1. What is ecological succession?
Ecological succession is the series of more or less predictable changes in ecological communities following a disturbance.
Ecologists once thought that succession in a given area would always proceed through the same stages, in the same order. When the process was completed, there would be a stable climax community.
Today, however, most ecologists agree that there is no one path through the events and processes of succession.
1. Define What is ecological succession?

2. What events set primary succession in motion?
Primary succession occurs in an area with no remains of an older community.
Primary succession can begin when a catastrophic event, such as a volcanic eruption, completely destroys an ecosystem, when new land surfaces emerge, or when a depression becomes filled in with water for the first time.
2. Sequence What kinds of event starts primary succession?

3. What events set secondary succession in motion?
Secondary succession occurs when a disturbance seriously disrupts an existing community, but does not destroy it. Unlike primary succession, soil is present at the start of secondary succession.
Secondary succession may follow a natural disturbance such as a hurricane, or it may follow human activities such as logging.
3. Compare and Contrast How does secondary succession differ from primary succession?
(contd.)

4. Secondary succession in aquatic ecosystems may eventually lead to the formation of a terrestrial ecosystem.

5. What processes cause succession to continue?
Both primary and secondary succession occur because every living organism changes its environment.
Lichens, for example, release acids that break down rocks and helps form soil.
Lichens are a pioneer species. A pioneer species is the first species to colonize a barren area.
4. Relate Cause and Effect How do lichens enable soil to form?
Lichens

6. How do the events of primary succession affect populations?
Events that start primary succession typically cause drastic reductions in the populations of many organisms. Some populations may be eliminated entirely.
After the event, conditions may be favorable for pioneer species.
For example, the eruption of Mount Saint Helens killed off entire populations of plants and animals. Lichens, however, could grow on the newly barren ground. The lichen population increased dramatically.
5. Infer After the eruption of Mount Saint Helens, why did the lichen population increase?

7. How do the events of secondary succession affect populations?
Like the events that cause primary succession, the events that cause secondary succession typically reduce populations dramatically.
However, these events rarely kill off entire populations.
Limiting factors may be eliminated, allowing some populations to increase. After a forest fire, for example, populations of sun-loving plants may increase.
6. Relate Cause and Effect Why can sun-loving plants thrive after forest fires?

8. How do the processes of succession affect populations?
Colonization and environmental change are two processes that continue throughout succession.
As each species colonizes an area, it changes environmental conditions.
The new conditions may cause some populations to increase and others to decrease.
7. Identify What are two processes that play a role in ecological succession?

9. How does competition affect populations?
As ecological succession proceeds, population density increases, making the process of competition among organisms more intense.
Eventually, competition can reduce the populations of early colonizers. As grassland gradually becomes forest, for example, tall trees may prevent sunlight from reaching the grasses.
8. Infer Why does increased population density intensify competition?

10. How do the events of succession affect species diversity?
The events that set off ecological succession, such as volcanic eruptions and hurricanes, usually reduce species diversity in the immediate area of the disturbance.
Events that start primary succession may eliminate species diversity entirely.
Ash and lava from Mount Saint Helens buried and destroyed plant life throughout a large area.
9. Define What is species diversity?

11. How do succession processes affect species diversity?
The processes of succession can have complex effects on species diversity.
As succession takes place, processes such as soil formation may gradually increase species diversity.
At some points in succession, the process of competition may decrease species diversity.
In general, mature communities have higher species diversity than areas in earlier stages of succession.
10. Relate Cause and Effect Why can soil formation increase species diversity?