The Ecoregions of the World project, and subsequent mapping, database development, and educational and scientific reports resulting from this endeavor have been a focus of the WWF-US Conservation Science Program for just over eight years. Developing the methods for delineating and classifying the biodiversity of the world into ecoregions has been an ongoing and evolving process in which we have enlisted the support and participation of thousands of regional experts and organizations from around the world. The project has been an exercise in working across country borders and has set the stage for improved conservation initiatives at the international, regional, and landscape levels. Importantly, it has changed the way we look at conservation issues and how we address them. It has also allowed us to assess the relative biological importance of different regions at global and regional biogeographic levels. And it has compelled us to ask several critical questions -- "What will it take to save all the pieces?" (representation); "How much is enough?" (minimum-area requirements for area sensitive species and ecological processes); "How do we maintain functionality across landscapes or seascapes?" (connectivity); and "Where do we need to act first?" (threat and opportunity analyses).

For a global ecoregion map to be a useful tool in conservation planning, it must have biogeographic units delineated at a scale useful for regional conservation planning, and each unit should be categorized within a standardized classification of biomes and realms. We define ecoregions as relatively large units of land or water 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. Others have defined ecoregions as areas of ecological potential based on combinations of biophysical parameters such as climate and topography. However, our ecoregions emphasize the distribution of distinct biotas, which often do not correspond to zones of ecological potential due to historical and chance events, or the complex differentiation of living communities in response to often subtle environmental conditions. In a technical sense, our ecoregions define areas within which one would expect to find a particular set of encounter probabilities of different biodiversity features, whether they are species or habitats or phenomena. As one moves into adjacent ecoregions the encounter probabilities change. The biogeographic resolution of ecoregions will depend upon which level of similarity among biotas (or encounter probabilities) one chooses to use to discriminate distinct biotas.

Three caveats are appropriate for all biogeographic mapping approaches. First, no single biogeographic framework is optimal for all taxa. Ecoregions reflect the best compromise for as many taxa as possible. Second, ecoregion boundaries rarely form abrupt edges; rather, ecotones and mosaic habitats bind them. Third, most ecoregions contain habitats that differ from their assigned biome. For example, rainforest ecoregions in Amazonia often contain small edaphic savannas. More detailed biogeographic analyses should map the less dominant habitat types that occur within the larger ecoregions (sub-ecoregions), and ecoregion conservation strategies should address their requirements.

The amount of information involved in producing the final map is staggering. We have consulted top biologists and biogeographers, studied hundreds of maps, conducted expert workshops around the world, developed species lists and databases, and enlisted field biologists and regional professionals to 'ground truth' the information gathered and write scientific reports. Finally, we used a geographical information system (GIS) and databases to synthesize these data into the maps and reports.

A primary goal of our work to date has been to map the terrestrial biodiversity of the world with sufficient resolution and standardization to allow us to assess and prioritize ecoregions for conservation action. We compare their biodiversity features, current status, level of threat, and representation value. Thus far, we have started conservation assessments for several of the Earth's realms. The results of these assessments are being published by Island Press, resulting in a series covering six major terrestrial realms: North America (Ricketts et al. 1999), Indo-Pacific (Wikramanayake et al. 2001), Africa (Burgess et al. In prep), Eurasia (Strand et al. In prep), Latin America and the Caribbean (Schipper et al. In prep; Dinerstein et al. 1995), and Oceania (Morrison et al. In prep). Work has also begun to delineate freshwater and marine ecoregions of the world (Abell et al. 2000; Thieme et al. in prep; WWF & TNC 2001). A global analysis of the world's ecoregions resulted in the Global 200 (Olson & Dinerstein 1998), a portfolio of the most outstanding and representative terrestrial, freshwater, and marine ecoregions that are priority for conservation action.

By prioritizing ecoregions, we attempt to maximize our own efforts to conserve the full expression of the world's biodiversity by making the most efficient use of available funding in the shortest time frame. The Global 200 has helped guide World Wildlife Fund and other organizations in deciding where to concentrate their conservation efforts. The World Bank has participated in these assessments to help in project development and funding so as to have the smallest ecological footprint while continuing its developmental goals (Castro et al. 2001). The Nature Conservancy, which has collaborated through these projects, is also using a similar ecoregion framework in its conservation efforts (Groves et al. 2000).

An important next step is the analysis of individual ecoregions, culminating in a long-term biodiversity vision that defines what conservation success should look like over several decades based on the best biological data and insights. Conservation visions should:

• characterize the biodiversity features of a region and highlight those that are truly outstanding at global and regional scales,
• identify those elements that are especially vulnerable and irreplaceable,
• clearly state transparent and defensible biodiversity goals (see below),
• identify thresholds and tradeoffs for biodiversity loss and persistence,
• map proposed representative systems of conservation areas of sufficient size, condition, and connectivity to maintain even the most sensitive species and ecological processes, and
• identify approaches for implementation that are most appropriate for the social, cultural, political, and economic situation in the region.

Ecoregion analyses are being conducted for several of WWF's priority ecoregions (Southwest Amazon moist forests, Valdivian temperate forests, the Congo Basin, Bering Sea, Mesoamerican Caribbean Reef, Lake Malawi, Atlantic forests, Chihuahuan desert, Northern Great Plains, Fynbos/Cape Floral Kingdom, Eastern Himalayas, Terai Arc, etc.). For further information on available publications, please continue to monitor the publications section of our Web site.

Literature Cited
Abell, R., D.M. Olson, E. Dinerstein, P. Hurley, J.T. Diggs, W. Eichbaum, S. Walters, W. Wettengel, T. Allnutt, C. Loucks, and P. Hedao. 2000. Freshwater ecoregions of North America: a conservation assessment. Island Press. Washington, DC, USA. 319 pp.

Burgess, N., J. D'Amico, E. Underwood, I. Itoua, and D.M. Olson. In press. Terrestrial ecoregions of Africa: a conservation assessment. World Wildlife Fund, Washington, DC, USA.

Castro, G., I. Locker, V. Russell, L. Cornwell, and E. Fajer. 2001. Mapping conservation investments: An assessment of biodiversity funding in Latin America and the Caribbean. Biodiversity Support Program, The World Bank, USAID, Washington, D.C. 79 pp.

Dinerstein, E., D.M. Olson, D.J. Graham, A.L. Webster, S.A. Primm, M.P. Bookbinder, and G. Ledec. 1995. A conservation assessment of the terrestrial ecoregions of Latin America and the Caribbean. The World Bank, Washington, DC, USA. 129 pp.

Morrison, J., E. Dinerstein, and D.M. Olson. Terrestrial and Freshwater Ecoregions of Oceania: a conservation assessment. World Wildlife Fund, Washington DC, USA.

Groves, C., L. Valutis, D. Vosick, B. Neely, K. Wheaton, J. Touval, and B. Runnels. 2000. Designing a Geography of Hope: A practitioner's handbook to ecoregional conservation planning. The Nature Conservancy, Arlington, VA.

Olson, D.M., and E. Dinerstein. 1998. The Global 200: a representation approach to conserving the Earth's most biologically valuable ecoregions. Conservation Biology 12:502-515.

Ricketts, T.H., E. Dinerstein, D.M. Olson, C.J. Loucks, W. Eichbaum, D. DellaSala, K. Kavanagh, P. Hedao, P.T. Hurley, K.M. Carney, R. Abell, and S. Walters. 1999. Terrestrial ecoregions of North America: a conservation assessment. Island Press. Washington, DC, USA. 485 pp.

Schipper, G.J., E. Dinerstein, and D.M. Olson. In preparation. Terrestrial Ecoregions of Latin America and the Caribbean: a conservation assessment. World Wildlife Fund, Washington DC, USA.

Strand, H., E. Dinerstein, and D.M. Olson. In preparation. Terrestrial Ecoregions of Eurasia: a conservation assessment. World Wildlife Fund, Washington DC, USA.

Thieme, M.L., R.A Abell, D.M. Olson, E. Dinerstein, M.L.J. Stiassny, J. D'Amico, and E.M. Underwood. In preparation. Freshwater ecoregions of Africa: a conservation assessment. World Wildlife Fund-US, Washington, DC, USA.

Wikramanayake, E., E. Dinerstein, C. Loucks, D. Olson, J. Morrison, J. Lamoreux, M. McKnight, and P. Hedao. 2001. Terrestrial ecoregions of the Indo-Pacific: a Conservation assessment. Island Press, Washington, DC.

WWF and TNC. 2001. Ecoregion-based Conservation in the Bering Sea: identifying important areas for biodiversity conservation. World Wildlife Fund and The Nature Conservancy of Alaska, Washington D.C.

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