228 ❯ STEP 4. Review the Knowledge You Need to Score HighSuccession
When something happens to a community that causes a shift in the resources available to
the local organisms, it sets the stage for the process of succession—the shift in the local
composition of species in response to changes that occur over time. As time passes, the
community goes through various stages until it arrives at a final stable stage called the
climax community.Two major forms of succession you should know about are primary
and secondary succession.
Primary succession occurs in an area that is devoid of life and contains no soil.
Apioneer species(usually a small plant) able to survive in resource-poor conditions takes
hold of a barren area such as a new volcanic island. The pioneer species does the grunt work,
adding nutrients and other improvements to the once uninhabited volcanic rock until future
species take over. As the plant species come and go, adding nutrients to the environment,
animal species are drawn in by the presence of new plant life. These animals contribute to
the development of the area with the addition of further organic matter (waste). This con-
stant changing of the guard continues until the climax communityis reached and a steady-
state equilibrium is achieved. Bare-rock successioninvolves the attachment of lichen to
rocks, followed by the step-by-step arrival of replacement species up to the climax commu-
nity.Pond successionis kicked off when a shallow, water-filled hole is created. As time
passes, animals arrive on the scene as the pioneer species deposit debris, encouraging the
growth of vegetation on the pond floor. Over time, plants develop whose roots are under-
water and whose leaves are above the water. As these plants begin to cover the entire area of
the pond, the debris continues to build up, transforming the once empty pond into a marsh.
When enough trees fill into the area, the marsh becomes a swamp. If the conditions are
appropriate, the swamp can eventually become a forest or grassland, completing the succes-
sion process. One trivia fact to take out of primary succession is that usually the pioneer
species is an R-selected species, while the later species tend to be K-selected species.
Secondary successionoccurs in an area that once had stable life but has since been dis-
turbed by some major force such as a forest fire. This type of succession is different from pri-
mary succession because there is already soil present on the terrain when the process begins.Trophic Levels
As we discussed earlier, an ecosystem consists of the individuals of the community and the
environment in which they exist. Organisms are classified as either producers or consumers.
The producers of the world are the autotrophs mentioned in Chapter 8, Photosynthesis.
The autotrophs you should recognize can be one of two types: photosynthetic or chemo-
synthetic autotrophs. Photoautotrophs(photosynthetic autotrophs) start the earth’s food chain
by converting the energy of light into the energy of life. Chemoautotrophs(chemosynthetic
autotrophs) release energy through the movement of electrons in oxidationreactions.
The consumers of the world are the heterotrophs. They are able to obtain their energy
only through consumption of other living things. One type of consumer is a herbivore,
which feeds on plants for nourishment. Another consumer, the carnivore,obtains energy and
nutrients through the consumption of other animals. A third consumer, the detritivore,
obtains its energy through the consumption of dead animals and plants. A special subcate-
gory of this type of consumer includes decomposers, which also consume dead animal and
plant matter, but then release nutrients back into the environment. The decomposer sub-
category includes fungi, bacteria, and earthworms.
Here comes another hierarchy for you to remember. The distribution of energy on the
planet can be subdivided into a hierarchy of energy levels called trophic levels.Take a lookKEY IDEABIG IDEA 4.C.4
The diversity of
species within an
ecosystem may
influence the
stability of that
ecosystem.
BIG IDEA 2.A.1
All living systems
require the constant
input of energy.