Humans: The Hyperkeystone Species

The last paper from one of the world’s greatest ecologists challenges his peers to think about humanity’s influence on the world.

Edgar Su / Reuters

In 1963, on the shore of Makah Bay, Washington, a tall, thirty-year-old biologist named Bob Paine started prising ochre starfish off the rocks with a crowbar, and hurling them into the sea. In doing so, he remade the beach. The animals that the starfish would have eaten bloomed in number and overran the shoreline—barnacles first, and then mussels. Limpets and algae were crowded out. Within a year, the number of species on the beach had halved.

By denuding the shore of starfish, Paine realised that some creatures are disproportionately influential. Entire communities of animals and plants depend on their presence, and can topple in their absence. He called them keystone species after the central stone that keeps an arch from crumbling. It’s an idea that has itself taken a central place in ecology.

But in analyzing the outcomes of his starfish experiment, Paine missed something obvious and important: his own part in them. Yes, the starfish were influential, but so was he. He, a single human, had reshaped a tiny corner of world. And yet, Paine left himself and his 7.4 billion peers out of the very framework that he had created. At a conference last October, Boris Worm, an ecologist who had known Paine for a few decades, asked him if he thought humans also counted as keystone species.

“Oh, we’re above that,” Paine replied. “We’re hyperkeystones.”

We are the influencer of influencers, the keystone species that disproportionately affects other keystone species, the ur-stone that dictates the fate of every arch.

Paine coined the term as a play on ‘hyperparasites’—organisms that parasitize other parasites. There are body-snatching wasps, for example, that lay eggs in the bodies of other insects, and other wasps that lay eggs in the eggs of those first ones—the latter are hyperparasites. So if hyperkeystone sounds grandiose, it’s not meant to; it’s almost the opposite.

The concept is clearest in the Pacific Northwest, the area where Paine and a large number of his students did most of their work. There, in the tidal zone, starfish control the numbers of mussels and barnacles. Sea otters keep kelp forests healthy by eating sea urchins that would graze the fronds down, and orcas hunt the otters. By migrating upstream, salmon carry nutrients from the sea into rivers; when they are killed by bears and wolves, their carcasses are dragged into the forests, where they fertilize the trees.

Bears, wolves, salmon, starfish, orcas, and sea otters: we influence the lot. Whether directly through hunting and fishing, or indirectly through light and noise pollution, climate change, or deforestation, we change the levels of keystone species everywhere.

“People now strongly influence all natural ecosystems,” says Julia Baum from the University of Victoria. “We do so to such an extent that as scientists we cannot even begin to understand how the ecosystems work if we do not first account for the ways in which people are changing them.”

Wait, aren’t we doing that? Isn’t that what ecology is all about?

Sort of, says Worm. He says that he and his peers have been increasingly obsessed with finding big patterns—how fish stocks rise and fall with time, or how communities of species change as temperatures rise. And in doing so, ecology had become a largely descriptive science. But in the meantime, it moved away from what Paine called “kick-it-and-see ecology”—experimental work that, like his starfish study, would reveal not only what is changing, but why and how.

“Bob felt that we had lost the way a little bit,” says Worm. “In the media, you read that we’re losing species, and birds are affected by plastic pollution, and tigers are dwindling, and that’s bad. That’s often the extent of it. But there’s a deeper and more profound side to this. Those losses set off these cascading interactions that we only know about in a few settings, largely because of people who are now in their seventies and eighties, like Bob.”

A few more recent examples hint at what we’re missing. In the northwest Atlantic, we overfished big sharks, releasing smaller sharks and rays from predatory control; they devoured shellfish and caused a century-old scallop fishery to collapse. In Ghana, we killed off lions and leopards in Ghana, allowing baboons to flourish; they then ate other primates, small antelopes, birds, and even crops, forcing local villagers to enlist school-age children as crop guards. In both cases, the direct consequences of our actions were clear, but it took a lot of work to understand everything that happened afterwards.

In the last few months, Worm and Paine wrote about the hyperkeystone concept in a new paper that’s meant to both galvanize and challenge their peers. Can they work out how exactly we are changing the world in our role as a hyperkeystone? We know about the Pacific Northwest, but what about the eastern seaboard, or East Africa, or Antarctica? And can we identify other hyperkeystones? Orcas might fit the bill: they, like us, are also far-ranging generalist predators. “Changes in such species are expected to have disproportionately large ecological impacts,” says Fiorenza Micheli from Stanford University. “Bob, as always throughout his career, has left us with a powerful concept.”

The hyperkeystone idea is a way of marrying the local domino effects that Paine observed with the global reach of our species. If fish stocks are falling, how does that affect the numbers of plankton or nutrients in the ocean, and how does ripple across ecosystems? To take just one example, when fish stocks fall in Ghanaian seas, hunting of bushmeat goes up and 41 land-based species go into decline. As hyperkeystones, we unite the entire world in a chain of falling dominoes. The question is how?

“We’re doing experiments all the time, by removing species in a replicated fashion,” says Worm. For example, he has shown that as fisheries throughout the Atlantic remove cod, shrimp and herring rise. “It was astonishing how predictable it was.” Other scientists are running large experiments, where they deliberately exclude or add animals to small patches of habitat to see what happens—a global version of Paine’s antics on Makah Bay.

Paine died of cancer last Tuesday, aged 83. By coincidence, the hyperkeystone paper was published on the same day. “It’s deeply moving to me,” says Worm.

“Bob was one of the most respected and admired marine scientists alive,” he adds. “But his original work ignored us and, of course, we cannot do ecology now by ignoring humans. Bob was part of that generation that treated humans as an externality. Very few of his papers had a human in the picture. We were the missing keystone. By adding us back, we make his insights relevant to the 21st century.”