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fh" 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.
2. Make a list of all the differences you notice between
the two scenes.
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Questions How would you describe
what happened during the time between the
two photographs? What questions do you
have about this process?
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How are primary and
secondary succession
different?
Reoding lip Before you read,
write a definition of what you
think the term succession might
mean. As you read, revise your
definition.
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1988, a huge fire raged through Yellowstone National
Park. The fire was so hot that it jumped from tree to tree
without burning along the ground between them. In an
instant, huge trees burst into flame from the intense heat. It took
weeks for the fires to burn themselves out. All that remained of
that part of the forest were thousands of blackened tree trunks
sticking out of the ground like charred toothpicks.
You might think it unlikely that Yellowstone could recover
from such a disastrous fire. But within just a few months, signs
of life had returned. First tiny green shoots of new grass appeared
in the black ground. Then small tree seedlings began to grow
again. The forest was coming back!
Fires, floods, volcanoes, hurricanes, and other natural disasters can change communities in a very short period of time. But
even without a disaster, communities change. The series of predictable changes that occur in a community over time is called
succession. This section describes two types of succession:
primary succession and secondary succession.
Primary Succession
Primary succession
in an area
where no ecosystem previously existed. Such an area might be a
new island formed by the eruption of an undersea volcano, or an
area of rock uncovered by a melting sheet of ice.
You can follow the series of changes an area might undergo
in Figure 26 below. These scenes show an area after aviolent volcanic eruption. At first there is no soil, just ash and rock. The
first species to populate the area are called pioneer species.
Pioneer species are often lichens and mosses carried to the area
bywind or water. These species can grow on bare rocks with little
or no soil. As these organisms grow they help break up the rocks.
\Mhen they die, they provide nutrients that enrich the thin layer
of soil that is forming on the rocks.
Over time, plant seeds land in the new soil and begin to grow.
The specific plants that grow depend on the biome of the area.
For example, in a cool, northern area, eatly seedlings might
include alder and cottonwood trees. As the soil grows older and
richer, these trees might be replaced by spruce and hemlock.
Eventually, succession may lead to a community of organisms
that does not change unless the ecosystem is disturbed. Re4ching
this stable community can take centuries.
d eAaZAn*4t
is the series of changes that occur
What are some pioneer species?
Figure 26 Primary succession
occurs in an area where no
ecosystem previously existed.
A. After a volcanic eruption, the
ground surface consists of ash
and rock. B. The first organisms
to appear are lichens and moss.
C. Weeds and grasses take root in
the thin layer of soil. D. Eventually,
tree seedlings and shrubs sprout.
Applying Concepts What determines
the particular species thot oppear
during succession?
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Figure
27
Secondary succession
occurs following a disturbance to
an ecosystem, such as clearing a
forest for farmland. When the farm
is abandoned, the forest gradually
returns. A. After two years, weeds
and wildflowers fill the field.
B. After five years, pine seedlings
and other plants populate the field.
C. After 30 years, a pine forest has
grown up. D. After I00 years, a
mixed forest of pine, oak, and
hickory is developing in the field.
Secondary Succession
The changes following the Yellowstone fire were an example of
secondary succession. Secondary succession is the series of
changes that occur after a disturbance in an existing ecosystem.
Natural disturbances that have this effect include fires, hurricanes, and tornadoes. Human activities, such as farming, logging, or mining, may also distuib an ecosystem. Unlike primary
succession, secondary suc'cession occurs in a place where an
ecosystem has previously existed.
Secondary succession occurs somewhat more rapidly than
primary succession. Consider, for example, an abandoned field
in the southeastern United States. Follow the process of succession in such a field in Figure 27. Aft"er a century, a hardwood forest
is developing. This forest is very stable and will remain for a long
time. Of course, the particular species that come and go in the
process of succession depend on the biome.
1. How are primary and secondary succession different?
2. What is a pioneer species?
3. Give two examples of natural disturbances and two
examples of human disturbances that can result in
secondary succession.
4. Thinking Critically Classifying Crass poking
through the cracks in a sidewalk is an example of
succession. ls this primary or secondary succession?
Explain.
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