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. The forest community in Yellowstone had begun to recover from the fire.
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. But even without a disaster, communities sometimes change
But even without a disaster, communities sometimes change. 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. sun-starved poplars. The maples eventually dominate the community,
Succession in Action
Imagine a catastrophic event: a forest fire rages through the Green Mountains of Vermont. The fires burn everything and leave behind a barren, rocky expanse.
The population of trees that once lived in this area can’t grow back because the fire has changed the ground composition. Without tree roots to act as anchors, rain washes away the soil and the ground becomes rocky and barren. This rocky ground, however, proves ideal to lichens, the pioneer population. The lichens colonize the rocks and thrive. As part of their life process, lichens produce acids that break down rock into soil. Lichens need solid places to survive: they are victims of their own success. Mosses and herbs are well suited to living in the shallow soil environment created by the lichen, and they replace the lichen as the dominant population.
The mosses and herbs continue to build up the soil. As the soil deepens, the conditions favor plants with longer roots, such as grasses. Eventually the land becomes suitable for shrubs and then for trees. The early dominant trees in the community will be species like poplar, which thrive in bright, sunlit conditions. As more trees
grow in the area, however, there is less sunlight, and maples, which grow in shade, supplant the
sun-starved poplars. The maples
eventually dominate the community,
because they don’t change the soil
composition and thrive in their own
shade. The community has reached
its climax community, with maple
as the dominant species. Don’t
forget that during all this, the
changing vegetation has brought
with it various changes in
animal populations.

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. 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.