Humans May Be the Most Crucial ‘Keystone’ Species of All

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The writer is a scientific commentator.

It’s fitting that a concept now considered somewhat slippery emerged in a rock pool. In the 1960s, Bob Paine, an ecologist at the University of Washington, pulled a species of purple starfish from a tide pool on a stretch of the US Pacific coast to see what would happen.

Years later, he discovered that mussels, normally controlled by carnivorous starfish, had invaded the territory, displacing other marine species. Paine called the starfish a “keystone species,” reflecting its enormous effect on its environment. The nickname spread like the Japanese knot, floating on the breeze of the nascent environmental movement and in the public consciousness.

Since then, more than 200 animal species have been described in academic literature as “cornerstone.” However, there is little agreement on the definition. And, as Quanta magazine reported last month, environmentalists are now reassessing the concept. Rethinking matters: These species are central to the idea of ​​rewilding, a modern form of conservation that promotes biodiversity by encouraging habitats to return to a more natural state. Without a clear idea of ​​which animal and plant species control the wheels of an ecosystem, environmentalists are reliving in the dark.

Keystone species are popularly thought to be the main predators of a territory, such as wolves or sharks, or as “modifying” species that alter their environment in ways that promote biodiversity. Beavers, for example, build dams and dig channels through floodplains, creating life-sustaining aquatic features in ponds. Bison “wallow” or roll on grassy plains, forming holes that retain rainwater.

The trickle-down effect of key species restoration can be seen in Yellowstone National Park in the United States, which reintroduced wolves in 1995. Today, wolf numbers have increased, as have moose and beaver populations. . The reintroduction triggered a “trophic cascade”: lurking wolves keep the moose on the move, which in turn means the willows are not overgrazed. This ensures enough food and building materials for the beavers.

But rewilding isn’t just about restoring charismatic species like wolves. It means understanding which species matter in which ecosystems, and this is not always obvious, even to experts.

Ishana Shukla, an ecologist at the University of California, Davis, assumed there would be a definitive list of all the keystone species ever recorded. There wasn’t. So she reviewed the literature and found 230 animal species, all labeled with the same “keystone” label, but based on different criteria.

“Generally, a keystone species is one whose impact on the community is disproportionately large relative to its abundance,” Shukla tells me. “However, there appears to be a lack of consensus on what ‘disproportionately large’ means.”

Sometimes biologists make that judgment mathematically by measuring the biomass of a species and then seeing how it increases or decreases as members enter and leave an ecosystem. But there are no defined numerical thresholds tied to the “key” (plus, he adds lightly, weighing wolves is complicated). Rather, researchers use their own judgment: If a species disappears and an ecosystem subsequently declines, for example, the keystone label is probably justified for that disappeared species.

However, the most important finding was that the 230 key animals were not just wolves and other alphas. They host species such as the cabbage butterfly, which affects vegetation; fish that feed on lower invertebrates; honey bees, which act as pollinators; and small mammals such as the black-tailed prairie dog, which turns over the soil. In short, cornerstones ran from the top of a food web to the bottom.

As Quanta notes, some ecologists favor a more holistic approach to conservation, focusing on entire habitats rather than individual species. But Shukla doesn’t want the key concept to be scrapped, arguing that flexible definitions accommodate the different ways animal and plant species contribute to nature. Plus, he points out, experiments like the one in Yellowstone seem to work.

There is one species that really plays a huge role in altering habitats. Just before Paine died in 2016, he and Dalhousie University ecologist Boris Worm wrote a paper describing humans as the “hyperkeystone species,” driving ecological change in every corner of the world. , from the oceans to the forests.

We may pay too much attention to wolves and sharks, but we often forget that we are also a species in the global ecosystem.

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