Within the biosphere—the total expanse of water, land, and atmosphere able to sustain life—the basic ecological unit is the ecosystem. An ecosystem may be as small as a tidal pool or a rotting log or as large as an ocean or a continent-spanning forest. Each ecosystem consists of a community of plants and animals in an environment that supplies them with raw materials for life, i.e., chemical elements and water. The ecosystem is delimited by the climate, altitude, water and soil characteristics, and other physical conditions of the environment.
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