Name ________________
Date _________________
ECOLOGICAL SUCCESSION
Ecological Succession is a process of ecological change in which a series of natural
communities are established and then replaced over time. Ecologists generally recognize
two kinds of succession, primary succession and secondary succession.
Primary succession takes place in an area that is originally completely empty of life.
Examples would be an area that has been covered by a flow of lava, or and area of bare
rock exposed by melting glaciers. With the right conditions, various kinds of organisms can
begin to grow in these areas. Over time, the variety of life-forms changes as succession
continues.
Primary Succession
One possible scenario for primary succession might begin with the appearance of simple
plants, such as lichens and mosses. Such plants are able to spring up in tiny cracks in the
rocks in which water and dissolved minerals collect. When these pioneer species die, they
decompose and begin to form soil in which other, more complex plants can begin to grow.
The second stage of plants might consist of grasses, herbs, and small shrubs. A
characteristic of these plants is that they devote a great deal of energy producing huge
numbers of seeds. They may live only one year, and spend the greatest part of their energy
ensuring that offspring will arise the following year. Species of this kind are known as
opportunist species. Grasses are a common example of opportunist species. Plants that
make up the early stages of succession also die, decompose, and contribute to the growing
layer of soil. This process takes place over hundreds or thousands of years. Eventually, the
soil is able to support larger plants, such shrubs and small trees including aspen, black
spruce, and jack pine. These plants gradually take over from earlier communities since they
are taller, have more leaves, and can capture more sunlight that was originally captured by
simpler plants.
In the final stages of succession, taller trees begin to grow. They, in turn, block out the
sunlight needed by smaller trees. The final stage of ecological succession is known as a
climax community which represents a relatively stable community that is environmentally
balanced. In the case of a forest, the species of trees in a climax community are large old
trees that are very tolerant to shade since the seedlings must grow under the thick canopy
of their parents.
Secondary succession is far more common. It occurs in an area where an ecosystem
currently exists but has then been disturbed. For example, imagine a forest that has been
destroyed by a wildfire. Again, for a period of time, no living organisms may exist in the
area. Before long, however, certain types of plants begin to reappear and, as in primary
succession, the nature of the plant communities changes over time ultimately resulting in a
stable climax community.
Secondary succession
The general trends that take place during secondary succession are similar to those for
primary succession. The main difference between the two is that in secondary succession
there was life there before so the soil is already present. Imagine that a forest has been
cleared for agriculture and then abandoned at a later date. In this case, a pioneer
community consisting of lichens and mosses is not needed. Soil, rich or not, is already
available. In such a case, the first plants to reappear might be annual (living one year)
weeds, such as crabgrass. At a somewhat later date, the weedy community might be
replaced by perennial (those that live year after year) weeds, and then by shrubs, a pine
forest, and finally a mature forest consisting of oaks, hickories, elms, and other large, longliving trees. As succession progresses, the trees and shrubs become more and more shade
tolerant eventually allowing their own seedlings to take root and grow resulting in a stable
climax forest.
Answer the following questions about succession:
1. A process of ecological change in which a series of natural communities are
established and then replaced over time is called __________________________.
2. Succession that takes place on an area that was originally completely empty of life
is called _____________________________________.
3. Succession that occurs in an area where an ecosystem exists but has been
disturbed is called ____________________________________.
4. Plants or communities that are the first to be established in an area previously
empty of life are called _____________________________________.
5. Plant species with short life-spans that devote most of their energy to producing
seeds are called _______________________________________.
6. A relatively stable community that is environmentally balanced is called
a__________________________________.
7. What are the most common pioneer plant species in primary succession?
8. What are the most common pioneer plant species in secondary succession?
9. Why are the pioneer species in primary and secondary succession different?