24.5 Use of Aquatic Ecological Units in Multiscale Description of Ecosystem Patterns and Processes
361
erned by local climate, geology, topography, and
potential plant cover (Minshall, 1994) and by specific ecosystem assessment needs. The geoclimatic
setting that immediately encompasses the hydrologic unit of intcrest defines its context (e.g., similar geoclimatic settings identify similar watershed
types). Geoclimatic patterns such as landtypes
(Table 24.2) divide watersheds into meaningful hydrologic response units and help us to understand
the functional components of the watershed. Infiltration and evapotranspiration, runoff and erosion,
and surface and subsurface flow are examples of
processes influenced by landtype-scale ecological
units.
Because the ecoregion maps of Bailey (1983,
1995) and Omernik (1987) both utilize geoclimatic
settings as defining criteria, there should be some
correlation between these ecoregions and aquatic
biotic patterns (e.g., fish distributions) at different
assessment scales. Hughes et aI. (1990) have shown
a coarse-scale correlation between ecoregions and
fish distributions over widely separated geographic
areas of the United States. However, both Lyons
(1989) and Poff and Allan (1995) have suggested
that an understanding of fish assemblages and distributions within ecoregions is improved considerably by using habitat variables and hydrology, respectively. Moreover, Bayley and Li (1992) have
explained that some of the inconsistency of ecoregions in predicting fish distributions can be attributed to the fact that the ecological potential of
aquatic ecosystems may be dominated by geoclimatic conditions in the headwaters of their watersheds, and not by the ecoregion in which they occur. These results reinforce the need to match
aquatic pattern predictions to appropriately scaled
biophysical environment templates in ecological
assessment efforts.
24.5 Use of Aquatic Ecological
Units in Multiscale
Description of Ecosystem
Patterns and Processes
Implementation of hierarchy theory in the description of ecological systems is achieved by explicitly
characterizing the scaled relations that exist between the patterns of interest and the ecological factors that determine such patterns, that is, the agents
of pattern formation (Urban et aI., 1987). This type
of ecosystem characterization is commonly called
pattern analysis (Bourgeron and Jensen, 1994) and
can be illustrated using fish distributions as the ecological pattern of interest. Fish species may be visualized as exhibiting different patterns of organization (e.g., individuals to metapopulations) that
follow different spatial and temporal scales (Figure
24.3a). The formal definition of the hierarchical
arrangement of fish distribution patterns is important because it requires explicit statements about
(1) the spatial and temporal bounds of each pattern
and (2) the order in which these patterns are nested.
Such an objective-specific exercise provides the basis for identification of pattern formation agents
(Urban et aI., 1987; Bourgeron et aI., 1994).
The agents of pattern formation can be organized
into different hierarchies of biotic processes (Figure 24.3b), disturbance processes (Figure 24.3c),
and environmental constraints or biophysical environments (Figure 24.3d). Biotic processes important to an understanding of fish distribution patterns
may include behavior or physiologic adjustment at
the channel unit or stream reach level, dispersal or
genetic exchange at the watershed level, and speciation or extinction at the river basin (or broader
scales) in this example. The specific spatial and
temporal relations that exist between fish distribution patterns and biotic processes are efficiently described through this type of characterization. In a
similar manner, the relation between fish distribution patterns and disturbances and environmental
constraints can be described if each agent is specified and its spatial and temporal bounds clearly
identified. At the watershed level, for example,
population or guild distributions may be viewed as
responding to mass wasting or flooding disturbance
processes, which in turn are a function of local climate, geology, and landform environmental constraints (Figure 24.3).
The aquatic biophysical variables (or ecological
units) described previously are very important to
the understanding of scaled relations between
aquatic ecosystem patterns and processes. The
aquatic processes that create habitat for species
(e.g., flooding, sedimentation, and temperature
loading) are most efficiently described and
mapped in most ecological assessment efforts
through an implicit relation to aquatic ecological
units. In a similar manner, the potential distribution patterns of aquatic species are commonly described through an understanding of aquatic ecological units and the ecological processes that they
constrain. The relations between aquatic ecological units, aquatic system processes, and species
distribution patterns are described more fully in
Chapters 25 and 26.
361
erned by local climate, geology, topography, and
potential plant cover (Minshall, 1994) and by specific ecosystem assessment needs. The geoclimatic
setting that immediately encompasses the hydrologic unit of intcrest defines its context (e.g., similar geoclimatic settings identify similar watershed
types). Geoclimatic patterns such as landtypes
(Table 24.2) divide watersheds into meaningful hydrologic response units and help us to understand
the functional components of the watershed. Infiltration and evapotranspiration, runoff and erosion,
and surface and subsurface flow are examples of
processes influenced by landtype-scale ecological
units.
Because the ecoregion maps of Bailey (1983,
1995) and Omernik (1987) both utilize geoclimatic
settings as defining criteria, there should be some
correlation between these ecoregions and aquatic
biotic patterns (e.g., fish distributions) at different
assessment scales. Hughes et aI. (1990) have shown
a coarse-scale correlation between ecoregions and
fish distributions over widely separated geographic
areas of the United States. However, both Lyons
(1989) and Poff and Allan (1995) have suggested
that an understanding of fish assemblages and distributions within ecoregions is improved considerably by using habitat variables and hydrology, respectively. Moreover, Bayley and Li (1992) have
explained that some of the inconsistency of ecoregions in predicting fish distributions can be attributed to the fact that the ecological potential of
aquatic ecosystems may be dominated by geoclimatic conditions in the headwaters of their watersheds, and not by the ecoregion in which they occur. These results reinforce the need to match
aquatic pattern predictions to appropriately scaled
biophysical environment templates in ecological
assessment efforts.
24.5 Use of Aquatic Ecological
Units in Multiscale
Description of Ecosystem
Patterns and Processes
Implementation of hierarchy theory in the description of ecological systems is achieved by explicitly
characterizing the scaled relations that exist between the patterns of interest and the ecological factors that determine such patterns, that is, the agents
of pattern formation (Urban et aI., 1987). This type
of ecosystem characterization is commonly called
pattern analysis (Bourgeron and Jensen, 1994) and
can be illustrated using fish distributions as the ecological pattern of interest. Fish species may be visualized as exhibiting different patterns of organization (e.g., individuals to metapopulations) that
follow different spatial and temporal scales (Figure
24.3a). The formal definition of the hierarchical
arrangement of fish distribution patterns is important because it requires explicit statements about
(1) the spatial and temporal bounds of each pattern
and (2) the order in which these patterns are nested.
Such an objective-specific exercise provides the basis for identification of pattern formation agents
(Urban et aI., 1987; Bourgeron et aI., 1994).
The agents of pattern formation can be organized
into different hierarchies of biotic processes (Figure 24.3b), disturbance processes (Figure 24.3c),
and environmental constraints or biophysical environments (Figure 24.3d). Biotic processes important to an understanding of fish distribution patterns
may include behavior or physiologic adjustment at
the channel unit or stream reach level, dispersal or
genetic exchange at the watershed level, and speciation or extinction at the river basin (or broader
scales) in this example. The specific spatial and
temporal relations that exist between fish distribution patterns and biotic processes are efficiently described through this type of characterization. In a
similar manner, the relation between fish distribution patterns and disturbances and environmental
constraints can be described if each agent is specified and its spatial and temporal bounds clearly
identified. At the watershed level, for example,
population or guild distributions may be viewed as
responding to mass wasting or flooding disturbance
processes, which in turn are a function of local climate, geology, and landform environmental constraints (Figure 24.3).
The aquatic biophysical variables (or ecological
units) described previously are very important to
the understanding of scaled relations between
aquatic ecosystem patterns and processes. The
aquatic processes that create habitat for species
(e.g., flooding, sedimentation, and temperature
loading) are most efficiently described and
mapped in most ecological assessment efforts
through an implicit relation to aquatic ecological
units. In a similar manner, the potential distribution patterns of aquatic species are commonly described through an understanding of aquatic ecological units and the ecological processes that they
constrain. The relations between aquatic ecological units, aquatic system processes, and species
distribution patterns are described more fully in
Chapters 25 and 26.