For example, think of a long high ridge. One natural community on the ridgetop may consist almost entirely of chestnut oak, while another might have a wider variety of oaks and shrubs, but all share some things in common. They all grow in relatively dry soil and are exposed to the elements on all sides—sun, wind, storms—as well as to fires that are the result of lightning strikes. The natural processes at work on this ridge help explain why certain natural communities, and not others, tend to cluster here. We call these clusters ecological systems.
Or think of a large river. There may be many different types of natural communities along the river—a forest of American sycamore and tuliptree, a shrubby willow community, a mucky alder swamp—but what unifies them? They all occur in areas prone to flooding by this river, which exposes them to a whole set of related natural processes. They are all part of one large river ecological system.
Ecological systems—and the natural processes that partly define them—provide a helpful context in which to understand natural communities and help predict what types of plants and animals might thrive there: You wouldn’t expect to find a water-loving, flood-adapted natural community on a dry ridgetop, nor would you expect to find a fire-adapted natural community along the banks of a river.