24.2 Basic Concepts
chical framework (Allen and Starr, 1982; Allen et
al., 1984; O'Neill et al., 1986). Recognizing this,
considerable recent attention has been directed at
describing the organization of ecological systems
for improved land management planning (Cleland
et al., 1997; Maxwell et aI., 1995). Hierarchy theory (Allen and Starr, 1982; O'Neill et aI., 1986)
suggests that multiscaled systems can be viewed as
a system of constraints in which a higher level of
organization provides to some extent the environment in which lower levels develop. A critical characteristic of hierarchical systems is the "whole/
part" duality of their components, whereby every
level is a discrete entity and at the same time part
of a larger whole (Koestler, 1967; Allen and Starr,
1982; Allen et al., 1984).
Hierarchy theory provides a needed framework
for ecological classification and mapping because
it facilitates scaled definitions of ecosystem components and identification of the linkages that exist between different scales of ecological organizations (Bourgeron and Jensen, 1994; Jensen et aI.,
1996). Hierarchical approaches to ecosystem characterization also simplify the description and prediction of complex ecological pattern/process relations at all relevant scales of system organization
(Forman and Godron, 1986; Urban et al., 1987;
Levin, 1992). For example, the types of channel
units found in a given environment are constrained
by the stream types in which they are nested, which
in tum are determined by the valley bottom types
and geoclimatic watershed types in which they are
nested (Maxwell et al., 1995). In a similar manner,
the physical/environmental processes that create
these aquatic patterns (e.g., channel scour, major
floods, glaciation) are also constrained hierarchically by scaled geoclimatic settings (e.g., lithologic
groups, landforms, climate zones) (Maxwell et aI.,
1995; Jensen et aI., 1997). Recognition of these relations greatly simplifies the prediction of finerscale patterns, such as channel units and stream
types, in classification and mapping of aquatic systems (Maxwell et al., 1995).
24.2.2 Hierarchical Framework for
Description of Aquatic Systems
Aquatic systems are commonly identified as riverine (streams and rivers), lacustrine (lakes and reservoirs), and groundwater (aquifer) systems (Cowardin et al., 1979). These systems are commonly
mapped directly (at appropriate scales) in most ecological assessment efforts (Maxwell et aI., 1995).
Understanding of these systems, however, is facil353
itated through description of the terrestrial biophysical environment, including zoogeographic
and geoclimatic properties, of the drainage basins
within which they are nested. The ECOMAP Working Group of the USDA, Forest Service, has developed a hierarchical framework (Maxwell et al.,
1995) useful for describing these aquatic ecosystems. A generalized summary of the ECOMAP approach follows.
Maps of the biophysical environment are used to
identify landscapes or aquatic habitats that behave
in a similar manner, given their potential ecosystem composition, structure, and function (Bailey et
aI., 1994). Such maps commonly delineate areas
with similar response potential and resource production capabilities and are constructed based on
landscape components that display low temporal
variability at a given scale of mapping (e.g., regional climate, geology, and landform). In designing biophysical environment maps, differentiating
criteria are selected to include those that exert primary control on the ecosystem patterns and
processes of interest in an assessment area (e.g.,
vegetation, flooding, and fire).
Biophysical environment maps are commonly
used to describe how the landscape could look or
function under historical, current, or potential future ecosystem process regimes (e.g., fire and successional pathway relations), as well as different
management scenarios. They provide a semipermanent map theme that can be used to extrapolate
ecosystem pattern/process relations from sampled
areas to unsampled locales and are useful to stratified sampling design strategies for environmental
monitoring purposes (Bailey et al., 1994). Ecological units (McNab and Avers, 1994; Cleland et al.,
1997), land units (Zonneveld, 1989), ecoregions
(Omernik, 1987), biogeoclimatic ecosystems (Meidinger and Pojar, 1991), and land systems (Christian and Stewart, 1968) are examples of mapping
systems that delineate ecologically homogeneous
biophysical environments at different spatial scales
based primarily on climatic, geomorphic, and biotic criteria. Hierarchical watershed/stream network maps are additional examples of biophysical
environment maps that are increasingly being used
in aquatic ecosystem assessment efforts (Maxwell
et al., 1995).
The biophysical environments that delineate
aquatic ecosystems include hydrologic units (drainage basins), geoclimatic settings (terrestrial ecological units), groundwater systems, riverine systems, and lake systems. All these environments are
hierarchically organized and are useful for the identification of aquatic ecological units (Figure 24.1).
chical framework (Allen and Starr, 1982; Allen et
al., 1984; O'Neill et al., 1986). Recognizing this,
considerable recent attention has been directed at
describing the organization of ecological systems
for improved land management planning (Cleland
et al., 1997; Maxwell et aI., 1995). Hierarchy theory (Allen and Starr, 1982; O'Neill et aI., 1986)
suggests that multiscaled systems can be viewed as
a system of constraints in which a higher level of
organization provides to some extent the environment in which lower levels develop. A critical characteristic of hierarchical systems is the "whole/
part" duality of their components, whereby every
level is a discrete entity and at the same time part
of a larger whole (Koestler, 1967; Allen and Starr,
1982; Allen et al., 1984).
Hierarchy theory provides a needed framework
for ecological classification and mapping because
it facilitates scaled definitions of ecosystem components and identification of the linkages that exist between different scales of ecological organizations (Bourgeron and Jensen, 1994; Jensen et aI.,
1996). Hierarchical approaches to ecosystem characterization also simplify the description and prediction of complex ecological pattern/process relations at all relevant scales of system organization
(Forman and Godron, 1986; Urban et al., 1987;
Levin, 1992). For example, the types of channel
units found in a given environment are constrained
by the stream types in which they are nested, which
in tum are determined by the valley bottom types
and geoclimatic watershed types in which they are
nested (Maxwell et al., 1995). In a similar manner,
the physical/environmental processes that create
these aquatic patterns (e.g., channel scour, major
floods, glaciation) are also constrained hierarchically by scaled geoclimatic settings (e.g., lithologic
groups, landforms, climate zones) (Maxwell et aI.,
1995; Jensen et aI., 1997). Recognition of these relations greatly simplifies the prediction of finerscale patterns, such as channel units and stream
types, in classification and mapping of aquatic systems (Maxwell et al., 1995).
24.2.2 Hierarchical Framework for
Description of Aquatic Systems
Aquatic systems are commonly identified as riverine (streams and rivers), lacustrine (lakes and reservoirs), and groundwater (aquifer) systems (Cowardin et al., 1979). These systems are commonly
mapped directly (at appropriate scales) in most ecological assessment efforts (Maxwell et aI., 1995).
Understanding of these systems, however, is facil353
itated through description of the terrestrial biophysical environment, including zoogeographic
and geoclimatic properties, of the drainage basins
within which they are nested. The ECOMAP Working Group of the USDA, Forest Service, has developed a hierarchical framework (Maxwell et al.,
1995) useful for describing these aquatic ecosystems. A generalized summary of the ECOMAP approach follows.
Maps of the biophysical environment are used to
identify landscapes or aquatic habitats that behave
in a similar manner, given their potential ecosystem composition, structure, and function (Bailey et
aI., 1994). Such maps commonly delineate areas
with similar response potential and resource production capabilities and are constructed based on
landscape components that display low temporal
variability at a given scale of mapping (e.g., regional climate, geology, and landform). In designing biophysical environment maps, differentiating
criteria are selected to include those that exert primary control on the ecosystem patterns and
processes of interest in an assessment area (e.g.,
vegetation, flooding, and fire).
Biophysical environment maps are commonly
used to describe how the landscape could look or
function under historical, current, or potential future ecosystem process regimes (e.g., fire and successional pathway relations), as well as different
management scenarios. They provide a semipermanent map theme that can be used to extrapolate
ecosystem pattern/process relations from sampled
areas to unsampled locales and are useful to stratified sampling design strategies for environmental
monitoring purposes (Bailey et al., 1994). Ecological units (McNab and Avers, 1994; Cleland et al.,
1997), land units (Zonneveld, 1989), ecoregions
(Omernik, 1987), biogeoclimatic ecosystems (Meidinger and Pojar, 1991), and land systems (Christian and Stewart, 1968) are examples of mapping
systems that delineate ecologically homogeneous
biophysical environments at different spatial scales
based primarily on climatic, geomorphic, and biotic criteria. Hierarchical watershed/stream network maps are additional examples of biophysical
environment maps that are increasingly being used
in aquatic ecosystem assessment efforts (Maxwell
et al., 1995).
The biophysical environments that delineate
aquatic ecosystems include hydrologic units (drainage basins), geoclimatic settings (terrestrial ecological units), groundwater systems, riverine systems, and lake systems. All these environments are
hierarchically organized and are useful for the identification of aquatic ecological units (Figure 24.1).