IRA FLATOW: This is Science Friday. I’m Ira Flatow. Now I want you to get a little philosophical, just for a second. And I’m going to ask you this. Why are there leaves on trees? Ever thought about that? You know, I don’t mean, oh, for the photosynthesis so the trees can make food. No.

What I’m getting at is this. Why aren’t those leaves just devoured by bugs? Why don’t the slugs and the caterpillars and the grasshoppers just explode in number and eat everything in sight? You know, I mean, something must be keeping them in check, right, from eating all of the leaves they see. But what is that?

These are the sorts of ecological head scratchers my next guest writes about in his new book, The Serengeti Rules– The Quest to Discover How Life Works and Why It Matters. And he comes up with a lot of beautiful examples of how nature regulates itself and keeps things in balance, like those tree leaves. And reading it, you might just look a little differently at the natural world around you.

Sean B. Carroll is the author of, The Serengeti Rules, and a professor of molecular biology and genetics at University of Wisconsin at Madison. And he joins us today from WAMU. Welcome back to the show.

SEAN B. CARROLL: Thanks for having me, Ira.

IRA FLATOW: You’re welcome. I want to just tell our listeners that we have an excerpt from your book and they can read a little bit of the book at ScienceFriday.com/rules. Now Sean, in the book you tell the story of this guy who’s peeling starfish off rocks and chucking ’em into the ocean! What’s going on there?

SEAN B. CARROLL: Yeah, it’s one of the most important experiments in the history of ecology. The man was Bob Paine, who’s currently still a professor at the University of Washington. As a young professor, he tested this idea of, do predators really do anything in ecosystems? And he found the perfect system, far up on the Pacific Northwest coast. And he found the perfect creature with these starfish that feed on mussels and barnacles and things like that.

And he thought, well, let’s find out what they do. So on one plot he left all the starfish. On another plot of rock, about six-foot-five Bob is. He’d go up with a crowbar, pry the starfish off and hurl ’em out into the ocean. And do that about once a month. And when he came back, it didn’t take long, several months before he saw some pretty dramatic effects.

IRA FLATOW: Wow. And what was this to prove? What was he trying to show?

SEAN B. CARROLL: It was testing the idea that you mentioned just a minute ago, which was the thinking in ecology, up through the early 1960s, was that sort of everything in the world was regulated from the bottom up. That plants gathered sunlight and produced the food that fed the herbivores, the plant-eaters. And then there are some carnivores that if there were enough plant eaters around they could feed on them.

But everyone thought all was flowing from the bottom up. And it was one of Bob Paine’s professors, Fred Smith, who put this notion in his head that maybe it works the other way around. Maybe the reason why the world is green is that predators are out there keeping the bugs and other things from eating all the greenery. And so the way he tested that in removing starfish was to see, that in fact, the starfish were keeping in check animals that would otherwise sort of overtake all the space and crowd out things like kelp and algae and things like that in the tidal edge of the Pacific Northwest.

IRA FLATOW: And in fact you tell that tale of one of the Bob Paine’s students who wants to study sea otter metabolism. And Bob says, no, no, no, you got it all wrong. What you really want to study is sea otters and the kelp.

SEAN B. CARROLL: That’s right.

IRA FLATOW: Kelp?

SEAN B. CARROLL: Yes, exactly. Jim Estes was a student, sort of just starting his thesis work. He’s now professor at the University of California Santa Cruz. And he met Bob on Amchitka Island in the Aleutians in the early 1970s. And he was going to study sea otters because they’re amazing creatures that stay out at sea. They have to keep their metabolism going with lots of food.

And Jim was thinking, well, I want to see how the kelp community supports these sea otters. And Bob said, you’ve got all wrong, Jim. You got to look at what the otters are doing to the kelp community. And the way Jim Estes checked that out was he went to some other Aleutian Islands that didn’t have any otters. And the appearance was remarkable.

Jim describes it as the most dramatic moment of learning in his life. That as soon as he put his head under the water in islands that didn’t have any otters, they were just a carpet of sea urchins with no kelp at all. So without the otters around to control the urchins, the urchins just mowed down the kelp forest. And the dramatic impact that has is that the kelp forest is habitat for all sorts of creatures, both in the water and outside.

IRA FLATOW: So what you say in your book is that Bob Paine’s students and Bob Paine’s experiments with the starfish show that there is a certain kind of a species– there is the most important species of an ecological system.

SEAN B. CARROLL: Yeah. Bob’s idea was the keystone. He got it from architectural. While he claims that he doesn’t know a lot about architecture, he knew enough to come up with this idea that a keystone species, like the keystone in a Roman arch– if you remove that last stone, or that top apex stone, then the sides of the arch collapse.

And what Bob saw with his experiments– and he repeated the experiments in other places in the world, such as New Zealand– is that when we removed this apex predator, this starfish from the community, the whole diversity of the community collapsed. And all that essentially remained of the initial 15 species he monitored was muscles that took over the whole space. So this keystone idea was born in the late 1960s, and over several decades ecologists have discovered all sorts of keystone species in all sorts of habitats around the world.

IRA FLATOW: So not only is there a keystone species, but this has now become a keystone in thinking about the ecology.

SEAN B. CARROLL: It’s a fundamental idea. Absolutely. And its strikingly different and surprising from the way we sort of thought the world worked. In Bob’s language, he says, all animals are equal but some animals are more equal than others. And it’s saying that some animals have a much stronger effect on other creatures in their ecosystems than others. And that’s a really important thing to understand as we try to manage these to supply our own needs.

IRA FLATOW: If you just joined us, I’m talking with Sean B. Carroll who was written a book called, The Serengeti Rules: The Quest to Discover How Life Works and Why It Matters. And now that you set up this whole idea about the keystone species, we now get to the concept of your book, The Serengeti Rules. You take this into the Serengeti, which is the real main character in your book.

SEAN B. CARROLL: Yeah, it was kind of the inspiration. I took a trip there in 2014 and I was just– even if I’d seen it many times on TV and read about and things like that, it was my first trip and I was just awestruck really by the numbers of animals. Just amazing– tens of thousands of mammals that you’re looking at.

And I thought, why are there a million wildebeest and a few thousand topi? Why are there 20,000 giraffes? And does anybody know? And I had no idea. So I started reading the literature as a scientist would do. And I started discovering five decades of work by all sorts of pioneers. And I thought, in naming the book, that I was looking for sort of these general rules of ecology. But I thought I’d name them the Serengeti rules and not like the Lake Erie rules or something like that. I think it’s better marketing. But we’ll find out.

IRA FLATOW: Well you do get into Lake Erie at a certain part of the book, but not about those kinds of animals, but algae blooms.

You have a diagram in the book connecting– and this sort of explains the interconnecting web of things– a cattle disease called rinderpest and to more giraffes on the Savannah– you relate the two. And it goes through the web. Give us an idea of how that works.

SEAN B. CARROLL: Well the general idea to have in mind is it one the major goals of biology is to understand the connections between things. And those connections are often really surprising. And this is certainly true on the Serengeti. What happened in the 1960s was that a virus that had been around for about seven decades that had come in with cattle was eradicated because of a vaccine.

And the sort of spillover effect of that was that it protected– the disappearance of the virus from cattle ended up protecting wildebeest and buffalo which were vulnerable to this virus. And wildebeest and buffalo numbers exploded beyond anyone’s imagination. So wildebeest mushroomed from about 200,000 animals to as many as 1.4 million. And as those wildebeest munched their way around the Serengeti in about a 600 mile circuit– that sort of flotilla of love lawn mowers– changes the whole ecosystem, mows the grass to a lower level, opens up habitat for butterflies. And by mowing the grass down, it reduces both the frequency and intensity of fires. OK.

IRA FLATOW: Right.

SEAN B. CARROLL: Here’s a domino effect. The virus goes away, more wildebeest, less grass. Less grass, now fewer fires. Fewer fires allows tree seedlings to take root and to grow. And as those trees grow up that’s more habitat for animals like the woodlands, but it’s also more food for things like giraffes that graze on the acacia. And so a virus that doesn’t affect giraffes directly, through wildebeest, through grass, through fires, et cetera, ends up affecting the number of giraffes on the Serengeti. And it’s those kinds of revelations that have really sort of spun our heads around as to the connections in nature.

IRA FLATOW: But you know, when you think about the influence that humans, that people have had on nature, you’re tempted to say that the humans are a keystone species also.

SEAN B. CARROLL: Oh it’s exactly what Bob Paine would say. In fact he’d say we’re the over-dominant keystones. And we better understand our role because we’ve inserted ourselves into food chains everywhere. Really what I’m talking about are sort of these extended food chains of one thing affects another thing which affects another thing. And we are dependent upon all sorts of food chains to supply our needs. And we therefore need to manage those food chains our best scientific understanding and our best management principles or things aren’t going to turn out so for us.

IRA FLATOW: But do we have a sub-keystone species in our own societies?

SEAN B. CARROLL: Do you mean in human societies?

IRA FLATOW: In human societies. You know, we have different cultural ideas, different economic plans. You know, I don’t think a wildebeest, it’s going to have any of that kind of stuff in its culture.

SEAN B. CARROLL: No. I mean, the Serengeti rules are sort of simple ecological rules of nature. And we try to suspend those rules for ourselves. I mean, we are always combating disease. We combat famine by the way we distribute food. So all of our cultural institutions have generally been, over the decades and centuries, insulating us from being so vulnerable to nature as our ancestors were.

And the dramatic effect of that is that the human population in the last 200 years has zoomed from one billion to seven and 1/2 billion. So these institutions are really successful. They’ve allowed us to live longer, and for many more of us to survive. But that means our biological footprint on the globe is ever more intense. So it’s a big trade-off.

IRA FLATOW: But we don’t have– just like the wildebeest and the other animals, we’re not going to be able to have infinite size herds either? Are we?

SEAN B. CARROLL: No, we’re not. We’re going to bump up against sort of the productive limits of the world. But we’re also going to bump up against our ability to manage these things. And there’s a lot a negative stories out there. But there’s a lot a positive experience of, once we learn something about, for example– say, take fisheries as an example. I mean, fisheries– you know, fish live in a fish eat fish world. And as we understand the way they’re connected, we understand how some of our fishing habits have these sort of chain reaction effects that we may not like, all the way from say, the open ocean to coastal waters and shellfish.

And if we understand why that is, then we’ve got a better chance to manage ourselves and make these fisheries as productive and sustainable as possible. And the good news is– and I think this is a really under-reported thing– is how resilient nature is. We have so many examples of populations of things that have been really reduced by human activity. And when we give them some chance to rebound, the rebound can be really dramatic. And so it’s just a combination, more easily said than done, but it’s a combination of good science and good management. And we got a good shot at feeding ourselves for a long time.

You write about the Serengeti, and you’re optimistic about it from the history that even the Serengeti was turned into a protected zone when it was noticed how beautiful and wonderful the animals that were living there.

SEAN B. CARROLL: Yeah. I mean, I think we kind of woke up early– there were pioneers, people like Julian Huxley, who had a different idea of how places like the Serengeti could be used, as opposed to just being hunting areas, but they could actually be protected. But what I’m really talking about in the book is not just the pretty places and not just about making places pretty, but really making them functional. Making them as productive as they can be, which means using our understanding of how they work to manage them wisely for the longer term.

And whether that’s crop management or even and how we manage freshwater lakes that we’re dependent upon for our water supply, we know better now. 50 years ago we didn’t know much about the connections between creatures but now we know better and we’re just going to have to be a little more prudent in our practices.

IRA FLATOW: Talking with Sean B. Carroll, author of the Serengeti Rules: The Quest to Discover How Life Works and Why It Matters on Science Friday from PRI, Public Radio International. Let’s see if we can get a quick phone call in before we have to go. Let’s go to Christopher in Oregon. Welcome to Science Friday.

CHRISTOPHER: Hey, how’s it going?

IRA FLATOW: Hey there.

CHRISTOPHER: Yeah, I just want to mention. You know, there is other interactions that we have in urban spaces that involve apex predators as well. Lyme disease, which is most associated with deer, on the east coast, because of the divided green space because basically the urban sprawl there, the lack of predators is effecting the mouse, creates larger populations which actually increases the incidence of Lyme disease and ticks. So there’s actually interactions that occur locally due to our own development that affects apex predators. And I just thought it was an interesting thing that when you were mentioning that about the starfish and how it actually relates, even to our urban infrastructure.

IRA FLATOW: Yeah. Good point. Sean?

CHRISTOPHER: That’s a great point, Christopher, and I would say, you know, we also know this on our crop fields that things like spiders and other predators that serve as natural enemies of pests– around the world you can see examples where having those predators intact is a much better form of bio-control than pesticides. Certain pesticides in fact kill off those natural enemies and allow certain plagues of insects to takeover. So sort of every places is a Serengeti. I use it as sort of the metaphor, but it turns out predators are everywhere, whether they’re tiny spiders or gigantic lions.

IRA FLATOW: One last thought– is climate change going to mix up the keystone species a little do you think?

SEAN B. CARROLL: Climate change is going to affect all ecosystems. In the book I really emphasized what we know is going on even before climate change becomes a factor. So the changes that are taking place in ecosystems because of human activity– hunting, fishing, development, et cetera, that’s sort of the baseline. And then now add climate change on top of that. But I think that it’s going to be a case by case basis, a region by region basis how well we’re going to be able to manage those effects.

IRA FLATOW: But you can see the effects beginning to take place.

SEAN B. CARROLL: Oh, sure. If you’re warming lake water, you’re going to get blooms a lot faster and a lot earlier in the year and those blooms are going to suffocate fish and make things undrinkable, unswimmable. There’s lots of some simple things that we can already observe, say going on the northern US.

IRA FLATOW: Sean, thanks for taking time to be with us today. Excellent, excellent book.

SEAN B. CARROLL: Thanks for having me, Ira.

IRA FLATOW: You write so well. Sean B. Carroll, author of The Serengeti Rules: The Quest to Discover How Life Works And Why It Matters. He’s a professor of molecular biology and genetics at the University of Wisconsin in Madison. And if you want to have a quick look at it, you can go to our website. It’s ScienceFriday.com/rules. Go to that website. It’s slash rules and you can read an excerpt from the book. So easy to read.

Copyright © 2016 Science Friday Initiative. All rights reserved. Science Friday transcripts are produced on a tight deadline by 3Play Media. Fidelity to the original aired/published audio or video file might vary, and text might be updated or amended in the future. For the authoritative record of ScienceFriday’s programming, please visit the original aired/published recording. For terms of use and more information, visit our policies pages at http://www.sciencefriday.com/about/policies.