Since the 1980s, there has been an increasing tendency to map biodiversity over "ecosystem regions" or "ecoregions". An ecoregion is "a relatively large unit of land or water containing a geographically distinct assemblage of species, natural communities, and environmental conditions" (WWF, 1999 [link]); thus, the ecosystems within an ecoregion have certain distinct characters in common (Bailey, 1998a [link]). Several standard methods of classifying ecoregions have been developed, with climate, altitude, and predominant vegetation being important criteria (Stein et al., 2000 [link]). Bailey's (1983, 1998a, b) classification is one of the most widely adopted. It is a hierarchical system with four levels: domains, divisions, provinces and sections.

Domains are the largest geographic levels and are defined by climate, e.g., polar domain, dry domain, or humid tropical domain. Domains are split into smaller divisions that are defined according climate and vegetation, and the divisions are split into smaller provinces that are usually defined by their major plant formations. Some divisions also include varieties of "mountain provinces". These generally have a similar climatic regime to the neighboring lowlands but show some altitudinal zonation, and they are defined according to the types of zonation present. Provinces are divided into sections, which are defined by the landforms present.

Because ecoregions are defined by their shared biotic and abiotic characteristics, they represent practical units on which to base conservation planning. Moreover, the hierarchical nature of Bailey's ecoregion classification allows for conservation management to be planned and implemented at a variety of geographical levels, from small scale programs focused on discrete sections, to much larger national or international projects that target divisions. Olson and Dinerstein (2002 [link]) identified 238 terrestrial or aquatic ecoregions called the "Global 200" that they considered to be priorities for global conservation. These ecoregions were selected because they harbor exceptional biodiversity and are representative of the variety of Earths ecosystems. For further discussion of ecoregions see the modules on Landscape ecology and Conservation planning on a regional scale.