The Global 200 : a representation Approach to Conserving the Earth’s Distinctive Ecoregions
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- Delineation of ecoregions
- Selection criteria
- Criteria for aggregating ecoregions
- Conservation status of the Global 200 ecoregions
MethodsAn objective hierarchy for selecting the Global 200 To maintain representation of biodiversity at a global scale, we first stratified ecoregions by realm (terrestrial, freshwater, and marine). We further divided realms by Major Habitat Types (MHTs) which describe different areas of the world that share similar environmental conditions, habitat structure, and patterns of biological complexity (e.g., beta diversity), and that contain communities with similar guild structures and adaptations. MHT classifications are roughly equivalent to biomes. We identified 14 MHTs in the terrestrial realm, seven in the freshwater realm, and nine in the marine realm (only five of these were assessed for this edition). Each MHT was further subdivided by biogeographic realm (e.g., Nearctic, Indian Ocean) in order to represent unique faunas and floras on different continents or ocean basins. Finally, we identified ecoregions, and in a few cases ecoregion complexes (see Appendix 4), that represent the most distinctive examples of biodiversity for a given MHT (Table 1). This hierarchy is based on biogeographic and ecological principles. First, to set priorities, we only want to compare the biodiversity value of ecoregions that share the same MHT. Employing species richness and endemism as a discriminator among ecoregions is much more powerful if the MHTs are analyzed separately, because the relative magnitude of these parameters varies widely among MHTs. For example, a comparison of tree richness between tropical moist forest ecoregions and desert or grassland ones would yield little useful information. Second, incorporating biogeographic realms addresses another important aspect of global representation: ecoregions that share the same MHT, but occur on different continents, support different species assemblages. To illustrate, the deserts of central and northwestern Australia support over 150 species of reptile, all of which are endemic to Australia (Cogger 1992). Using biogeographic realms also addresses higher-order taxonomic diversity. For example, the Chihuahuan and Sonoran deserts include the centers of diversity for cacti, a family absent from African and Asian deserts. Also, continental ecoregions of Asia are some of the richest on Earth for terrestrial mammals (Lidicker 1989, Corbett & Hill 1992, Wikramanayake et al. in prep.), but include no species in the mammalian orders of marsupials (Peremelina and Diprotodontia). In contrast, the marsupials are a dominant order of mammals in the ecoregions of New Guinea and Australia, where eutherian species are less abundant. Third, assessments of the relative intactness and degree of threat (conservation status) of ecoregions can be made more accurately if ecoregions are categorized within a framework of MHTs. This allows us to better address the patterns of biodiversity, ecological dynamics, and responses to disturbance that are specific to different MHTs. Delineation of ecoregionsTerrestrial Ecoregions The boundaries of terrestrial ecoregions for the Global 200 are taken from intensive regional analyses of biodiversity patterns across five continents undertaken by the World Wildlife Fund (WWF) Conservation Science Program and others (e.g. Victor 1955, Freitag 1971, Zohary 1973, Miyawaki 1975, Yim 1977, Chinese Vegetation Map Compilation Committee 1979, New Zealand Department of Conservation 1987, Noirfalise 1987, Changchun Institute of Geography & Chinese Academy of Sciences 1990, Kurnaev 1990, Bohn 1994, Krever et al. 1994, Dinerstein et al. 1995, Ecological Stratification Working Group 1995, Gallant et al. 1995, Hilbig 1995, Omernik 1995, Thackway & Cresswell 1995, Mongolian Ministry for Nature and the Environment et. al. 1996, European Topic Centre on Nature Conservation 2000, Ricketts et al. 1999, WWF/IUCN 1994, 1995, 1997, Bohn & Katenina 1996, S. Gon, pers. comm., Wikramanayake et al. unpublished data). These assessments were conducted in collaboration with hundreds of regional experts and included extensive literature reviews. The resulting Terrestrial Ecoregions of the World map (in press) is a compilation of the separate regional maps. Thus, the Global 200 is a selected subset of this global map of all ecoregions. Ecoregions are most finely delineated in tropical moist forests. Complexity is high within and among tropical moist forest ecoregions, as is turnover of species along environmental gradients or with distance (beta diversity). By contrast, tundra and boreal forests show only slight longitudinal or latitudinal variation and are therefore delineated as larger units. Freshwater ecoregions Separate analyses of freshwater and terrestrial ecoregions were conducted because the distribution of freshwater biodiversity in many cases diverges from terrestrial patterns. Freshwater ecoregions were based on several regional analyses and consultations with regional experts (Hocutt & Wiley 1986, Frest & Johannes 1991, WCMC 1992, Maxwell et al. 1995, Oberdorff et al. 1995, Kottelat & Whitten 1996, Olson et al. 1999, Abell et al. 2000, Thieme et al. in prep.). Marine ecoregions Marine ecoregions delineated by the Global 200 are nested within a large marine ecosystem framework, derived from several global and regional analyses (e.g., Hayden et al. 1984, IUCN 1988, Sherman 1990, Croom et al. 1992, Ray & Hayden 1993, Kelleher et al. 1995, Groombridge & Jenkins 1996, Ormond et al. 1997, Sullivan & Bustamante 1996) and review of the available literature. The delineation of marine ecoregions is intended to highlight general regions within which characteristic animals, plants, ecological interactions, and biophysical processes occur. Relative to most terrestrial ecoregions, these are more spatially and temporally dynamic ecological and biogeographic units (Sherman et al. 1990). Selection criteriaFor five of the continents, detailed regional priority-setting analyses have been conducted by the WWF Conservation Science Program in collaboration with a host of regional experts and supplemented with extensive literature reviews. Thus, the final Global 200 is not just an interpretation of coarse global reviews. It draws heavily from the results of intensive regional analyses of biodiversity conducted over the last several years (Krever et al. 1994, Dinerstein et al. 1995, Abell et al. 2000, Burgess et al. in prep., Olson et al. 1999, Ricketts et al. 1999, Wikramanayake et al. in prep). Within each MHT and biogeographic realm, ecoregions are classified by their biological distinctiveness at one of four levels: globally outstanding, regionally outstanding (e.g., Neotropics), bioregionally outstanding (e.g., Caribbean), or locally important. Biological distinctiveness, as a discriminator, evaluates the relative rarity of different units of biodiversity. It can be used to estimate the urgency of action based on the opportunity for conservation that exists. The criteria we used to prioritize ecoregions for the Global 200 are similar to those used for the regional assessments. On a global scale, and within each biogeographic realm, we chose the set of ecoregions with the greatest biological distinctiveness based on the following parameters: species richness, endemism, higher taxonomic uniqueness (e.g., unique genera or families, relict species or communities, primitive lineages), extraordinary ecological or evolutionary phenomena (e.g., extraordinary adaptive radiations, intact large vertebrate assemblages, presence of migrations of large vertebrates), and global rarity of MHT Biodiversity features were weighted and measured in the regional analyses as illustrated in Appendix 1 using the method employed for terrestrial ecoregions of North America. The weight assigned to these parameters also varied by MHT. Families and genera comprise higher hierarchical levels—above the species level—in the taxonomy of living organisms. Accordingly, the presence of an endemic higher taxon would contribute more to an ecoregion’s biotic distinctiveness than would an endemic species. Naturally rare representatives of relict or primitive genera, families, or orders also contribute to the distinctiveness of an ecoregion’s biota, and its urgency for conservation action. In other words, some ecoregions are noted for biotas that contain unique taxa at higher taxonomic levels than species (Vane-Wright et al. 1991, Williams 1991, Gaston & Williams 1993, Forey et al. 1994, Williams & Humphries 1994). For example, most families and genera of birds and mammals in Australia are unique to the continent. Moreover, the moist forests of northeastern Australia, northern New Zealand, and New Caledonia are recognized as having a number of the most primitive lineages of conifers and flowering plants in the world. Madagascar, another long-isolated island, is often considered a seventh continent from a biogeographic perspective because of its uniqueness at higher taxonomic levels. Species richness and endemism are poor discriminators among the more depauperate terrestrial ecoregions found in boreal forests and tundra, the two northernmost terrestrial MHTs. For these MHTs and for some ecoregions of sub-Saharan Africa, we gave greater weight to extraordinary ecological phenomena—in these cases, examples of extensive intact habitats and large vertebrate assemblages, particularly those that still sustain top predators, large herbivores, and larger frugivores that are highly sensitive to human disturbance and fragmentation. Large blocks of natural habitat where species populations and ecological processes still fluctuate within their natural range of variation are rapidly disappearing around the world (WCMC 1996, Bryant et al. 1997, Dinerstein et al. 1997). Therefore, remaining intact ecosystems represent rare opportunities for conservation, even though this aspect of their distinctiveness was once widespread but is now rare due to the prevalence of human disturbance around the world. In assessing the intactness of habitat and faunal assemblages for ecoregions, the presence of larger blocks of habitat is emphasized because principles of landscape ecology and conservation biology suggest that biodiversity is best maintained within such areas (Noss & Cooperrider 1994). Strategies that emphasize the conservation of large vertebrates and the full complement of migratory species help conserve intact faunal assemblages (e.g., Balmford et al. 1995, Dinerstein et al. 1997). Naturally occurring extraordinary ecological phenomena, such as long-distance caribou migrations or the tremendous seasonal fish migrations and fish frugivory in the flooded forests of the Amazon (varzea forests) (Goulding 1980, Goulding et al. 1996), were also given due recognition. Unusual evolutionary phenomena such as the extraordinary adaptive radiations seen in Hawaiian plants, birds, and insects, the radiation of Galapagós finches, relict taxa of the Greater Antilles, and the radiation of cichlids in Rift Valley lakes of Africa—also elevated some ecoregions to the Global 200. Some level of radiation within taxa characterizes the biotas of many ecoregion, but here we highlight extensive adaptive radiations of species in one or more higher taxa. All ecoregions in globally rare MHTs are highly distinctive at this analytical scale. This criterion encompasses ecological and evolutionary phenomena, but it also addresses those characteristics at the scale of whole ecosystems and biotas, as well as structural features of ecosystems and habitats. The species that manage to live in these habitats often have unusual adaptations to specialized conditions. For this reason, their community structures, assemblages, and ecological processes are highly distinctive at a global scale. We counted only naturally occurring rarity for the distinctiveness analyses, although human-induced rarity is an important condition to assess when developing conservation strategies. Examples of rare MHTs include the Mediterranean woodlands and scrub (there are six worldwide, and all of limited area) and the temperate rain forests that occur in seven relatively localized areas around the world. Paramos, or wet tropical alpine shrublands, occur in restricted distributions in the Andes, on a few East African mountain peaks, and in New Guinea. For ecoregions with similar biological distinctiveness in the same MHT, we selected the ecoregions that had relatively more intact habitats and biotas based on assessments of their conservation status (Dinerstein et al. 1995, Wikramanayake et al. in prep., Burgess et al. in prep., Ricketts et al. 1999). Ecoregions that were identified as being relatively more intact in these comparisons did not necessarily exhibit a degree of intactness that would qualify them as displaying unusual ecological phenomena, the fourth distinctiveness criteria. We did not use ecological function, conservation feasibility (i.e., political, social, economic, cultural factors), or human utility as discriminators to identify the Global 200 for reasons explained in Appendix 3. We reiterate that the Global 200 is focused on biological values as the critical first step in setting global conservation priorities. Criteria for aggregating ecoregionsThe Global 200 map guides users to the general location of relatively large geographic units of biodiversity, defined here as ecoregions. At this scale, the specific location and configuration of boundaries of some ecoregions do not present an exact target area for a regional conservation strategy. The regional analyses upon which much of the Global 200 is based depict ecoregions at a higher level of resolution (see Krever et al. 1994, Dinerstein et al. 1995, Abell et al. 2000, Burgess et al. in prep., Olson et al. 1999, Ricketts et al. 1999, Wikramanayake et al. in prep.) Given the goal of representing the Earth’s diverse ecosystems and species assemblages, keeping the number of priority ecoregions to a manageable number required aggregating a few ecoregions that were delineated for regional conservation assessments (for the Neotropics, Russia, North America, Africa, and Asia). The ecoregions that were lumped are adjacent, related by habitat type, and are biogeographically similar at a global scale. Conservation status of the Global 200 ecoregionsEcoregions vary greatly not only in their biological distinctiveness, but also in their conservation status. Conservation status represents an estimate of the current and future ability of an ecoregion to maintain viable species populations, to sustain ecological processes, and to be responsive to short and long-term environmental changes. Conservation status assessments of the Global 200 ecoregions were based on landscape or aquascape-level features, such as total habitat loss, the degree of fragmentation, water quality, and estimates of future threat. From a practical perspective, conservation status sheds light on the relative opportunity we have to conserve biodiversity in a particular ecoregion or major habitat type, as well as on the urgency, kinds of conservation activities, and level of effort needed. Again, we drew heavily from regional conservation assessments to estimate conservation status (Krever et al. 1994, BSP et al. 1995, Dinerstein et al. 1995, Harcourt et al. 1996, MacKinnon & Bunting 1996, Bryant et al. 1997, Dinerstein et al. 1997, Dobson et al. 1997, Abell et al. 2000, Burgess et al. in prep., Ricketts et al. 1999, Wikramanayake et al. in prep.). For the Global 200, we confined our assessment to terrestrial ecoregions. We classified ecoregions into one of three broad categories: critical/endangered, vulnerable, or relatively stable/relatively intact over the next forty years. For terrestrial ecoregions, the most prominent contributor to conservation status is habitat loss, followed by the size of remaining habitat blocks, degree of fragmentation, degree of degradation, and degree of protection. For a more a detailed discussion of scoring ecoregions for conservation status, see Appendix 1, Dinerstein et al. (1995), and Ricketts et al. (1999). Other conservation targets The conservation of large-scale ecological phenomena, such as bird and butterfly migrations, often requires hemispheric coordination of activities that transcends ecoregion-level conservation efforts. Habitat conservation within Global 200 ecoregions can contribute to this effort, but identification of critical stopover, breeding, feeding, wintering, and resting sites for migratory birds, bats, butterflies, and cetaceans is necessary. Effective habitat conservation within Global 200 ecoregions can help conserve regional-scale terrestrial mammal migrations, such as those of caribou and wildebeest, and altitudinal movements of birds, insects, bats, and some larger mammals. Widespread and dynamic pelagic ecosystems have not been adequately mapped up to this point (Angel 1993), nor have hydrothermal vent communities and other abyssal ecosystems and cave and groundwater ecosystems (Gage & Tyler 1991, Grassle 1991, Grassle & Maciolek 1992, D. Culver, pers. comm.). 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