Ecosystems.

The ecosystem is essentially an abstract organizing unit superimposed on the landscape to help ecologists study the form and function of the natural world. An ecosystem consists of one or more communities of interacting organisms and their physical environment. Ecosystems have no distinct boundaries; thus, the size of any particular ecosystem should be inferred from the context of the discussion. Individual lakes, streams, or strands of trees can be described as distinct ecosystems, as can the entire North American Great Lakes region. Size and boundaries are arbitrary because no ecosystem stands in complete isolation from those that surround it. A lake ecosystem, for example, is greatly affected by the streams that flow into it and by the soils and vegetation through which these streams flow. Energy, organisms, and materials routinely migrate across whatever perimeters the ecologist may define. Thus, investigators are allowed considerable latitude in establishing the scale of the ecosystem they are studying. Whatever the scale, though, the importance of the ecosystem concept is that it forces ecologists to treat organisms not as isolated individuals or species but in the context of the structural and functional conditions of their environment.