Terrestrial Ecoregions of the World: A New Map of Life on Earth

Abstract

The tapestry of life on Earth is unraveling as humans increasingly dominate and transform natural ecosystems. Scarce resources and dwindling time force conservationists to target their actions to stem the loss of biodiversity—a pragmatic approach, given the highly uneven distribution of species and threats (Soulé and Kohm 1989, Olson and Dinerstein 1998, Mace et al. 2000, Myers et al. 2000). Unfortunately, the ability to focus strategically is hindered by the absence of a global biodiversity map with sufficient biogeographic resolution to accurately reflect the complex distribution of the Earth's natural communities. Without such a map, many distinctive biotas remain unrecognized. In this article, we address the disparity in resolution between maps currently available for global conservation planning and the reality of the Earth's intricate patterns of life. We have developed a detailed map of the terrestrial ecoregions of the world that is better suited to identify areas of outstanding biodiversity and representative communities (Noss 1992). We define ecoregions as relatively large units of land containing a distinct assemblage of natural communities and species, with boundaries that approximate the original extent of natural communities prior to major land-use change. Our ecoregion map offers features that enhance its utility for conservation planning at global and regional scales: comprehensive coverage, a classification framework that builds on existing biogeographic knowledge, and a detailed level of biogeographic resolution. Ecoregions reflect the distributions of a broad range of fauna and flora across the entire planet, from the vast Sahara Desert to the diminutive Clipperton Island (eastern Pacific Ocean). They are classified within a system familiar to all biologists—biogeographic realms and biomes. Ecoregions, representing distinct biotas (Dasmann 1973, 1974, Udvardy 1975), are nested within the biomes and realms and, together, these provide a framework for comparisons among units and the identification of representative habitats and species assemblages. Although our ecoregions are intended primarily as units for conservation action, they are built on the foundations of classical biogeography and reflect extensive collaboration with over 1000 biogeographers, taxonomists, conservation biologists, and ecologists from around the world. Consequently, ecoregions are likely to reflect the distribution of species and communities more accurately than do units based on global and regional models derived from gross biophysical features, such as rainfall and temperature (Holdridge 1967, Walter and Box 1976, Schulz 1995, Bailey 1998), vegetation structure (UNESCO 1969, deLaubenfels 1975, Schmidthüsen 1976), or spectral signatures from remote-sensing data (Defries et al. 1995, Loveland and Belward 1997). None of these other approaches emphasizes the importance of endemic genera and families (higher taxa), distinct assemblages of species, or the imprint of geological history, such as the strong influence of past glaciations or Pleistocene land bridges, on the distribution of plants and animals. Existing maps of global biodiversity have been ineffective planning tools because they divide the Earth into extremely coarse biodiversity units. These units are typically well beyond the size of landscapes tractable for designing networks of conservation areas, the largest of protected areas, or the 50,000 km2 threshold for restricted-range species (Stattersfield et al. 1998) that are of particular concern (Stuart Pimm [Center for Environmental Research and Conservation, Columbia University, NY], personal communication, 2000). The average size of our ecoregions is roughly 150,000 km2 (median 56,300 km2), whereas the biotic provinces of Udvardy (1975) have an approximate mean of 740,000 km2 (median 306,000 km2) and the biodiversity hotspots of Myers et al. (2000), which represent threatened regions with high concentrations of endemic species, have an approximate mean of 787,760 km2 (median 324,000 km2). We subdivided the terrestrial world into 14 biomes and eight biogeographic realms (Figure 1). Nested within these are 867 ecoregions (Figure 2). This is roughly a fourfold increase in resolution over that of the 198 biotic provinces of Dasmann (1974) and the 193 units of Udvardy (1975). The increased resolution is most apparent in the tropics (between the Tropics of Cancer and Capricorn) where Dasmann (1974) and Udvardy (1975) identify 115 and 117 units, respectively, compared with 463 found in the ecoregion map. Biodiversity assessments that employ large biotic provinces or hotspots often fail to discern smaller but highly distinctive areas, which may result in these areas receiving insufficient conservation attention. The island of New Guinea is illustrative. Dasmann and Udvardy treat the island as a single unit, whereas the new terrestrial map distinguishes 12 ecoregions: four lowland and four montane broadleaf forest