Abstract

We present a new map depicting the first global biogeographic regionalization of Earth's freshwater systems. This map of freshwater ecoregions is based on the distributions and compositions of freshwater fish species and incorporates major ecological and evolutionary patterns. Covering virtually all freshwater habitats on Earth, this ecoregion map, together with associated species data, is a useful tool for underpinning global and regional conservation planning efforts (particularly to identify outstanding and imperiled freshwater systems); for serving as a logical framework for large-scale conservation strategies; and for providing a global-scale knowledge base for increasing freshwater biogeographic literacy. Preliminary data for fish species compiled by ecoregion reveal some previously unrecognized areas of high biodiversity, highlighting the benefit of looking at the world's freshwaters through a new framework.

Growth of the human population, rising consumption, and rapid globalization have caused widespread degradation and disruption of natural systems, especially in the freshwater realm. Freshwater ecosystems have lost a greater proportion of their species and habitat than ecosystems on land or in the oceans, and they face increasing threats from dams, water withdrawals, pollution, invasive species, and overharvesting (MEA 2005, Revenga et al. 2005). Freshwater ecosystems and the diverse communities of species found in lakes, rivers, and wetlands may be the most endangered of all (MEA 2005).

These stressed systems support an extraordinarily high proportion of the world's biodiversity. In terms of area, freshwater ecosystems occupy only 0.8% of Earth's surface, but they are estimated to harbor at least 100,000 species, or nearly 6% of all described species (Dudgeon et al. 2006). Each year, new freshwater species are described. For South America alone, about 465 new freshwater fish species have been described in the last five years (Eschmeyer 2006), a figure that corresponds to a new species every four days. The presence of species confined to small ranges is also unusually high in freshwater ecosystems; for example, 632 animal species have been recorded as endemic to Lake Tanganyika (Groombridge and Jenkins 1998).

Despite this combination of extraordinary richness, high endemism, and exceptional threat, few broadscale conservation planning efforts have targeted freshwater systems and their dependent species. This relative inattention derives in part from an acute lack of comprehensive, synthesized data on the distributions of freshwater species (Revenga and Kura 2003). The most exhaustive recent global inventory of freshwater taxa acknowledges serious survey gaps and assigns species distributions only to the level of continent (Lévêque et al. 2005). Such inventories are valuable for highlighting research priorities and providing a global picture of how taxonomic diversity compares across continents, but they have limited utility for conservation planning efforts, for which the largest planning unit is often the river basin or ecoregion.

A global freshwater regionalization

Ecoregions are a widely recognized and applied geospatial unit for conservation planning, developed to represent the patterns of environmental and ecological variables known to influence the distribution of biodiversity features at broad scales (Groves et al. 2002). Building on the work of Dinerstein and colleagues (1995), we define a freshwater ecoregion as a large area encompassing one or more freshwater systems with a distinct assemblage of natural freshwater communities and species. The freshwater species, dynamics, and environmental conditions within a given ecoregion are more similar to each other than to those of surrounding ecoregions, and together form a conservation unit. Ecoregion boundaries are not necessarily determined by the turnover of species ranges (McDonald et al. 2005) but are intended to describe broad patterns of species composition and associated ecological and evolutionary processes.

Ecoregion delineation benefits from the best available data describing species and systems ecology, but can proceed with imperfect information (Wikramanayake et al. 2002). Global ecoregion frameworks have already been developed for the terrestrial and, more recently, marine realms, both of which are characterized by their own data limitations (Olson et al. 2001, Spalding et al. 2007). In this article we demonstrate how the ecoregion concept has been applied to freshwater systems, and present the first global map of freshwater ecoregions—a starting point for conservation planning anywhere on Earth.

Ecoregions have typically been delineated to represent patterns of potential vegetation (Olson et al. 2001) and have at times been used to characterize regional differences in water quality as well (Omernik 1987). Terrestrial ecoregions are delineated largely on the basis of climate, physiography, and vegetation types, but different features are often dominant in shaping the broadscale distributions of freshwater species. As Tonn (1990) described, the species occurring in a given river reach, lake, spring, or wetland will be a function of a hierarchy of continental-scale filters (including mountain building, speciation, and glaciation) that have defined large biogeographic patterns; regional-scale filters (such as broad climatic and physiographic patterns, and dispersal barriers such as regional catchments); and subregional and finer-scale habitat filters (e.g., distinct physiographic types and macrohabitats) acting on the regional species pool. Freshwater ecoregions capture the patterns generated primarily by continental- and regional-scale filters.

Of these filters, dispersal barriers in the form of catchment divides (also called watersheds) are distinctive to freshwaters. Unlike terrestrial species or those with aerial or wind-dispersed life stages, obligate freshwater species—those confined to the freshwater environment and unable to move via land, air, or sea—generally cannot disperse from one unconnected catchment to another. Furthermore, all species dependent on freshwater systems, whether or not they are confined to the aquatic environment, are to some extent affected by the hydrological and linked ecological processes of the catchments where they live. As a result, catchments strongly influence broad freshwater biogeographic patterns in most regions. There are exceptions, however. Tectonic movements have in some cases separated once-joined catchments, allowing for further speciation. Also, natural drainage evolution over geological time includes river piracy, which severs connections and provides new interdrainage links that reconform systems. The freshwater ecoregions of the world presented here reflect both the hydrological underpinning of freshwater fish species distributions as well as historical shifts in landmasses and consequent evolutionary processes.

Ecoregion delineation and species list compilation

No global biogeographic framework for freshwater species was available as the foundation for our map. The applicability of Wallace's (1876) and Udvardy's (1975) zoogeographic realms to most freshwater taxa is unresolved (Berra 2001, Vinson and Hawkins 2003), and these divisions are too large for conservation planning endeavors. Several examinations of global freshwater biogeography (e.g., Banarescu 1990) provided information at somewhat finer scales but could not be clearly translated into seamless ecoregion delineations. Where appropriate, we adapted previous continental efforts. For North America, Africa, and Madagascar, we updated regionalizations outlined in two previously published volumes (Abell et al. 2000, Thieme et al. 2005), but we excluded a prior delineation for Latin America and the Caribbean (Olson et al. 1998) because the approach differed markedly from our current methodology, and data have improved substantially since its development (e.g., Reis et al. 2003). We examined but chose to exclude the 25 European regions of Illies's impressive Limnofauna Europaea 1978 because the approach for delineating those regions differed considerably from ours: those regions were based on the distributions of 75 different taxonomic groups and were drawn without reference to catchments. Moreover, neither ecological nor evolutionary processes figured in those delineations. A complete list of all references and experts consulted in the process of delineating ecoregions is available online (www.feow.org).

We assembled our global map of freshwater ecoregions using the best available regional information describing freshwater biogeography, defined broadly to include the influences of phylogenetic history, palaeogeography, and ecology (Banarescu 1990). We restricted our analyses to information describing freshwater fish species distributions, with a few exceptions for extremely data-poor regions and inland seas, where some invertebrates and brackish-water fish were considered, respectively. We focused on freshwater fish for several reasons. On a global scale, fish are the best-studied obligate aquatic taxa. Detailed information exists for other freshwater taxa in regions like North America and Europe, but the consideration of such groups in a global analysis would be difficult, given the wide variation in available data (Balian et al. 2008). Freshwater dispersant fish species—those unable to cross saltwater barriers—are better zoogeographic indicators than freshwater invertebrates, which can often disperse over land, survive in humid atmospheres outside water, or be transported between freshwaters (Banarescu 1990). Finally, the distributions of obligate aquatic invertebrate groups in general respond to ecological processes at localized scales that are too small to be meaningful for ecoregion delineation (Wasson et al. 2002). Therefore, fish serve as proxies for the distinctiveness of biotic assemblages. We recognize that analyses of other taxonomic groups would almost certainly reveal different patterns for some regions, and that our results are scale dependent (Paavola et al. 2006). Our near-exclusive focus on fish is a departure from earlier continental eco-regionalization exercises (Abell et al. 2000, Thieme et al. 2005), and we have updated the ecoregion delineations accordingly.

The available data for describing fish biogeography vary widely. In the United States, it is possible to map presence/absence data for all freshwater fish species to subbasins averaging about 2025 square kilometers (km2) in size (NatureServe 2006). But for many of the world's species, occurrence data are limited to a small number of irregularly surveyed systems. Large parts of the massive Congo basin remain unsampled, for instance, with most sampling occurring near major towns and most taxonomic studies of the region dating from the 1960s. Problems with taxonomy and species concepts hamper broadscale analyses even where systems have been reasonably well sampled (Lundberg et al. 2000). Although addressing many of these problems is beyond the scope of this project, in our analyses we have attempted to minimize nomenclatural errors by normalizing species names with Eschmeyer's Catalog of Fishes (2006; www.calacademy.org/research/ichthyology/catalog/).

Freshwater fish patterns were analyzed separately for different regions of the world to account for data variability. The geographic scope of major information sources largely defined those regions (table 1). Information sources were typically taxonomic works, some of which included biogeographical analyses. Leading ichthyologists delineated ecoregions primarily by examining the distributions of endemic species, genera, and families against the backdrop of an area's dominant habitat features and the presence of ecological (e.g., large concentrations of long-distance migratory species) and evolutionary (e.g., species flocks) phenomena. More than 130 ichthyologists and freshwater biogeographers contributed to the global map by either delineating or reviewing ecoregions.

Data gaps and biogeographic drivers resulted in the use of slightly different criteria among and even within some regions (table 2, box 1). Where fish species data were reasonably comprehensive and available at subbasin or finer scales, we attributed species distributions to catchments to facilitate evaluation of biogeographic patterns in a bottom-up approach. For example, a new high-resolution hydrographic dataset (HydroSHEDS; www.wwfus.org/freshwater/hydrosheds.cfm.) for South America provided fine-scale catchment maps that, in conjunction with newly synthesized species data (Reis et al. 2003), aided in the assessment of biogeography. In regions without extensive species data, or where major basins support highly similar faunas as a result of recent glaciation, a top-down analysis used qualitative expert knowledge of distinctive species and assemblages to map major biogeographic patterns (table 2). Ecoregional boundaries resulting from either approach, therefore, largely coincide with catchment boundaries.

Whereas overall there is correspondence between catchments and ecoregion boundaries, unconnected neighboring catchments were in some cases grouped together, where strong biogeographic evidence indicates that landscape or other features overrode contemporary hydrographic integrity. For example, owing to historic drainage evolution and similarities in fauna, Africa's southern temperate highveld combines headwaters of coastal basins that drain to the Indian Ocean with those of the Atlantic-draining Orange basin. Considerable faunal exchange of the headwaters of the Orange River system with that of the coastal systems may have occurred as the coastal rivers eroded their basins at a faster rate than the adjacent Orange tributaries (Skelton et al. 1995). These and other examples demonstrate that historical geographic events and current hydrology may have conflicting effects on the fish fauna of a particular region and thereby argue for different boundaries. The decision to weigh some effects more strongly than others was made on a case-by-case basis, and it is acknowledged that additional data may favor alternative delineations.

With the exception of islands, individual freshwater ecoregions typically cover tens of thousands to hundreds of thousands of square kilometers (Maxwell et al. 1995). Ecoregion size varies in large part because of landscape history. Regions with depauperate faunas resulting from recent glaciation events tend to have large ecoregion sizes, as do those dominated by very large river systems (e.g., much of South America). Regions with recent tectonic activity or smaller, more isolated freshwater systems often are divided into smaller ecoregions. For example, central Mexico has experienced intermittent isolation and exchange between basins owing to active mountain-building processes leading to small, fragmented systems with distinct faunas. We acknowledge that data quality may also influence the size of ecoregions; for instance, the entire Amazon is currently divided into only 13 ecoregions, but better data on species occurrences within major subbasins would most likely support finer delineations.

The process of delineating ecoregions required compiling and synthesizing information on the distributions of fish species. A logical and practical extension of the delineations was the compilation of fish species lists for each ecoregion. For the United States, NatureServe provided presence/absence data for individual species, coded to eight-digit hydrologic unit codes (HUCs); these HUC occurrences were then translated to ecoregions, and the data were manually cleaned of erroneous occurrences derived from introductions and problematic records. These species lists were then merged with those from Canada and Mexico for transnational ecoregions. For all other ecoregions, data came from the published literature, as well as from gray literature and unpublished sources (see table 1; a full bibliography is available at www.feow.org). In all cases, experts served as gatekeepers of these data to ensure that lists were based on the best available information, both in terms of distributions and nomenclature. Introduced species were removed from the tallies presented here, as were undescribed species. Confirmed extinct species (Ian J. Harrison, American Museum of Natural History, New York, personal communication, 29 March 2007) were excluded, but extirpated species were included to acknowledge restoration opportunities. Endemic species, defined as those occurring only in a single ecoregion, were identified first by experts and cross-checked using a species database constructed for this project, which includes more than 14,500 described fish species. Species were coded as freshwater, brackish, or marine using data from FishBase (www.fishbase.org), and species with only brackish or marine designations were omitted from the richness and endemism totals reported here.

Freshwater ecoregional map and species results

Our map of freshwater ecoregions contains 426 units, covering nearly all nonmarine parts of the globe, exclusive of Antarctica, Greenland, and some small islands (figure 1; a full legend is available at www.feow.org). There is large variation in the area of individual ecoregions. Large ecoregions, such as the dry Sahel (4,539,429 km2), tend to be found in more depauperate desert and polar regions exhibiting low species turnover. Smaller ecoregions are typically found in noncontinental settings where systems are by nature smaller and species turnover is higher, as in the Indo-Malay region. The smallest ecoregion, at 23 km2, is Cocos Island (Costa Rica); the average ecoregion size is 311,605 km2. Ecoregions ranged from those encompassing only 1 country to those straddling 16 countries (central and western Europe ecoregion).

In total, we assigned more than 13,400 described freshwater fish species to ecoregions, of which more than 6900 were assigned to single ecoregions (i.e., endemic). Examination of the fish species data synthesized by ecoregion confirms some well-known patterns and highlights others unknown to many conservationists, managers, and policymakers working at regional or global scales (figures 2a–2d). In agreement with previous global assessments (Groombridge and Jenkins 1998, Revenga et al. 1998), our analysis identifies as outstanding for both fish richness and endemism systems that include large portions of Africa's Congo basin, the southern Gulf of Guinea drainages, and Lakes Malawi, Tanganyika, and Victoria; Asia's Zhu Jiang (Pearl River) basin and neighboring systems; and large portions of South America's Amazon and Orinoco basins. Areas confirmed for globally high richness include Asia's Brahmaputra, Ganges, and Yangtze basins, as well as large portions of the Mekong, Chao Phraya, and Sitang and Irrawaddy; Africa's lower Guinea; and South America's Paraná and Orinoco. When richness is adjusted for ecoregion area, additional systems such as the Tennessee, Cumberland, Mobile Bay, Apalachicola, and Ozark highlands in the southeastern United States; portions of Africa's Niger River Basin; the islands of New Caledonia, Vanuatu, and Fiji; China's Hainan Island; and large parts of Sumatra and Borneo, among many other areas, are also especially noteworthy.

Numerous systems previously identified as highly endemic for fish were confirmed, as measured by either numbers of endemic species or percentage endemism. A subset includes highland lakes in Cameroon along with Africa's Lake Tana; northwestern and eastern Madagascar; freshwaters from Turkey's central Anatolia region, the northern British Isles, the Philippines, Sri Lanka, India's western Ghats, the southwestern Balkans, and northwest Mediterranean; southwestern Australia and nearly the entire island of New Guinea; Eurasian lakes, including Baikal, Inle, and Sulawesi's Lake Poso and Malili system; Death Valley in the United States and Mexico's Pánuco system; and South America's Iguaçu River, Lake Titicaca, and the freshwaters of both the Mata Atlántica and the continent's northwestern Pacific coast. Additionally, newly available data show that some systems previously recognized for high endemism, such as those of South America's Guianas, also exhibit exceptional richness.

Because our ecoregions cover all nonmarine waters, and because they often exist as subdivisions of major river basins, our results also highlight a number of smaller systems for the first time in global analyses. Using finer-resolution data allowed us to identify the high richness of the Congo's Malebo Pool and Kasai basin. Cuba and Hispaniola stand out for endemism, along with the Amazon's western piedmont and the Tocantins-Araguaia systems. The Tocantins-Araguaia, as well as the highly endemic São Francisco, were defined as units of analysis in Revenga and colleagues (1998), but fish data were unavailable for those basins when that study was done. Systems never before analyzed globally but recognized in our results as exceptionally rich for fish include those of the Malay Peninsula's eastern slope and Japan. A large number of ecoregions are identified for the first time for highly endemic faunas, measured as percentage endemism. Newly identified ecoregions with at least 50% endemism include Africa's Cuanza, Australia's Lake Eyre Basin, Mexico's Mayrán-Viesca, and New Zealand, as well as a large number of highly depauperate ecoregions such as Africa's karstveld sink holes, Turkey's Lake Van, the Oman Mountains, western Mongolia, and Hawaii.

Each of the biodiversity analyses that we offer here emphasizes different sets of ecoregions, suggesting that a single measure of species diversity might overlook ecoregions of important biodiversity value. In a comparative analysis of biodiversity value, ecoregions are probably best evaluated against others within the same region, with similar historical and environmental characteristics, and of similar size to account for the typically positive relationship between river discharge and fish species richness (Oberdorff et al. 1995). Nonetheless, some systems, such as the Amazon and many of Africa's Rift Valley lakes, stand out by nearly any measure of fish biodiversity and are indisputable global conservation priorities.

Conservation applications

The ecoregion map and associated species data summarized here have a number of conservation applications. At global and regional scales the ecoregion map can be used to distinguish distinct units of freshwater biodiversity to be represented in conservation efforts. The Convention on Wetlands, for instance, requires that sites nominated as wetlands of international importance—with wetlands defined to include all freshwaters—be evaluated against a “biogeographic regionalization” criterion (Ramsar Bureau 2006). Lack of a global biogeographic scheme has stalled the application of this criterion, but our global map and database may provide a necessary framework for identifying broadscale gaps in protection. Similarly, progress toward the establishment of representative networks of freshwater protected areas, as called for by the third IUCN World Conservation Congress, the fifth World Parks Congress, and the seventh Meeting of the Conference of the Parties to the Convention on Biological Diversity, can now be measured using ecoregions as a proxy for finer-scale global species or habitat distribution data. At a regional level, the freshwater ecoregion map may be used as supplementary information for implementation of the European Union's Water Framework Directive (2000/60/EC), which requires a characterization of surface water bodies and currently uses regions defined by Illies (1978).

A primary use of ecoregions is as conservation planning units (Higgins 2003). Our attribution of freshwater fish species data to ecoregions is an important first step for data-poor regions. Organizations or agencies with regional mandates may choose to compare biodiversity values across ecoregions in the process of setting continental priorities (Abell et al. 2000, Thieme et al. 2005). At the basin scale, ecoregions can help to introduce biodiversity information into water-resource or integrated-basin management activities (Gilman et al. 2004). Where major basins are divided among multiple freshwater ecoregions, whole-basin exercises can use ecoregions as stratification units to ensure adequate representation of distinct biotas. Where unconnected drainages are combined into a single freshwater ecoregion, planners may choose to consider a counterintuitive planning unit to incorporate biogeographic patterns. Freshwater ecoregions defined in previous exercises have already been put to use by the Nature Conservancy and WWF in numerous conservation planning efforts across North America (e.g., Upper Mississippi; Weitzell et al. 2003), South America (e.g., the Pantanal; de Jesus 2003), and Africa (e.g., the Congo basin; Kamdem-Toham et al. 2003).

Caveats and limitations

Ecoregions are delineated based on the best available information, but data describing freshwater species and ecological processes are characterized by marked gaps and variation in quality and consistency. Data quality is generally considered high for North America, Australia, New Zealand, Japan, western Europe, and Russia; moderate for Central America, the southern cone of South America, southern and western Africa, Oceania, and the Middle East; and poor for much of southeastern Asia, central and eastern Africa, and South America north of the Paraná River basin.

Freshwater ecoregions are not homogeneous units. Within individual ecoregions there will be turnover of species along longitudinal gradients of river systems and across different habitats such as flowing and standing-water systems. The inclusion of multiple macrohabitat types within a given freshwater ecoregion is a marked departure from terrestrial ecoregions, which typically encompass a single vegetation-defined biome (e.g., deciduous forests, evergreen forests, or scrub; Wikramanayake et al. 2002).

Ecoregions are imperfect units for highlighting certain highly distinct and highly localized assemblages occurring at subecoregion scales. Examples include many peat swamps or subterranean systems. Underground systems such as caves and karsts may require their own planning framework, as ground-water catchments may not correspond with the surface-water catchments upon which our ecoregions are built.

For reasons of practicality and scale, our ecoregion framework does not take into account the distributions of freshwater species such as invertebrates, reptiles, and amphibians. This is a limitation of the ecoregional approach presented here, which is especially problematic for places such as isolated islands where freshwater fish provide little information to inform biogeographic delineation. We hope this taxonomic omission will serve as motivation for generating and synthesizing global data for other taxonomic groups to provide complementary information for conservation planners, particularly when working at subecoregional scales. We recognize that improved information in the future may warrant map revisions, and we highlight areas of greatest data uncertainty in part to encourage enhanced research investment in those places. We believe that the critical state of freshwater systems and species argues against waiting for ideal biodiversity data to be developed before generating urgently needed conservation tools like the ecoregion map.

Shifting transition zones for species are common, and we recommend that ecoregions be viewed as logical units for more detailed analyses and strategies. Ecoregions are intended to depict the estimated original extent of natural communities before major alterations caused by recent human activities, but original distributions can be difficult to reconstruct. As new species are described, our understanding of distribution patterns may also change. Ecoregional delineation is an iterative process, and changes to ecoregion boundaries should be incorporated as new information becomes available.

There is no definitive, error-free data source for classifying fish species as freshwater, brackish, or marine. We chose to use the global FishBase habitat assignments, which are derived from the literature, to ensure that any given species in our database would be classified consistently wherever it occurred. We recognize that errors of omission or commission may derive from inaccuracies in the FishBase assignments as well as from the habitat plasticity of some species. All species information provided to us by experts, regardless of habitat assignment, is retained in our database for future analyses.

The preliminary richness and endemism numbers presented here are in some cases markedly different from existing estimates in the literature. For example, our tally for Lake Malawi contains 431 described fish species, but other estimates run as high as 800 or more (Thieme et al. 2005). Our omission of undescribed species, as well as the conservative approach taken by experts in using only robust species occurrence data, account for many of these lower-than-expected numbers. Numbers of endemics may in some cases be higher than expected because endemics were identified strictly through a database query for unique occurrences, and many species lists are undoubtedly incomplete or use synonyms. We anticipate that many tallies will change with further refinement of species lists but that the broad patterns presented here will hold.

Conclusions

The newly available species data attributed to ecoregions has important implications for prioritizing conservation investments. As one illustration, in 2005 the Global Environment Facility (GEF), which spends more than $1 billion each year on environmental projects, adopted a new resource allocation framework. Terrestrial ecoregion maps and biodiversity data were notable inputs to the framework, but parallel freshwater information to help guide investments was lacking. The GEF framework fortunately leaves open the possibility of incorporating freshwater ecoregions and biodiversity data at a later date (GEF 2005).

In addition to providing data for scientific and conservation purposes, we aim to give the largest possible number of people access to the ecoregion-level information collected in association with the global map. The information will be freely available on the Internet (www.feow.org) as well as in brochures, posters, and other publications. The freshwater ecoregion map covers virtually all land surfaces on Earth, so people around the globe will have the opportunity to learn about the freshwater systems where they live.

For most policymakers, water resource managers, and even conservationists, freshwater biodiversity is more of an afterthought than a central consideration of their work. The freshwater ecosystem services that support the lives and livelihoods of countless people worldwide are a far larger concern. Yet freshwater biodiversity and ecosystem services are linked through ecological integrity, and better-informed efforts to conserve freshwater biodiversity should benefit human communities as well. The freshwater ecoregions of the world map and associated species data begin to improve access to previously dispersed and difficult to access freshwater biodiversity information. We hope that this set of products catalyzes additional work toward a better understanding of freshwater species distributions and—of equal if not more importance—leads to a ramping up of freshwater conservation activity and success.

Acknowledgements

The authors would like to thank the dozens of scientists who contributed to development of the ecoregion map and synthesis of fish species data: E. K. Abbam, Vinicius Abilhoa, Angelo Agostinho, James Albert, Hector Samuel Vera Alcazar, Claudio Baigun, Eldredge Bermingham, Tim Berra, Vinicius Bertaco, Richard Biggins, Flavio Bockmann, Paulo Buckup, Noel Burkhead, Brooks Burr, Mary Burridge, Lauren Chapman, Lindsay Chatterton, Barry Chernoff, Lynda Corkum, Ian Cowx, William Crampton, Alain Crivelli, Carolina Joana da Silva, Tim Davenport, Luc De Vos, Ignacio Doadrio, Carlos DoNascimiento, Luis Fernando Duboc, Brian Dyer, Carlo Echiverri, Jean Marc Elouard, Joerg Freyhof, Christopher Frissell, German Galvis, Angus Gascoigne, Abebe Getahun, A. Gopalakrishnan, Michael Goulding, Jon Harding, Tan Heok Hui, Liu Huanzhang, Leonardo Ingenito, Michel Jégu, Howard Jelks, Aaron Jenkins, Wolfgang Junk, Ad Konings, Friedhelm Krupp, Philippe Lalèyè, Carlos Alcala Lasso, Christian Lévêque, Flávio C. T. Lima, Cas Lindsey, Jorge Liotta, Marcelo Loureiro, Carlos Lucena, Margarete Lucena, Paulo Henrique Lucinda, Antonio Machado-Allison, Christopher Magadza, Luis Malabarba, Mabel Maldonado, Maria Cristina Dreher Mansur, Larry Master, Don McAllister, Robert McDowall, J. D. McPhail, Geraldo Mendes dos Santos, Naércio A. Menezes, Roberto Carlos Menni, Jose Ivan Mojica, Peter Moyle, Thierry Oberdorff, Javier Maldonado Ocampo, Mike K. Oliver, Hernan Ortega, Mark Oswood, Vadim E. Panov, Carla Simone Pavanelli, Christine Poellabauer, David Propst, Edson Pereira, Saul Prada, Francisco Provenzano, Gordon McGregor Reid, Anthony J. Ribbink, Francisco Antonio Rodrigues Barbosa, Ricardo S. Rosa, Norma J. Salcedo-Maúrtua, Jansen Alfredo Sampaio Zuanon, Robert Schelly, Michael Schindel, Uli Schliewen, Juan Jacobo Schmitter Soto, Martin Schneider-Jacoby, Uwe Horst Schulz, Lothar Seegers, Ole Seehausen, Scott Smith, John S. Sparks, Don Stewart, Donald Taphorn, Christopher Taylor, Guy Teugels, Louis Tsague, Denis Tweddle, Paul Van Damme, D. Thys van den Audenaerde, Stephen J. Walsh, Claude Weber, Robin Welcomme, James D. Williams, Phillip Willink, and Stamatis Zogaris. Additionally, William Eschmeyer and Stan Blum provided critical support toward improving our fish species database. The importance of biological collections and the work of taxonomists are basic to all biogeographic mapping projects, and so we acknowledge and highlight the fundamental contribution of collections and taxonomy to this effort and to conservation generally. Institutions and organizations that have generously provided data and assistance include the American Fisheries Society's Endangered Species Committee, the American Museum of Natural History, Belgium's Royal Museum for Central Africa, the California Academy of Sciences, FishBase, Fundación La Salle de Ciencias Naturales, Instituto Nacional de Pesquisas da Amazônia, IUCN, Museu de Ciências e Tecnologia PUCRS, Museo de Zoologia de la Universidad Central de Venezuela, Museo de Zoologia de la Universidad Nacional de los Llanos Occidentales, Museu Nacional do Rio de Janeiro, Nature-Serve, South African Institute for Aquatic Biodiversity, and the Zoological Museum of the University of Copenhagen. Ezequiel Zamora, George Ledec, Douglas Graham, and Gonzalo Castro were instrumental in the earliest stages of this project. We also thank Nasser Olwero for his development of the FEOW Web site; Eric Dinerstein for his guidance and review of an earlier manuscript; and many additional former and current WWF and Nature Conservancy staff acknowledged on the FEOW Web site, including but not limited to Jamie Pittock, Allison Pease, Brian Blankespoor, and Tucker Gilman. This work was supported in part by grants to WWF from the Coca-Cola Company and JohnsonDiversey Inc. Additional support was generously provided to the Nature Conservancy by Bill Barclay, Ofelia Miramontes, and John Mordgridge. Work in South America was supported in part by the US Agency for International Development through award number EDG-A-00-01-0023-00 for the Parks in Peril Program.

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Table 1.

Regional information sources used for ecoregion delineations.

Table 1.

Regional information sources used for ecoregion delineations.

Table 2.

Basic ecoregion delineation approaches for individual regions.

Table 2.

Basic ecoregion delineation approaches for individual regions.

Box 1. Example of criteria applied to ecoregion delineation: South America.

The delineation process for South America followed a stepwise process of subdivision of the continent's major drainage systems. Delineation started with the historically recognized major ichthyographic provinces exemplified in Gery (1969) and Ringuelet (1975) and proceeded with subdivision at finer scales using regionalized data on fish distributions.

The criteria for determining the merit of delineating an ecoregion were not uniform across the continent as a result of localized faunistic differences. In some areas, delineations were based on family-level data, whereas in others, faunistic turnover at lower taxonomic levels was the criterion. For instance, astroblepid catfishes are distinct components of high-elevation freshwaters along the Andes forefront, and that family's distribution was critical to informing the delineation of the high Andean ecoregions. On the other side of the continent along the Atlantic coast, we used the presence or absence of endemic assemblages of the genus Trichomycterus, several genera of the subfamily Neoplecostomatinae, and the presence or absence of annual killifish genera and species to distinguish distinct drainage complexes from one another.

In the piedmont zones and in contact areas between lowlands and geologic shield areas, we used indicator groups to determine where along the elevation/slope gradient the fauna was changing. The distribution of lowland forms was matched with forms found in higher-gradient systems to establish where one group was dropping out and the other started occurring. This transition zone was then established as the operational boundary between connecting ecoregions.

For areas like Patagonia, the Titicaca altiplano, and the Maracaibo basin, the uniqueness of the fauna, often occurring within clearly defined geographic areas, permitted reasonably straightforward delineations. In the larger river basin systems where there are no clear boundaries, the ecoregional limits are the best approximation, given the current data.

Figure 1.

Map of freshwater ecoregions of the world, in which 426 ecoregions are delineated. An interactive version of this map that includes additional information is available at www.feow.org.

Figure 1.

Map of freshwater ecoregions of the world, in which 426 ecoregions are delineated. An interactive version of this map that includes additional information is available at www.feow.org.

Figure 2.

Preliminary freshwater fish species data for ecoregions: (a) species richness, (b) number of endemic species, (c) percentage endemism, and (d) species per ecoregion area. Numbers may be adjusted on the basis of an ongoing process to correct nomenclatural errors. Natural breaks (Jenk's optimization) was the classification method used for panels (a)–(c). This method identifies breakpoints between classes using a statistical formula that identifies groupings and patterns inherent in the data.

Figure 2.

Preliminary freshwater fish species data for ecoregions: (a) species richness, (b) number of endemic species, (c) percentage endemism, and (d) species per ecoregion area. Numbers may be adjusted on the basis of an ongoing process to correct nomenclatural errors. Natural breaks (Jenk's optimization) was the classification method used for panels (a)–(c). This method identifies breakpoints between classes using a statistical formula that identifies groupings and patterns inherent in the data.

Author notes

Robin Abell (e-mail: robin.abell@wwfus.org), Michele L. Thieme, Rebecca Ng, Nikolai Sindorf, and Eric Wikramanayake are with WWF in Washington, DC. Carmen Revenga, Mark Bryer (Bethesda), James Robertson, Eric Armijo (Bolivia), Jonathan V. Higgins (Chicago), Thomas J. Heibel, and Paulo Petry (Boston) are with the Nature Conservancy, headquartered in Arlington, Virginia. Paulo Petry is also an associate in ichthyology at the Museum of Comparative Zoology at Harvard University in Massachusetts. Maurice Kottelat is an independent consultant in Switzerland and an honorary research associate at the Raffles Museum of Biodiversity Research at the National University of Singapore. Nina Bogutskaya and Alexander Naseka are senior researchers at the Zoological Institute of the Russian Academy of Sciences in St. Petersburg. Brian Coad is a research scientist at the Canadian Museum of Nature in Ottawa. Nick Mandrak is a research scientist at the Great Lakes Laboratory for Fisheries and Aquatic Sciences, Fisheries and Oceans Canada, Burlington, Canada. Salvador Contreras Balderas is professor emeritus of the Universidad Autónoma de Nuevo Leon in Monterey, Mexico. William Bussing is professor emeritus at the Universidad de Costa Rica. Melanie L. J. Stiassny is the Axelrod Research Curator of Ichthyology at the American Museum of Natural History and an adjunct professor at Columbia University in New York City. Paul Skelton is managing director of the South African Institute for Aquatic Biodiversity and professor at Rhodes University in Grahamstown, South Africa. Gerald R. Allen is a research associate at Western Australian Museum in Perth. Peter Unmack is a postdoctoral associate in the Department of Integrative Biology at Brigham Young University in Utah. David Olson is director of science and stewardship at Irvine Ranch Conservancy in California. Hugo L. López is head of the vertebrate zoology department at the Museo de La Plata, assistant professor in the Facultad de Ciencias Naturales y Museo, and researcher at CIC (Buenos Aires) in Argentina. Roberto E. Reis is a professor at Católica do Rio Grande do Sul in Porto Alegre, Brazil. John G. Lundberg is chair and curator of ichthyology, and Mark H. Sabaj Pérez is collection manager, at the Academy of Natural Sciences in Philadelphia.

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