Animals and plants live together
in communities. These communities create the habitat
that is home to all the organisms. The plants and animals are
mutually interdependent and create a dynamic and lively balance
of life forms that share and recycle the available resources.
Communities on earth have three kinds of members.
- Primary Producers make food from inorganic materials and
some source of energy.
- Consumers eat the producers, (and sometimes other consumers)
keeping their numbers in check.
- Decomposers break down waste products and the tissues of
dead animals and plants, making it possible to recycle the materials
necessary for life.
The complexity of the community depends on the availability
of resources. Factors include:
- the availability of water
- the availability of light or some other energy source
- the availability of necessary chemical elements
- the degree of variety and complexity in land forms and life
forms, making niches available.
In the ocean, water is not a problem. The water environment
buoys up life forms and supports their bodies: it provides fairly
stable temperatures that change slowly. Its currents circultate
oxygen, distribute nutrients, and carry food particles.
Light does not penetrate
far into water. About 45% of the sun's light is absorbed by the
first three feet of water, and darkness rules where water is
more than about 300 feet deep. Water plants live around the shorelines
of land masses. Continental shelves provide rocks and sand that
are covered by shallow water near the shore. As the shelves slope
away toward the open ocean, the water gradually grows deeper.
Plants can only grow where there is light, so the only primary
producers over much of the ocean are tiny phytoplankton.
Chemicals necessary for life come and go in the ocean. Organic
material may wash down from the land or be carried to the surface
by upwelling currents of water. When these nutrients become available
water plants respond rapidly, using up the supply. Animals are
busy eating the plants, so when the nutrients are used up
the numbers in populations
of organisms go down. Surviving individuals then wait for the
next food bonanza.
Niches are areas of specialization.
Specialization allows a life form to fit into a particular role
in a community, and the better that life form is adapted to (specialized
for) that niche, the more securely it fits into the community.
However, if something in the environment changes, the niche may
change or disappear, and the specialized life form may become
Communities have specialists
Generalists are able
to adapt their behavior to changing conditions. They eat a wide
variety of foods and their bodies tend to have a sort of basic
generic structure. They are often small to medium-sized. If there
is a sudden change in the environment, the generalists generally
have the best chance of survival because they can adapt in many
If the environment is stable,
specialists may outcompete the generalists. Specialists often
have specialized body structures or unusual shapes. The sea horse,
for example, is a specialized life form. Specialists have
adapted to their niches so well that they can exploit its resources
very efficiently, but their special adaptations make adapting
to change difficult.
Two species cannot share a niche.
They will compete, and one or the other will win. In challenging
environments a niche may be broad -- in the desert, for instance,
a little rodent may eat many different plants. In a complex and
rich environment, such as the tropical rain forest, niches may
be small and highly specialized. Animals may exist that eat only
one or two kinds of plants. These animals are vulnerable to environmental
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