In that sense, in microbial communities, the keystone species concept is context-dependent. A keystone in one microbiome might not be a keystone in another. “I feel that this aspect has not been highly appreciated by ecologists,” Liu said.

Ecologists are now grappling with this contextual nature of keystone species beyond microbes and pondering whether, and how, the concept matters amid the reality of biodiversity loss.

Reassessing the Metaphor

Menge has dedicated his career to understanding ecological community structure, continuing the emphasis on rocky shores from his graduate work with Paine. He’s found that Paine’s iconic purple star isn’t a keystone species everywhere. It has stronger keystone-ness in some places, for example in tide pools more intensely beaten by waves. “In fact, in more sheltered places, the sea star isn’t really much of a keystone at all,” he said.

Paine came to accept this too. Up in Alaska, where the mussel preferred by more southern purple stars is absent, the predator is “just another sea star,” Power recalled Paine saying.

The fact that keystone species are context-dependent and that they vary in space and time is “missed in short-term studies,” Menge said.

Still, Srivastava isn’t ready to discard the concept. While the focus on keystones and single species may have distracted policymakers and conservationists from more holistic approaches to conservation, protecting and restoring a single species can sometimes benefit many other species in an ecosystem. “It doesn’t mean we rush to save keystone species and ignore the diversity of the system as a whole,” she said.

Srivastava also emphasized that keystones are not the only way systems are stabilized. “Ecologists now think that some of the most important interactions in terms of stability are actually relatively weak interactions,” she said. “If you have a high number of species that are weakly interacting, it’s kind of like having a lot of tent pegs tying down your tent in a windstorm. It dissipates some of the perturbations.”

Menge largely agrees. Amid a global loss of species, the main focus should be protecting habitats and biodiversity, not individual species, he said. “If those two things were done in enough places, then I’m not sure that the keystone-species idea is all that critical.”

Maybe one keystone matters more than the rest. In one of Paine’s final papers, published in 2016 on the day of his death, he and ecologist Boris Worm proposed that humans are a “hyperkeystone species” — one that exerts profound effects through exploitation of other keystones.

Humans can’t be removed from the system like starfish to quantify our impact. But we can learn how to reduce our keystone-ness through effective conservation practice and policy, Salomon said. “We also have the ability to learn to steward ourselves.”

That’s one reason why ecologists continue to redefine and reconsider keystone species. The powerful symbol isn’t going anywhere, but with an improved definition, people could learn how to apply it better.

Paine knew this. Salomon likes to share his words with her students: “You can’t manage out of ignorance. You have to know what species do, whom they eat, what role these prey species play. When you know that, you can make some intelligent decisions.”