Basic Concepts and Management Issues of Rivers - River Dynamics and Integrated River Management - page 28

Fig. 1.22 Some species of benthic macro-invertebrates living in gravel bed rivers: (a) leech ( Hirudinea ); (b) mayfly
( Ephemeridae ); (c) caddis fly ( Trichoptera ); and (d) Dytiscidae
Fish —Fish are ecologically important in the stream ecosystems because they are usually the largest
vertebrates and often are the apex predator in the aquatic systems. The numbers and species composition
of fish in a given stream depends on the geographic location, evolutionary history, and such intrinsic
factors as physical habitat (current, depth, substrates, riffle/pool ratio, wood snags, and undercut banks),
water quality (temperature, DO, suspended solids, nutrients, and toxic chemicals), and biotic interactions
(exploitation, predation, and competition).
Indicator species —Landres et al. (1988) defined an indicator species as an organism whose characteristics
(e.g., presence or absence, population density, dispersion, reproductive success) are used as an index of
attributes or environmental conditions of interest, which are difficult, inconvenient, and expensive to
measure for other species. Ecologists and management agencies have used aquatic and terrestrial
indicator species as assessment tools for many years. In many cases benthic invertebrates are selected as
indicator species for assessment of river ecology. 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.
Ecological stress— Ecological stresses are defined as the disturbances that bring changes to river
ecosystems. The ecological stresses are natural events or human-induced activities that occur separately or
simultaneously. Either individually or in combination, the ecological stresses have the potential to alter
the structure and impair the ability of the river ecosystem to perform key ecological functions. A stress
occurring within or adjacent to a river typically produces a causal chain of effects, which may permanently
alter one or more characteristics of a stable system.
Stream restoration— According to the U.S. National Research Council (NRC, 1992), restoration should
involve the return of a given ecosystem to a state approximating that in which it existed prior to disturbances.
Restoration of the impaired river ecosystem is necessary for most of the worldrivers.