Ecosystems are not frozen. After a fire, a flood, a volcano or an abandoned farm field, the community rebuilds in stages. This change over time is ecological succession, and the climate of the biome decides where it ends up.
Key idea
Primary succession starts where there is no soil, as on new lava rock or ground left bare by a glacier. Secondary succession starts where soil remains after a disturbance, as after a forest fire or on an abandoned field. Secondary succession is faster.
Why it works
Each stage prepares the ground for the next. Lichens grip bare rock and release acids that crumble it; when they die they add organic matter, and a thin soil starts. Mosses and grasses can then root, adding more soil and holding water. Taller plants shade out the early ones. After a fire, the soil, seeds and roots are already there, so recovery skips the slow soil-making stages.
Worked example: Which kind?
Compare two places: land buried under fresh volcanic rock by the 1980 eruption of Mount St. Helens, and a forest that burns in a wildfire but keeps its soil. Which shows primary succession?
- Fresh volcanic rock has no soil: life must start almost from scratch.
- The burned forest kept its soil, seeds and roots.
Answer: The fresh volcanic rock shows primary succession; the burned forest shows secondary succession.
Common mistake
Students think succession always starts from bare rock, or that an ecosystem stays the same forever once it forms. Ask: is there soil? If yes, it is secondary succession, and it is faster. Even mature forests keep changing with storms, fires and new species.
Glacier Bay in Alaska is a classic study site: as the glacier melted back over about 200 years, it left a row of sites of different ages, from bare ground with mosses to spruce and hemlock forest.