When starfish are removed from rocky coastal ecosystems, mussels take over. Without their main predator keeping them in check, mussels spread aggressively across rock surfaces, smothering and crowding out other species. In one landmark experiment, 6 of 20 species vanished within eight months, and within 15 months mussels had monopolized the majority of available space. This single change, the removal of one predator, triggers a cascade of effects that reshapes entire ecosystems from tide pools to kelp forests.
The Experiment That Changed Ecology
In the 1960s, ecologist Robert Paine removed all ochre sea stars from a 400-square-foot patch of rocky shoreline on the Washington coast and left an adjacent patch untouched. The results were immediate and stark. The area without starfish quickly became dominated by California mussels, which are the starfish’s preferred prey. Over five years, the mussel line advanced down the rock face by an average of nearly 3 feet toward the low tide mark, monopolizing most available space and pushing all other species out completely.
Seven species that had previously shared the rock surface, including barnacles, limpets, and algae, were crowded out by the expanding mussel beds. Paine coined the term “keystone species” to describe animals like the ochre sea star: species whose influence on an ecosystem is wildly out of proportion to their numbers. Remove the keystone, and the whole structure collapses into something far simpler.
Why Mussels Take Over
California mussels are fierce competitors for space. They grow densely, anchor themselves firmly to rock, and can outmuscle nearly every other organism trying to live on the same surface. The only thing that normally prevents them from covering every inch of available rock is predation. Ochre sea stars pry mussels open and eat them, particularly along the lower edges of mussel beds where the two species overlap. This constant pruning opens up patches of bare rock where barnacles, anemones, seaweed, and other organisms can establish themselves.
Without that pruning, mussels expand downward and outward until they form a thick, continuous sheet. The rock surface underneath becomes a monoculture. Species that need open rock to attach to or graze on simply have nowhere left to live.
Biodiversity Drops, Then Shifts
The relationship between starfish and biodiversity is more nuanced than it first appears. On the exposed rock surface, diversity plummets when starfish disappear. But mussels themselves are three-dimensional structures, full of crevices, gaps, and layered shells. Those mussel beds provide habitat for over 300 smaller associated species: tiny crabs, worms, snails, and other invertebrates that live tucked inside the bed itself. So while the visible community on the rock face simplifies dramatically, the mussel bed creates a different, hidden ecosystem within it.
This doesn’t offset the loss of the original community so much as replace it with something structurally different. The open, diverse mosaic of algae, barnacles, limpets, and anemones gives way to a dense mussel carpet with its own set of inhabitants. The ecosystem becomes less varied on the surface but more complex in a narrow, specific way.
Kelp Forests and the Urchin Problem
A different species of starfish, the sunflower sea star, plays an equally dramatic role in a separate ecosystem. Sunflower stars are voracious predators of purple sea urchins, the small spiny grazers that feed on kelp. When sunflower stars are present, they keep urchin populations low enough that kelp forests thrive. When the stars disappear, urchins multiply unchecked and begin eating kelp faster than it can grow.
This is exactly what happened starting around 2013, when a disease called sea star wasting syndrome swept the Pacific coast from Mexico to Alaska. The epidemic killed billions of sea stars across more than 20 species, but sunflower stars were hit hardest. Their populations collapsed, and the consequences were visible within a few years. Purple urchin numbers exploded. Kelp forests that had stood for decades were grazed down to bare rock, creating what ecologists call “urchin barrens”: underwater landscapes stripped of nearly all plant life.
In healthy kelp forests, urchins mostly feed on bits of kelp that naturally break off and drift to the seafloor. But when kelp becomes scarce for any reason, urchins shift their behavior and start actively grazing on living kelp and young plants trying to establish themselves. This creates a feedback loop: less kelp means more direct grazing, which means even less kelp, until the forest collapses into a stable barren state that can persist for years.
How Fast Ecosystems Recover
The good news is that these changes can reverse, but only if starfish return in sufficient numbers. In one three-year removal experiment, mussels shifted their lower boundary downward on the rocks. When sea star predation resumed, the mussel line retreated back to its original position within about three years. The ecosystem essentially reset itself once the predator was back on the job.
Recovery from larger-scale losses is slower and less certain. More than a decade after sea star wasting syndrome began, sunflower star populations remain severely depleted along much of the Pacific coast. Researchers recently identified the bacterial pathogen responsible for the disease, which may eventually help with recovery efforts. But in the meantime, many former kelp forests remain urchin barrens, and the fish, invertebrates, and marine mammals that depended on those forests have had to move on or decline alongside them.
What This Means for How Ecosystems Work
The starfish removal story fundamentally changed how scientists think about food webs. Before Paine’s experiment, the prevailing view was that ecosystems were primarily shaped from the bottom up: by sunlight, nutrients, and the plants that captured them. The idea that a single predator at the top could control the structure of an entire community was radical. It showed that predation doesn’t just reduce prey numbers. It actively maintains diversity by preventing any one species from monopolizing resources.
This concept, called top-down control, has since been documented in ecosystems around the world. Wolves in Yellowstone, sharks on coral reefs, and otters in kelp forests all play similar roles. But the ochre sea star remains the original and clearest example: one predator, removed from one stretch of coastline, turning a thriving community of 15 or more species into a wall of mussels in just over a year.