Introduction To Ecology

figure 18-3

These flowers represent a population of California poppies, Eschscholzia californica, living in California.

figure 18-3

These flowers represent a population of California poppies, Eschscholzia californica, living in California.

Ecosystems

The biosphere is composed of smaller units called ecosystems. An ecosystem (EK-oh-sis-tuhm) includes all of the organisms and the nonliving environment found in a particular place. Consider a pond ecosystem. It contains a variety of living things, such as fish, turtles, aquatic plants, algae, insects, and bacteria. These organisms interact in ways that affect their survival. For instance, insects and fish eat aquatic plants, and turtles eat fish. The pond ecosystem also includes all the nonliving (physical and chemical) aspects of the pond that influence its inhabitants. The chemical composition of the pond—its pH, its levels of dissolved oxygen and carbon dioxide, and its supply of nitrogen—helps to determine what kinds of organisms live in the pond and how abundant they are. A very important physical factor is the amount of sunlight the pond receives, because sunlight is the ultimate source of energy for the pond's inhabitants.

Communities, Populations, and Organisms

Whereas an ecosystem contains both living and nonliving components, a community includes only species of organisms. A community is all the interacting organisms living in an area. For instance, all the fish, turtles, plants, algae, and bacteria in the pond described above make up a community. Although it is less inclusive than an ecosystem, a community is still very complex, and it may contain thousands of species. Ecologists studying a community often focus on how species interact and how these interactions influence the nature of the community. Remember that the word community has a specific meaning in biology that differs from its everyday meaning.

Below the community level of organization is the population level, where the focus is on the members of a single species. A population includes all the members of a species that live in one place at one time. An example of a population of flowers is shown in Figure 18-3. The simplest level of organization in ecology is that of the organism. Research at this level concentrates on the adaptations that allow organisms to overcome the challenges of their environment.

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