Concepts of Ecological Classification
While the need for broad ecological regionalization has long been recognized, attempts at developing a North America ecological classification based on a holistic interpretation of ecosystems are relatively recent. Some of the earliest such studies between Canada and the United States were in response to such issues as acid rain and protected areas. The focus of the initial work lay along the 49th parallel, later moving north to the Yukon and Alaska. Ultimately, the entire area of each country was the focus. These studies arose from the need to have a common basis for state of the environment reporting, particularly one that would encourage the application and use of an ecological approach to sustainable resource use.
Ecological classifications have evolved considerably over the past thirty years. Early pioneering works in North America evolved from forest and climate classifications and were often climate driven (Hills 1961; Flores et al. 1971; CETENAL (now INEGI) 1976; Bailey 1976). The use of more holistic classifications is more recent. Several more broadly based regional ecological classifications emerged during this period (Oswald and Senyk, 1977; Lopoukhine et al. 1979; Strong and Leggart 1980; Hirvonen 1984). The first national compilations of ecological classifications emerged in the mid-1980s (Wiken, comp. 1986; Omernik 1987). These were holistic approaches that recognized the importance of considering a full range of physical and biotic characteristics to explain ecosystem regionality. Equally, they recognized that ecosystems of any size or level are not always dominated by one particular factor.
Ecological land classification is a process of delineating and classifying ecologically distinctive areas of the Earth’s surface. Each area can be viewed as a discrete system which has resulted from the mesh and interplay of the geologic, landform, soil, vegetative, climatic, wildlife, water and human factors which may be present. The dominance of any one or a number of these factors varies with the given ecological land unit. This holistic approach to land classification can be applied incrementally on a scale-related basis from very site-specific ecosystems to very broad ecosystems.
The classification can be produced following various approaches. The two used for this project were:
- opinions were sought from ecologists and other scientists on the relevant features for each region; and
- a data matrix was produced that could be used to build each ecological level.
Because the underlying dynamics of the ecosystems produce complex, multiple patterns of correlation among the biotic, abiotic, and human factors, these two approaches tended to produce a converging depiction of regions. The focus for this project was to develop ecological land classifications suitable for use in continental, national and regional/ local environmental reporting and assessment. A similar hierarchical ecological classification of oceanic areas in Canada has been published (Hirvonen et al. 1994; CCEA 1995); however, integration of these with oceanic areas in the United States and Mexico has not yet taken place.
How Mapped Areas are Derived
Diagnostic criteria for individual mapped areas are based on “enduring” components of the ecosystems contained therein. These components are relatively stable, such as soil, landform, or major vegetation types: that is, features that do not change appreciably over ecological time. Climate is also considered but, unlike the other stable components, it needs to be assessed by looking at long-term records. Enduring components are attributes that can be determined, either visually (e.g., from aerial photographs or satellite imagery) or from pertinent field studies or resource sector maps. For any level of ecological generalization, the mosaic of components may vary from one ecological area to the next. Ecological classification is science-based, but, in a way, it is also an art because ecological cycles, characteristics and interactions are not readily apparent and need to be interpreted from soil, vegetation and landform characteristics or other factors. Thus a mapped area must be considered a partial abstraction of real ecosystems. Maps depict where major ecological areas exist as a result of major ecological interactions but they do not readily illustrate the more dynamic aspects of ecosystems. More intangible characteristics, like changing weather patterns, species dynamics and soil chemical processes, are all vital in understanding ecosystems.
Which parameter is initially used to define an area often depends on the background of the scientist doing the analysis and on those indicators that person finds contribute most incisively to understanding the nature of the ecosystem. If vegetation serves this function, then vegetation types, forms and/or composition might initially be used. Ultimately, through the interpretive process, the broad range of ecological characteristics, including climate, soils, physiography and water bodies would be considered. Boundaries bisect transition areas, distinguishing one ecological area from another. When these transition areas are abrupt, delineation is relatively straightforward. At other times, the transition zone may be diffuse and extend for hundreds of kilometers. In these situations, boundary delineation becomes more subjective.
Current land use and other human influences are characteristics that have not been commonly accepted as useful for delineating ecological areas. However, in this study these attributes were found to be relevant and sometimes even essential to the description. In situations where human use has historically been pervasive, it may significantly and irreversibly influence the ecological processes and attributes of that area. Examples could be the Great Plains and the Temperate Sierras, where land use and human activities serve as an important interpretive parameter because they have largely transformed the regions. On the other hand, some of the larger ecosystems, like the Arctic, have not been significantly transformed by humans over long periods of time.
Key Points in Mapping Ecological Regions
- Ecological classification incorporates all major components of ecosystems: air, water, land, and biota, including humans.
- It is holistic (“the whole is greater than the sum of its parts”).
- The number and relative importance of factors that are helpful in the delineation process vary from one area to another, regardless of the level of generalization.
- Ecological classification is based on hierarchy—ecosystems are nested within ecosystems.
- Such classification integrates knowledge; it is not an overlay process.
- It recognizes that ecosystems are interactive—characteristics of one ecosystem blend with those of another.
- Map lines depicting ecological classification boundaries generally coincide with the location of zones of transition.
The Ecological Regions of North America
“Ecological region” refers to any one of the ecological areas that were mapped and described in this project. In a technical sense, they represent many things: a concept, a mapped and classified area, and an area of land with distinctive biological, physical and human characteristics. Determining ecological regions at a continental level is a challenging task. It is difficult, in part, because North America is ecologically diverse and because a nation’s territorial boundaries are a strong hindrance to seeing and appreciating the perspectives across the land-mass of three countries. Ecosystems vary in composition. The interactions that occur within and among them are many and complex. Mapped areas must reflect this complexity in a “workable” and understandable manner for planning and communication purposes. Delineating an ecological area serves to “capture” its general ecological composition as well as the links between the ecosystems it contains.
What the Maps Depict
For planning and reporting purposes, maps are essential. The level of generalization of delineated ecosystems respects different levels of planning and reporting needs. In the context of North America, ecological regions are depicted at three levels of mapping. All three levels depict the spatial distribution of ecosystems. In some cases these are simple and fairly homogeneous, but often they are heterogeneous aggregations. The actual processes underlying ecosystems are not easily reflected on maps, and nor are the specific characteristics themselves. The intent is to illustrate the net product of many interacting ecological processes and functions of living organisms. Accompanying descriptions and other supplementary information, as provided in this report, are required to depict more fully the dynamism and complexity, both spatial and temporal, of real-world ecosystems.
As an example, the Great Plains ecological region has characteristics that are easily defined in a geographic sense. They include expanses of prairie soils, plains, areas of cereal grain production and grassland communities. In contrast, other characteristics that have a major influence on prairie ecology may not readily be seen. For example, although weather and hydrological patterns may be reflected in the types of vegetation and soil that are present, they require formal instrumentation and monitoring for their assessment and evaluation.
The names used for the level I and II ecological regions are generally those in standard use in the individual countries. This was done to maintain as much continuity in nomenclature as possible. However, the names of some of the transboundary regions were adapted to respect the broader geographical coverage of this study. Names were generally intended to describe the overall character of the regions but, in other cases, they reflect prominent biophysical features such as mountain ranges or forest types. Each region is identified by a unique color and numerical code on the accompanying maps.
LEVEL I Ecoregions
North America has been broken down into 15 broad, level I ecological regions. These highlight major ecological areas and provide the broad backdrop to the ecological mosaic of the continent, putting it in context at global or intercontinental scales. Viewing the ecological hierarchy at this scale provides a context for seeing global or intercontinental patterns. Level I ecological regions are: Arctic Cordillera, Tundra, Taiga, Hudson Plains, Northern Forests, Northwestern Forested Mountains, Marine West Coast Forests, Eastern Temperate Forests, Great Plains, North American Deserts, Mediterranean California, Southern Semi-Arid Highlands, Temperate Sierras, Tropical Dry Forests and Tropical Humid Forests. Brief narrative descriptions of each level I region can be found in Section III. These descriptions—each of which is divided into sections describing the physical setting, biological setting and human activities therein—provide an overview of the principal attributes of each region. The intent is to provide a sense of the ecological diversity, the human interactions taking place and how each region differs from adjacent ones.
Level I can be characterized as follows:
- number of ecological regions: 15
- scale of presentation: approximately 1:50 million
- continental perspectives
- determination of the areas composing the regions through satellite imagery and appropriate natural resource source maps at broad scales (approximately 1:40 million – 1:50 million)
LEVEL II Ecoregions
The 52 level II ecological regions that have been delineated are intended to provide a more detailed description of the large ecological areas nested within the level I regions. For example, the Tropical Humid Forests of level I is the region covering coastal portions of the United States and Mexico, and is composed of six level II regions. Level II ecological regions are useful for national and subcontinental overviews of physiography, wildlife, and land use. Three level I regions (Hudson Plains, Marine West Coast Forests and Mediterranean California) have no level II delineations. The Great Plains, Tropical Dry Forests and Tropical Humid Forests level I regions, on the other hand, each have six level II subdivisions. The table on the reverse of the level II map provides a synopsis of the major physical and biological attributes along with human activities associated with each of the level II ecological regions.
Level II can be characterized as follows:
- number of ecological regions: 52
- scale of presentation: 1:30 million
- nested within level I regions
- national/regional perspectives
- determination of the areas composing the regions through satellite imagery and appropriate natural resource source maps at broad scales (approximately 1:20 million – 1:30 million)
LEVEL III Ecoregions
Level III mapping describes smaller ecological areas nested within level II regions. These smaller divisions will enhance regional environmental monitoring, assessment and reporting, as well as decision-making. Because level III regions are smaller, they allow locally defining characteristics to be identified, and more specifically oriented management strategies to be formulated.
Level III can be characterized as follows:
- number of ecological regions: approximately 200
- scale of presentation: approximately 1:5 – 1:10 million
- nested within level II regions
- regional perspective
- determination of the areas composing the regions through remote sensing techniques and appropriate regional natural resource source maps (at scales of approximately 1:2 – 1:4 million)
- Alvarez, S., M. Ticul, and E. González. 1987. Fauna. In Atlas cultural de México. SEP-INAH. Mexico City: Ed. Planeta. ISBN: 9684060874
- Alvarez, T., and F. de la Chica. 1974. Zoogeografía de los vertebrados de México. In El escenario geográfico-recursos naturales. A. Flores-Díaz, L. Gonzalez, Q.T. Alvarez and F. de la Chica, eds., 219–335. SEP-INAH.
- Anderson, J.R. 1970. Major land uses (map). In The national atlas of the United States of America. Reston, VA: US Geological Survey.
- Bailey, R.G. 1976. Ecoregions of the United States (map). Ogden, UT: US Department of Agriculture, US Forest Service, Intermountain Region.
- Bailey, R.G. 1989. Ecoregions of the continents (map). Environmental Conservation. 16(4): 307–10.
- Bailey, R.G. 1995. Description of the ecoregions of the United States. 2d ed.(map). Miscellaneous Publication No. 1391. Washington, D.C.: US Department of Agriculture, US Forest Service.
- Baldwin, J.L. 1974. Climates of the United States. Washington, D.C.: US Department of Commerce, National Oceanic and Atmospheric Administration.
- Banfield, A.W.F. 1977. Les mammifères du Canada. Quebec: Les Presses de l’Université Laval et l’Université de Toronto. (Original English edition: A.W.F. Banfield. 1974. The mammals of Canada. Published for the National Museum of Natural Sciences, National Museums of Canada, by University of Toronto Press, Toronto.). ISBN: 0802021379
- Barbour, M.G. and W.D. Billings, eds. 1988. North American terrestrial vegetation. Cambridge: Cambridge University Press.
- Barnes, C.P. and F.J. Marschner. 1933. Natural land-use areas of the United States (map). Washington, D.C.: US Department of Agriculture.
- Bocco, G. 1995. The Tijuana River Watershed GIS: A tool for shared management (Project overview). In R. Wright, K. Ries, and A. Winckell. Identifying priorities for a GIS for the Tijuana River Watershed. San
Diego: SDSU. ISBN: 0925613142
- Campbell, J.A., and W.W. Lamar. 1989. The venomous reptiles of Latin America. Ithaca: Comstock Publishing Co. ISBN: 0801420598
- Canadian Council on Ecological Areas (CCEA). 1995. Overview of Canada’s marine ecosystem framework. Ottawa: CCEA.
- CETENAL (now INEGI). 1976. La información CETENAL en la Zonificación Agropecuaria y Forestal, con fines de un manejo mejor aprovechamiento de los recursos naturales. México, D.F.
- Commission of the European Communities. 1993. Multilingual illustrated dictionary of aquatic animals and plants. Oxford: Blackwell’s. ISBN: 9282660249
- Conant, R., and J.T. Collins. 1991. Reptiles and amphibians; eastern and central North America. 2d ed. Peterson Field Guides. Boston: Houghton Mifflin Co. ISBN: 0395583896
- Corbet, G.B. and J.E. Hill. 1991. A world list of mammalian species. New York: Oxford University Press. ISBN: 0198540175
- Cuanalo de la Cerda, H., E. Ojeda, A. Santos, and C. Ortiz Solorio. 1989. Provincias, regiones y subregiones terrestres de México. Montecillo, México: Colegio de Postgraduados. Centro de Edafológia.
- Dunbier, R. 1968. The Sonoran Desert. Its geography, economy and people. Tucson: The University of Arizona Press.
- Ecological Stratification Working Group. 1996. A national ecological framework for Canada. Ottawa: Environment Canada, State of Environment Directorate, and Agriculture and Agri-Food Canada, Research Branch, Centre for Land and Biological Resources Research.
- Environment Canada. 1993. Canadian climate normals. 1960–1991. 6 vols. Ottawa: Atmospheric Environment Service. ISBN: 0660518996, ISBN: 0660526522, ISBN: 0660518104, ISBN: 0660518198, ISBN: 0660526530, ISBN: 0660518112
- Environment Canada. 1978–1986. Northern land use information map series, Districts of Keewatin, Mackenzie and Franklin, N.W.T. Ottawa.
- Environment Canada. 1986. Climate atlas climatique Canada: Map series 2 — Precipitation. Ottawa: Atmospheric Environment Service.
- Escalante, P., A.M. Sada, and J.R. Gil. 1996. Listado de nombres comunes de las aves de México. México: Comisión Nacional para el Conocimiento y Uso de la Biodiversidad.
- Ezcurra E., P. Rump, and R.N. Phillip. 1993. North American workshop on environmental information, 19–22 October 1993, Mexico City.
- Fenneman, N.M. 1938. Physiography of the eastern United States. New York: McGraw-Hill.
- Fenneman, N.M. 1946. Physical divisions of the United States (map). Reston, VA: US Geological Survey.
- Fernald, M.L. 1950. Gray’s manual of botany. New York: American Book Co. Ferrusquia-Villafranca, I. 1993.
- Geology of Mexico: a Synopsis. In Biological diversity of Mexico: Origins and distribution. T.P. Ramamoorthy, R. Bye, A. Lot and J. Fa, eds. Oxford: Oxford University Press. ISBN: 019506674X
- Flores, M.G., J. Jimenez, X. Madrigal S., F. Moncayo R., and F. Takaki. 1971. Memoria y mapa de los tipos de vegetación de México. México: SARH.
- Flores-Villela, O., and P. Gerez. 1994. Biodiversidad y conservación en México: vertebrados, vegetación y uso del suelo. México: CONABIO-UNAM.
- Flores-Villela, O. 1991. Analisis de la distribución de la herpetofauna de México. Tesis doctoral, Facultad de Ciencias. Mexico City: UNAM.
- Gallant, A.L., T.R. Whittier, D.P. Larsen, J.M. Omernik, and R.M. Hughes. 1989. Regionalization as a tool for managing environmental resources. EPA/600/3-89/060. Corvallis: US Environmental Protection Agency.
- García, E. 1991. Modificaciones al sistema de clasificación climática de Koppen. México.
- Gotch, A.F. 1995. Latin names explained: A guide to the scientific classification of reptiles, birds, and mammals. London: Blandford Press. ISBN: 0816033773
- Government of Canada. 1996. Conserving Canada’s natural legacy (the state of Canada’s environment). CD-ROM version.
- Griffith, G.E., J.M. Omernik, C.M. Rohm, and S.M. Pierson. 1994. Florida regionalization project. EPA/600/Q-95-002. Corvallis: US Environmental Protection Agency, Environmental Research Laboratory.
- Griffith, G.E., J.M. Omernik, T.F. Wilton, and S.M. Pierson. 1994. Ecoregions and subregions of Iowa: A Framework for water quality assessment and management. Journal of the Iowa Academy of Science 101(1): 5–13.
- Hall, E.R. 1991. The mammals of North America, 2d edition. 2 vols. New York: John Wiley and Sons. ISBN: 0826037550
- Hammond, E.H. 1970. Classes of land-surface form (map). In The national atlas of the United States of America. Washington, D.C.: US Geological Survey, 62–63.
- Hills, G.A. 1961. The ecological basis for land-use planning. Research Report No. 46. Toronto: Ontario Department of Lands and Forests.
- Hirvonen, H. 1984. The Atlantic region of Canada: An ecological perspective. Dartmouth, NS: Environment Canada. Lands and Integrated Programs Directorate.
- Hirvonen, H. 1992. The development of regional-scale ecological indicators: A Canadian approach. In Ecological Indicators. 2 vols. Daniel McKenzie, D. Eric Hyatt and V. Janet McDonald, eds., 901–15. London: Elsevier Applied Science.
- Hirvonen, H., L. Harding, and J. Landucci. 1994. A national marine ecological framework for ecosystem monitoring and SOE reporting. In Proceedings of the Second International Conference on Science and Management of Protected Areas, 16–21 May 1994, Halifax, N.S.
- Hirvonen, H., and J. J. Lowe. 1995. Integration of Canada’s forest inventory with the National Ecological Framework for State of Sustainability Reporting. Paper presented at IUFRO XX World Congress, 6-12 August
1995, Tampere, Finland.
- Hornig, C.E., C.W. Bayer, S.R. Twidwell, J.R. Davis, R.J. Kleinsasser, G.W. Linam, and K.B. Mayes. 1995. Development of regionally based biological criteria in Texas. In Biological assessment and criteria: tools for water resource planning and decision-making. W. Davis and T.P. Simon, eds., 145–52. Boca Raton, FL: Lewis Publishers. ISBN: 0873718941
- Howell, S.N.G., and S. Webb. 1995. A Guide to the birds of Mexico and northern central America. New York: Oxford University Press. ISBN: 0198540124
- Hughes, R.M. 1995. Defining biological status by comparing with reference conditions. In Biological assessment and criteria: tools for water resource planning and decision-making. W.S. Davis and T.P. Simon, eds., 31–47. Boca Raton, FL: Lewis Publishers. ISBN: 0873718941
- Hughes, R.M., S.A. Heiskary, W.J. Matthews, and C.O. Yoder. 1994. Use of ecoregions in biological monitoring. In Biological monitoring of aquatic systems. S.L. Loeb and A. Spacie, eds., 125–51. Boca Raton, FL: Lewis Publishers. ISBN: 0873719107
- Hughes, R.M., D.P. Larsen, and J.M. Omernik. 1986. Regional reference sites: A method for assessing stream potentials. Environmental Management 10(5): 629–35.
- Hunt, C.B. 1979. Surficial geology (map). Reston, VA: US Geological Survey. INEGI. 1970. Serie de cartas temáticas (geología, suelo, clima, uso del suelo y vegetación, fisiografía) escalas 1:250,000; 1:1,000,000; 1:4,000,000.
- INEGI. 1970. Serie de cartas topográficas, escalas 1:250,000; 1:1,000,000 y 1:4,000,000.
- INEGI. 1988. Carta climática. Atlas nacional del medio físico. México. ISBN: 9688922781
- INEGI. 1988. Carta edafológica. Atlas nacional del medio físico. México. ISBN: 9688922781
- INEGI. 1988. Carta geológica. Atlas nacional del medio físico. México. ISBN: 9688922781
- INEGI. 1989. Carta topográfica. Atlas nacional del medio físico. México. ISBN: 9688922781
- INEGI. 1991. Datos básicos de la geografía de México. ISBN: 9688920045
- INEGI. 1991. XI Censo general de población y vivienda. Ags. Méx. ISBN: 968892489X
- INEGI. 1995. Serie espaciomapas 1:250 000; 1:1,000,000; 1:4,000,000.
- INEGI. 1995. Estadísticas del medio ambiente. México.
- Jaeger, E.C. 1957. The North American deserts. Palo Alto: Stanford University Press.
- Jaeger, E.C. 1965. The Californian deserts. 4th ed. Palo Alto: Stanford University Press.
- King, P.B., and H.M. Biekman. 1974. Geologic map of the United States. Reston, VA: US Geological Survey.
- Krajina, V. 1969/70. Ecology of western North America. 2 (no. 1 and 2). Vancouver: University of British Columbia, Department of Botany.
- Krane, W. 1986. Five-language dictionary of fish, crustaceans, and molluscs. Hamburg: Behr Verlag. ISBN: 3925673016
- Kuchler, A.W. 1964. Potential natural vegetation of the conterminous United States. Special Publication No. 36, New York: American Geographical Society.
- Kuchler, A.W. 1970. Potential natural vegetation (map). In The National atlas of the United States of America. Washington, D.C.: US Geological Survey, 89–91.
- Leopold, A.S. 1959. Wildlife of Mexico: The game birds and mammals. Berkeley: University of California Press. ISBN: 0520007247
- Leopold, A.S. 1977. Fauna silvestre de México: Aves y mamíferos de caza. México: Editorial PAX.
- Leopold, A.D., R.J. Gutiérrez, and M.T. Bronson. 1982. North American game birds and mammals. New York: Charles Scribner’s Sons. ISBN: 0684179237
- Lopoukhine, N., N..A. Prout, and H.E. Hirvonen. 1979. The ecological land classification of Labrador: A reconnaissance. Halifax: Lands Directorate, Fisheries and Environment Canada.
- Loveland, T.R., J.W. Merchant, D.O. Ohlen, J.F. Brown, B.C. Reed, P. Olson, and J. Hutchinson. 1995. Seasonal land-cover regions of the United States. Annals of the Association of American Geographers 85(2): 339–55.
- Macura, P. 1979. Elsevier’s dictionary of botany. Amsterdam: Elsevier Scientific Publishing Co.
- Matthews, L.H. 1971. Les mammifères. Paris/Montreal: Bordas.
- McGinnies, W.G., B.J. Goldman, and P. Paylores, eds. 1968. Deserts of the world. Tucson: The University of Arizona Press.
- Omernik, J.M. 1987. Ecoregions of the conterminous United States. Annals of the Association of American Geographers 77(1): 118–25.
- Omernik, J.M. 1995. Ecoregions: A spatial framework for environmental management. In Biological assessment and criteria: tools for water resource planning and decision-making. W. Davis and T. Simon, eds. Boca Raton, FL: Lewis Publishers, 49–62. ISBN: 0873719107
- Omernik, J.M., and A.L. Gallant. 1990. Defining regions for evaluating environmental resources. In Global natural resource monitoring and assessments. Proceedings of the International Conference and Workshop. Venice, Italy, 936–47. ISBN: 0944426298
- Omernik, J.M., and G.E. Griffith. 1991. Ecological regions versus hydrological units: frameworks for managing water quality. Journal of Soil and Water Conservation 46(5): 334–40.
- Oswald, E.T., and J.P. Senyk. 1977. Ecoregions of Yukon Territory. Publication No. BC-X-164. Victoria, BC: Environment Canada, Canadian Forestry Service.
- Ouellet, H., M. Gosselin, and J.P. Artigau. 1990. French nomenclature of North American birds. Ottawa: Minister of Supply and Services Canada. ISBN: 0660558246
- Preston, Richard J. 1961. North American trees. Ames: The Iowa State University Press.
- Rowe, J.S. 1972. Forest regions of Canada. Publ. No. 1300. Ottawa: Department of Environment, Canadian Forest Service.
- Robbins, C.S., B. Brunn, and H.S. Zim. 1983. A guide to field identification of birds of North America. New York: Golden Press.
- Rzedowski, J. 1978. La vegetación de México. México: Editorial Limusa.
- Rzedowski, J. 1993. Diversity and origins of the phanerogamic flora of Mexico. In T.P. Ramamoorthy, R. Bye, A. Lot and J. Fa, eds: Biological diversity of Mexico: Origins and distribution. Oxford: Oxford University Press. ISBN: 019506674X
- Saint-Laurent, Agnès, ed. 1986. Faune et flore de l’Amérique du Nord. Montréal: Reader’s Digest Ltd.
- Seely, M. 1993. Desiertos. Barcelona (Spain): Plaza y Janes Editores, S.A. Shreve, G., and I.L. Wiggins. 1964. Vegetation and flora of the Sonoran Desert. Palo Alto: Stanford University Press. ISBN: 0804701636
- Stebbins, R.C. 1985. Western reptiles and amphibians. Peterson Field Guides, Boston: Houghton Mifflin Co. ISBN: 039538253X
- Strong, W.L., and K.R. Leggart. 1980. Ecoregions of Alberta. ENR Report Number 143. Edmonton: Alberta Department of Energy and Natural Resources.
- Tamayo, J.L. 1981. Geografía moderna de México. México: Editorial Trillas.
- Toledo, V.M., and M.J. Ordomez. 1993. The biodiversity scenario of Mexico: a review of terrestrial habitats. In T.P. Ramamoorthy, R. Bye, A. Lot and J. Fa, eds. Biological diversity of Mexico: Origins and distribution. Oxford University Press. ISBN: 019506674X
- Tornes, L.H., and M.E. Brigham. 1994. Nutrients, suspended sediment and pesticides in waters of the Red River of the North basin, Minnesota, North Dakota and South Dakota, 1970-1990. Water-Resources Investigations Report 93-4231. Mounds View, MN: US Geological Survey.
- Universidad Nacional Autónoma de México. 1990. Atlas nacional de México. México: UNAM, Instituto de Geografía. ISBN: 9683615864
- US Department of Agriculture, Forest Service. 1970. Major forest types (map). In The national atlas of the United States of America. Reston, VA: US Geological Survey, 154–55.
- US Department of Agriculture, Agriculture Experiment Station of the North Central Region. 1957. Soils of the north central region of the United States (map). Publication No. 76, Bulletin 544. Madison, WI: USDA.
- US Department of Agriculture, Agriculture Experiment Stations of the Southern States and Puerto Rico Land-Grant Universities, and USDA Soil Conservation Service. 1973. Soils of the southern states and Puerto Rico (map). In Soils of the southern states and Puerto Rico. Southern Cooperative Series Bulletin No. 174. Forth Worth, TX: USDA.
- US Department of Agriculture, Agriculture Experiment Stations of the Western States Land-Grant Universities and Colleges and US Department of Agriculture, Soil Conservation Service. 1964. Soils of the western United States (exclusive of Hawaii and Alaska) (map). Pullman, WA: USDA.
Disclaimer: This article is taken wholly from, or contains information that was originally published by, the Commission for Environmental Cooperation. Topic editors and authors for the Encyclopedia of Earth may have edited its content or added new information. The use of information from the Commission for Environmental Cooperation should not be construed as support for or endorsement by that organization for any new information added by EoE personnel, or for any editing of the original content.