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<br />RESTORATION OF REGULATED RIVERS
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<br />foci for huinan activities within the catchment basin (Amoros e/ al., 1987; Petts et al., ] 989; Wissmar er ai"
<br />1994)
<br />Additional data are needed to confinn explicitly the pattern of biodiversity hypothesized in Figure 2 for a
<br />spectrum of rivers world-wide, but the importance of alluvial zones as biological 'hot spots' within river con-
<br />tinua is very clear (e,g, riparian plants: Junk e/ al., 1989; Gregory et aI., 1991; benthic insects: Zwick, 1992;
<br />Roth et al., in press; fishes: WeJcomme, 1979; Rieman and McIntyre, 1995). Moreover, metapopulation the-
<br />ory suggests that core populations are critical for persistence of metapopulations with core-satellite struc-
<br />tures (Schoener 1991; Harrison 1991, 1994). Core populations are relatively large populations occupying
<br />high quality habitat. In rivers, large alluvial reaches may support core populations of fishes (Lichatowich
<br />and Mobrand, 1995), These productive populations can serve as stable sources of dispersers that can reco-
<br />lonize peripheral habitats where less productive satellite populations have undergone local extinctions (Har-
<br />rison, 1991, 1994; Reiman and McIntyre 1993; Li e/ aI., 1995; Schlosser and Angermeier, 1995); or, core
<br />populations may 'rescue' from eKtinction satellite populations whose abundance has been severely reduced
<br />(Brown and Kodrick-Brown, 1977; Gote1li 1991; Stacey and Taper, 1992), Thus, core populations can buffer
<br />metapopulations against environmental change and contribute to resiliency of regional fish production, Cer-
<br />tain riparian plant species also appear to exist as meta populations with cores on alluvial floodplains,
<br />(Decamps and Tabacchi, 1994), Therefore, we propose that alluvial reaches should also be foci for large river
<br />conservation and restoration.
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<br />THE RIYER D1SCONTINUUM: HUMAN AL TERA nON OF LARGE RIVER ECOSYSTEMS
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<br />Humans vastly reduce the capacity of river ecosystems to sustain natural biodiversity and bioproduction by
<br />severing or compromising the dynamic interactive pathways of the river continuum, As described above,
<br />native biota of rivers display life history traits that allow populations to survive within a certain range of
<br />environmental variation that characterizes a particular river. ]f this range of variation changes, organisms
<br />must locally adapt to the new range of environmental conditions or be extirpated, Recolonization of extir-
<br />pated areas may occur over time as environmental constraints ameliorate and/or as a consequence of immi-.
<br />gration of suitably adapted populations. However, human-mediated environmental change can be so rapid
<br />and so severe as to exceed the ability of biota to adapt. The interactive pathways of the river continuum too
<br />often are permanently severed by human activities, and native biodiversity and bioproduction decline.
<br />Pervasive human perturbations that uncouple important ecological processes linking ecosystem compo-
<br />nents in large river basins can be lumped into three broad classes: (a) water pollution of all types; (b)
<br />food-web manipulation by harvest, stocking and exotic invasions; and (c) alteration of water, temperature
<br />and materials flux by dams, diversions and revetment. Human land use creates direct and diffuse inputs of
<br />water-borne wastes from the catchment and its airshed(Hynes, 1966; Warren, 1971), accelerates erosion and
<br />sediment loading related to deforestation and road building (Waters, 1995), alters flux rates of materials in
<br />rivers (e,g, eutrophication, acidification) and uncouples lotic food-webs by toxic effects. Harvest of fishes and
<br />invertebrates, and the purposeful and accidental introduction of non-native species, induces strong interac-
<br />tions that alter food-webs by causing biomass and bioproduction shifts, species replacements and other
<br />trophic effects (Mooney, 1986) that may cascade through all trophic levels and even involve terrestrial spe-
<br />cies that feed on aquatic biota (Spencer et aI., 1991), Pollution and food-web manipulation are inleractive
<br />with stream regulation effects in most catchments, However, alteration of flow regimes and associated sever-
<br />ing of connectivity in the three spatial dimensions of riverine ecosytems perhaps are the most strikingly per-
<br />vasive influence of humans on river landscapes world-wide (Dynesius and Nilsson, 1994).
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<br />Three first principles ~r the ecology of stream regulation
<br />At least three fundamental commonalties emerge from the large literature on the ecology of regulated
<br />rivers (reviewed by Baxter, 1977; Ward and Stanford, 1979, 1987; Lillehammer and Saltveit, 1984; Petts,
<br />1989; Calow and Petts, 1992). These principles must be recognized in the derivation of large river restora-
<br />tion strategies,
<br />J, Habitat diversity is subs/antially reduced, Large storage dams world-wide inundate piedmont or
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