Keystone species are organisms that hold their ecosystems together despite being relatively low in number. Remove one, and the entire community of plants and animals around it shifts dramatically. The concept comes from a 1966 experiment in which ecologist Robert Paine pulled a single species of predatory sea star off rocky tidal shores and watched mussels take over, crowding out seven other species that had previously shared the habitat. Since then, researchers have identified keystones across every type of ecosystem, from ocean floors to grasslands. Here are the most well-documented examples and what makes each one so important.
Sea Stars in the Rocky Intertidal
The ochre sea star (Pisaster ochraceus) along the Pacific coast of North America is the original keystone species, the one that inspired the term. These predators feed heavily on mussels, which are aggressive competitors for space on rocks. When Paine removed the sea stars from tide pools in Washington State, mussels spread unchecked and displaced seven other species that had been clinging to the same surfaces.
The twist is that mussels themselves are what ecologists call a foundation species. Their dense beds create three-dimensional habitat for over 300 smaller organisms, from worms to crabs, that live in the crevices between shells. So while the sea stars reduce mussel dominance, the mussels that remain still support a rich community. The sea star’s role is to keep mussels from monopolizing everything, preserving room for algae, barnacles, and other organisms that would otherwise be smothered.
Sea Otters and Kelp Forests
Sea otters along the Pacific coast are a textbook keystone predator. They eat sea urchins, which graze on kelp. Without otters, urchin populations explode and devour kelp forests down to bare rock, creating what marine biologists call “urchin barrens,” stretches of seafloor with almost no plant life. With otters present, urchin numbers stay low enough for multilayered kelp forests to grow, supporting dozens of algae and animal species.
Research off Vancouver Island documented this cascade in real time. When otters first arrived at sites where they had been absent, they depleted urchin populations so quickly that the change was visible between annual surveys. At those sites, researchers almost never found red urchins and giant kelp coexisting in meaningful numbers once otters were established. The kelp recovery that followed brought back fish, invertebrates, and marine mammals that depend on the forest canopy for shelter and food.
Wolves in Yellowstone
Gray wolves were reintroduced to Yellowstone National Park in 1995 after a 70-year absence, creating one of the most studied trophic cascades in ecology. Wolves prey on Rocky Mountain elk, which had been overgrazing streamside vegetation for decades. With wolves back in the picture, elk changed both their numbers and their behavior, spending less time lingering near rivers and streams where they were vulnerable to attack.
The results were dramatic. A 20-year study running from 2001 to 2020 found that willow crown volume along Yellowstone’s streams increased by roughly 1,500 percent. Willows that had been browsed to stubby shrubs grew tall enough to shade streams and stabilize riverbanks. That vegetation recovery rippled outward: songbirds returned to nest in the willows, beaver colonies re-established themselves using the regrown woody plants, and stream temperatures cooled enough to benefit native fish. One predator’s return reshaped the park from the top of the food chain to the riverbed.
Beavers as Ecosystem Engineers
Beavers reshape landscapes more visibly than almost any animal besides humans. By felling trees and building dams, they flood valleys, create ponds, and convert fast-flowing streams into slow, marshy wetlands. These wetlands become biodiversity hotspots. Research has found that beaver sites support higher species richness and abundance of breeding birds than unmodified watercourses, with 27 percent of recorded bird species found exclusively at beaver-altered sites.
The influence doesn’t stop at the water’s edge. Studies show that the beaver’s effect on bird communities extends to terrestrial habitats up to 100 meters from the water, where species richness and abundance are measurably higher than in comparable areas without beaver activity. Beaver lodges themselves function as local diversity hotspots, particularly in autumn, likely because they offer food, shelter, and warmer microclimates. Globally, ecosystem engineers like beavers contribute to roughly a 25 percent increase in biodiversity compared to sites without them.
African Elephants
African elephants are the largest land animal and one of the most powerful ecosystem engineers on Earth. They break branches, push over trees, uproot shrubs, and strip bark from woody plants. This sounds destructive, and for individual trees it is. But at the landscape level, elephant foraging prevents woodlands from becoming too dense and can convert closed canopy into open grassland and savanna mosaic.
That structural variety matters. Open patches created by elephants allow grasses and low shrubs to flourish, providing habitat for grazing animals, ground-nesting birds, and reptiles that can’t survive in thick forest. In Kenya’s Samburu and Buffalo Springs reserves, researchers documented how elephant-driven reductions in riverine tree density shifted which plant species dominated, favoring several drought-tolerant and disturbance-adapted trees. The cascading effects touch everything from soil nutrients to fire patterns to the viability of protected areas for other species. Both African elephant species are now classified as threatened by the IUCN, with the forest elephant listed as Critically Endangered, making their conservation a high-stakes issue for entire ecosystems.
Gopher Tortoises
In the southeastern United States, the gopher tortoise plays a keystone role not through what it eats but through what it builds. These tortoises dig deep burrows, sometimes 10 feet down and 40 feet long, that stay cool in summer and warm in winter. Over 60 vertebrate species and more than 300 invertebrate species use these burrows for shelter, foraging, and nesting. Residents include indigo snakes, burrowing owls, gopher frogs, and hundreds of beetle and spider species that have no other reliable underground refuge in the sandy, fire-prone longleaf pine habitat where tortoises live.
Because so many species depend on these burrows, the decline of gopher tortoises from habitat loss and development threatens an entire community of animals that may have no alternative shelter.
Prairie Dogs
Black-tailed prairie dogs are a grassland keystone across the western United States. Their colonies, called towns, can cover hundreds of acres. The burrows provide shelter, foraging grounds, and nesting habitat for a wide range of animals, including burrowing owls, rattlesnakes, and the critically endangered black-footed ferret, which feeds almost exclusively on prairie dogs. Above ground, prairie dog grazing keeps vegetation short and nutrient-rich, attracting pronghorn and bison that prefer the fresh growth found on colony sites. Raptors hunt over prairie dog towns because the short grass makes prey visible. When colonies disappear, this entire web of species loses its foundation.
Pollinators
Bees, particularly wild bee species alongside managed honeybees, function as keystone mutualists. They don’t control other species through predation or reshape terrain through engineering. Instead, they enable plant reproduction. Bees provide pollination services for over 30 percent of the human food supply, and the percentage of wild flowering plants that depend on insect pollination is even higher. Without bee pollination, many plant species would fail to reproduce, collapsing the base of food webs that support insects, birds, and mammals far removed from any flower.
What Makes a Species “Keystone”
The defining trait of a keystone species is a disproportionate effect relative to its abundance. A keystone doesn’t have to be the most common organism in its ecosystem. In fact, it usually isn’t. It exerts an outsized, stabilizing influence on biodiversity, and removing it triggers changes that ripple through the entire community. This distinguishes keystones from foundation species, which are typically abundant primary producers (like corals or large trees) that physically form the habitat itself.
Keystones fall into several functional categories. Predators like wolves, sea otters, and sea stars control prey populations that would otherwise dominate. Ecosystem engineers like beavers, elephants, and gopher tortoises physically create or modify habitats. Mutualists like bees facilitate relationships between species. The common thread is that without them, their ecosystems look fundamentally different: less diverse, less stable, and less resilient to disturbance.