River Ecology and Restoration - River Dynamics and Integrated River Management - page 663

10.3 Assessment of River Ecosystems

10.3.1 Indicator Species
Complete measurement of the state of a river ecosystem, or even a complete census of all of the species
present, is not feasible. Thus, good indicators of the system conditions are efficient in the sense that they
summarize the health of the overall system. The current value of an indicator for an impaired river
ecosystem can be compared to a previously measured, pre-impact value, a desired future value, an
observed value at an “unimpaired” reference site, or a normative value for that class of river ecosystems.
For example, an index of species composition based on the presence or absence of a set of sensitive
species might be generally correlated with water quality. If a river is polluted some species may be
absent and the number of species may be less than that before the pollution. An index of indicator species
itself provides no information on how water quality should be improved. However, the success of
management actions in improving water quality could be tracked and evaluated through iterative
measurement of the index.
An indicator species group is defined as a set of organisms whose characteristics (e.g., number of
species, presence or absence, population density, dispersion, reproductive success) are used as an index
of attributes or environmental conditions of interest, which are too difficult, inconvenient, or expensive
to measure for other species (Landres et al., 1988). The 1970s-1980s is a peak interest period using aquatic
and terrestrial indicator species for assessment of ecosystems. During that time, Habitat Evaluation
Procedures (HEP) were developed by the U.S. Fish and Wildlife Service, and the use of management
indicator species was mandated by law with passage of the National Forest Management Act in 1976.
Since that time, numerous authors have expressed concern about the ability of indicator species to meet
the expectations expressed in the above definition. Landres et al. (1988) critically evaluated the use of
vertebrate species as ecological indicators and suggested that rigorous justification and evaluation are
needed before the concept is used.
Indicator species have been used to predict environmental contamination, population trends, and
habitat quality. The assumptions implicit in using indicators are that if the habitat is suitable for the
indicator it is also suitable for other species and that wildlife populations reflect habitat conditions.
However, because each species has unique life requisites, the relationship between the indicator and its
guild may not be completely reliable. It is also difficult to include all the factors that might limit a
population when selecting a group of species that an indicator is expected to represent.

10.3.1.1 Selection of Indicator Species
Several factors are important to consider in the selection process of indicator species ( FISRWG, 1997) :
(1) Sensitivity of the species to the environmental attribute being evaluated. When possible, data that
suggest a cause-and-effect relation are preferred to mere correlation (to ensure the indicator reflects the
variable of interest).
(2) Indicator accurately and precisely responds to the measured effect. High variation statistically
limits the ability to detect effects. Generalist species do not reflect change as well as more sensitive
endemics. However, because specialists usually have lower populations, they might not be the best for
cost-effective sampling. When the goal of monitoring is to evaluate on-site conditions, using indicators
that occur only within the site makes sense. However, although permanent residents may better reflect
local conditions, the goal of many riparian restoration efforts is to provide habitat for migratory birds. In
this case, residents such as cardinals or woodpeckers might not serve as good indicators for migrating
warblers.
(3) Size of the species home range. If possible, the home range should be larger than that of other
species in the evaluation area. Game species are often poor indicators simply because their populations