he concept of community encompasses only living organisms and
ignores the physical and chemical environment in which those organ-
isms live. If we include these features, including the underlying rock and
soil, the water moving through the habitat, and the atmosphere permeat-
ing the soil and surrounding the vegetation, then we have an even more
complex, interactive system that is called the ecosystem. Whereas the
idea of community concentrates on the different species found in associa-
tion with one another, the concept of the ecosystem is largely concerned
with the processes that link different organisms to one another.
Two fundamental ideas underlie the ecosystem concepti these are
energy flow and nutrient cycling. Energy, initially fixed from solar radia-
tion into a chemical form by green plants, moves into herbivores as a
result of their feeding on plants, and then moves on into carnivores as the
-herbivores are themselves consumed. Since herbivores rarely consume all
the available plant material, and since carnivores do not eat every individ-
ual of their prey organisms, some living tissues are allowed to die naturally,