Sources of Ecoregion Data

This volume includes an examination of the climatic characteristics of ecoregions under three different systems in North America. The "Graphical Displays" section lists the ecoregions included in this analysis with Level I ecoregions (shown in figure 1) listed in capital letters, Level II ecoregions listed in mixed case (only first letters of names are capitalized), and Level III ecoregions listed in italicized mixed case (Level I is the coarsest scale, and Level III is the finest scale). The author's original name and spelling for each ecoregion are used in this volume (for example, Bailey [1997] uses 'tayga' instead of 'taiga'). The WWF ecoregion designations differ somewhat between their book (Ricketts and others, 1999) and their GIS coverage (see URL in the Internet Availability section). We chose to use the names from the GIS coverage in this volume. The Bailey and World Wildlife Fund ecoregion systems are both organized in a three-level hierarchy, with the Bailey system having 4 "domains" in North America at the coarsest level (Level I), 28 midlevel Level II "divisions," and 63 finest scale Level III "provinces." The World Wildlife Fund ecoregion system in North America has 6 Level I categories, 12 Level II "major habitat types" (MHTs), and 156 Level III "ecoregions." Küchler (1985) did not explicitly label his categories in a hierarchy. However, there are levels of specificity on his map which appear to correspond with the three levels of the other two systems. There are 4 "Level I," 11 "Level II," and 114 "Level III" categories on the Küchler map for the 48 contiguous States of the United States.

Küchler Ecoregions
A.W. Küchler (1985) reconstructed the "potential natural vegetation" for North America, and we digitized this map for inclusion in our ecoregion analysis. Küchler (1985) defined potential natural vegetation ". . . as the vegetation that would exist today if man were removed from the scene and if the plant succession after his removal were telescoped into a single moment."

Bailey Ecoregions
R.G. Bailey (1997) of the USDA Forest Service defined a three-tiered hierarchy of ecoregions for North America and, under this system, the domains (Level I) and divisions (Level II) are based on broad climatic zones defined by Köppen (1931) and Trewartha (1968), with the divisions representing seasonality or the degree of aridity or coldness. Finer scale categories within the divisions (again largely based on climatic differences) are called provinces, that ". . . largely correspond to major plant formations . . ., which are delimited on the basis of macro features of the vegetation by concentrating on the life-forms of the plants" (Bailey, 1998, p. 2). Mountain regions with significant altitudinal zonation of climate and vegetation are classified as "mountain provinces."

World Wildlife Fund Ecoregions
As described in Ricketts and others (1999), the World Wildlife Fund (WWF) ecoregions for North America are based in large part on Dinerstein and others (1995) for Latin America, Omernik (1987, 1995a, 1995b) for the contiguous United States, ESWG (1995) for Canada, and Gallant and others (1995) for Alaska. The WWF ecoregions were not defined in the same manner as those of Bailey (1997, 1998). Instead the researchers considered suites of climatic, physiographic, geologic, ecologic, and other parameters to define areas where unique combinations of these environmental parameters occur. There are three levels in the WWF ecoregion scheme. The broadest scale categories have no formal designation (and we refer to them as Level I categories). The middle scale categories are referred to as "Major Habitat Types" (MHTs), which are subdivided into ecoregions. "MHTs are not geographically defined units; rather, they refer to the dynamics of ecological systems and to the broad vegetative structures and patterns of species diversity that define them. In this way they are roughly equivalent to biomes" (Ricketts and others, 1999, p. 13–14). At the finer scale, "An ecoregion is defined as a relatively large area of land or water that contains a geographically distinct assemblage of natural communities. These communities (1) share a large majority of their species, dynamics, and environmental conditions and (2) function together effectively as a conservation unit at global and continental scales . . . ." (Ricketts and others, 1999, p. 7).