A、A community of plants. B、A community of animals. C、The study and understanding of plants and animals. D、The interacting physic

No living creature, plant or animal, can exist in complete isolation. An animal is bound to depend on other living creatures, ultimately plants, for its food supply; it must also depend upon the activities of plants for a continued oxygen supply for its respiration. Apart from these two basic relationships, it may be affected directly or indirectly in countless different ways by other plants and animals around it. Other animals prey on it or compete with it for the same food; plants may provide shelter, concealment or nesting material, and so on. Similarly, the animal will produce its own effects on the surrounding plants and animals: some it may cat or destroy, for others it will provide food; and through its contribution of manure it may influence the texture and fertility of the soil.
    This dependence on other living things is not confined to animals. Though plants manufacture their own food, they are dependent on animal respiration for at least a part of the carbon dioxide which they use as raw material in this process.  Moreover, despite the apparently peaceful relationships in plant communities, there is intense competition going on for water, nutrient salts, and above all, for light.
    The whole complex of the plants and animals forming a community, together with all the interaction physical factors of the environment, really forms a single unit, which has been called an ecosystem. It will be seen that the final aim of ecology -- the complete understanding of ecosystems -- is an ideal one we can scarcely hope to attain. It is nevertheless an ideal well worth pursuing and valuable progress has been made towards it.