Ecology is an important component of a basic biology course. Odum (1963) defines it as "the science of the living environment." The main ideas of ecology include: (1) organization at the community level, (2) organization at the population level, (3) factors that limit and regulate ecological systems, (4) biogeochemical cycles, and (5) energy in ecological systems. This paper describes one way to study in the classroom the ideas of organization at the community level. All biology students should

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... realize that the concept of the ecosystem is a complex one involving an inseperable relationship between the living and nonliving components of the system. Thus, in this activity where we want to study a lotic (running-water) community, we are focusing an assemblage of organisms normally found living in a stream. Ignoring the physical and chemical parameters such as water chemistry, flow rate, and temperature while studying organisms is impossible. In addition, in this activity the student confronts the problem of working with a "model," a very important scientific tool, but an imperfect representation of the real thing.