Sean B. Carroll
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And one of my favorite examples is the influence of salmon in western rivers on tree growth. And you're like, what? Yes, trees need salmon. So who would have thought that?
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Let's start with one rule, which is really some animals are more important than others, or if you want to use the Orwell saying, are more equal than others. There's sort of a poetic description sometimes of nature, that everything matters and everything has its role. But it turns out that some creatures have a really outsized role in the diversity and stability of their ecosystems.
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When ecologists started to discover this, it was even to their own surprise, well, the consequences are great. So that if you think about, for example, people may be familiar with the story of both the
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eradication and then the later reintroduction of wolves in the yellowstone ecosystem so in the early part of the 20th century there was a great effort to eliminate wolves virtually everywhere in the lower 48 states you know because they were Disliked by humans, their effect on livestock, they were scary, et cetera, et cetera.
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What we didn't understand what we were doing was we were taking a creature that had a really outsized impact on the stability and diversity of its ecosystem. We were taking it out of the picture and this had huge consequences. One of those consequences we can still see all over the place, which is, for example, the enormous numbers of deer that are in many states.
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those deer in turn have enormous consequences on uh plant diversity because they mow down the plants and forests etc etc so the discovery that some creatures have these outsized effects um was a surprise it's sort of they were about these sort of hidden connections in in ecosystems and then we understand that if we can replace some of these things maybe we can reverse
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some of the some of the changes that um some of the unwanted changes that have happened in places so it's i give you the wolf example but you know in the tide pools on the pacific coast that certain starfish in the serengeti it's the wildebeest that the massive numbers of wildebeest just munching the grass have an enormous impact on the diversity and stability of that ecosystem.
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So this gives us insight into really how these systems work and how we can better manage them in the future.
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Well, I mean... You know, nature was around a long, you know, we're late comers to nature, right? Humans and human population growth is very, very recent. So most of these places, you know, have operated on their own without human intervention for hundreds of thousands or millions of years.
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And then we came along and, you know, in either in settling these places or in wanting to exploit them, for example, think of things like the fur trade and things like this, where we took specific creatures because we wanted we wanted them for our own gain. We've disrupted the balance in in those systems. So from nature's point of view, these creatures are all good.
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From our point of view, they were either bad or desirable for other reasons. And we You know, we eliminated them. And then we've really sort of, you know, tip these systems into a different place. So it's rethinking our relationship to nature, you know, not to not to make ourselves out as the, you know, as bad and evil or anything like that. It's just we're doing this unwittingly.
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We just didn't understand our impact when we, for example. eliminated most of the sea otters, for example, from the Pacific coast. And what that did amazingly to kelp forests, you know, you'd sort of think, I mean, why would, why would sea otters have any influence on kelp forests? Well, it turns out they eat the things that eat the kelp forests.
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And if you don't have sea otters, the kelp forests can't grow. And those kelp forests are great habitat for fish. And those fish are great food for bald eagles, et cetera, et cetera. So there are all these domino effects that removal of these, what are called keystone species, uh,
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Keystones, dubbed after the name of the keystone in a Roman arch, removal of these keystones have really outsized effects on systems. And so their restoration can have outsized effects on the restoration of the health of these ecosystems.
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It's a very competitive world out there, yeah. And creatures are competing with each other of different kinds and of their own kind for resources, et cetera. And these are very complex web of interactions that's going on anywhere, in a coastal system, in a forest, in a pond, out on a plains.
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But that web, there are certain components of those systems that when you remove them, the web sort of collapses. And that's what humans were doing unwittingly for, I think, a good part of the 19th and 20th centuries.
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And it was a bunch of biologists, a bunch of ecologists studying these systems over long periods of time in the last four or five decades of the 20th century that put together, essentially from studying different systems, an understanding of these rules that sort of knit life on a large scale together. And understanding these rules then, of course, empowers us with a better understanding of life.
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What's our best long-term behavior?
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Yeah, let's go with one. So one rule are these certain species that have these outsized effects called keystone species. Another rule is there's sometimes really strong indirect effects between species. And one of my favorite examples is the influence of salmon in Western rivers on tree growth. And you're like, what? Yes, trees need salmon. How does that work?
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Well, it turns out that those rivers, think of things like the Columbia or Snake, sort of majestic rivers in the Pacific Northwest, those rivers don't carry a lot of nutrients.
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But salmon, when they come upstream to spawn, and when those salmon are then taken, for example, by predators and eaten on stream banks and all that sort of stuff, a lot of the nutrients that they bring from the ocean get into the forest, and you can map those nutrients right into the growth of those trees.
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So it turns out that salmon are like a conveyor belt for nutrients coming from the ocean into these trees.
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forests alongside these rivers so who would have thought that right I mean it's just sort of stunning to think that trees are somehow influenced by salmon yeah and as you know a lot of those rivers have been dammed that's blocked off the salmon well that has effect on the rivers that has effect on the forests around those rivers and so again there's a lot now there's a huge movement
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There's a lot of dams being taken down in the West and biologists are monitoring what's going on. And as the salmon populations come back, you can actually see the nutrients moving up those rivers again. So these strong indirect effects, the idea is that salmon would influence trees or that otters would influence the kelp forest. For example, the number of
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will the beast on the serengeti influences the numbers of giraffe i can go through that again that explanation as well these strong indirect effects these domino effects that that ripple through a system they're they really were unexpected and they it you know it's it so almost everyone is a sort of just surprised when it's discovered that um these the dependence of species on each other are not obvious uh you have to sort of map these out
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yeah that's part that's partly why they're doing well but the hunting really wouldn't have been that significant until the deer herds exploded in the early part of the 20th century so deer when you know when wolves roamed the lower 48 and abundant and you know in large numbers uh deer numbers were i'm going to take a stab at maybe a 50th or a hundredth of what they are now i spent a lot of time in the state of wisconsin
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And I mean, the annual deer hunt is a huge thing culturally and the deer herd is enormous, but some of the pioneering ecologists of the 20th century understood that that was a huge change from say that from the 19th century and that deer exploded when you took away their predators.
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so predators have a big role in controlling the size of populations of their prey and take away the top predators and you can have an explosion in the number of their prey so really deer numbers are abnormal all over the country because they have no essentially no significant natural predators i mean there are a few you know wolves are coming back in various places but they're not making a dent
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now deer have spread over all these you know suburban and urban populations and they're making a living on our gardens right so um yeah so so deer you can i know they're cute uh but they are essentially an invasive species um or at least they're a species that's out of control um out of natural control so you know i think deer are with us to stay in these large numbers but
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That's a great point, Mike. In fact, essentially the third rule. And that third rule is that the density of populations is sort of self-limiting. So when there's a lot of food, abundant food and populations are small, they will grow explosively.
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But as that sort of population fills up the space and as food starts to become limiting, that will slow or in fact sort of reverse that to a population decline. So lots of things are density regulated. And so deer are, you know, have expanded their habitat. They're filling, you know, our suburban areas and things like that. But there's still limits on deer in terms of food abundance.
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And there's also, you know... there's cars you know as you know there's lots of collisions and things like this so there's various things that are um constraining the deer population as well as disease, you know, because they have certainly acquired some diseases now that they're in close contact with humans.
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So, yeah, so this, the general concept is that in no area will be, there is a limit to how much can be carried by, the population can be carried in any given area, you know, limited particularly by food supply.
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That's a hugely important, maybe sort of philosophical question. I think, you know, you said earlier about how nature kind of works these things out. Let's take something like a beaver and you could say, all right, a beaver builds a dam and that dam diverts some water. And, you know, does that impact other creatures? It certainly does.
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um and and maybe it impacts you know the ability of fish to get around in some places and things like that but it also in creating these wetlands you see all sorts of plant life that thrives in the wetlands and then creatures that invade these places and um or exploit these places and so
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beavers you know are often referred to as you know one of nature's engineers you know their net effect is you know to create habitat for lots of different things so we you know in a lot of places we took out beaver and we straightened out rivers we straightened out waterways we didn't want beaver dams on our you know on our on our waterways um we wanted you know unfettered access to this but
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You know, it's hard for me to see the beaver in a negative light because, you know, it has done this particular job for eons and all sorts of creatures became dependent upon that job. And then we came in and essentially took the beavers out of the picture and lots of things, you know, suffered for that.
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So it's, it's often the case, you know, it's, I mean, I'm going to try to see if I can come up with some other example where you might wonder about, I'll jump through to elephants. Okay. You can look at elephants are incredibly majestic creatures, but they're pretty tough on forests. Elephants are, We'll knock down a lot of trees.
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And I've been in areas in Africa where it's pretty astounding what elephants can do to a forest and probably can do it in a fairly short period of time. And if you're knocking down trees, you're probably taking away nesting sites from birds. You may be taking away food from other animals. So.
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the knock-on effects of some of these creatures are negative for for other creatures but you know take away the elephants and then you don't get the turnover of the forests and you don't get uh you know fertilization you know through their dung and that supports all sorts of other communities and you know and so on and so forth so it it takes some
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um thinking through the myriad effects of you know creatures in these ecosystems to sort of you know weigh these things out but i think i think on balance we see that you know ecosystems the systems themselves are healthier when the components that have been there for eons are are there and thriving and working and you know these forests that produce you know oxygen and timber and um you know
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rivers that give us fresh water and glaciers and rivers that give us fresh water and oceans that produce fish and things like this. These are all more productive when they when their diversity is intact. So I think on balance, it's a long way to get around to your question and say, you know, do these animals have negative impacts?
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I would say, you know, yes, in probably a narrow view, but in the broader view, they've evolved as parts of these systems and the integrity of these systems depend on them.
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Well, we've hit three of the really big ones, but I think the most important rule, Mike, and probably the most important message I could deliver is that nature is incredibly resilient. That given a chance...
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given time and and space and taking pressure off these places they can rebound and they can rebound spectacularly populations can rebound species that have been pushed right to the brink of extinction can recover spectacularly habitats that look incredibly degraded can come roaring back and they can come roaring back before our eyes not you know not in centuries but i mean in years to decades
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And this resilience of nature, I mean, just let's give it a minute to sort of sink in. I'll give some examples. You know, fisheries, which we pay close attention to for their commercial significance, a good number of fisheries in North America have been fished to a critical state.
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And then the fisheries regulators step in and moratoria are put up, and many of those fisheries have rebounded and rebounded well.
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uh it just turns out that the oceans are incredibly productive given a chance and populations will will rebound species that are familiar to us like the bald eagle people may know that in the 1960s because of widespread use of ddt bald eagles and peregrine falcons were devastated they had a lot of trouble with the viability of their chicks so populations crashed
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We were down to fewer than 500 mating pairs of bald eagles, the national symbol, by the late 60s. Today, that number is over 70,000 breeding pairs, and the total population is even bigger than that. So when we took the pressure off, when we took away DDT, which was compromising their ability to reproduce, the population came roaring back.
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And you can tell the same story of manatees in the waters off Florida, of grizzly bears in the Rockies, of sea otters on the Pacific coast. So individual species can come roaring back.
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and um you know we alligators the american alligator people may not know in the late 60s alligators were extremely threatened and you know goodness go to florida now and try not to see an alligator you know they're everywhere well i like the way you have framed this as the rules of life how we're all interconnected and intertwined and interdependent on each other and it's not something i think most of us think about but
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Thanks, Mike. It's always a pleasure.