Aquatic Communities

Living Together

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

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 extinct.

Communities have specialists and generalists.

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 ways..

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 change.

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