26.2 Biotic Patterns in Streams
scales at which land-use impacts on stream ecosystems should be evaluated. Their work demonstrated
that catchment-wide land use (measured as percent
area in agricultural land use) explained a vastly
larger share of site to site variation in stream habitat conditions and biotic integrity of fish populations than did indices of local riparian condition.
The strong implication of similar research (e.g.,
Steedman, 1988; Corkum 1989, 1990; Quinn and
Hickey, 1990; Fore et aI., 1996; Richards et aI.,
1996) is that site-specific environmental assessments, which fail to account for the larger-scale,
cumulative, basin-wide consequences of catchment
alteration (by anthropogenic or natural processes),
will not accurately characterize biotic response to
environmental change.
Environmental assessments do not make a credible and comprehensive effort to address incremental or local effects of activities or projects in
the context of catchment-wide processes and conditions (Schlosser, 1990; Stanford and Ward, 1992;
Frissell and Bayles, 1996). Indeed, there is a recognized scarcity of decision-making mechanisms
to avert the consequences of catchment-scale effects, which often encompass and implicate many
landowners and jurisdictions. Widespread recognition that the effects of nonpoint sources of water
pollution are hampering or overriding efforts to
achieve water quality goals under the U.S. Clean
Water Act is but one manifestation of this problem
(Karr, 1991; Hunsaker and Levine, 1995; Naiman
et aI., 1995; Allan et aI., 1997).
26.2.2 Indexes of Biotic Integrity
The index of biotic integrity (lBI) approach is becoming widely accepted as a standard method for
ecological assessment. This approach, based on biological indicators of ecosystem state, is not intrinsically hierarchical in a geographical sense (although nothing would preclude it from being so
adapted). As commonly applied, the IBI approach
rests on the identification and sampling of representative reference sites within a larger ecoregion
(Hughes et aI., 1986; Omernik, 1987; Fore et aI.,
1996). Within the ecoregion, all other streams of
roughly the same basic physical context (stream order, drainage area, elevation, etc.) are assumed to
have the potential to exhibit the same biotic structure observed at reference sites. Following comprehensive biotic sampling of streams with a range
of known conditions and environmental stresses, an
index of biotic integrity (Steedman, 1988; Karr,
1991) is developed for a particular segment of the
biota (typically, for fish or macroinvertebrates,
393
sometimes algae). These multimetric indexes
(sensu Karr and Chu 1997) incorporate data from
several independent methods and biological levels
of organization (Table 26.1). Fish IBIs, for example, often combine data on species richness, proportional abundance of certain taxa or life stages
known to be sensitive to prevalent environmental
stresses, and the incidence of external disease
symptoms on individual fish as an indicator of sublethal stress. Sampling and data are typically organized at the level of a stream reach (sensu Frissell
et aI., 1986) or site. These data are combined in the
form of a numerical index whose overall rating provides a relative ranking of how far the observed biotic community diverges from the community that
we would expect to observe at an unimpacted reference site. Furthermore, the particular aspects of
the index that contribute to a low rating can provide some insight into the nature of the environmental stress (e.g., absence of thermally tolerant
species or depletion or proliferation of certain
trophic specialists such as algivores) (Muntkittrick
and Dixon, 1989). Most variations of the IBI have
been developed for streams, but similar applications to lakes and other ecosystems are feasible
(Minns et aI., 1996; Karr and Chu, 1997).
Where they have been developed, tested, and
widely applied, IBIs and similar multimetric
bioindicator approaches can serve as convenient
and effective monitoring tools. However, IBIs cannot be widely applied without careful regional calibration to accommodate geographical variation in
species distribution and ecological tolerances
(Fausch et aI., 1984; Steedman, 1988). Tailoring
and testing IBIs for each ecoregion can be an intensive and time-consuming process. Many nuances and criticisms of the use of multimetric indices in IBI-type applications are discussed in
detail by Karr and Chu (1997).
In application to assessments of regional scope,
certain limitations of the IBI approach sometimes
are overlooked. First, some ecoregions are so pervasively altered by human activity that true reference sites are impossible to locate. The nature of
unimpaired communities may be unknown, and the
magnitude of biodiversity loss from past impacts
will go unmeasured. Second, some ecoregions, particularly those in montane areas, where lithology is
diverse or where karst groundwaters occur, have
highly heterogeneous stream habitats. In such a region, it is not clear that even a large set of reference sites is sufficient to provide a reliable indication of a potential biotic community that could be
expected to occur across the region. The more detailed stratification and interpretation of biotic data
scales at which land-use impacts on stream ecosystems should be evaluated. Their work demonstrated
that catchment-wide land use (measured as percent
area in agricultural land use) explained a vastly
larger share of site to site variation in stream habitat conditions and biotic integrity of fish populations than did indices of local riparian condition.
The strong implication of similar research (e.g.,
Steedman, 1988; Corkum 1989, 1990; Quinn and
Hickey, 1990; Fore et aI., 1996; Richards et aI.,
1996) is that site-specific environmental assessments, which fail to account for the larger-scale,
cumulative, basin-wide consequences of catchment
alteration (by anthropogenic or natural processes),
will not accurately characterize biotic response to
environmental change.
Environmental assessments do not make a credible and comprehensive effort to address incremental or local effects of activities or projects in
the context of catchment-wide processes and conditions (Schlosser, 1990; Stanford and Ward, 1992;
Frissell and Bayles, 1996). Indeed, there is a recognized scarcity of decision-making mechanisms
to avert the consequences of catchment-scale effects, which often encompass and implicate many
landowners and jurisdictions. Widespread recognition that the effects of nonpoint sources of water
pollution are hampering or overriding efforts to
achieve water quality goals under the U.S. Clean
Water Act is but one manifestation of this problem
(Karr, 1991; Hunsaker and Levine, 1995; Naiman
et aI., 1995; Allan et aI., 1997).
26.2.2 Indexes of Biotic Integrity
The index of biotic integrity (lBI) approach is becoming widely accepted as a standard method for
ecological assessment. This approach, based on biological indicators of ecosystem state, is not intrinsically hierarchical in a geographical sense (although nothing would preclude it from being so
adapted). As commonly applied, the IBI approach
rests on the identification and sampling of representative reference sites within a larger ecoregion
(Hughes et aI., 1986; Omernik, 1987; Fore et aI.,
1996). Within the ecoregion, all other streams of
roughly the same basic physical context (stream order, drainage area, elevation, etc.) are assumed to
have the potential to exhibit the same biotic structure observed at reference sites. Following comprehensive biotic sampling of streams with a range
of known conditions and environmental stresses, an
index of biotic integrity (Steedman, 1988; Karr,
1991) is developed for a particular segment of the
biota (typically, for fish or macroinvertebrates,
393
sometimes algae). These multimetric indexes
(sensu Karr and Chu 1997) incorporate data from
several independent methods and biological levels
of organization (Table 26.1). Fish IBIs, for example, often combine data on species richness, proportional abundance of certain taxa or life stages
known to be sensitive to prevalent environmental
stresses, and the incidence of external disease
symptoms on individual fish as an indicator of sublethal stress. Sampling and data are typically organized at the level of a stream reach (sensu Frissell
et aI., 1986) or site. These data are combined in the
form of a numerical index whose overall rating provides a relative ranking of how far the observed biotic community diverges from the community that
we would expect to observe at an unimpacted reference site. Furthermore, the particular aspects of
the index that contribute to a low rating can provide some insight into the nature of the environmental stress (e.g., absence of thermally tolerant
species or depletion or proliferation of certain
trophic specialists such as algivores) (Muntkittrick
and Dixon, 1989). Most variations of the IBI have
been developed for streams, but similar applications to lakes and other ecosystems are feasible
(Minns et aI., 1996; Karr and Chu, 1997).
Where they have been developed, tested, and
widely applied, IBIs and similar multimetric
bioindicator approaches can serve as convenient
and effective monitoring tools. However, IBIs cannot be widely applied without careful regional calibration to accommodate geographical variation in
species distribution and ecological tolerances
(Fausch et aI., 1984; Steedman, 1988). Tailoring
and testing IBIs for each ecoregion can be an intensive and time-consuming process. Many nuances and criticisms of the use of multimetric indices in IBI-type applications are discussed in
detail by Karr and Chu (1997).
In application to assessments of regional scope,
certain limitations of the IBI approach sometimes
are overlooked. First, some ecoregions are so pervasively altered by human activity that true reference sites are impossible to locate. The nature of
unimpaired communities may be unknown, and the
magnitude of biodiversity loss from past impacts
will go unmeasured. Second, some ecoregions, particularly those in montane areas, where lithology is
diverse or where karst groundwaters occur, have
highly heterogeneous stream habitats. In such a region, it is not clear that even a large set of reference sites is sufficient to provide a reliable indication of a potential biotic community that could be
expected to occur across the region. The more detailed stratification and interpretation of biotic data