The number and relative importance of factors helpful in delineating ecological units varies from one area to another, regardless of the level of generalization. It is based on a hierarchy with ecosystems nested within ecosystems. It involves integration of knowledge and is not simply an overlay process. It recognizes that ecosystems are interactive and that characteristics of one ecosystem blend with those of another. It recognizes that map lines generally depict the location of zones of transition.
Ecosystems are numerous and complex.
The challenge is to make the ecological map units workable and understandable to reflect this complexity. It is equally important to recognize that while ecological land classification is science-based, it is also an art in the sense that ecological cycles, characteristics and interactions are not always readily apparent or measured and therefore need to be interpreted from the development of vegetation, soil, and landform characteristics or other factors. A hierarchical system permits the choice of detail that suits management objectives and the proposed use. Because management and other decision-making deal at various levels, from local to regional to national and even to international, one of the prerequisites of ecological land classification is to portray ecosystems at a level, scale, and intensity appropriate to the need. The dominance or importance of any one factor may vary considerably in defining the spatial expression of an ecosystem at each level of generalization.