these regions indicate rocky soil conditions,
forming reservoirs of water for taproots. They
occur there because of the ground water condition; not because of the climate. In these cases, a
map of climate-based ecoregions ignores such
areas and relegates edaphically controlled
ecosystems to a lower level of classification and
more detailed maps. The emphasis in this kind of
mapping is on understanding pattern in term of
process.
Regarding land use, the ecoregion
delineations described in this book are based
solely on biophysical factors (i.e., climate, landform, and vegetation). This is an important difference from the systems described above in
which land use is one of the delineators. One of
the major advantages of the biophysical approach
(as opposed to directly mapping land cover) is its
ability to predict the potential character of sites
where natural ecosystems have been profoundly
modified (e.g., by land clearance or fire) or
replaced by exotic species regarded as pests and
weeds. Because land use has not been considered
in drawing up regional boundaries, exact correspondence may not occur between these
biophysical-based ecoregions and existing vegetation patterns.
The system in this book also stands in contrast
to other systems that use the distribution limits of
species and races of plants and animals as criteria
for determining the boundaries of ecoregions.
Sometimes, the distribution limits for several
species might coincide with an ecoregion boundary if that boundary follows some barrier that
prevents range expansion, such as where plains
meet mountains. Often, however, the range of a
species does not stop abruptly at the border of an
ecoregion but continues for a distance into the
adjacent ecoregion. The reason for this seems to
be that some isolated areas of suitable habitat
usually occur in the adjacent region. Furthermore, because, at small map scales, physiognomy (life-form) is the best expression of
ecological conditions (Ku ¨chler 1973; Gosz and
Sharpe 1989), floristic and faunistic differences
are best left to maps with other purposes.
Because physiognomy is basic and applicable
without exception anywhere on the Earth, it was
selected to serve as the source for the criteria
necessary to establish the basis for regional
differences. These criteria permit a uniform
approach throughout the world and put the various parts of the world on a comparable basis.
In this book, no attempt is made to develop
separate systems to define freshwater and terrestrial ecosystem units. In some other systems,
there is a false dichotomy being drawn between
aquatic and terrestrial systems. Ecosystems are
not aquatic or terrestrial; they are geographic
system that include both aquatic and terrestrial
components that are integrated, or combined, in a
particular geographic area. The streams and
rivers that drain an ecological unit have common
biological and physical characteristics that are
controlled by the nature of the ecological unit
they are embedded in (Omernik and Bailey
1997; Bailey 2009).
Finally, the boundaries of ecoregions on some
maps are very irregular; thus implying accuracy
and precision. In contrast, the view of ecoregions
promulgated in this book looks on the boundaries
between ecoregions as representing climatic
gradients that reflect gradual change and tend to
be indistinct. Such boundaries cannot be located
precisely. The complex boundary lines on
ecoregion maps give a false sense of accuracy
and precision. More details are presented in
Chap. 11 and Bailey (2004).
References
Bailey RG (1976) Map: ecoregions of the United States.
USDA Forest Service, Intermountain Region, Ogden,
UT, Scale 1:7,500,000
Bailey RG (1988) Problems with using overlay mapping
for planning and their implications for geographic
information systems. Environ Manage 12:11–17
Bailey RG (1989) Explanatory supplement to ecoregions
map of the continents. Environ Conserv 16:307–309,
with separate map at 1:30,000,000
Bailey RG (1994) Map: ecoregions of the United States
(rev.). USDA Forest Service, Washington, DC, Scale
1:7,500,000
References
43
forming reservoirs of water for taproots. They
occur there because of the ground water condition; not because of the climate. In these cases, a
map of climate-based ecoregions ignores such
areas and relegates edaphically controlled
ecosystems to a lower level of classification and
more detailed maps. The emphasis in this kind of
mapping is on understanding pattern in term of
process.
Regarding land use, the ecoregion
delineations described in this book are based
solely on biophysical factors (i.e., climate, landform, and vegetation). This is an important difference from the systems described above in
which land use is one of the delineators. One of
the major advantages of the biophysical approach
(as opposed to directly mapping land cover) is its
ability to predict the potential character of sites
where natural ecosystems have been profoundly
modified (e.g., by land clearance or fire) or
replaced by exotic species regarded as pests and
weeds. Because land use has not been considered
in drawing up regional boundaries, exact correspondence may not occur between these
biophysical-based ecoregions and existing vegetation patterns.
The system in this book also stands in contrast
to other systems that use the distribution limits of
species and races of plants and animals as criteria
for determining the boundaries of ecoregions.
Sometimes, the distribution limits for several
species might coincide with an ecoregion boundary if that boundary follows some barrier that
prevents range expansion, such as where plains
meet mountains. Often, however, the range of a
species does not stop abruptly at the border of an
ecoregion but continues for a distance into the
adjacent ecoregion. The reason for this seems to
be that some isolated areas of suitable habitat
usually occur in the adjacent region. Furthermore, because, at small map scales, physiognomy (life-form) is the best expression of
ecological conditions (Ku ¨chler 1973; Gosz and
Sharpe 1989), floristic and faunistic differences
are best left to maps with other purposes.
Because physiognomy is basic and applicable
without exception anywhere on the Earth, it was
selected to serve as the source for the criteria
necessary to establish the basis for regional
differences. These criteria permit a uniform
approach throughout the world and put the various parts of the world on a comparable basis.
In this book, no attempt is made to develop
separate systems to define freshwater and terrestrial ecosystem units. In some other systems,
there is a false dichotomy being drawn between
aquatic and terrestrial systems. Ecosystems are
not aquatic or terrestrial; they are geographic
system that include both aquatic and terrestrial
components that are integrated, or combined, in a
particular geographic area. The streams and
rivers that drain an ecological unit have common
biological and physical characteristics that are
controlled by the nature of the ecological unit
they are embedded in (Omernik and Bailey
1997; Bailey 2009).
Finally, the boundaries of ecoregions on some
maps are very irregular; thus implying accuracy
and precision. In contrast, the view of ecoregions
promulgated in this book looks on the boundaries
between ecoregions as representing climatic
gradients that reflect gradual change and tend to
be indistinct. Such boundaries cannot be located
precisely. The complex boundary lines on
ecoregion maps give a false sense of accuracy
and precision. More details are presented in
Chap. 11 and Bailey (2004).
References
Bailey RG (1976) Map: ecoregions of the United States.
USDA Forest Service, Intermountain Region, Ogden,
UT, Scale 1:7,500,000
Bailey RG (1988) Problems with using overlay mapping
for planning and their implications for geographic
information systems. Environ Manage 12:11–17
Bailey RG (1989) Explanatory supplement to ecoregions
map of the continents. Environ Conserv 16:307–309,
with separate map at 1:30,000,000
Bailey RG (1994) Map: ecoregions of the United States
(rev.). USDA Forest Service, Washington, DC, Scale
1:7,500,000
References
43