2.7 Ecological Succession

Primary succession happens where there's no soil at all: on new volcanic rock, on land exposed when a glacier retreats, or on a newly formed island. Nothing much can grow until soil forms, so it's slow and can take hundreds to thousands of years.

The first organisms to arrive are pioneer species, usually lichens and mosses. Lichens release acids that slowly break down rock, and when they die they add organic matter. Together with weathering, this builds a thin layer of soil. Then grasses and small plants move in, followed by shrubs and finally trees. Each stage changes the conditions, often making them better for the next set of species than for itself. Over very long times, pioneer populations that settle a brand-new habitat, like a young volcanic island, can adapt to its conditions so much that they become new species.

Secondary succession happens where a disturbance removes most of the plants but leaves the soil, like after a forest fire, a hurricane, a logged forest or an abandoned farm field. Because soil, seeds and roots are still there, it's much faster than primary succession, often taking decades to a century or two.

A typical example in the eastern U.S.: an abandoned cornfield fills with weeds and grasses in the first year or two, then shrubs, then fast-growing sun-loving trees like pines, and eventually shade-tolerant hardwoods like oaks and maples.

A mature, relatively stable community at the end of succession is often called a climax community, though disturbances like fires and storms mean many ecosystems never fully settle.

• Total biomass rises as plants get bigger and more numerous, especially when trees take over.
• Species richness generally increases as soil develops and more habitats form.
• Net productivity rises quickly in early and middle stages, when plants grow fast. In a mature community it often levels off, because the large amount of living tissue uses much of the captured energy in respiration.
• Soil gets deeper and richer in organic matter and nutrients.

A keystone species has a much larger effect on its ecosystem than its numbers would suggest. Remove it and the community changes dramatically. When ecologist Robert Paine removed a predatory sea star from tide pools in the 1960s, mussels took over and the number of species in the pools dropped by about half. Sea otters, beavers and prairie dogs are other keystone species. Beavers build dams that create ponds and wetlands used by many other species.

An indicator species is one whose presence, absence or health tells you about environmental conditions. Mayfly, stonefly and caddisfly larvae are sensitive to pollution and need high dissolved oxygen, so finding many of them in a stream means the water is clean. Lichens are sensitive to air pollution, and amphibians, with their thin, permeable skin, are sensitive to water pollution.