In the last 60 years, ecologists have discovered that specific animals have an outsized impact on the health of their communities. The functioning of these ecosystems are sometimes entirely dependent upon certain individual species or small groups of species than others, says biologist Sean B. Carroll, who codified the laws of nature into a set of rules called The Serengeti Rules.

One of the chief points of The Serengeti rules is that some species are more integral in striking this balance than others. That’s important knowledge because if we lose those species, those communities can collapse, and if those communities are compromised, reintroducing or boosting those lost species can have positive effects on the overall health of the ecosystem.

For example, the 70-year absence of wolves in Yellowstone was contributing to stunted trees. In the Pacific Northwest, trees along the river rely on nutrients from salmon carcasses to grow tall. Sean B. Carroll explains the hidden rules of interconnectivity, and why sometimes the smallest detail is fundamental to the functioning of our vast world.

SEAN B. CARROLL: So here's a shocking fact: In the Pacific Northwest, the trees need salmon. What? The trees need salmon! Do you want me to unpack this a little bit as how I got to that "aha!"? So something that's generally true about nature, including our bodies, is that: And what I mean by regulated, it means that in our bodies, there are all sorts of substances, some that are in very small amounts, some of them in sort of medium amounts, some of them in large amounts. And that those levels, those amounts, are kept in check by some of the body systems, by all sorts of regulatory systems, some of that is feedback regulation. And that's important because sometimes if we have too much of one thing, that's a disease, too little of something, that's also a disease. So, for scientists to figure out the rules of regulation in the body to know what the right level of insulin is, or what the right level of red blood cells are, that's been really important to managing health and managing all sorts of situations. So, what's true for the body is also true in broader nature. It turns out there's rules in nature that govern the relative abundance of plants and animals. And when we understand those rules, we know better how to manage those places for sustainability, for their health and our health. While society's been humming along and enjoying all these advances in agriculture and medicine, in the last 50 or 60 years, ecologists have learned a lot about how nature works. I've codified these into a set of rules called the 'Serengeti Rules.' I call 'em the Serengeti Rules 'cause you can see them in operation in the magnificent Serengeti. You can see them in operation in all sorts of other places like Lake Erie, but I don't think 'Lake Erie Rules' would've sold as many books. So, a couple of those rules that are really important is that some animals are more important than others, meaning they have an outsized impact on the communities in which they live. There's sort of this poetry about nature of, you know, every creature has its place and its purpose and everything is equally important. That's not true. The functioning of these ecosystems are sometimes much more dependent upon certain individual species or small groups of species than others. That's really important knowledge because if we lose those species, those communities can collapse. And if those communities are somehow compromised, reintroduction or boosting those species can have great knock-on effects on the overall health of the ecosystem. So for example, in Yellowstone National Park in the United States, it's a pretty famous story of reintroduction of wolves, which had been exterminated from the Lower 48, and the last wolf was killed in Yellowstone in about 1924. So there have been no wolves for 70 years in Yellowstone. And you might say, "So what?" We can get along without these large predators. Well, that's been shattered by ecological discoveries. These large predators play a really important role. And in the case of Yellowstone, essentially, trees need wolves. So think about that for a second. Trees need wolves. How do you draw, you know, how do you draw that connection in your head? Well, it's by wolves controlling the browsing and grazing of deer and elk. That was absent for 70 years. It was stunting plant and tree life in Yellowstone, and so wolves were reintroduced in 1995. And within a decade, you could see changes in the landscape of Yellowstone. And those changes led to increased abundance of other sorts of creatures. Similarly, we know that some species have really strong indirect effects on others. So for example, in the Pacific Northwest, the trees need salmon. Rivers don't have a lot of nutrients. A lot of the nutrients in those rivers comes in from the ocean in the form of salmon bodies. And when those salmon are taken by all sorts of carnivores, and when those carcasses are left on the stream banks, the nutrients from those carcasses fertilize the trees, and we can actually trace those nutrients from the ocean into the trees in the Pacific Northwest. Who would've thought of such things? As we've learned about all sorts of hidden connections and ecosystems, we're realizing, first of all, why were some of our actions so damaging? But flip side of that coin is why a little bit of action now can be so rewarding, can be so positive, and that these systems, these sort of webs of life, can be restitched just by the introduction of certain key creatures. This is really important because we are nature's manager now, whether you like it or not, we've taken it over. We're trying to manage nature for the long run so that we have the things that we need and hopefully nature can persist. So we need to keep exploring these interactions, whether they're interactions within the body or they're interactions out there in nature, to manage our future. You see the payoffs in medicine, every discovery, every new medicine, the way we manage so many diseases, that's entirely the fruit of the last 50 years of understanding these basic rules. We need that same sort of mindset to be taken outdoors into nature so that we can manage the planet better. If we managed our bodies the way we managed the planet, there would be a revolution.