The High Costs — And Potential Gains — Of Migration, Both Animal And Human

Science journalist Sonia Shah, herself the child of Indian immigrants, has long been fascinated with the way animals, people and even microbes move. She received acclaim for her 2016 book "Pandemic," which tracked contagions. Shah’s latest book, "The Next Great Migration: The Beauty and Terror of Life on the Move," delves into the history and science of migration. Speaking with Shannon Henry Kleiber for "To The Best Of Our Knowledge," she said that migration is both a crisis and an opportunity.

This transcript has been edited for brevity and clarity.

Shannon Henry Kleiber: Your book is called "The Next Great Migration." Are we already in it and how do we know?

Sonia Shah: I think we are in it. We know that around 80 percent of wild species are shifting their ranges, so they're on the move right now, moving into new habitats, toward the poles and up into the heights. And we know that even seemingly stationary, sedentary parts of the natural world are also moving. Forests are moving; coral reefs are moving. So we have this large scale movement that we are tracking already, and it's probably just starting.

SHK: I love how there are certain animals and insects in your book that are like characters, like the checkerspot butterflies. Can you give me some examples about how we can learn from animals about how we move too?

SS: Well, the checkerspot butterfly is an interesting example because this is just a very small nondescript little butterfly. The particular subspecies is one that lives on the western coast of North America. It was considered to be doomed. They were widely understood by scientists who study them as sedentary. That is that they didn't move very much from their habitat. They were thought to be exquisitely adapted to this very particular kind of environment they lived in. They relied on these little native host plants. They seemed very fragile and sedentary. And because of climate change ... the southern part of their range was shrinking and the top part of their range was also shrinking down because of urban expansion in San Francisco and Los Angeles. So this butterfly was thought of as doomed to become extinct. And what happened is just the opposite, that it actually started to move in response to climate change. And that was tracked early on by a scientist named Camille Parmesan.

And when she realized that she started to get in touch with other scientists, too, who wanted to look at their data and look at those animals that they were studying, and they found that those all moved also.

So it was this one study of these checkerspot butterflies that kind of sparked this much larger inquiry that revealed that species around the world are moving in response to climate change. So this little checkerspot butterfly had an outsized effect, even though it itself is this rather nondescript little creature.

SHK: You call this butterfly a "homebody." It likes to be where it feels comfortable, which makes sense. But migration is inherently dangerous and difficult. And so why do we do it if we aren't forced to do it or does it take being forced to do it to happen?

SS: I don't think we know the answers to that question. But what I try to expose in the book is that there must be huge benefits because we know the costs, as you mentioned, are huge. 

We’re living through some of the costs of movement right now with this pandemic. And yet what we're discovering is that there's so much movement going on right now in the present among wild species and humans, but also in the past. And so that really begs the question of what are the benefits of this mobility that could outweigh these very heavy costs that we've studiously delineated? We know in wild species that migrating is a way to adapt to environmental variation.

So species that live in habitats that are more exposed to environmental change — like bird species that rely on seasonally available fruits — are more likely to migrate compared to bird species that feed on insects in the interior forests. Arthropods that live in seasonal ponds are more likely to migrate and to have wings than those that live in the interior forests. So that gives us some clues. Bats that live in trees migrate more than bat species that live in caves, for example. We know that ecosystems rely on migrants to connect them. You look at just seed dispersal. For example, 90 percent of the trees in rainforests rely on animals on the move to carry their seeds around. And we know a lot of plants have evolved special ways to attract animals on the move to carry them around ... whether it's the birds that stick to your dog's fur or the beautiful fruit that we all want to eat and carry around and disperse seeds around.

SHK: Do you think there's a fundamental need for change in the way humans interact with wildlife?

SS: We’re living in a moment where we've destroyed so much wildlife habitat. Humans and our cities, our towns, our mines, our farms have swallowed up over half of the planet's surface with all of our stuff and our infrastructure. And it means that there's much less habitat left for wild species, which is why we're having this huge biodiversity crisis that we're living through now where we're losing 150 species every day. We're seeing the effects of that. The species that remain have to crowd into smaller fragments of wildlife habitat that we leave behind for them.

And it means that their microbes can more frequently spill over into our bodies. And we see the effect of that now with this coronavirus pandemic and emerging infectious diseases that are coming from animal bodies. 

We need to rethink all of that not just because of pandemics, but also because of the climate disasters. These are all things that are slow motion catastrophes of our own making. And they have to do with how we have interacted with nature. We’re seeing the biological catastrophe of overuse of natural resources right now, as well as the atmospheric catastrophe of climate change at the same time.

SHK: Wow. The slow-moving catastrophe. That's pretty amazing to think about. Your last book was called "Pandemic." I wanted to talk about that for a little bit. How are pandemics and migration related?

SS: Well, roughly speaking, contagions generally are a function of animals, peoples and microbes moving around in different ways and colliding in different ways. If you look at Ebola, for example, as a virus in bats, it spills over into humans. That happens in some forest area in West Africa, and then people carry that into cities. And so we have the movement of the bats relating to the movement of the microbes and then the people move it around. 

So contagion is one of the effects of our mobile planet. It's one of the disruptive effects of people, animals and microbes moving in new ways. And then they collide in new ways. And sometimes that provides opportunities for microbes to find new habitats to live inside of our bodies. And that sometimes can make us very sick. That's what we're seeing right now. I think much of my work in the past has been about those disruptive effects of biota on the move.

SHK: How will the way we move, whether we're in airplanes or we're on foot, shape the future of the COVID-19 pandemic?

SS: Well, one thing that's very striking is how we instantly decided "let's close the borders." As soon as this thing erupted, countries around the world decided to close their borders. And I think that was another expression of our underestimation of the scale and meaning of our own mobility. Because by that time, 7 million people had already left Wuhan, China and they'd carried that microbe around the world. 

I think policymakers around the planet really underestimated the scale of movement that's always going on. It's beneath our notice, except when something really destructive like this pandemic happens.

SHK: What about the idea that we belong in a place, that a place is our home? Is that concept changing or is it changed forever?

SS: I think there's a tension there because I think we want to belong to certain places. But at the same time, we also need to reckon with the fact that we are very mobile and we've moved a lot over the course of our history.

Almost all of us are only a few generations removed from an active long distance migration except for people in parts of Africa. When we migrated out of Africa into new places, it wasn't empty. There were already human populations there. And we mixed with them and we mingled with them. And their DNA is now in our DNA.

We're seeing this picture of kind of continuous movements all along. So I think this idea of "where do you belong" is really an artifact of a time when we didn't understand ourselves as a migratory species, a species that moves a lot and all the time.

SHK: Some of that, too, strikes me as racist and xenophobic. You don't belong to that place, the "us" and the "them." Do you think that's changing?

SS: I would say it should change, but I don't think it's changing. I think it's more real today than ever. 

I trace it back to Carl Linnaeus, who is the father of modern taxonomy, and he decided where everything belonged. He named the living world and thousands and thousands of species. He came up with a system for naming them and he decided that, OK, people in Africa belong in Africa. People in Europe belong in Europe. People in Asia belong in Asia. People in the Americas belong in the Americas — to such an extent that he said they were biologically isolated from each other and biologically alien to each other, that people who lived on different continents were different subspecies of humans. And in fact, people from Africa, he said, were almost subhuman, that they were not fully homo sapiens. 

To this day, with the killing of George Floyd, the first thing I thought of when you look at that man's face as he has his knee on this other person's neck, and he has this nonchalant expression. His hands are in his pockets and it's very much like he doesn't think that person under his knee is fully human. He's treating him like he's subhuman, like he's not fully a person. And that is a very old idea. And we see it's still so alive in our politics to this day.

SHK: How can we move on from that? What can we do?

SS: Understanding ourselves as having a shared humanity means that we're going to have to recover our lost history of migration and the ways we are all connected. The white nationalists have this thing where they chug milk, that's a big white supremacy thing. It’s supposed to indicate that they're Europeans and only Europeans are able to digest milk because they have a special gene that allows them to do that. But what they forget is that same gene exists in people from nomadic people, from parts of Africa, from parts of the Middle East, camel herders. They think that Europeans have these special biological attributes that are different from other peoples because they've been so isolated in their borders of Europe for so, so, so long. And they want to highlight that difference. But the fact is that those differences are fuzzy, they're nebulous. That same gene is present in people who they would never accept as like them. 

But we do all have this shared ancestry because we have moved around. So those connections that migrants create, that's what we need to recover. It's part of overcoming this old idea that we are naturally split into continental races that are color-coded by each continent: the Black people in Africa, the white people in Europe, the yellow people in Asia, the red people in the Americas. This is what Linnaeus said. And it still is so deeply ingrained in how we think today. 

But science has moved on since Linnaeus, and we know that we have more in common across those boundaries of race than within them because we've all moved around so much. And whenever people move into a new place, they mix with the people who are there. We meld very easily. So the genes in migrants’ bodies spread into the host populations. That's what creates the similarities between us. And that's what creates our bonds. And that's what makes us all human, is that we have this shared ancestry and this shared experience of migration.

SHK: It just makes me think that animals are doing such a better job than we are.

SS: Yeah. It's easy to think that, especially these days when we really don't seem to be doing well at all.

SHK: You mentioned the pandemic can also have a side that's not negative. Can you tell us a little bit more about that? And then can you also tell us about migration as a crisis or an opportunity or is it a mix of both?

SS: Pandemics are a social and political phenomenon that we should try our damnedest to prevent. And I think there's a lot of ways we can prevent them and also prepare better for them so that when they do occur we don't have as much death and destruction as we're seeing today.

But in terms of migration, I think human mobility has been fundamental to our biological resilience. Over the course of our long history, you look at our capacity to cooperate across geographic borders, and we have all these barriers. We have mountains and oceans that separate us. And yet we share our technology, we share our insights into how medicine works and our health works. There are all these things that we are able to share, and those ties are built by migrants.

The cultural connections forged by people on the move inject diversity into societies that would otherwise be insular, whether it's genetic diversity, but also cultural diversity and innovation. It's really this dynamic process that contributes to our resilience. That's the kind of fungibility and flexibility that arises in creatures that move into new places. It's why the human body can thrive in the Tibetan plateau as well as in the Brazilian rainforest. We can live in the desert, and we can live by the sea.

We can live in all these different places because our bodies are adaptable and they're sensitive to the environment. One example that I like is sweat glands, that we're all born with the same number of sweat glands. But the number of sweat glands that actually become functional depends on the climatic conditions around us during the first few years of life. 

So we're shapeshifters. We shift our bodies depending on what environment we find ourselves in. And that's the kind of ability that evolves in creatures that move.