Why couldn’t the insects come on shore until the plants did?
  The insects needed plants to feed on.

• It takes more than 100 years for an abandoned field to become a forest.

If there are no trees in the abandoned field to begin with, how do trees come to grow there?
 Seeds are transported by wind or other vector into the area and begin growing.

• Did all forest ecosystems start out as forest ecosystems?
• What happens to an area that has been devastated by fire or other natural disaster?

Science Background
Succession in Aquatic Ecosystems Like terrestrial ecosystems, aquatic ecosystems undergo a gradual change in their vegetative makeup over time as less competitive species are edged out by more competitive species.
In a pond ecosystem, for example, increased silt deposits on the pond floor make the water more shallow. As a result, floating plants such as pond lilies may move in. Eventually reeds and grasses will take root around the water’s edge. Over time, the pond may dry up completely as the pond fills in with plants, becomes more and more shallow, and eventually is unable to collect rainwater.

Investigate

Teaching Tips
Point out that students should sprinkle their seeds randomly as this action represents how seeds blow into a new community naturally.

Draw Conclusions

1. Sample response: The seeds sprouted; the pond dried up partially or completely; the pond filled up with soil and plants.
2. Sample response: In both, soil enters the pond, the water dries up if there is no rain, and plants begin to grow in and around the pond.
3. Sample response: Students may infer that a pond may shrink and dry up as plants begin to grow around and then in it.

Investigate Further

Supply students with additional materials that will make their pond models more realistic. They may create a larger model and add algae, pond lilies and other plants, as well as aquatic insects.

SCIENCE IDEAS
Help students focus on supporting details for this main idea: Gradual change in an ecosystem is called succession.

• Over how many years does succession occur? many years, sometimes thousands
• What is primary succession? succession that takes place on new, bare land
• How are new lands formed? when glaciers melt, or when volcanoes erupt, for example
• What can you infer about the characteristics of pioneer plants? They are probably hearty, requiring little in the way of soil or nutrients.

DEVELOP SCIENCE VOCABULARY
succession, p. B92—Use the word “succeed” in a sentence, as in “Henry I succeeded William II as King of England in 1100.” Point out how each vegetative stage succeeds the next in the ecological process of succession.
pioneer plants, p. B92—Ask students to explain how pioneer plants are like the American pioneers. (The pioneer plants also moved into "uninhabited" country to settle and grow in numbers. Point out that the new lands were in fact inhabited by Native Americans but that the pioneers were nevertheless entering unknown territory.)

USE PROCESS SKILLS
Classify Have students look at the pictures on these two pages. Challenge them to classify the plants described according to their form.

• What are some general characteristics of pioneer plants? They are small, herbaceous (not woody), and hearty.
• What are some general characteristics of climax community plants? They are generally trees—large woody plants.
• What sort of plants come after pioneer plants but before climax community plants? medium-sized plants, such as grasses and bushes

SCIENCE IDEAS
Help students focus on supporting details for this main idea: Soil is formed as plants die and are mixed back into the ground material.

• How do dead plants create and enrich soil? Dead plant material breaks down and releases its nutrients to the ground, which eventually becomes soil. New plants growing in this soil use the nutrients for their own needs.
• What can you infer about the soil quality of the climax community? It must be rich as many generations of plants have died and then been incorporated back into the soil.

DEVELOP SCIENCE VOCABULARY
climax community, p. B93—From the Greek word meaning ladder. A climax community is the highest level community that can be achieved. Ask students to describe when the climax of a movie occurs. (at the height of action or suspense, when the villain is slain or the victim is saved)

SCIENCE IDEAS
Help students focus on supporting details for this main idea: Secondary succession is the return of a damaged ecosystem to its natural climax community.

• How are ecosystems destroyed? Erupting volcanoes can cover an area with hot ash and mud that kills all vegetation. Fires can also destroy almost all of the vegetation in an area.
• How does secondary succession begin? Seeds and roots underground begin to sprout and produce new plants. Also, seeds may blow in from neighboring areas and start growing in the new rich soil.

Critical Thinking Lead students to conclude how secondary succession differs from primary succession.

• How are primary and secondary succession alike? How are they different? Both primary and secondary succession involve the progression of vegetation from tiny plants, to grasses, to shrubs, to small trees, to large trees—a climax community. But, while primary succession occurs on areas that have never been inhabited by plants or animals, secondary succession occurs in areas where the local plants and animals were wiped out by a natural disaster.

VISUAL LEARNING
Have students look at the pictures on this page. Ask questions about what they observe and infer.

• How do you think the fallen trees affected the ecosystem around Mount St. Helens? Once the tall trees were down, more sunlight could reach the grasses and smaller plants on the forest floor. Also, as the logs decomposed, they added valuable nutrients to the soil which sprouting seeds used to grow and flourish.
• What are some of the different habitats you see in the Yellowstone park picture? old growth forest, new growth forest, a meadow with grasses

REACHING ALL LEARNERS
Kinesthetic Learners Have students act out the process of primary succession in the classroom. Assign different vegetative roles to each student. Then have the lichens and mosses stand in a marked-out area. Slowly have the students playing grasses and bushes come and tap the lichens and mosses on the shoulder and take their places. Continue until the area is populated with mainly large trees. Colorful costumes can be used to enrich this activity.

INVESTIGATION CHALLENGE
Hands-On Activity: Eruption Destruction
Use this idea for a hands-on activity to extend students’ understanding of key lesson concepts.
Have students recreate the Mount St. Helens event by covering a small plot of land near the school with a thick layer of cooled ash. The ash should completely cover the smaller grasses, and partially cover larger plants. Encourage students to return to the site every day to see how wind and rain have begun to wash away the ash. Have them look for evidence of new growth under the dwindling ash layer.
If you prefer a more controlled environment, have students grow grass seed on a tray and then cover the tray with ash in the classroom. Simulating a light rain every few days, students should record any changes they see taking place in their artificial environments.

LINKS TIPS
Math Link The alders appear in 1800, the spruce takes over around 1920, and the spruce-hemlock forest takes over in 2000. Have students record the birthdates of their ancestors on the time line if they know them. Then challenge students to determine how many generations it took for this climax community to form from a receding glacier. (about 6 or 7)
Writing Link Students may want to research both positions before writing their letters. They can then include a summary of each position in their letters and ask the ranger to explain the park’s policies in light of these arguments.
Art Link Students may need to do some research alongside their observations in order to have enough information to draw their pictures.