Successional Changes In Communities

During the summer and early fall of 1988, fires burned large areas of Yellowstone National Park. Approximately 320,900 hectares (793,000 acres) were affected. If you visit Yellowstone today, you will find that regrowth is well underway in the burned areas. In time, if no further major disturbances take place, the burned areas of Yellowstone National Park will undergo a series of regrowth stages. The gradual, sequential regrowth of a community of species in an area is called ecological succession (EK-uh-LAHJ-i-kuhl suhk-SESH-uhn). You can see early stages of succession in vacant lots, along roads, and even in sidewalks or parking lots where weeds are pushing up through cracks in the concrete. Figure 20-11 shows the successional changes that took place after the eruption of Mount St. Helens in 1980.

Ecologists recognize two types of succession. Primary succession is the development of a community in an area that has not supported life previously, such as bare rock, a sand dune, or an island formed by a volcanic eruption. In primary succession, soil is not initially present. Secondary succession is the sequential replacement of species that follows disruption of an existing community. The disruption may stem from a natural disturbance, such as a forest fire or a strong storm, or from human activity, such as farming, logging, or mining. Secondary succession occurs where soil is already present.

Any new habitat, whether it is a pond left by heavy rain, a freshly plowed field, or newly exposed bedrock, is an invitation to many species that are adapted to the new conditions. The species of organisms that predominate early in succession are called pioneer species. Pioneer species tend to be small, to grow quickly, and to reproduce quickly, so they are well suited for invading and occupying a disturbed habitat. They also may be very good at dispersing their seeds, which enables them to quickly reach disrupted areas.

Early Stage Succession Pioneer Plant
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figure 20-12

Ecologists study the process of primary succession by examining a variety of areas at different successional stages. These photos were taken at different locations at Glacier Bay, Alaska; the changes that occur between the stage shown in (a) and the stage shown in (c) take about 200 years. Shown in (a) is lifeless glacial "till" (pulverized bare rocks) left in the wake of the retreating glacier. Shown in (b) is an early stage of succession in which small plants and shrubs are growing on the site. A mature forest is shown in (c), an end stage of succession.

Primary Succession

Primary succession often proceeds very slowly because the soil is too thin or lacks enough minerals to support plant growth. For example, when glaciers last retreated from eastern Canada, about 12,000 years ago, they left a huge stretch of barren bedrock from which all the soil had been scraped. This geologic formation, called the Canadian Shield, was a place where plants and most animals could not live. Repeated freezing and thawing broke this rock into smaller pieces. In time, lichens—mutualistic associations between fungi and either algae or cyanobacteria—colonized the barren rock. Acids in the lichens and mildly acidic rain washed nutrient minerals from the rock. Eventually, the dead organic matter from decayed lichens, along with minerals from the rock, began to form a thin layer of soil in which a few grasslike plants could grow. These plants then died, and their decomposition added more organic material to the soil. Soon, mosses and then larger plants began to grow. Today, much of the Canadian Shield is densely populated with pine, balsam, and spruce trees, whose roots cling to soil that in some areas is still only a few centimeters deep. A similar series of changes has been documented at Glacier Bay, Alaska, shown in Figure 20-12.

Secondary Succession

Secondary succession occurs where an existing community has been cleared by a disturbance, such as agriculture, but the soil has remained intact. In secondary succession, the original ecosystem returns through a series of well-defined stages. In eastern temperate regions, secondary succession typically begins with weeds, such as annual grasses, mustards, and dandelions, whose seeds may be carried to the site by wind or by animals. If no major disturbance occurs, succession in these regions proceeds with perennial grasses and shrubs, continues with trees such as dogwoods, and eventually results in a deciduous forest community. The complete process takes about 100 years.

figure 20-12

Ecologists study the process of primary succession by examining a variety of areas at different successional stages. These photos were taken at different locations at Glacier Bay, Alaska; the changes that occur between the stage shown in (a) and the stage shown in (c) take about 200 years. Shown in (a) is lifeless glacial "till" (pulverized bare rocks) left in the wake of the retreating glacier. Shown in (b) is an early stage of succession in which small plants and shrubs are growing on the site. A mature forest is shown in (c), an end stage of succession.

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www.scilinks.org Topic: Succession Keyword: HM61475

www.scilinks.org Topic: Succession Keyword: HM61475

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Maintained by the rPMJL'C National Scianca f TO. Taachan Association

figure 20-13

A recently abandoned agricultural field is being pioneered by weeds. Eventually, taller plants and shrubs will compete with the pioneers. If no further disturbances occur, a forest of pine or cottonwood may follow, succeeded by a hardwood forest. The whole process will take about 100 years.

figure 20-13

A recently abandoned agricultural field is being pioneered by weeds. Eventually, taller plants and shrubs will compete with the pioneers. If no further disturbances occur, a forest of pine or cottonwood may follow, succeeded by a hardwood forest. The whole process will take about 100 years.

Human disturbances such as mining, logging, farming, and urban development often start the process of succession. A recently abandoned farm field is shown in Figure 20-13. After areas such as woodlands and prairies are cleared by humans, grasses and weeds often begin to grow, thus beginning the process of secondary succession.

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