Ecological succession

Ecological succession is the process by which the mix of species in an area changes over years to centuries after new ground appears or an existing community is disturbed. Primary succession begins where there is no soil at all, such as on a fresh lava flow or on rock left bare by a retreating glacier. Secondary succession follows a disturbance such as a wildfire, storm, flood or abandoned farm field that clears much of the vegetation but leaves the soil, seeds and roots in place, so it moves much faster.

The first colonists, called pioneer species, are hardy organisms such as lichens, mosses and fast-growing weeds. They break down rock, trap dust and add dead organic matter, slowly building soil. In doing so they change conditions in ways that let other species move in, and the newcomers, first shrubs and then trees, eventually shade out or outcompete them. Nitrogen-fixing plants such as alders often speed this up by enriching the soil. A community may settle into a fairly stable state, traditionally called a climax community, although many ecologists now see most communities as always shifting with new disturbances.

Old fields turning back into forest are a common example of secondary succession, and the land exposed by shrinking glaciers in Alaska shows the primary kind.