Scientists recognize a hierarchy of different levels of organization within organisms. Each organism is composed of one or more organs. Each organ is composed of tissues, which, in turn, are composed of cells, and so on. Likewise, ecologists recognize a hierarchy of organization in the environment, as illustrated in Figure 18-2.
Each level has unique properties that result from interactions between its components, so a complete study of ecology would look at all levels. But for practical reasons, ecologists often focus their research on one level of organization while recognizing that each level is influenced by processes at other levels.
The broadest, most inclusive level of organization is the biosphere (BIE-oh-SFIR), the thin volume of Earth and its atmosphere that supports life. All organisms are found within the biosphere. It is about 20 km (13 mi) thick and extends from about 8 to 10 km (5 to 6 mi) above the Earth's surface to the deepest parts of the oceans. In comparison, the Earth's diameter is about 12,700 km (7,900 mi), or more than 600 times the thickness of the biosphere. If Earth were the size of an apple, the biosphere would only be as thick as the apple's skin. Ecologists often describe the biosphere as a thin film of life covering an otherwise lifeless planet. Living things are not distributed evenly throughout the biosphere. Many organisms are found within a few meters of the surface of the land or oceans.
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