A map of the Amazon rainforest ecoregions. The yellow line encloses the ecoregions per the World Wide Fund for Nature.
A map of North America's bioregions
An ecoregion (ecological region) is an ecologically and geographically defined area that is smaller than a bioregion, which in turn is smaller than an ecozone. All three of these are either less or greater than an ecosystem.
Ecoregions cover relatively large areas of land or water, and contain
characteristic, geographically distinct assemblages of natural communities and species. The biodiversity of flora, fauna and ecosystems
that characterise an ecoregion tends to be distinct from that of other
ecoregions. In theory, biodiversity or conservation ecoregions are
relatively large areas of land or water where the probability of
encountering different species and communities at any given point
remains relatively constant, within an acceptable range of variation
(largely undefined at this point).
Three caveats are appropriate for all bio-geographic mapping
approaches. Firstly, no single bio-geographic framework is optimal for
all taxa. Ecoregions reflect the best compromise for as many taxa as
possible. Secondly, ecoregion boundaries rarely form abrupt edges;
rather, ecotones and mosaic habitats bound them. Thirdly, most ecoregions contain habitats that differ from their assigned biome.
Biogeographic provinces may originate due to various barriers. Some
physical (plate tectonics, topographic highs), some climatic
(latitudinal variation, seasonal range) and some ocean chemical related
(salinity, oxygen levels).
History
The history of the term is somewhat vague, and it had been used in many contexts: forest classifications (Loucks, 1962), biome classifications (Bailey, 1976, 2014), biogeographic classifications (WWF/Global 200 scheme of Olson & Dinerstein, 1998), etc.
The concept of ecoregion of Bailey gives more importance to
ecological criteria, while the WWF concept gives more importance to
biogeography, that is, distribution of distinct biotas.
Definition and categorization
The Ötztal Alps, a mountain range in the central Alps of Europe, are part of the Central Eastern Alps, and can both be termed as ecoregions.
A conifer forest in the Swiss Alps (National Park).
An ecoregion is a "recurring pattern of ecosystems associated with characteristic combinations of soil and landform that characterise that region".
Omernik (2004) elaborates on this by defining ecoregions as: "areas
within which there is spatial coincidence in characteristics of geographical phenomena associated with differences in the quality, health, and integrity of ecosystems". "Characteristics of geographical phenomena" may include geology, physiography, vegetation, climate, hydrology, terrestrial and aquatic fauna,
and soils, and may or may not include the impacts of human activity
(e.g. land use patterns, vegetation changes). There is significant, but
not absolute, spatial correlation among these characteristics, making
the delineation of ecoregions an imperfect science. Another complication
is that environmental conditions across an ecoregion boundary may
change very gradually, e.g. the prairie-forest transition in the
midwestern United States, making it difficult to identify an exact
dividing boundary. Such transition zones are called ecotones.
Ecoregions can be categorized using an algorithmic
approach or a holistic, "weight-of-evidence" approach where the
importance of various factors may vary. An example of the algorithmic
approach is Robert Bailey's
work for the U.S. Forest Service, which uses a hierarchical
classification that first divides land areas into very large regions
based on climatic factors, and subdivides these regions, based first on
dominant potential vegetation, and then by geomorphology and soil characteristics. The weight-of-evidence approach is exemplified by James Omernik's work for the United States Environmental Protection Agency, subsequently adopted (with modification) for North America by the Commission for Environmental Cooperation.
The intended purpose of ecoregion delineation may affect the method used. For example, the WWF
ecoregions were developed to aid in biodiversity conservation planning,
and place a greater emphasis than the Omernik or Bailey systems on
floral and faunal differences between regions. The WWF classification
defines an ecoregion as:
A large area of land or water that contains a geographically distinct assemblage of natural communities that:
- (a) Share a large majority of their species and ecological dynamics;
- (b) Share similar environmental conditions, and;
- (c) Interact ecologically in ways that are critical for their long-term persistence.
According to WWF, the boundaries of an ecoregion approximate the
original extent of the natural communities prior to any major recent
disruptions or changes. WWF has identified 867 terrestrial ecoregions,
and approximately 450 freshwater ecoregions across the Earth.
Terrestrial Ecoregions of the World (Olson et al. 2001, BioScience)
Importance
The
use of the term ecoregion is an outgrowth of a surge of interest in
ecosystems and their functioning. In particular, there is awareness of
issues relating to spatial scale in the study and management of landscapes.
It is widely recognized that interlinked ecosystems combine to form a
whole that is "greater than the sum of its parts". There are many
attempts to respond to ecosystems in an integrated way to achieve
"multi-functional" landscapes, and various interest groups from agricultural researchers to conservationists are using the "ecoregion" as a unit of analysis.
The "Global 200" is the list of ecoregions identified by WWF as priorities for conservation.
Ecologically based movements like bioregionalism
maintain that ecoregions, rather than arbitrarily defined political
boundaries, provide a better foundation for the formation and governance
of human communities, and have proposed ecoregions and watersheds as the basis for bioregional democracy initiatives.
Terrestrial
WWF terrestrial ecoregions
Terrestrial ecoregions are land ecoregions, as distinct from freshwater and marine ecoregions. In this context, terrestrial is used to mean "of land" (soil and rock), rather than the more general sense "of Earth" (which includes land and oceans).
WWF (World Wildlife Fund) ecologists currently divide the land surface of the Earth into 8 major ecozones containing 867 smaller terrestrial ecoregions.
The WWF effort is a synthesis of many previous efforts to define and
classify ecoregions. Many consider this classification to be quite
decisive, and some propose these as stable borders for bioregional democracy initiatives.
The eight terrestrial ecozones follow the major floral and faunal
boundaries, identified by botanists and zoologists, that separate the
world's major plant and animal communities. Ecozone boundaries generally
follow continental boundaries, or major barriers to plant and animal distribution, like the Himalayas and the Sahara. The boundaries of ecoregions are often not as decisive or well recognized, and are subject to greater disagreement.
Ecoregions are classified by biome
type, which are the major global plant communities determined by
rainfall and climate. Forests, grasslands (including savanna and
shrubland), and deserts (including xeric shrublands) are distinguished by climate (tropical and subtropical vs. temperate and boreal climates) and, for forests, by whether the trees are predominantly conifers (gymnosperms), or whether they are predominantly broadleaf (Angiosperms) and mixed (broadleaf and conifer). Biome types like Mediterranean forests, woodlands, and scrub; tundra; and mangroves host very distinct ecological communities, and are recognized as distinct biome types as well.
Marine
Marine ecoregions
are: "Areas of relatively homogeneous species composition, clearly
distinct from adjacent systems….In ecological terms, these are strongly
cohesive units, sufficiently large to encompass ecological or life
history processes for most sedentary species." They have been defined by The Nature Conservancy (TNC) and World Wildlife Fund (WWF) to aid in conservation activities for marine ecosystems. Forty-three priority marine ecoregions were delineated as part of WWF's Global 200 efforts.
The scheme used to designate and classify marine ecoregions is
analogous to that used for terrestrial ecoregions. Major habitat types
are identified: polar, temperate shelves and seas, temperate upwelling,
tropical upwelling, tropical coral, pelagic (trades and westerlies),
abyssal, and hadal (ocean trench). These correspond to the terrestrial biomes.
The Global 200 classification of marine ecoregions is not
developed to the same level of detail and comprehensiveness as that of
the terrestrial ecoregions; only the priority conservation areas are
listed.
See Global 200 Marine ecoregions for a full list of marine ecoregions.
In 2007, TNC and WWF refined and expanded this scheme to provide a
system of comprehensive near shore (to 200 meters depth) Marine
Ecoregions of the World (MEOW). The 232 individual marine ecoregions are grouped into 62 marine provinces, which in turn group into 12 marine realms,
which represent the broad latitudinal divisions of polar, temperate,
and tropical seas, with subdivisions based on ocean basins (except for
the southern hemisphere temperate oceans, which are based on
continents).
Major biogeographic realms, analogous to the eight terrestrial ecozones,
represent large regions of the ocean basins: Arctic, Temperate Northern
Atlantic, Temperate Northern Pacific, Tropical Atlantic, Western
Indo-Pacific, Central Indo-Pacific, Eastern Indo-Pacific, Tropical
Eastern Pacific, Temperate South America, Temperate Southern Africa,
Temperate Australasia, Southern Ocean.
A similar system of identifying areas of the oceans for
conservation purposes is the system of large marine ecosystems (LMEs),
developed by the US National Oceanic and Atmospheric Administration (NOAA).
Freshwater
The Amazon River in Brazil.
A freshwater ecoregion is a large area encompassing one or more
freshwater systems that contains 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. Freshwater systems include rivers, streams, lakes, and wetlands.
Freshwater ecoregions are distinct from terrestrial ecoregions, which
identify biotic communities of the land, and marine ecoregions, which
are biotic communities of the oceans.
A map of Freshwater Ecoregions of the World, released in 2008,
has 426 ecoregions covering virtually the entire non-marine surface of
the earth.
World Wildlife Fund
(WWF) identifies twelve major habitat types of freshwater ecoregions:
Large lakes, large river deltas, polar freshwaters, montane freshwaters,
temperate coastal rivers, temperate floodplain rivers and wetlands,
temperate upland rivers, tropical and subtropical coastal rivers,
tropical and subtropical floodplain rivers and wetlands, tropical and
subtropical upland rivers, xeric freshwaters and endorheic basins, and
oceanic islands. The freshwater major habitat types reflect groupings of
ecoregions with similar biological, chemical, and physical
characteristics and are roughly equivalent to biomes for terrestrial
systems.
The Global 200,
a set of ecoregions identified by WWF whose conservation would achieve
the goal of saving a broad diversity of the Earth's ecosystems, includes
a number of areas highlighted for their freshwater biodiversity
values. The Global 200 preceded Freshwater Ecoregions of the World and
incorporated information from regional freshwater ecoregional
assessments that had been completed at that time.
