Extinction, the bromide goes, is forever; but if ecosystems really can get stuck in new arrangements, it would mean that not only species, but whole natural systems, could be irrevocably lost. And such losses could be grave for a large number of people. Each year, humanity captures and keeps about 80 million tons of fish from the sea. It's a number worth staring at: 160,000,000,000 pounds, which provides almost half the world with at least a fifth of its animal protein. In many places, it is a resource without which humans cannot survive.
According to formerly dominant paradigms in oceanography and marine ecology, impacts—even our very substantial impacts—were not supposed to cascade through oceanic ecosystems, and such durable transformations were not anticipated. More broadly still, the ecological rearrangements, and especially the way scientists are now talking about responding to them, mark a shift in our basic relationship with the ocean. So this is a story of conversion not only in the sea, but also in the way we think of it.
One way to think of an ecosystem is with a diagram of its food web. In such a picture, each oval represents a species, or a natural grouping of them, and each arrow signifies what ecologists would refer to as transfer of biomass, but which you could just as well call eating.
What ecologists call trophic cascade has, I'm afraid, entered popular consciousness mainly through managerial missteps. Macquarie Island is a recent example. Halfway between New Zealand and Antarctica, Macquarie is a World Heritage Site, on account of a rich unusual flora and millions of nesting sea birds. Aiming to protect those birds, managers eradicated thousands of feral housecats. At first the plan worked: bird colonies grew. But soon, exotic rabbits, which had previously been hunted by housecats, began to do what rabbits are famous for. Proliferating, they mowed down the vegetation, facilitating erosion, which swamped and undermined bird colonies. This illustrates trophic cascade because a change at the top of the food web—trophe is Greek for food—cascaded down to levels below: the cat oval vanished; the rabbit oval swelled; the plant oval contracted. The mudslides are more of a metaphorical flourish on the basic concept.
Happily, not every example is so grim. In Yellowstone, the reintroduction of wolves stopped Elk from browsing down all the trees along riverbanks, and thus helped to restore healthy riverine ecosystems. But in both kinds of story—dispiriting and heartening alike—it's worth noting a common pattern: The population on any given floor of the food web behaves in contrast with those on adjacent floors, but in concert with those that are two floors away. Cats, on floor three, decline, and so do plants, on floor one. This pattern can hold true even in ecosystems with many levels, and ecologists look for it to see how an ecosystem works. If they find it, they say an ecosystem is subject to top-down control. For instance, cats or wolves, up above, determine the abundance of everyone below.
But that's all on land. Marine scientists have not traditionally thought in terms of trophic cascades and top-down control. Rather, they've assumed that the abundance on any given floor of the food web is determined mainly by the supply of nutrients from the floors below, and not by the impact of consumers up above. There are good reasons for supposing as much. In some parts of the ocean, food supply is patchy and sporadic; when there is food, there's too much for consumers to make a dent. Also, in a lot of oceanic ecosystems, many different consumers feast on any given floor of the food web. As a result, a contraction of one population near the top, such as yellowfin tuna, could be compensated for by expansion of another, such as albacore; and in that case, only changes in productivity coming from below will alter the overall abundance on the floors overhead.