1. Succession
Read the lessons title aloud.

2. Learning Objectives
Describe how ecosystems recover from a disturbance.
Compare succession after a natural disturbance with succession after a human-caused disturbance.
Click to show each learning objective.
Read each objective aloud or ask for a volunteer to do so.
Describe or show photographs of a local area, familiar to students, that was recently disturbed by a natural event or human actions. Ask students to predict how the area would look in ten years if it were left undisturbed. Tell them they will learn what happens to such disturbed areas in this lesson.

3. Ecological Succession
A series of more-or-less predictable events that occur in a community over time
Tell students that the drawing represents landscape in Glacier Bay, Alaska, after a retreating glacier exposed bare rock.
Click to reveal the green arrow and the drawing of the hemlock and spruce forest.
Point out that over 100 years, the barren rock landscape became the hemlock and spruce forest seen in the area today.
Emphasize that ecosystems change over time, especially after disturbances, as new species move in, populations change, and some species die out.

4. Primary Succession
Succession that begins in areas that have no remnants of an older community
Point out that the change in Glacier Bay from barren rock landscape to hemlock and spruce forest is an example of primary ecological succession.
Describe another example of primary ecological succession:
In 1883, the volcanic island of Krakatau in the Indian Ocean was destroyed by an eruption. The tiny island that remained was completely barren. Within two years, however, grasses were growing. Fourteen years later, there were 49 plant species, along with lizards, birds, bats, and insects. By 1929, a forest containing 300 plant species had grown. Today, the island is blanketed by mature rain forest.
Ask students to describe how the stories of the two regions are similar. Guide students to understand that in both cases, the landscapes had experienced events that led to barren landscapes with little to no life.
Click to reveal the definition of primary succession.
Be sure students understand that the changes the picture shows are not a matter of plants just growing larger and reproducing. The changes are actually in the type and number of species.
Ask: Are the species in the first few panels the same as those in the later panels?
Answer. No, over time there are different species in the area.
Ask: Based on the drawings, describe the relationship and diversity of species and succession.
Answer: Species diversity typically increases over the course of succession.

5. Pioneer Species The first species to colonize barren areas Lichen
Point out that this image focuses on early primary succession, showing the first two panels from the diagram on the previous slide. Remind students that the hemlock-spruce forest of Glacier Bay took more than 100 years to develop, and that hemlock and spruce were not present in the early stages of succession.
Explain that the changes in the kind and number of species over time is due to the conditions that can be tolerated by various species. Hemlock and spruce cannot grow on barren rock. For these and many other plant species to inhabit an area, there first needs to be adequate soil, among other resources. Only a small number of species can tolerate the harsh barren conditions of early primary succession. These initial species are called pioneer species, and their presence helps develop conditions that other organisms can tolerate.
Explain that one ecological pioneer that grows on bare rock is lichen—a mutualistic symbiosis between a fungus and a blue-green bacterium.
Click to reveal the label for lichen.
Over time, lichens fix atmospheric nitrogen to synthesize organic material. Lichens also break down rock and begin to form soil.
Click to reveal the definition of pioneer species.
Emphasize that every organism changes its environment. As pioneer species alter their environment, external environmental factors change in ways that make it easier for other species to survive. As lichens add organic matter and form soil, for example, mosses and other plants can colonize and grow. As trees grow, their branches and leaves shade and protect the ground and provide shelter and food for animals. These processes increase the complexity of the environment, enabling more plant and animal species to find homes and food. As succession continues, therefore, species diversity usually increases.

6. Secondary Succession
Succession that occurs when a disturbance affects an existing community but doesn’t completely destroy it
Point out at that succession doesn’t always start from bare rock. Sometimes, a disturbance affects an existing community but doesn’t completely destroy it, leaving remnants of former ecosystems, such as soil and even some plants. In such as case, secondary succession occurs.
Click to reveal the definition of secondary succession.
Explain that this series of pictures shows changes taking place in abandoned fields of the Carolinas’ Piedmont. Over the last century, these fields have passed through several stages and matured into oak forests. Changes will continue for years to come.
Draw student attention to the timeline below the diagram.
Click to reveal the highlighting box.
Ask: How does this timeline differ from the timeline shown in the diagram for primary succession?
Answer: The timeline for secondary succession is much shorter.
Tell students that secondary succession proceeds more quickly than primary succession, in part because bits of the old community survive and can regrow rapidly. On land, secondary succession often follows a wildfire, hurricane, or other natural disturbance. We think of these events as disasters, but many species are adapted to them. Although forest fires kill some trees, for example, other trees are spared, and fire can stimulate their seeds to germinate. Secondary succession can also follow human activities like logging and farming.

7. Climax Community
A specific, uniform, and stable community at the “end” of succession
However . . .
Recent research shows climax communities are not always uniform and stable.
Explain that years ago, ecologists thought that succession in a given area would always proceed through the same stages to produce a specific, uniform, and stable climax community like the spruce-hemlock forest that is developing in Glacier Bay.
Click to reveal the first point.
Point out that recent studies have shown that succession doesn’t always follow the same path, and that climax communities are often not uniform and stable.
Click to reveal: However . . .
Click to reveal the second point.

8. Succession After Natural Disturbance
Patches of climax communities may be in different stages of secondary succession due to multiple disturbances.
Tell students that these photos show El Yunque National Forest in Puerto Rico, immediately following Tropical Storm Jeanne in September 2004 (left), and then again in May 2007 (right).
Explain that many communities regularly experience natural events that disturb patches of varying sizes. Healthy coral reefs and tropical rain forests recover from storm damage, as shown here. Healthy temperate forests and grasslands recover from wildfires.
Ask: What kind of succession occurred in this rain forest? How do you know?
Answer: Secondary succession occurred, since soil and a few plants remained in the area after the storm.
Emphasize that events and processes during secondary succession in healthy ecosystems following natural disturbances often reproduce the original climax community. But many climax communities look more like patchwork quilts than uniform blankets. This is because natural disturbances often disturb patches of those communities at different times, so different patches may be in different stages of secondary succession. Some climax communities are disturbed so often that they can’t really be called stable.
September 2004
May 2007

9. Succession After Human Disturbance?
Explain that in North America, land cleared for farming and then abandoned often passes through succession that restores the original climax community.
Click to reveal the arrow pointing to the “normal” climax community.
Emphasize, however, that this is not always the case. Ecosystems may not recover from extensive human-caused disturbances.
Click to reveal the arrow pointing to the “unknown” climax community.
Emphasize that ecosystems may or may not recover from extensive disturbances. Tell students that clearing and farming of tropical rain forests, for example, can change the structure and community of microorganisms in soil in ways that prevent regrowth of the original community.
Point out that random factors can influence the outcome of succession. For example, you might discuss such factors as wind direction, rainfall, and the organisms that happen to be actively breeding immediately after a disturbance.

10. Studying Succession
Tell students about the eruption of Mount St. Helens: In 1980, when volcanic activity blew up Mount St. Helens in Washington State, all the trees for hundreds of square miles of mature forest around the volcano were scorched, burned to the ground, or toppled like matchsticks. More than 20 square miles were buried to an average depth of 150 feet by the largest debris avalanche in recorded history. The U.S. Congress acted quickly to protect thousands of acres of the destroyed forest from human intervention so scientists could study ecological succession in the unique living laboratory the volcano created. Soon, scientists from more than 70 universities and government agencies had started long-term research projects on Mount St. Helens. Since then, their research has made major contributions to the science of succession. The results of their research are also being applied to help restore other disturbed forests.
Ask: What type of succession has occurred in the area since the eruption?
Answer: primary succession
Click to reveal the photos of a ranger studying organisms.
Explain that ecologists study succession by comparing different cases and looking for similarities and differences. Researchers who studied Mount St. Helens as soon as it was safe, for example, could compare it with Krakatau. In both places, primary succession proceeded through stages. In both cases, pioneer species had arrived via seeds, spores, or adult species that traveled over long distances. These hardy pioneers helped stabilize loose volcanic debris, enabling later species to take hold. Historical studies in Krakatau and ongoing studies on Mount St. Helens confirm that early stages of primary succession are slow and that chance can play a large role in determining which species colonize at different times.

11. Overview: Succession
Ask: Does this diagram represent secondary succession or primary succession?
Answer: secondary succession.
Ask: What are two ways the diagram would be different if it represented primary succession?
Answer: In the far-left (first) panel, there would be only rock, no living things. The timeline would also likely be much longer.
Have volunteers read the statements aloud, verbally filling in with the correct terms.
Click to reveal the correct answers.
1. Secondary succession in healthy ecosystems following a
disturbance often reproduces the original
are the first species to colonize barren areas.
climax community
Pioneer species