Description of the Ecoregions of North America (CEC)

August 19, 2011, 8:19 pm

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:

  1. opinions were sought from ecologists and other scientists on the relevant features for each region; and
  2. 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)

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Cooperation, C. (2011). Description of the Ecoregions of North America (CEC). Retrieved from http://www.eoearth.org/view/article/151701

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