Transcript Succession: Equilibrium in Ecosystems

Succession: Equilibrium in
Ecosystems
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?
• 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.
• 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.
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
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
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.
http://www.sparknotes.com/testprep/books/sat2/biology/chapter10section4.rhtml
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.
http://www.sparknotes.com/testprep/books/sat2/biology/chapter10section4.rhtml
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.