Ecoregion - Definition and Categorization

Definition and Categorization

An ecoregion is a "recurring pattern of ecosystems associated with characteristic combinations of soil and landform that characterise that region" Omernik (2004), elaborates on this by defining ecoregions as: “areas within which there is spatial coincidence in characteristics of geographical phenomena associated with differences in the quality, health, and integrity of ecosystems” “Characteristics of geographical phenomena” may include geology, physiography, vegetation, climate, hydrology, terrestrial and aquatic fauna, and soils, and may or may not include the impacts of human activity (e.g. land use patterns, vegetation changes). There is significant, but not absolute, spatial correlation among these characteristics, making the delineation of ecoregions an imperfect science. Another complication is that environmental conditions across an ecoregion boundary may change very gradually, e.g. the prairie-forest transition in the midwestern United States, making it difficult to identify an exact dividing boundary. Such transition zones are called ecotones.

Ecoregions can be categorized using an algorithmic approach or a holistic, “weight-of-evidence” approach where the importance of various factors may vary. An example of the algorithmic approach is Robert Bailey’s work for the U.S. Forest Service, which uses a hierarchical classification that first divides land areas into very large regions based on climatic factors, and subdivides these regions, based first on dominant potential vegetation, and then by geomorphology and soil characteristics. The weight-of-evidence approach is exemplified by James Omernik’s work for the United States Environmental Protection Agency, subsequently adopted (with modification) for North America by the Commission for Environmental Cooperation.

The intended purpose of ecoregion delineation may affect the method used. For example, the WWF ecoregions were developed to aid in biodiversity conservation planning, and place a greater emphasis than the Omernik or Bailey systems on floral and faunal differences between regions. The WWF classification defines an ecoregion as:

A large area of land or water that contains a geographically distinct assemblage of natural communities that:

(a) Share a large majority of their species and ecological dynamics;

(b) Share similar environmental conditions, and;

(c) Interact ecologically in ways that are critical for their long-term persistence.

According to WWF, the boundaries of an ecoregion approximate the original extent of the natural communities prior to any major recent disruptions or changes. WWF has identified 867 terrestrial ecoregions, and approximately 450 freshwater ecoregions across the Earth.