One of the most exciting scientific findings of the past half century has been the discovery of widespread trophic cascades. A trophic cascade is an ecological process which starts at the top of the food chain and tumbles all the way down to the bottom, and the classic example is what happened in the Yellowstone National Park in the United States when wolves were reintroduced in 1995. Now, we all know that wolves kill various species of animals, but perhaps we’re slightly less aware that they give life to many others. It sounds strange, but just follow me for a while. Before the wolves turned up, they’d been absent for 70 years. The numbers of deer, because there was nothing to hunt them, had built up and built up in the Yellowstone Park, and despite efforts by humans to control them, they’d managed to reduce much of the vegetation there to almost nothing, they’d just grazed it away. But as soon as the wolves arrived, even though they were few in number, they started to have the most remarkable effects. First, of course, they killed some of the deer, but that wasn’t the major thing. Much more significantly, they radically changed the behavior of the deer. The deer started avoiding certain parts of the park, the places where they could be trapped most easily, particularly the valleys and the gorges, and immediately those places started to regenerate. In some areas, the height of the trees quintupled in just six years. Bare valley sides quickly became forests of aspen and willow and cottonwood. And as soon as that happened, the birds started moving in. The number of songbirds, of migratory birds, started to increase greatly. The number of beavers started to increase, because beavers like to eat the trees. And beavers, like wolves, are ecosystem engineers. They create niches for other species. And the dams they built in the rivers provided habitats for otters and muskrats and ducks and fish and reptiles and amphibians. The wolves killed coyotes, and as a result of that, the number of rabbits and mice began to rise, which meant more hawks, more weasels, more foxes, more badgers. Ravens and bald eagles came down to feed on the carrion that the wolves had left. Bears fed on it too, and their population began to rise as well, partly also because there were more berries growing on the regenerating shrubs, and the bears reinforced the impact of the wolves by killing some of the calves of the deer.

Whales in the southern oceans have similarly wide-ranging effects. One of the many post-rational excuses made by the Japanese government for killing whales is that they said, “Well, the number of fish and krill will rise and then there’ll be more for people to eat.” Well, it’s a stupid excuse, but it sort of kind of makes sense, doesn’t it, because you’d think that whales eat huge amounts of fish and krill, so obviously take the whales away, there’ll be more fish and krill. But the opposite happened. You take the whales away, and the number of krill collapses. Why would that possibly have happened? Well, it now turns out that the whales are crucial to sustaining that entire ecosystem, and one of the reasons for this is that they often feed at depth and then they come up to the surface and produce what biologists politely call large fecal plumes, huge explosions of poop right across the surface waters, up in the photic zone, where there’s enough light to allow photosynthesis to take place, and those great plumes of fertilizer stimulate the growth of phytoplankton, the plant plankton at the bottom of the food chain, which stimulate the growth of zooplankton, which feed the fish and the krill and all the rest of it. The other thing that whales do is that, as they’re plunging up and down through the water column, they’re kicking the phytoplankton back up towards the surface where it can continue to survive and reproduce. And interestingly, well, we know that plant plankton in the oceans absorb carbon from the atmosphere — the more plant plankton there are, the more carbon they absorb — and eventually they filter down into the abyss and remove that carbon from the atmospheric system. Well, it seems that when whales were at their historic populations, they were probably responsible for sequestering some tens of millions of tons of carbon every year from the atmosphere.