What are keystone species, and why do they matter?

This article has been updated.

What happens when a top predator is removed from an ecosystem? The population it preyed upon, no longer kept in check, explodes, putting huge strain on the next organism down in the food chain. And so it goes on. Right?

In fact, it’s more complicated than that, as Robert Paine, the scientist who first described the keystone species concept in the 1960s, found.

Some organisms help to maintain balance and diversity in a complex ecosystem, and losing them doesn’t just affect the next species in the chain – everything looks different.

In short, keystone species enable other species to survive, occupying a key role in the ecosystem they are part of. Without them, their ecosystems would be dramatically different or even cease to exist.

For example, Paine conducted experiments on a patch of rocky coastline in the Pacific Northwest of the United States in which he took out the top predator, the starfish.

Within months, one species of barnacle, and then another type of barnacle, and then later a mussel, had become the dominant species. The succession of proliferating species wiped out their main food source, algae, prompting limpets and other species to migrate away from the area due to a lack of prey and space. Overall, biodiversity plummeted – the number of species went from 15 to 8.

In a seminal 1966 research paper, Paine explained what had happened by designating the starfish as a keystone species, which influences lower levels of the food web, preventing species from monopolizing resources including space and food.

A keystone species can be an animal, plant or microorganism. Coral is instrumental in creating diverse reef ecosystems. In the plant kingdom, keystone species include mangrove trees, which support shorelines from erosion and provide protective habitats for small fish and other organisms.

Other keystone species include the sea otter, which eats sea urchins, and they in turn feed on kelp. Without the sea otter, which prevents sea urchins from overbreeding, kelp forests – which sustain a variety of other species – would be severely depleted.

The grey wolf was largely wiped out of the contiguous United States in the early 1900s, resulting in a surge in the number of elk. In Yellowstone National Park, elk and deer overgrazed on trees, grasses, reeds and other plants, putting intense pressure on animals dependent on plant life, including fish, beavers and birds. Wolves also indirectly feed eagles, coyotes and bears who scavenge leftover carcasses.

A successful reintroduction effort in the 1990s increased their numbers and they are now protected under the Endangered Species Act.

In addition to predators, there are other keystone species responsible for maintaining or producing distinct habitats. These “ecosystem engineers” include beavers, which remove dead trees along riverbanks and create dams that divert water creating wetland habitats.

Elephants are another example. They trample forests and dense grasslands, supporting the growth of smaller species. And they travel vast distances, dispersing seeds in their dung, thus supporting vegetation growth. Some research suggests elephants could disperse seeds up to 65km, which helps to maintain the genetic diversity of many tree species and prevents local inbreeding.

Keystone species can be found in a huge variety of regions and ecosystems.

In the ocean, sharks are not only top of the food chain, but also act as keystone predators, keeping other populations in check with their eating habits.

In the United States, the Mojave desert tortoise is an essential part of the ecosystem, its burrows offering shelter to other species, like roadrunners and burrowing owls. While the saguaro cactus provides food for mammals, birds and insects, as well as crucial nesting spots for hawks and woodpeckers.

The boreal forest or taiga is home to many keystone species, including wild red raspberries, the snowshoe hare and aspen trees, which "contribute significantly to the species diversity of forest landscapes", whether alive or dead.

Another significant keystone flora is the fig tree, which in tropical rainforests "feeds more than 1,200 types of birds, bats and other species year-round". While the western lowland gorilla, native to the Congo basin, eats such quantities of roots, leaves and fruit that they end up shaping entire plant communities and maintaining the 'lungs of the planet' - as well as dispersing seeds.

The concept of the keystone species helps ecologists explain the delicate, intricate web of interactions which sustain any given ecosystem.

It also helps conservationists argue in favour of mitigating human impacts in order to protect species and the complex ecosystems they hold together. With biodiversity loss and ecosystem collapse considered the world's third biggest threat over the coming decade, according to the World Economic Forum's Global Risks Report 2024, protecting keystone species is more vital than ever.