1. Succession: Equilibrium in Ecosystems

2. What Happened Here?
The two photographs at the right show the same area in Yellowstone National Park in Wyoming. Photograph A was taken soon after a major fire. Photograph B was taken a few years later. Observe the photographs carefully.
What are all the differences you notice between the two scenes?

3. A community is all the different organisms that live together in an area.
A community in an ecosystem is in equilibrium, or a state of balance, when the numbers and species of organisms in it do not change suddenly.
Events such as fires, floods, volcanoes, and hurricanes disrupt the equilibrium of a community by changing it drastically in a very short time.

4. Succession- What is it?
Succession is the series of predictable changes that occur in a community over time. After a fire, volcano, or other disaster, succession enables an ecosystem to recover. There are two main types of succession: primary and secondary.

5. Primary Succession
Primary succession is the series of changes that occur in an area where no ecosystem previously existed.
For example: a new island formed by the eruption of an undersea volcano, or an area of rock uncovered by a melting sheet of ice.
The first species to populate the area are called pioneer species. Pioneer species are often lichens and mosses carried to the area by wind or water. These species can grow on bare rocks with little or no soil. As these organisms grow, they help break up the rocks. When they die, they enrich the thin layer of soil that is forming.
As the soil gets richer and richer, new plant species will grow. Over time primary succession may lead to a community of organisms in equilibrium, which does not change drastically unless the ecosystem is disturbed. Reaching this stable community can take centuries.
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6. Secondary Succession
Secondary succession is the series of changes that occur after a disturbance in an existing ecosystem.
Both natural and human activities can cause secondary succession.
Examples:
Natural disturbances: fires, hurricanes, and tornadoes
Human activities: farming, logging, or mining
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7. Succession in Action

8. Succession in a Pond
Succession in a pond follows a similar pattern. Originally, the pond will contain protozoa, some small fish, and algae. As individual organisms die and water runs into the pond, sediment builds up at the bottom and the pond grows shallower. The shallower pond becomes marsh-like and fills with reeds and cattails. The standing water eventually disappears, and the land is merely moist: grasses and shrubs dominate. As the land grows even less moist, it becomes woodland and as trees come to dominate, the climax community will arise from a species that
can grow in the shade of its neighbors.

9. Pond Secondary Succession

10. Compare Primary and Secondary Succession
Primary Succession
Happens ONLY with brand new areas never before colonized by life.
Takes centuries to become a stable environment at equilibrium.
Secondary Succession
Happens ONLY with an area that was once colonized by life, but has been destroyed.
Takes about one century to become a stable environment at equilibrium.
Both
Lead to a community of organisms in equilibrium.
Pioneer species move in first, followed by a series of vegetation which eventually leads to the "climax forest".
Plant types depend on the biome.