How Ecosystems Change

as you read

You'll Learn

■ Explain how ecosystems change over time.

■ Describe how new communities begin in areas without life.

■ Compare pioneer species and climax communities.

It's Important

Understanding ecosystems and your role in them can help you manage your impact on them and predict the changes that may happen in the future.

9 Review Vocabulary ecosystem: community of living organisms interacting with each other and their physical environment

New Vocabulary

• succession

• pioneer species

• climax community

Ecological Succession

What would happen if the lawn at your home were never cut? The grass would get longer, as in Figure 1, and soon it would look like a meadow. Later, larger plants would grow from seeds brought to the area by animals or wind. Then, trees might sprout. In fact, in 20 years or less you wouldn't be able to tell that the land was once a mowed lawn. An ecologist can tell you what type of ecosystem your lawn would become. If it would become a forest, they can tell you how long it would take and predict the type of trees that would grow there. Succession refers to the normal, gradual changes that occur in the types of species that live in an area. Succession occurs differently in different places around the world.

Primary Succession As lava flows from the mouth of a volcano, it is so hot that it destroys everything in its path. When it cools, lava forms new land composed of rock. It is hard to imagine that this land eventually could become a forest or grassland someday.

The process of succession that begins in a place previously without plants is called primary succession. It starts with the arrival of living things such as lichens (LI kunz). These living things, called pioneer species, are the first to inhabit an area. They survive drought, extreme heat and cold, and other harsh conditions and often start the soil-building process.

Figure 1 Open areas that are not maintained will become overgrown with grasses and shrubs as succession proceeds.

64 ♦ E CHAPTER 3 Ecosystems

Jeff Greenberg/Visuals Unlimited

Figure 1 Open areas that are not maintained will become overgrown with grasses and shrubs as succession proceeds.

Succession Ferns GrassPlants Breaking Down Rocks
Figure 2 Lichens, like these in Colorado, are fragile and take many years to grow. They often cling to bare rock where many other organisms can't survive. Describe how lichens form soil.

New Soil During primary succession, shown in Figure 2, soil begins to form as lichens and the forces of weather and erosion help break down rocks into smaller pieces. When lichens die, they decay, adding small amounts of organic matter to the rock. Plants such as mosses and ferns can grow in this new soil. Eventually, these plants die, adding more organic material. The soil layer thickens, and grasses, wildflowers, and other plants begin to take over. When these plants die, they add more nutrients to the soil. This buildup is enough to support the growth of shrubs and trees. All the while, insects, small birds, and mammals have begun to move in. What was once bare rock now supports a variety of life.

Secondary Succession What happens when a fire, such as the one in Figure 3, disturbs a forest or when a building is torn down in a city? After a forest fire, not much seems to be left except dead trees and ash-covered soil. After the rubble of a building is removed, all that remains is bare soil. However, these places do not remain lifeless for long. The soil already contains the seeds of weeds, grasses, and trees. More seeds are carried to the area by wind and birds. Other wildlife may move in. Succession that begins in a place that already has soil and was once the home of living organisms is called secondary succession. Because soil already is present, secondary succession occurs faster and has different pioneer species than primary succession does.

r^L rrjrr i-i~rwj ~tji ^ Which type of succession usually starts without soil?

Scien

Topic: Eutrophication

Visit booke.msscience.com for Web links to information about eutrophication (yoo truh fih KAY shun)—secondary succession in an aquatic ecosystem.

Activity Using the information that you find, illustrate or describe in your Science Journal this process for a small freshwater lake.

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