After a disturbance, such as an intense forest fire, disrupts an established biological community, or when a new substrate, such as ash from a volcano, is laid down, microorganisms, plants, and animals begin to colonize the affected area. As these organisms establish themselves, they modify the environment, starting a process of ecological change called succession, which is the gradual change in a community over time following a disturbance.

Succession on a bare geologic surface, such as a recent lava flow, is called primary succession. It is generally a slow process, requiring hundreds or thousands of years, because it begins in the absence of well-developed soil. One example of this is the succession that takes place on the stones and gravel exposed by a retreating glacier (Figure 4.14). The earliest colonizers must be tolerant of low nutrient conditions. Many of these “pioneer” species, including plants and lichens (an association between a fungus and cyanobacteria), do not depend entirely on soil nutrients because they are capable of getting nitrogen from the air (for more on the nitrogen cycle, see Chapter 7, page 192).

The earliest community to develop during succession, called a pioneer community, is made up mainly of species that are tolerant of exposure to full sun, capable of living under conditions that are stressful to many other species. These pioneer species also generally have high reproductive rates and good dispersal abilities. Gradually, as these early organisms grow and shed detritus, which is consumed by fungi, worms, and other detritivores, nutrient-rich soil forms. As soil builds, conditions become suitable for the establishment of grasses and herbs, then shrubs and trees. In time, a forest could develop on the site.

Succession following a disturbance of an established community that doesn’t destroy all living creatures or the soil is called secondary succession, because the process occurs on a landscape already much modified by organisms. For example, after an agricultural field is abandoned, it may gradually become forested again. Early in succession, annual plants—those that germinate from seeds, then grow, set seeds, and die within one year—with high reproductive rates and rapid dispersal ability will dominate the plant community. Later, perennial herbs, grasses, and shrubs, with more restricted environmental requirements, will establish themselves, displacing many of the earliest, pioneer plant species. Next, we might see fast-growing pine trees. Eventually, the pines will be overgrown and replaced by slower-growing deciduous trees, such as oak and hickory. These will become the foundation species of the climax community, which will remain stable until the next disturbance (Figure 4.15). The species making up the climax community will, of course, depend on the biome in which succession occurs.

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As succession unfolds, the sequence of species that arrive have different life histories (see Chapter 3, page 71). The communities during the early stages of succession are dominated by the r-selected species (many offspring, wide dispersal) characteristic of disturbed environments, while the later stages of succession favor K-selected species (few offspring, slow growth). Between these two endpoints, mid-successional communities are formed from a mixture of r- and K-selected species. As a result, the middle stages of succession are generally richer in species than either early or late successional stages (Figure 4.16).