Succession: Equilibrium in Ecosystems

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Succession Equilibrium in Ecosystems
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• 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?

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• 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.

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• 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.

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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.
• Prentice Hall

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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
• Prentice Hall

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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
cant 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 dont change the
soil composition and thrive in their own shade.
The community has reached its climax community,
with maple as the dominant species. Dont
forget that during all this, the changing
vegetation has brought with it various changes
in animal populations.
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ogy/chapter10section4.rhtml
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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.
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ogy/chapter10section4.rhtml
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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.

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