Ecological succession is the change in the composition of species over time. The traditional view of succession describes how one community with certain species is gradually and predictably replaced by another community consisting of different species. As succession progresses, species diversity (the number of species in a community) and total biomass (the total mass of all living organisms) increase. Eventually, a final successional stage of constant species composition, called the climax community, is attained. The climax community persists relatively unchanged until destroyed by some catastrophic event, such as a fire. Succession, however, is not as predictable as once thought. Successional stages may not always occur in the expected order, and the establishment of some species is apparently random, influenced by season, by climatic conditions, or by which species happens to arrive first. Furthermore, in some cases, a stable climax community is never attained because fires or other disturbances occur so frequently.

Succession occurs in some regions when climates change over thousands of years. Over shorter periods of time, succession occurs because species that make up communities alter the habitat by their presence. In both cases, the physical and biological conditions which made the habitat initially attractive to the resident species may no longer exist, and the habitat may be more favorable to new species. Some of the changes induced by resident species are listed below:

1. Substrate texture may change from solid rock, to sand, to fertile soil, as rock erodes and the decomposition of plants and animals occurs.

2. Soil pH may decrease due to the decomposition of certain organic matter, such as acidic leaves.

3. Soil water potential, or the ability of the soil to retain water, changes as the soil texture changes.

4. Light availability may change from full sunlight to partly shady, to near darkness as trees become established.

5. Crowding, which increases with population growth, may be unsuitable to certain species.

Succession is often described by the series of plant communities that inhabit a region over time. Animals, too, take up residence in these communities but usually in response to their attraction to the kinds of resident plants, not because of any way in which previous animals have changed the habitat. Animals do, however, affect the physical characteristics of the community by adding organic matter when they leave feces or decompose, and the biological characteristics of the community when they trample or consume plants or when they disperse seeds. But because animals are transient, their effects on succession are often difficult to determine. The plants and animals that are first to colonize a newly exposed habitat are called pioneer species. They are typically opportunistic, r-selected species that have good dispersal capabilities, are fast growing, and produce many progeny rapidly. Many pioneer species can tolerate harsh conditions such as intense sunlight, shifting sand, rocky substrate, arid climates, or nutrient-deficient soil. For example, nutrient-deficient soils of some early successional stages harbor nitrogen-fixing bacteria or support the growth of plants whose roots support mutualistic relationships with these bacteria.

As soil, water, light, and other conditions change, r-selected species are gradually replaced by more stable K-selected species. These include perennial grasses, herbs, shrubs, and trees. Because K-selected species live longer, their environmental effects slow down the rate of succession.

Once the climax community is established, it may remain essentially unchanged for hundreds of years. There are two kinds of succession, as follows:

1. Primary succession occurs on substrates that never previously supported living things. For example, primary succession occurs on volcanic islands, on lava flows, and on rock left behind by retreating glaciers. Two examples follow:

• Succession on rock or lava usually begins with the establishment of lichens. Hyphae of the fungal component of the lichen attach to rocks, the fungal mycelia hold moisture that would otherwise drain away, and the lichen secretes acids which help erode rock into soil. As soil accumulates, bacteria, protists, mosses, and fungi appear, followed by insects and other arthropods. Since the new soil is typically nutrient deficient, various nitrogen-fixing bacteria appear early. Grasses, herbs, weeds, and other r-selected species are established next. Depending upon local climatic conditions, r-selected species are eventually replaced by K-selected species such as perennial shrubs and trees.

• Succession on sand dunes begins with the appearance of grasses adapted to taking root in shifting sands. These grasses stabilize the sand after about six years. The subsequent stages of this succession can be seen on the dunes of Lake Michigan. The stabilized sand allows the rooting of shrubs, followed by the establishment of cottonwoods.

Pines and black oaks follow over the next fifty to one hundred years. Finally, the beech-maple climax community becomes established. The entire process may require a thousand years.

2. Secondary succession begins in habitats where communities were entirely or partially destroyed by some kind of damaging event. For example, secondary succession begins in habitats damaged by fire, floods, insect devastations, overgrazing, and forest clear-cutting and in disturbed areas such as abandoned agricultural fields, vacant lots, roadsides, and construction sites. Because these habitats previously supported life, secondary succession, unlike primary succession, begins on substrates that already bear soil. In addition, the soil contains a native seed bank. Two examples of secondary succession follow:

• Succession on abandoned cropland (called old-field succession) typically begins with the germination of r-selected species from seeds already in the soil (such as grasses and weeds). The trees that ultimately follow are region specific. In some regions of the eastern United States, pines take root next, followed by various hardwoods such as oak, hickory, and dogwood.

• Succession in lakes and ponds begins with a body of water, progresses to a marshlike state, then a meadow, and finally to a climax community of native vegetation. Sand and silt (carried in by a river) and decomposed vegetation contribute to the filling of the lake. Submerged vegetation is established first, followed by emergent vegetation whose leaves may cover the water surface. Grasses, sedges, rushes, and cattails take root at the perimeter of the lake. Eventually, the lake fills with sediment and vegetation and is subsequently replaced by a meadow of grasses and herbs. In many mountain regions, the meadow is replaced by shrubs and native trees, eventually becoming a part of the surrounding coniferous forest. Part II: Subject Area Reviews with Sample Questions and Answers