It’s hard to be an optimist these days. We are living through what biologists call the sixth mass extinction, a time of dramatic depletion of species, from frogs to rhinos and butterflies. By the end of the century, it is estimated that one in six species will be extinct. The causes—human population growth, habitat loss, climate change—are complex and interlocking, fueling each other in an ever faster destructive spiral.

Talking from his home in Massachusetts, Wilson explains why despair is not the answer to Earth’s problems, how most of Earth’s species have not even been identified, and why setting aside half the planet would actually be good for business. [Explore a war-weary national park with E.O. Wilson.]

View Images Courtesy of W. W. Norton & Company

How bad is the situation facing our planet today?

There were five great extinction episodes during past geologic history, the last being 65 million years ago. I think it is correct to call what's happening now due to human activity the sixth. If we allow it to continue, we could drive down the diversity of life on Earth pretty close to what it was in the last big extinction at the end of the Mesozoic. We have a lot of good measurements, and though they're approximations, they've been carefully done by teams of scientists, who put the extinction rate at between 100 and 1,000 times greater than it was before the coming of humanity. I actually believe that it's probably closer to 1,000 right now than it is to 100.

Proponents of what’s known as “new conservation” say we should stop trying to save the planet. Or as environmental scientist Erle Ellis put it, “Nature is gone. We now live in the Anthropocene.” Do you disagree?

This is defeatism that is both irrational and dangerous. What they're saying is that there's no stopping extinction, that humanity is a juggernaut which is essentially mindless, and that we should concentrate on saving the species we most value, like elephants and other big mammals. They seem unaware of the vastness of biological diversity, which runs into the millions of species, many of which are still unknown to science. It's a combination of ignorance and despair. The other element that comes into it is a proposal that if we save the DNA, we can clone these species later and bring them back to life. That’s an absurdity.

We have no need to surrender to nihilism. But if we continue to allow species to go extinct, by the end of the century we will have lost a large part of biological diversity. If, on the other hand, we take the right measures, we can save the greater part of it. I believe the choices are clear-cut and that the optimistic one is doable.

Last year broke global warming records by an unprecedented amount. How is this linked to the destruction of the biosphere?

It's fundamental, just as it is to human existence. The extinction process can be summarized with the word HIPPO. Each of the letters from H to O designates one of the causes of extinction. So H is habitat; I is the impact of invasive species; the first P is continuing high levels of population growth; and the second P is pollution. [O stands for overharvesting.]

H is number one—and that's where climate change comes in. Changing climatic conditions destroys habitat wholesale. There are parts of the Earth where, as habitats are affected, the fauna and flora have no way to retreat. Notable is southern Africa, which has a magnificent fauna and flora. Other parts of the world will also see very large extinction of species and habitat because of climate warming. The arctic and subarctic regions, for instance, contain a rich fauna and flora and those are in imminent danger. We all know about the polar bears. But there are also large numbers of small invertebrate animals, including insects, which are the foundations of the ecosystem. And they are also at high risk.

View Images A mother polar bear and her cub are stranded on an iceberg in the open sea. Polar bears are among the many species whose habitat is shrinking due to climate change. Photograph by Paul Nicklen, National Geographic Creative

The title of your book refers to setting aside half the planet as nature reserves. Surely that’s just a pipe dream.

It would seem to be upon first hearing. But I've seen a lot of data and talked with senior ecologists, who are best informed on biodiversity and the extinction process, to conclude that it's easier than you think. In the 1960s, I proposed a theory with an ecologist named Robert MacArthur that we called the theory of island biogeography. One of its major tenets is the close relationship between the area of a habitat and the number of species of plants and animals that can live sustainably there. In the simplest form, if you were to be able to save half of the surface of the Earth, then you would come close to saving between 80 and 90 percent of all species.

Let's start with the ocean. Currently, 3 percent of the ocean has been set aside for reserves. The reserves are usually along the coasts of coastal nations, and we want to get that to 50 percent. That might seem almost impossible. But two recent, independent studies by experts on marine organisms show that if we could set aside the entire blue water outside the Exclusive Economic Zones of the coastal nations, and prohibit fishing throughout the open sea, we would actually increase, not decrease, the global growth of fisheries. There would be richer fisheries and faster growth in coastal fishing areas. It would be to our benefit to set aside the ocean, the open sea. That's a remarkable, unintended consequence of the half earth concept.

View Images A fisherman holds up a Patagonian toothfish. Wilson argues that prohibiting fishing in the open sea would boost fisheries in coastal waters. Photograph by Paul Sutherland, National Geographic

One of the surprises reading your book is how much we still don’t know about life on earth. Explain why it is so important to catalogue all living species.

Our lack of knowledge of biological diversity is one of the great scandals of the biological sciences. Those of us that work in biodiversity, going back to the time of Linnaeus, have catalogued a little over two million species of all kinds of organisms. But the estimated number of plants, animals, and microorganisms in the world is in the vicinity of 10 million. In other words, roughly 80 percent of the species on Earth remain undiscovered. We live on a little-known planet. We still have only begun the process of mapping the biodiversity on Earth. That's why we have such difficulty in making projections and why we urgently need to reopen a grand survey of Earth's biodiversity.

Pope Francis recently said that Catholics shouldn’t go on multiplying “like rabbits.” Why is population control so important to saving what is left of the planet?

I think most people know intuitively that the world is already overpopulated and that the more land we take over, the less the rest of life has to exist on. But I would like to quote something else Francis said: “Every tree, every pond, every member of every species is unique and special to God.” That kind of blessing from a major religious leader is a powerful force to work.

We can actually be a bit optimistic about population because current demographic trends show that in every developed country there's a substantial middle class. Most importantly, where women have some degree of economic freedom, the birthrate plummets. According to United Nations estimates, population will peak by the end of the century at somewhere around 10-11 billion, about 50 percent more than now, but then it should slowly come down.

The philosopher Francis Bacon coined the term Lord of Creation. You suggest the opposite—that human beings are actually “weak and dependent.” Unpack that idea for us.

We have reached unimaginable heights in our scientific and technological knowledge. But that knowledge is mostly confined to our own immediate welfare, particularly the biological aspects of medicine. We have a very powerful instinct to explore, we know a great deal now about astrophysics, but when we look at the biological world—what the resources are and how are we going to manage our future with or without a certain number of species living with us—it's pretty much a blank slate. We are fragile and if we continue present trends in the utilization of the Earth we could have a catastrophe.

Technology and conservation appear to be opposites. But you don’t see it that way, do you? Explain why the digital revolution can help save the planet.

I've taken the opposite tack from some of my colleagues, who uphold the classical opposition between human progress and technology versus Nature. I see it the opposite way. In the digital age, with the fundamental economic incentives humans have to get the most from the least, the economy evolves almost automatically towards less material, less energy, and more efficiency. This trend, combined with a move toward alternative energy sources, creates the potential of reducing what is called our ecological footprint: the average amount of space required by each human being for his or her livelihood. So I believe our techno-scientific civilization has the potential, unintended consequence of making the proposal to set aside half the Earth a practical possibility.

View Images Two of the main causes of extinction are human population growth and pollution. But even teeming Bangkok, where this traffic jam occurred, has managed to clean up its air. Photograph by Jodi Cobb, National Geographic

Let’s talk about the specifics of the Half Earth idea—how would it actually be implemented? Would the earth literally be divided in half? How much of the earth’s surface is currently given over to conservation?

Currently, 15 percent of the Earth's surface and 3 percent of the sea has been conserved. The U.S. National Park system has devised a number of ways to show how you can achieve half the Earth in steps. They include things like national natural landmarks: areas that have special value for the nation, either in terms of nature or history. There would be no change in ownership or rules against using this land. But when people have property in a national natural landmark, they will tend to have pride in it and make sensible use of it. This is just one of many steps that have been devised within our current parks system in the United States. Then there are the landmark areas officially recognized by the United Nations. We can do it stepwise.

You end your book by saying, “Only a major shift in moral reasoning…can meet the greatest challenge of the century.” Are you optimistic or pessimistic that we can make it happen?

I'm quite optimistic. I think one of the main reasons we haven't moved more quickly is that we haven't made that moral shift where people value what they call wildlife. Not just the big animals, but the rest of life. Once people get a sense of what that is and how important it is for nature’s ecosystems and, ultimately, for them, I believe a shift towards the preservation of the rest of life would follow, to our all-around benefit.

I belong to a worldwide group of amateurs and professional entomologists, headquartered in the United Kingdom, who have an interest in dragonflies and damselflies. They go out on trips, like birders. I think it wouldn't be difficult for many other invertebrate groups to attract a similar interest and caretaker group. I tend to stop short of spiders. [Laughs] I'm a little bit of an arachnophobe myself. But who knows?

This interview was edited for length and clarity.