New Book Argues Migration Isn't A Crisis — It's The Solution

DAVE DAVIES, HOST:

This is FRESH AIR. I'm Dave Davies, in today for Terry Gross. Our guest today, science writer Sonia Shah, has authored several books, including one that's made her a sought-after interview guest lately. That book, published in 2016, is called "Pandemic." She explored the increasing threat of viral outbreaks and her own personal experience with contagious infections. We'll talk about that today and about her new book, which explores another subject of intense interest in recent years - immigration.

While the Trump administration and some European governments are hardening their borders and cracking down on immigrants, Shah argues that mass migration is nothing new in human history or, for that matter, the history of plants and animals as well. For centuries, she writes prominent intellectuals have regarded migrants as menacing deviants, defying the natural order that places everyone in a singular, natural setting. But she says science now shows that migration among all species is a biological imperative as important as breathing. Sonia Shah's new book is "The Next Great Migration: The Beauty And Terror Of Life On The Move." I spoke to her from my home in Philadelphia. She was at her home in Baltimore.

Well, Sonia Shah, welcome back to FRESH AIR. In an interview at the end of January of this year, you said that COVID-19 will be bigger, more dangerous and more economically damaging than the SARS epidemic. You were right. How did you know?

SONIA SHAH: Well, it seemed - early on, it was clear that this pathogen had kind of found the sweet spot between how deadly it was on one hand - which is not terribly deadly compared to all the pathogens out there that we've had - but it was just deadly enough and transmissible enough to spread really widely and cause a lot of death and disruption.

So the first SARS was a lot more deadly but less transmissible, so it kind of burnt itself out on its own. But this is something that can really sustain itself because it doesn't kill everyone. You know, it has a lot of a symptomatic spread and all of those. So the basic parameters of this virus made it clear pretty early on that this could be the next big one. And of course, now we know what it is.

DAVIES: Right. And you've had a couple of months' more experience to see this unfold. What has surprised or impressed you about the shape of this pandemic?

SHAH: Most of it is pretty similar - the way we're responding, the process by which the pathogen is moving through populations. I mean, there's this confrontation between human population that's had no experience with this thing and, suddenly, it coming into population. So we've seen that before in history.

I think what's really been so surprising and unexpected about the way this pandemic is unfolding that, really, I don't think anyone predicted was the U.S. response being so chaotic and dysfunctional. And I don't think anyone really predicted that. You know, no one really predicted that the CDC would become an institution that not everyone trusted anymore, you know, that our - the U.S. government would not put forward a vigorous sort of federally led response. So that has been very surprising, and I think that has changed how the pandemic and the response has unfolded.

DAVIES: I believe you wrote in the last book about efforts to try and predict where the next dangerous microbe might appear. You want to just talk about that and what became of those efforts?

SHAH: Yeah. Well, I mean, that was sort of the whole, you know, approach of that book was to look at, well, how does a microbe that is basically harmless and benign to humans, living in the body of some bat or, you know, some other primate or wherever it is - how does it turn into a pandemic-causing pathogen and cause all this death and disruption? And the reason I was able to write a whole book about that is because we know that process. We don't know which microbe's going to cause the next pandemic, but we know the basic sort of outline of how this process occurs and unfolds.

So we know that 60% of the pathogens that are emerging today come out of the bodies of animals. Most of those are wild animals. And so - and then they - you know, and that's because we are invading wildlife habitats, squeezing these creatures to live in smaller and smaller fragments of habitat in closer proximity to where we live. And that provides opportunities for the microbes that live in their bodies to spill over into ours and then, you know, adapt to our bodies and become pathogenic.

And then, of course, we spread them to each other in our social interactions. We carry them around the world on our flight network. We amplify them in our slums and cities that are overcrowded and don't have a lot of, you know, infrastructure in them. And, you know, so this process has been pretty well laid out, and, you know, that's what I wrote the book about. So given all that, you can - you know, if you know how a pathogen emerges, then you can predict where it's most likely to occur.

And so disease ecologists have actually come up with maps of the world where, you know, they can look at, well, these are the disease hotspots. This is where - we can't surveil every microbe in the whole world. There's, you know - obviously. But in certain places, where there is a lot of invasion of wildlife habitat, a lot of slums, a lot of factory farms, a lot of flight connections, you know, or some combination of all of those factors, you could do active surveillance from microbes in those places to look and see - how are they changing? And are they evolving in ways that might allow them to become human pathogens? And that was work that the U.S. government was funding up until a couple of years ago.

DAVIES: And any idea why it - why the funding ended?

SHAH: I think it's part of the whole kind of anti-science bias of the administration. The justification they gave was that this was a program that was housed under USAID and that that was not a good fit, and I think there was a spokesperson who actually said out loud that, you know, they were not comfortable funding that kind of, quote, "cutting-edge science."

DAVIES: Yeah, that's the U.S. Agency for International Development, which has been an agency that applied foreign aid for decades. You've been interviewed a lot about the coronavirus, and one thing I've heard you say is that how - what we take out of this will depend upon the stories that we tell ourselves.

And when you look at the divergent and competing narratives that we see, it's pretty striking. You have one side saying, well, this came from China or may have come from China, even perhaps a Chinese government lab, and that the administration's response was swift and effective, and the death toll is probably inflated, so it's not actually that bad. And then another side that has a completely different view. I'm wondering, are we going to end up with an accepted understanding of what's happened here?

SHAH: I mean, I think all the stories that we're telling right now conform to a basic narrative, which is that we have been invaded by this intruder. You know, whether we say it's the Chinese virus or it's from Wuhan or, you know, from some otherworldly place - maybe it's a bat or some strange eating habit or, you know, something outside of us - there's this sense of us being the passive victims of these invasive germs.

And I think that kind of paradigm of, you know, I call it kind of microbial xenophobia, it's - you know, it's rooted in our history of how we've treated contagious diseases. It's rooted in germ theory, the advent of scientific medicine and all of that. And it's helped us in a lot of ways with antibiotics and vaccines. You know, we think of the microbe as this invasive contaminant, and then we target it with killing chemicals. But what it obscures is our own role in creating the opportunities that allow microbes to turn into pathogens and to spread between us.

And the problem with that is right now we have this new pathogen here. We're not going to have drugs and vaccines to save us from it in the short term. What we need to do is change our behavior. But the way we talk about the pathogen really obscures the role of human behavior in all of this. So I think that is something that we need to kind of grapple with.

DAVIES: Right. And so how do we need to change our behavior?

SHAH: A lot of ways. I mean, I think we need to look more deeply at the way we're interacting with nature. We need to look more deeply at the crisis of biodiversity, which is really the fundamental driver of all of these spillover pathogens coming into human populations. I mean, it's not just the novel coronavirus; it's also Ebola and Zika and HIV in the 1980s and West Nile virus and new kinds of Lyme disease, tick-borne disease.

You know, we have a whole sort of host of these pathogens that are coming out of animal populations into humans because we are destroying wildlife habitat at such a huge rate. You know, we're losing 150 species every day. So this biodiversity crisis is the fundamental driver. So we need to, I think, look at that more deeply and consider human health to be connected to the health of our livestock, our wildlife and our ecosystems more generally.

DAVIES: We need to take a break here. I'm just going to reintroduce you. Sonia Shah is a science writer and the author of several books. Her latest is "The Next Great Migration: The Beauty And Terror Of Life On The Move." We'll continue our conversation in just a moment. This is FRESH AIR.

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DAVIES: This is FRESH AIR, and we're speaking with Sonia Shah. She's a science writer and the author of several books, including "Pandemic," published in 2016. Her latest book is "The Next Great Migration: The Beauty And Terror Of Life On The Move."

One of the things you deal with in the book is the long-held idea that people on the move are aberrant, they're deviant, that it's kind of a threat. And it's interesting because you were born in the United States, but both your parents emigrated from India. And I'm interested in what your own experience was being in a country, in the United States, while you still had a lot of cultural ties and extended family in India, what it, you know, meant for your own sense of yourself and identity.

SHAH: I mean, I think I adopted the sense I got from everyone around us that I was somehow out of place, that it's somehow, you know, strange and anomalous for this body that traces its ancestry to the South Asian subcontinent to be present in North America. Like, I - you know, I would tell people where I'm - you know, I'm from. I'm from New York City. That's where I was born. And they say, no, no, no, where are you really from me? You know, and that's a very common experience of many people of color in the United States, that we're not believed. And we say, no, you know, we're American; we were born here.

You know, really, most of us are not more than one or two generations distant from an act of long-distance migration. And so - and all of us are migrants on some time scale or another, except for maybe a very few people in parts of Africa. And so, really, the - if anyone's anomalous, it's not me; it's the people who stayed still.

DAVIES: (Laughter) Right. Right. You write that for a long time, you would feel like, you know, I don't really have a right to root for the Boston Red Sox because, you know, I'm this kind of hybrid American or something like that. You said writing the book and researching the book changed your perspective on this. How?

SHAH: I mean, I just had, you know, that idea of the imposter syndrome, right? Like, I think I always felt as, like, an imposter American. I remember as a kid thinking - you know, being ashamed that I didn't like chocolate ice cream, for example, because I had some idea that real American kids liked chocolate ice cream. I liked strawberry ice cream, and that seemed very suspicious to me. And every place I've gone - you know, I've - I lived in Boston for over 10 years, but I still didn't feel entitled to say I'm a Bostonian.

You know, when the bombings happened at the Boston Marathon, I was living in Baltimore at the time, but I felt very close to the city of Boston because, you know, both my boys were born there, and I lived there for many years. But I had - you know, I noticed on my social media feed, like, all these people saying, I - you know, Boston strong and, you know, really identifying with the city of Boston at that time, and I did, too. But I didn't feel entitled to claim Boston as my own place, even though I saw other people who had lived there for less time than I had, you know, very easily doing that.

So, you know, all of that just brings to a point that you realize, like, you have to feel entitled to take up space, you know. And that's what writing this book has done for me, personally, is just feeling entitled to take up space wherever I am.

DAVIES: You know, you write that for centuries, leading intellectuals regarded migrating people and migrating animals even as aberrant, like, kind of out of the natural order of things, you know, that particular people, races, ethnic groups and animal species had a natural homeland or habitat; that's where they belong. Who propagated these ideas? And how did they connect to the power structures of those - of the time?

SHAH: Well, I traced most of these ideas back to Linnaeus, Carl Linnaeus, who's considered the father of modern taxonomy. And he's this 18th-century Swedish naturalist, and he kind of decided for all of us, well, where does everything belong? He named everything. And he - you know, he came from a very Christian household, and like most naturalists of the time, he was very religious. And he thought of nature as an expression of God's perfection. So everything was in its rightful place for him, you know. So wherever he found things, that's where they, quote, unquote, "belonged."

And that extended to his human taxonomy. So he decided that, you know, the people in Africa belong in Africa, the people in America, they belong in America, etc., to such an extent that he decided that all of these different peoples on different continents were not - you know, didn't have a shared ancestry, a shared migration history, but were actually separate subspecies of humans. And in fact, he called Africans even less than human, that they were sort of a hybrid between real humans and this other archaic mythical human that he called troglodytes.

But those ideas were incredibly influential because we see them today in our ideas about race and about where people belong and where wild species belong. You know, when a wild creature crosses from, you know, a different place into a new territory, we think of it as an invasive; we call it an alien. And, you know, we see hints of all of that in the way we make policy around immigration and newcomers in places around the world.

DAVIES: In the early 20th century, in the United States, when the country was experiencing a lot of immigration from Europe, there was a whole bunch of intellectuals who saw great danger in mixing people of different races or ethnicities - you know, the threat of the hybrid.

SHAH: Yeah.

DAVIES: Madison Grant, Henry Fairfield Osborn - people that you write about who wrote very influential stuff. What sort of science or alleged science was behind these theories?

SHAH: I mean, this was a huge endeavor in scientific inquiry, is to kind of map out the different human subspecies, you know. And it was called race science, and the idea was to try to figure out, like, what are the biological indicators that show that this person is a different subspecies from this person. And, you know, the subspecies concept came to be called, you know, what we now understand of as race.

And so they really felt like immigration would cause a biological catastrophe because they thought that, you know, people from Africa or, you know, Polynesia or, you know, elsewhere in the world were biologically distinct. And so if they came into our country and started to partner with native people, with local people, that they would have these hybrid children that would be, like, deformed, essentially, degenerated and deformed.

And they - you know, they did all this science to try to prove that, which, you know, by modern standards is not scientific as we understand it at all. But they were scientists, so they had that, you know, kind of authority of being people who are looking into this really deeply. But even, you know, leading people in the American Public Health Association and, you know, the president - President Calvin Coolidge actually wrote about what he called biological laws according to which divergent people could not mix or blend.

And, you know, the Public Health Association said that if we allowed immigrants in - and these are people who are of different subspecies - to mix with native people, that that would lead to hybrid generations that would, you know, lead to - I think the quote was absolute ruin for American society.

DAVIES: Sonia Shah is a science writer. Her 2016 book is "Pandemic: Tracking Contagions From Cholera To Ebola And Beyond." Her latest book is "The Next Great Migration: The Beauty And Terror Of Life On The Move." We'll talk some more after a short break. I'm Dave Davies, and this is FRESH AIR.

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DAVIES: This is FRESH AIR. I'm Dave Davies, in today for Terry Gross. We're speaking with science writer Sonia Shah, whose latest book deals with the history of mass migration among humans, plants and animals. She writes that, while fear and hatred of immigrants has flourished at times, scientific research shows migration is more likely to bring benefit than harm. Her book is "The Next Great Migration: The Beauty And Terror Of Life On The Move."

It's interesting that in the early 20th century, you know, there were a lot of racist theories, which held that immigration was a terrible thing, that it would dilute the purity of Americans. And along with this came the idea that animals, too, didn't migrate, right? I mean, animals had a place that they belonged. And, you know, I guess the status of scientific and natural research wasn't yet in a position to really test that. You write that over the subsequent decades, we've come to understand just how much mass migration there is. You want to give us some examples of the distances that animals traverse routinely in migration?

SHAH: Well, I think this is just starting to get mapped right now because we've had this absolute revolution in our ability to track the way animals move. I mean, in the past, we only could see animal movement sort of episodically in glimpses, you know? We didn't have batteries that lasted long enough to follow them around. And, you know, even if you were able to follow an animal around, you wouldn't have funding for it because animal movement was considered sort of instinctual and robotic. And, you know, there wasn't a lot of scientific interest in it.

But what scientists are discovering now is that - you know, we're able to use new technology to study the way animals move. And so with solar technology and GPS, we can track animals 24/7 over the course of their lifetimes sort of continuously. So you can see the full picture of the way they move. And what they're finding is that these creatures are moving farther and faster and in more complex and responsive ways, dynamic ways, than anyone ever thought before.

And, you know, what's funny is, like, we've created all these parks and reservations to kind of protect animals. And when we've actually now studied, well, where do they actually go? It turns out, well, the giraffes, you know, they're supposed to stay in the park in Ethiopia that we set aside for them. In fact, they're crossing borders and going, you know, much farther than that. The turtles are swimming well beyond the boundaries of the marine-protected zones we've made up for them.

So - and we see animals are moving sort of en masse now because of the climate crisis, you know, the climate change, that tens of thousands of species are moving towards the poles and up into the heights, including, you know, creatures that we don't think of as mobile, you know? We think of coral reefs, for example, as very still, as stone walls, essentially. And coral reefs are moving.

Coral polyps around Japan, for example, have moved about 14 kilometers every year since the 1930s. Forests are moving up the Himalayan mountains, climbing uphill 19 meters a decade. And, you know, there's some animals that have moved even farther, like Atlantic cod, which have shifted more than 200 kilometers every decade. So there's just this sense that we're just getting - just being revealed to us of a planet that's on the move and in a very dynamic, responsive way.

DAVIES: Right. And then you have plenty of animals that have long, regular migration routes. I mean, these little, tiny monarch butterflies, you know, go from New England to Mexico - thousands of miles. Eels cross the Atlantic. And I'm wondering what your sense - what's the lesson for us about this constant motion among animal species? What's the lesson for our understanding of human migration?

SHAH: I think what we can see in animals is that their mobility allows them to adapt to environmental change. So we see in bird species, for example, if they rely on seasonally available fruits, they migrate more often than bird species that, say, feed on insects that are available year-round. Bats that live in trees, they migrate more than bat species that live in caves. Arthropods that live in seasonal ponds migrate more than those who live inside forests. So the more exposed you are to environmental change and variation, the more likely you are to migrate.

And whole ecosystems depend on animals on the move, you know? We see that - and this is something that's a complete reversal to how scientists used to think of wild migrants, as sort of parasites, you know, as disruptors and parasites. What we now understand is that, you know, over 90% of the trees and rainforests, for example, rely on the movement of birds and other animals to disperse their seeds. So wild migrants and wild movements really build the scaffolding for a lot of our ecosystems.

So we can see that in the natural world. So what does that mean about the way we move? Well, we know that humans are not - our bodies are not attuned and specifically adapted to specific niches, you know, the way that Linnaeus and the race scientists of the early 20th century thought of the human body as very specifically adapted to a certain place. So the African body was adapted to African environments, the European body to temperate, European climates, et cetera. And that's not what we now know about the human body.

I mean, we - our genes are not - you know, they don't robotically dictate how our bodies function and our - how our bodies develop. And we're very fungible. Our genes are very responsive to environmental signals around the genes themselves from the diets of our mothers, from the climates we're exposed to. And that kind of fluidity and responsiveness, it's what allows our bodies to thrive in such widely variable places from the Tibetan Plateau to the middle of rainforests.

And it's the kind of thing that doesn't evolve in species that are staying in one place and that are highly adapted to one specific location. Our bodies are just the opposite. We're very fungible. And so our bodies really are built to migrate. So you have to think about, like, the whole picture of, why did we evolve this way? And how did our - how did migration come to be such a prominent part of our history? It's because its benefits outweighed its risks over the long-term. So this whole idea of migration as a crisis is what I'm trying to kind of interrogate. And it seems to me that it could be just the opposite, that migration isn't the crisis, migration is the solution.

DAVIES: You know, on the subject of animals moving around and plants, you attack the idea of invasive species, you know? There's the idea that - there are often these panics that some species of plant or animal is coming. And it's going to, you know, create havoc in our local ecosystems. And I've got to say, this is a pretty persistent idea. I mean, there was, you know, kudzu, this sort of vine that took over a lot of areas in the south. Now we've got the spotted lanternfly, which the Pennsylvania Agriculture Department wants us to step on whenever we see it because it's going to damage trees. And then the latest thing is the Asian giant hornets, which destroy beehives and present such a hazard. Are we all wrong about this stuff?

SHAH: I mean, I don't deny that those disruptions are occurring. But I dispute how - you know, what the reason is behind it. There's so many species that are moving around. And to think that every newcomer who comes into your territory is sort of alien to it, as opposed to the natives who belong there, I think that's where we go wrong. I mean, it's interesting with the hornets, you know? We - this is very typical of how we respond to novel creatures that we think are out of place.

I mean, we're calling these hornets murder hornets, which is a very pejorative way to talk about anything, really. We don't - you know, lions also are predators. We don't call them murder cats. But, you know, so this is a pejorative way of looking at them. And what they're really threatening is, you know, honeybees and stuff, which are also not native either. They're from Europe. So you know, I'm not saying the disruption doesn't exist. But it's the way we think about it.

What we know is that only 10% of species that move into new places are able to establish themselves. And then only 10% of those are - become sort of pesty, you know, cause disruptions, unwanted effects on either human health or economies or on already-resident species. So we're talking about 1% of all the species that are moving around actually causing these problems of what we call invasiveness.

And yet, we have this approach, you know, that's embedded in our conservation policies, like the Convention on Biological Diversity, which recommends that we, you know, detect species that are new in our environments early, that we repel them and that we eradicate them before they are able to establish themselves. And the idea there is that you get rid of them because you're presuming that they're going to cause problems.

And so that presumption, I think, is what is problematic because - and especially right now because the climate is changing. And we need species to move into new places. And we don't want to repel them as invaders or aliens just because they're moving into new places, which is what's going to allow them to survive.

DAVIES: We're speaking with Sonia Shah. She's a science writer and the author of several books. Her latest is "The Next Great Migration: The Beauty And Terror Of Life On The Move." We will continue our conversation in just a moment. This is FRESH AIR.

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DAVIES: This is FRESH AIR, and we're speaking with Sonia Shah. She's a science writer and the author of several books, including "Pandemic," published in 2016. Her latest book is "The Next Great Migration: The Beauty And Terror Of Life On The Move."

I was fascinated reading the book that another source of anti-immigration sentiment was "The Population Bomb." That was a book written in 1968 by a guy named Paul Ehrlich, you know, warning of the dangers of global overpopulation - that this actually stoked anti-immigration feelings. You want to just tell us about this book and its impact?

SHAH: So this book was highly influential at the time in the 1970s. And it wasn't just Paul Ehrlich. There was also a whole host of population ecologists and other people in the biological sciences who were raising the alarm about birth rates going up and death rates falling. And this was tied into the Second World War. So after World War II, we saw, you know, antibiotics and fertilizers and all of these great, new technological advancements that allowed people to live longer, you know? We didn't have to die from infectious diseases as much.

And that was happening even in places like India and China, that death rates were falling. At the same time, birth rates were going up in places like the U.S. and Europe after the war because of what we now understand as the baby boom. But the sense of these ecologists at the time was that this is going to be a disaster. And what they saw was what the 18th century cleric Malthus - Thomas Robert Malthus - had saw. And he said in the 18th century that if we start helping the poor and giving them food and clothing that - you know, that's not charity because if we assist the poor and if we make their lives better, then they'll just keep - they'll survive more. And there'll be more and more of them. And that will lead to scarcity and conflict and this sort of permanent state of crisis.

Now, Malthus was proved wrong in the centuries after he came up with that theory. In fact, as we conquered infectious diseases and our mortality rates fell, our birthrates fell, too. But these same concerns came up again in the post-war era with this whole real scare about what they called overpopulation. Major magazines having covers with all these women with babies hanging all over them - mostly brown women - focused on Third World women's reproduction, that they were just going to have too many babies.

And so that's what Paul Ehrlich was talking about in his book "The Population Bomb." It came out in the 1960s. And it was a huge hit. He was on Johnny Carson a bunch of times. He started an organization called Zero Population Growth, which was hugely influential and popular across college campuses and, ultimately, really inspired the founder of what is now the modern anti-immigration movement, John Tanton, who was a huge enthusiast of "The Population Bomb."

And he thought the main problem was that if we opened up our immigration laws, if we opened up our borders as we had done in 1965, that that would be a huge crisis because all of those hungry, you know, overpopulated peoples would start flooding the United States. And so these concerns of overpopulation sort of bled into this new push to close the border because we didn't want those overpopulated societies to come and ransack our society.

DAVIES: Well, and it was striking to me that environmentalists who talked about the ecological damage that population growth was doing, many of them embraced the idea that immigration had to be curtailed as well.

SHAH: Yeah. I mean, John Tanton started out as an environmentalist. He was a conservationist. He was active in the Sierra Club and other environmental conservation organizations. So major environmental organizations of that time agreed that immigration was a problem, that if we had too many people here, that we too would become, quote, unquote, "overpopulated" and that that would be environmentally really disruptive and damaging. And so it was only as the kind of more racial implications of the overpopulation debate became clear that the environmental movement kind of steered off from these anti-immigration stances.

DAVIES: You know, for this book, you spent some time in a lot of places, including parts of the United States, the U.S.-Mexico border and places in Europe where immigrants, often fleeing desperate circumstances, come up against government efforts to keep them out or make their journeys harder or more dangerous. Would you just share some of what you observed and how it informed your thinking for this book?

SHAH: Yeah, so I did reporting in different parts of the world, where I was looking at how people move and what happens to them. And what was really striking to me is that our attempts to repel movement, to bar and arrest movement, have such an outsized effect, you know. And we think of it as, you know, well, if we put up this wall, if we make it harder for them, we demand documents, do all these things, that it'll mean less people will move around. And in fact, it's - that's not what happens. It doesn't arrest movement.

These borders, these walls, these - all of these barriers we put up to movement, in fact, just deflect it. So people start moving deeper into the desert in order to get around the border checks. People start going into, you know, the more turbulent waters to get to shores that they can land on. And so, you know, right now we have more borders around the world fortified with walls and other barriers than ever before. And what that's done is it hasn't repelled movement. It hasn't made people stay home; it's just made migration a lot more deadly.

So people are risking their lives now to find refuge from bombs and beheadings and poverty and the rest of it. So that was what was most striking to me. Another thing about reporting on migration is you basically can go anywhere because there's migration happening sort of behind the scenes everywhere you go. So what I wanted to do is look at, well, where are people getting stuck, you know, and whether it's refugee camps or border checks or you know dying in the desert on the U.S.-Mexico border.

DAVIES: So let me ask the contrarian question here. If we grant that migration is sort of a natural condition of life on Earth for, you know, millions of years, one could argue that things have changed since our ancestors mastered agriculture and created cities and, eventually, these institutions, however unnatural, nation-states. And so we have these situations where people in a given area are governed by common systems of laws and economic regulations and that accord some benefits - you know, education, public safety to a degree. Is it necessarily wrong for governments to impose limits on the number of people who are fleeing countries that can't or won't provide the basic kind of economic and physical security that their own countries do?

This is not to excuse some of the policies that we've seen, like separating kids at the border or crazy rules for people who seek asylum. But just on the basic principle, do nation-states mean that a different kind of laws makes sense?

SHAH: Of course, human societies can decide how people should behave when they're part of their group - who can come in and how they should act once they come in, you know. And we do that, and that's fine. You know, people can move into new societies, and they're still subject to all of those rules. I think what we don't absorb enough in our policymaking is that people assimilate so quickly. Assimilation really works. If you let it happen, it really works.

Within a generation and within two, you don't see any difference in our major indicators between, you know, native peoples and newcomers. And that starts to change immediately, as Franz Boas found when he measured the bodily dimensions of all of these first-generation immigrant children. You know, we start to change immediately because we can move into different places and successfully. So if we allow people to assimilate, if we don't hold them apart, then we can absorb them into our societies.

And that doesn't absolve any newcomers from obeying the rules that we've come up with. But it does mean that we can think of them coming into our societies in a whole new way. You know, we don't have to think of it as, like, well, they're going to come here and disrupt us, which is how our rule-making is - you know, that's how we make our rules right now. But if we had a new idea about what migration means, then we could rethink all of that.

DAVIES: We're speaking with Sonia Shah. She's a science writer and the author of several books. Her latest is "The Next Great Migration: The Beauty And Terror Of Life On The Move." We will continue our conversation in just a moment. This is FRESH AIR.

(SOUNDBITE OF TERRY SLINGBAUM'S "WATER GAMES - RAVEL RE-IMAGINED")

DAVIES: This is FRESH AIR, and we're speaking with Sonia Shah. She's a science writer and the author of several books, including "Pandemic," published in 2016. Her latest book is "The Next Great Migration: The Beauty And Terror Of Life On The Move."

You write in the book that the next great migration is upon us. Is this the migration driven by climate change?

SHAH: Yeah. That's what's happening with wild species. Up to 80% of wild species that have been tracked are moving towards the poles and up into altitude. And we see also migration patterns in humans are also changing. And so, you know, given what we understand, that migration is a response to environmental change and a way to adapt to environmental change, we can predict that, as the climate changes, that we're going to continue to see, you know, a new kind of migration happening.

DAVIES: Right. And one estimate, I think, you quote is that there could be 200 million climate refugees by the middle of this century. To a lot of people, that's a very scary number (laughter).

SHAH: Yeah. And I think I am reluctant to throw the numbers around because some of the estimates are done in order to cause alarm because they sound like a really big number. But I think the thing to remember is that there's so many people moving around already, continuously, that we absorb without really thinking about it.

Even if we have a lot of people who do have to move because of climate change, it doesn't mean it has to be catastrophic. So sea level rise, for example - it doesn't all happen all at once and people suddenly say, oh, my God, I have to leave. It's - you know, there's indicators that are happening slowly. As the fields dry up or there's saltwater intrusion or - you know, things happen slowly.

And if we had open avenues, if we had legal pathways for people to move, we might see much more organic moving away from less habitable places to more habitable places in a way that wasn't, you know, an army of refugees who suddenly kind of form out of nowhere like a tsunami, which is sort of the alarmist vision of it when people throw around these big numbers of how many people are going to have to move because of climate change.

DAVIES: Yeah. Another way of looking at it is 200 million people is, like, a quarter of 1% of the world's population. So, I mean, it's a lot of people, but it's a big world, too. One of the points you make, I think, is that people tend to - that climate change migration tends to occur not at a big moment of crisis like a tsunami but gradually over time.

SHAH: Yes. And I think that's something we should encourage.

DAVIES: I mean, that's - we're sort of not in a political climate that embraces that idea right now, to say the least.

SHAH: (Laughter).

DAVIES: I mean, there's a lot of demonization that goes on. Do you see a way for things to change?

SHAH: I mean, it's hard to know. It's hard to know. I mean, I think as more of us do need to move, you know, the people in power are still sort of cocooned from the worst effects of this. But one thing I talk about in the book is this idea of a migration ratio, that if the time it takes for you to rear the next generation exceeds the amount of time you can expect your habitat to be stable, then you're more likely to encourage your next generation to migrate. So that's sort of an ecological scenario that migration experts have come up with to explain why migration evolves in different species, and it's sort of this ratio.

And if you think about that for us, you know, that's becoming true for an increasing number of us, not just people who live on faraway islands in the Pacific but also people who live in California and in the middle of Australia and, you know, around the Chesapeake Bay and - you know, all these places that we thought of as really stable, you know, that these are places that are going to be the same for our kids and their kids and on and on. And that formula is changing. You know, that calculus is going to change. And I can imagine that, as that happens, that we will also start to come to a new understanding of what migration really means for us.

DAVIES: Well, Sonia Shah, thank you so much for speaking with us again.

SHAH: Thank you.

DAVIES: Sonia Shah is a science writer. Her 2016 book is "Pandemic: Tracking Contagions From Cholera And Ebola And Beyond." Her latest book is "The Next Great Migration: The Beauty And Terror Of Life On The Move."

On tomorrow's show, we'll talk about the protests over the death of George Floyd and the police response with Wes Moore. He's the author of a book about the 2015 death of Freddie Gray in the back of a Baltimore police van and the uprising that followed. Moore is president of the anti-poverty organization the Robin Hood Foundation and author of the bestselling book "The Other Wes Moore." I hope you can join us.

FRESH AIR's executive producer is Danny Miller. Our technical director and engineer is Audrey Bentham. We had additional engineering help from Mike Villers. Our interviews and reviews are produced and edited by Amy Salit, Phyllis Myers, Sam Briger, Lauren Krenzel, Heidi Saman, Therese Madden, Thea Chaloner and Seth Kelley. Our associate producer of digital media is Molly Seavy-Nesper. Roberta Shorrock directs the show. For Terry Gross, I'm Dave Davies.

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