Along the rocky coast of Washington State in the 1960s, a young ecologist named Robert Paine began prying purple and orange sea stars off the rocks and tossing them into the sea. What happened next changed how ecologists think about communities: removing one predator did not simply leave more food for everyone else. It unraveled the whole shoreline.

Competition

When two species need the same limited resource, they compete. In the early 1930s the Russian biologist Georgii Gause grew two species of the single-celled Paramecium together in tubes with a fixed food supply. Each species did well alone, but together one always drove the other out. His results gave rise to the competitive exclusion principle: two species competing for exactly the same limited resources cannot coexist indefinitely. One will be slightly better and will eventually win.

Yet nature is full of similar species living side by side. How? In 1958 Robert MacArthur studied five species of warblers in the spruce forests of New England. They looked as if they were competing for the same insects, but careful watching showed that each warbler species fed mostly in a different part of the same trees, some near the tips, some near the trunk, some high, some low. This dividing up is called resource partitioning. Competition also shapes where species can live. On Scottish shores in the late 1950s, Joseph Connell showed that one barnacle species could survive lower on the rocks but was crowded out there by a stronger competitor, a clear case of a realized niche smaller than the fundamental one.

Predation and partnership

Predators kill and eat prey; herbivores eat plants, often without killing them; parasites live on or in a host and harm it, usually slowly. Each can drive an evolutionary arms race. Not all interactions are hostile. Mutualism is an interaction in which both species benefit: bees get nectar and flowers get pollinated; fungi wrapped around plant roots trade minerals for sugars. In commensalism, one species benefits while the other is barely affected, like a bird nesting in a tree. Predators also affect prey simply by being feared. Prey that sense danger may avoid open ground, feed less, or keep watch more, which ecologists sometimes call a landscape of fear.

Keystone species

Paine's sea star was Pisaster ochraceus, which eats mussels. On the stretch of shore where he removed it, mussels spread across the rocks and crowded out barnacles, limpets and algae. Within a few years, mussels had taken over and the number of species there had roughly halved, from fifteen to eight in his first report. In 1969 Paine borrowed an architectural term to name such a predator. A keystone species is one whose effect on its community is far larger than its abundance would suggest, like the central stone that holds up an arch.

Paine later popularized another phrase. A trophic cascade occurs when a predator's effects ripple down through more than one feeding level. In the 1970s, James Estes and John Palmisano compared islands in the Aleutian chain. Where sea otters were present, kelp forests thrived; where otters had been hunted out, sea urchins multiplied and grazed the kelp down to bare rock, leaving what divers call urchin barrens. Beavers, whose dams create wetlands, shape communities in another way and are called ecosystem engineers. Critics warn that the keystone label is now used so loosely that it loses meaning, and that many communities have no single species holding them together.

Top down or bottom up?

Why is the world green, when there are so many plant eaters? In 1960 three American ecologists, Nelson Hairston, Frederick Smith and Lawrence Slobodkin, answered that predators keep herbivores in check, so plants survive. This is top-down control. Others stressed bottom-up control: plant defenses such as thorns and toxins, and the supply of nutrients, limit what herbivores can do. Most ecologists now see both forces at work, with their balance varying from place to place.

The most famous modern test is Yellowstone National Park, where wolves returned in 1995. Some researchers credit the wolves with a cascade that let willows and aspens regrow. Other researchers argue that the recovery is patchier than the popular story says, and that elk also declined because of human hunting, bears, cougars and drought.

Worked example: wolves in Yellowstone

Wolves were wiped out of Yellowstone by the mid-1920s. Without them, elk browsed heavily on young willow and aspen. In 1995 and 1996, 31 wolves from Canada were released in the park. Elk numbers on the northern range fell sharply over the next fifteen years. Ecologists William Ripple and Robert Beschta reported that willows and aspens were growing taller in places, beavers were returning, and stream banks were stabilizing, partly because elk avoided risky spots. Other studies pushed back. Some found little evidence that elk avoided risky areas strongly enough to spare trees. Experiments showed that willows often stayed short even when protected from browsing, because the loss of beaver dams had lowered water tables. A 2022 study argued that earlier sampling had overstated aspen recovery. The likeliest answer is that wolves contributed to real changes, but alongside many other causes, and that removing a species for seventy years cannot be undone simply by returning it.