from the atmosphere, and so on. If so, then why should these services not also be provided by so-called non-native species? There’s no clear evidence that the old ones are better than new arrivals at doing these ecological jobs. The fact that new species are becoming established in our new, disturbed environmental conditions, suggests that non-native species could actually be better at these jobs. If you were to say, “My standpoint is that ecosystems are degraded by the loss of the former diversity,” then you might think that the ecosystem service has declined. But that argument doesn’t follow once you take into account the balance in gains.

Close Up: Look close and species are flourishing everywhere, says Chris Thomas. On his own land in York, England, “I have counted 22 butterfly species, 15 types of dragonflies and damselflies, 92 bird species, and 19 different kinds of mammals,” he writes in Inheritors of the Earth.

Tell us why you dug a hole on your property.



I live in the Vale of York, a flat area of Northeastern England. The farther down you dig, the further back you go in history. When I got down a meter or so, past layers of sand, I hit this goopy clay. It turns out this clay was laid down at the bottom of a lake that was at the edge of the ice sheets 15,000 years ago. This was a community that might have had the odd polar bear swimming in it. It would have had Arctic fish, and you could imagine early hunters might have gone fishing and tried to kill reindeer to get fur and meat. But when the climate started to warm, the ice melted, and this resulted in the whole lake draining very rapidly.

The take-home message is that every biological community that’s been on this land for at least the last 10,000 years has itself been a transient assemblage of species. As humans are changing the world and warming the climate, a new transient set of species are arriving, and they will in turn be replaced by others. The replacement of species is a normal operation of our biological planet. Species survive by moving from one place to another, not necessarily by surviving in exactly the same place in the long term. Yet many of our environmental strategies are about trying to keep things as they are. The way nature survives is often by moving around, not by staying as it is.

“Species come and go with the vagaries of the Earth’s climate,” you write. That sounds so nonchalant. Let’s look at a species that gets people fired up: the polar bear. Do we not agree its habitat is under threat from a warming planet, that its food sources are dying?

We do indeed. And I gave an assessment as to whether the polar bear should be listed as threatened as an endangered species in North America. I argued that it should. Although I’m pointing out that many species are successful, I also point out that many are not being successful, and the critical thing to do is to reduce our greenhouse gas emissions to minimize the amount of climate warming that takes place. That’s what’s going to reduce the number of species that are endangered from climate change. Everything else is like putting a sticking plaster on a gaping wound.

Couldn’t climate skeptics use your statements to defend their views that global warming is overblown?

But you’re right, the polar bear is the poster child of endangerment from climate change. What do we do about it? Well, the polar bear is a recently evolved type of bear, having come into existence during the Ice Ages of the last couple of million years. It can still hybridize with the brown bear. Let’s say, though, you want preserve the polar bear. Are you going to shoot brown bears so they don’t hybridize with the polar bears? That’s gonna be a tricky thing to do. Are you going to move them to Antarctica where they would be very happy? There would be ecological Armageddon. They would eat their way through the penguin and seal colonies.

There are going to be some types of species in the future that are probably just not going to hack it. We have to decide what resources we’re going to put into trying to keep them going. In the case of the polar bear, we know the overwhelming threat is climate warming. We know that it is a result of human-generated greenhouse gases, and we know how we can at least reduce the rate at which it declines and minimize the risk of its final extinction.

Short of arresting global warming, are you saying, “We just have to stand back and reconcile ourselves with the fact that the polar bear may hybridize into a brown bear. There’s nothing we can do about that.”

Yes. In the case of polar bears, there’s nowhere north of the North Pole they can go. They’re kind of stuffed. Fortunately there aren’t so many species that live so far north. Far more species threatened from climate change live on tropical mountains. If you were to imagine species that were restricted to mountains somewhere in Central America, one could ask, “What proportion, if any of them, would be able to establish elsewhere?” They can’t get down to the hot lowlands and then go up another mountain range somewhere else, say one which is a bit higher and cooler for them. They can’t do it on their own, so could we as humans transport a fraction of those species and reduce the extinction rate? That tends to fall outside traditional conservation thinking. They would be seen as non-native species, and potentially detrimental. But if we take this longer-term view, then we can potentially reduce the extinction rate compared to what it would be otherwise.

Do you personally think that’s a good idea?

I personally do, yes. In the long-run, the world’s ecosystems—and we—rely on the species that currently live on Earth and their descendants. Since we have no real idea what people will do to the planet over the next thousand years or so, keeping the present species alive somewhere, on the off-chance that they will be suited to the new conditions, seems like a reasonable long-term biological insurance policy. It is entirely logical that we should try to maintain as many of them alive as possible for future generations, whether or not they turn out to be important.

Let’s stay with climate change for a second more. You write, “Given how many species live in the hottest parts of the world, it is not surprising that the average biological diversity per square kilometer of the world goes up when the climate warms.” I have to say, climate skeptics could easily use statements like that to defend their views that global warming is overblown.

They could indeed.

So what would you say to climate skeptics who would use your book to buttress their views?

What I would say is climate change has paradoxical effects on biological diversity. It’s threatened a large number of species. There’s a strong consensus on that in the scientific community, although there is no consensus as to exactly what percentage are endangered. People seem to think it’ll be 10 or so percent of the world’s species that might be endangered, and many of these are these mountain species with nowhere to run.

However, there are more species close to the equator than elsewhere, and they will spread. Just this morning I saw a little egret fishing on the edge of my university lake in Yorkshire. It only started to breed in Britain a couple of decades ago, and it can already be seen in half the country. Southerners like the egret moving north as the climate warms. The result? The biodiversity of some areas of the world is starting to increase because the rate of arrival of new species slightly exceeds the rate of departure of those for which it is no longer suitable.

Life is just what happens, and unfortunately, or fortunately, it has no ultimate purpose.

So why should we be concerned? Well, the loss of the cold-adapted species is potentially a long-term concern, because if the climate ever becomes cooler again, and we’re still in a long-term Ice Age, these are the species that will thrive and become common in the future. They will be the key players in future ecosystems. Secondly, if we lose 10 percent of the species to climate change, then we have thrown 10 percent of the biological opportunity of the world for future generations. It’s a paradoxical thing. Gains in some places, but extinctions of others.

If extinction of whole species does not convince skeptics, I’d just say, “New Orleans and London, not to mention the farmers of Bangladesh.” They are going to be in deep trouble once the sea levels rise seriously. Although that doesn’t happen very fast within an individual lifetime, it is an inexorable process that once it has started cannot be stopped for thousands of years. Therefore we are effectively sealing the fate of humanity to live on a smaller area of land on the planet.

The Hybrid: “The more people look, the more it seems hybridization is the norm,” Chris Thomas writes. That goes for the polar bear. “Alaskan grizzlies have polar bear genes in every cell of their bodies,” Thomas notes. Polar bears may one day breed with a bear species in a warm climate.

You tell us this loss of around 10 percent of species “falls far below the level of extinction (75 percent plus) required to match one of the previous ‘Big Five’ mass extinctions in the geological past.” Is the Sixth Extinction overrated?



Well, I agree there has been a huge acceleration of the extinction rate in the human epoch, and that if we keep up the current rate of extinction for the next 10,000 years, we end up with a mass extinction of 75 percent of species going extinct. That is equivalent to the percentage that went extinct when the dinosaurs died out. And 10,000 years or even 100,000 years is an extraordinarily short period in geological history.

But could it be that we’re spurring evolution too fast? Maybe the rate at which the change is going on now, compared to the past, is too fast for there to be corrections.

Evolution is a process. It can’t be too fast or too slow. It just happens. Under some circumstances it’s faster and slower. By and large, the faster the environment changes, the faster evolution takes place, because some lineages are lost and some are able to diversify under the new conditions. Yes, there could be a rate of change that was so fast that evolutionary adaptations were incapable of keeping up with it. But there isn’t any evidence at the moment that we are beyond some kind of tipping point beyond which evolution can’t hack it any longer. Look at how fast things evolve resistance to pesticides, or how many species manage to colonize new parts of the world that we have disturbed.

Why do you call Earth “Anthropocene Park?”

To focus our thinking on the fact that we are both the inmates of the park and its managers. And to take issue with the dream of unaffected nature. A park is a modified environment and so calling our world Anthropocene Park gets us away from thinking there are any places unaffected by us. Every decision we make, or every time we decide not to intervene, has an influence on the biodiversity of a particular place and on the planet as a whole. Like it or not, we are managing the world species and ecosystems. We have to get used to that. We are part of the system.

Are there monsters in the park?

There’s a certain pink primate in the park. Well, I don’t mean “pink” because that’s a thoroughly narrow view of the world. I would like to unsay it. I’ll say “naked” primate. But, yes, absolutely, there are monsters. There are pathogens. Biodiversity is not a one-way street to a beautiful, happy world. There are elements of biodiversity that are harmful to us and other things. I am happy to rejoice at the extinction of smallpox and other diseases that affect us and our livestock because they seriously interfere with the capacity of humans to live on the planet.

You’ve just got to get real that every biological system is modified by us.

You write, “Humans are natural within the Earth system, so anything we do is also a natural part of the evolutionary history of life.” Well, what do you mean by “natural?” That would imply there is an “unnatural.” How do you differentiate the two?

You spotted the fatal flaw! But if I’m not allowed to use the word “natural,” and I accept it has no special meaning, I would just have to say that the Earth’s system is simply what it is: It now contains the human species as well as other species. That is the system we have, but we cannot distinguish between the human parts from the inhuman parts. Humans evolve within the system, so we are part of the system. Unless you go deep into rocks or some such location, you cannot identify locations where the impacts of humans are zero.

OK, but wouldn’t you agree that a lot of the natural things we do are awfully destructive? Are you saying they’re just part of a natural way?

Sure, humans have changed the world a lot, and some of the changes do look like destruction. But destruction is a bit of a subjective word. Normally we change one type of habitat into another. You’ve just got to get real that every biological system is modified by us. I’d argue that conservationists should throw off their apparent reluctance to do so-called unnatural things. Let’s ensure that positive things happen to the natural world as well, however that might be achieved. Why tie our hands behind our backs when we try to fight for nature? The forces of industry, some of whom are less concerned about nature, are not tying their hands behind their backs.

We’re the species with the big brain. We have the technology and ability to shape the natural world to our own ends. Do you think we have an ethical responsibility to conserve nature?

Not really, but human society as a whole does tend to quite like a lot of things about wildlife and the environment. And that goes for me too. From our perspective, we would rather live in a green world full of animals and plants. Therefore we might wish to intervene to ensure that that is the case.

But I don’t feel there is any ethical duty to do this. Life is just what happens, and unfortunately, or fortunately, it has no ultimate purpose. It came into existence, it will exist for a while, and eventually it will disappear. Conservation is more about humanity’s perspective on the now and the relatively near future, by which I might mean millions of years. But in terms of conserving life in the universe, I don’t think there is any essential ethical requirement for us to do it.

Why do you say that conservation and the environmental movement are “backward-facing?”

If you want to measure how the state of the world is changing, and that’s a valuable thing to do, you need some kind of reference point. It’s natural we take some past point and monitor things into the future. But when you reference back to some time in the past to identify how things are changed, it tempts us to think that this baseline is a more idealized way the world was or should be. Then you tend to think that the environment is deteriorating, when it may just be changing.

I also think that there’s the human social perspective. Many of us are reflecting on our childhoods, or maybe things that we have read about. In England we have loads of cultural traditions about snowy Christmases because there was a string of snowy years when Charles Dickens was a child, and he wrote about them. It’s well-known in social science circles that people often imagine a time, in the relatively recent-to-medium past, that was an idealized state of the world.

The difficulty is time goes forward. The biological processes of the birth and death of individuals, the better survival of some species than others, the evolution of new genetic types—all of the biological processes that take place on the planet—are dynamic ones. The expectation that things should stay exactly as they are is not a realistic expectation of the biological systems of the world.

Looking backward is really informative. It’s extremely valuable. Don’t get me wrong. But we should also, in our aspirations, look forward to how we might make things better for ourselves, even if that “better for ourselves” and whatever we care about, such as wildlife, becomes less like it was in the past.

How did a pessimistic narrative get so ingrained in environmentalism and conservation?

Well, you may not be able to quote this, but shit happens. Species have gone extinct, and we as humans have caused those extinctions. We nearly killed off the great whales just as we killed off many of the world’s great land mammals. North America doesn’t have ground sloths wandering around in it today because our human ancestors killed them. Those bad things are really clear, and they leave a very strong impression on us. There are positive trends, but the positive trends do not generate additional inconvenience to us, so we don’t respond to them very much.

You want conservationists and science and environmental writers to do a better job. How should we?

I think we should ask, “How could I increase biodiversity or promote biological gains?” whenever we consider a question like “How can I stop something declining?” These are complementary questions. Both are valid. It’s clear to me that we also need a lot more systematic scientific analysis of the balances of gains and losses. We need to know this for different metrics of biological diversity, on different timescales and for different spatial scales, and also in different parts of the world. That information is starting to come together, but that process of bringing it together is far from complete at the moment.

This might be a shock, but every scientist is actually a human, and they come with their own personal baggage. Although we might agree on the data, very often people will disagree about the interpretation, not because of a data difference, but because of the personal lens through which they are seeing it. So science writers should always be aware of where a particular scientist is coming from.

That’s great advice. Where are you coming from?

I walked into that one. The confusing answer is from all sorts of directions at once! I still accept many of the dire messages of environmental doom. But now I hold another set of generally more hopeful views. Some of them are “as well” opinions, others have replaced what I previously thought. The first time I came to the conclusion that humans might increase the number of species in the world made me shudder—I still remember the moment—because it was so shocking to my previous beliefs. For me, both as a person and as a scientist, I’m now really happy about this. Changing my mind on all sorts of previously cherished opinions means I’ve really learned something. Whether it has led to any new clarity of thought I leave others to judge.

Footnotes

1. Dov Sax at Brown University has done important research. He started with the Hawaiian islands and U.S. states, where, on average, the number of species in each region has been tending to increase—more species arrive, often through human transport, than have been disappearing. In fact, the numbers of incoming species has increasing in most parts of the world. I am also aware of recent studies by Mark Vellend and Maria Dornelas suggesting that if you monitor local diversity in many parts of the world, you find that it hasn’t been going down. Which species are present in a particular location has been changing, but the actual number of species has not been declining.

Then, from the evolutionary perspective, I had been reading studies by geneticists, people like Jim Mallet and Richard Abbott, interested in the process of speciation. Ever since Darwin, geneticists have been besotted by speciation. They’ve been trying to understand the processes by which populations evolve into new species and new hybrids come into existence. People have been asking for years, “What’s the extinction rate?” Everyone knows it’s high. But they never stepped back and asked, “What’s the speciation rate?” The moment you ask that question, it becomes fairly obvious it’s increasing.