On the morning of May 18, 1980, Mount St. Helens in Washington State blew out its north side. In minutes, a blast of hot rock and gas flattened forest across some 600 square kilometers and buried valleys under pumice. What scientists saw return over the next forty years bears on one of the oldest debates in ecology.

The ecosystem idea

The British botanist Arthur Tansley coined the word ecosystem in 1935. He wanted a term for the living community and its physical environment treated as a single system, linked by flows of energy and cycles of matter. He also used it to push back against colleagues who spoke of plant communities as if they were living organisms.

Succession

Communities change over time. Ecological succession is the gradual and fairly predictable change in the species of a community after a disturbance or on new ground. Ecologists distinguish two kinds. Primary succession begins on new ground with no soil, such as fresh lava, rock left behind by a retreating glacier, or a new volcanic island. Hardy pioneer species such as lichens and mosses arrive first, slowly building soil as they grow and die. Secondary succession begins where soil remains, after a fire, a flood, logging, or the abandonment of a farm. In the eastern United States, an abandoned field typically passes from weeds and grasses to shrubs, then to pines, and eventually to hardwoods such as oak and hickory.

Organism or accident?

Early in the twentieth century, the American ecologist Frederic Clements argued that succession was orderly and directional, ending in a stable climax community set by the regional climate. In his 1916 book Plant Succession he wrote that "as an organism the formation arises, grows, matures, and dies." In his view, a plant community develops like an organism toward a stable climax. In 1926 another American botanist, Henry Gleason, challenged this picture. He argued that each species responds individually to conditions, so communities are loose and partly accidental collections, shaped by which seeds happen to arrive and which conditions happen to suit them. For decades Clements's view dominated. Today most ecologists lean toward Gleason, recognizing that disturbance is common and that no single climax is guaranteed, while still finding real and repeatable patterns in how communities change.

The role of disturbance

Fires, storms, floods and landslides are not always catastrophes from an ecosystem's point of view. Many systems depend on them. Lodgepole pines bear cones sealed with resin that open only in the heat of a fire, releasing seeds onto freshly cleared ground. In 1978 Joseph Connell proposed the intermediate disturbance hypothesis: diversity tends to be highest where disturbance is neither too frequent nor too rare. Too much disturbance leaves only the hardiest pioneers; too little lets a few dominant competitors take over. The idea became a textbook staple, though later reviews found the evidence mixed, and some ecologists now argue that it should be retired.

Biomes

Travel from the equator to the Arctic and you pass through broad zones of distinct vegetation. Biomes are large regions defined by their characteristic vegetation and shaped mainly by temperature and rainfall. On land they include tropical rainforest, savanna, desert, temperate grassland, temperate forest, the vast boreal forest of conifers across Canada and Russia, and the treeless tundra. Aquatic systems include oceans, coral reefs, estuaries, lakes and rivers. Elevation works like latitude. When Alexander von Humboldt climbed the Andean volcano Chimborazo in 1802, he recorded how vegetation changed with height, and later drew it as a famous diagram. Climbing a high mountain passes through zones much like traveling toward the poles. As climate shifts, biome boundaries move too.

What ecosystems do for people

The Millennium Ecosystem Assessment of 2005, a large international review, sorted the benefits people get from nature into four groups. Provisioning services are products such as food, fresh water and timber. Regulating services are processes such as flood control by wetlands, pollination and water purification. Cultural services include recreation and spiritual meaning, and supporting services include soil formation and nutrient cycling. Supporters say pricing these services makes nature visible to decision makers. Critics answer that a price tag can cut both ways: if a forest is worth less than a mine, the same logic says dig.

Worked example: life returns to Mount St. Helens

Ecologists expected the blast zone to follow a slow, orderly primary succession, with wind-blown seeds arriving first at the edges. Instead, recovery began from within. Pocket gophers survived underground, plants beneath snowbanks, and fish in ice-covered lakes. These survivors, called biological legacies, seeded recovery from scattered patches. On the barren Pumice Plain, a few prairie lupines were found in 1981. Lupines can fix nitrogen, so their patches built soil that let other plants follow. Chance played a large role: where a seed landed, whether insects found a lupine patch, and how the ground eroded. The result supported Gleason's view that history and accident matter, while still showing the general pattern of soil building and colonization that Clements would have recognized.