Ecoregions of the continents are based on macroclimate (i.e., the climate that lies just beyond the local modifying irregularities of landform and vegetation). The theory behind the approach is that macroclimates are among the most significant factors affecting the distribution of life on Earth. As the macroclimate changes, the other components of the ecosystem change in response. Macroclimates influence soil formation and help shape surface topography, as well as affecting the suitability for human habitation.
Four Domains were defined: Polar, Humid temperate, Humid tropical and Dry. The combination of temperature and rainfall to indicate major climatic zones was based on Köppen and Trewartha's work, where dry climates were treated as a separate entity from Tropical humid and Temperate humid. However, the Köppen system defines an addtional "Subtropical" division at this level.
The next level in the Bailey system is the Divisions, and these are also climate - based, for example in the Humid temperate Domain there is Hot continental, Warm continental, Subtropical, Marine, Prairie and Mediterranean, all with Mountain variants (i.e., a total of 12 Divisions in this Domain). There are a total of 30 of these.
The third and last level are the Provinces, which are based on physiognomy of vegetation, modified by climate. For example, the Forest-Meadow of Eastern Oceanic (Monsoon climate). There are a total of 98 of these subdivisions.
The global map has been digitised and converted to a geographic (lat/long) projection by the WCMC, Cambridge, UK. It is also available on CD from NOAAs National Geophysical Data Center in Boulder, Colorado as part of their Global Ecosystem Database Project. http://www.ngdc.noaa.gov/Store/.Click for a larger image
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