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<br />1164 Issessill" Ih-rhvlq"m 111 craliou Ric[uerrl.:i. I
<br />dos conto 'pi•c,-inzpacto" ), post-izzzpacto, "lo gzze generG 64 11zdiccidores de AtternciGzz Hidrologica. Se pretezzde
<br />gue este znetodo sea utilizado en conjunto con ostros en lnventarios tie integridad de ecosistenzas, en la pia-
<br />neacion de actividades de znanejo de ecosisteznas y en la definicidn ), zzzedlcitin de avances en las nzetas de
<br />conservaciem o restauraciGn.
<br />Introduction
<br />A goal of ecosystem management is to sustain ecosystem
<br />integrity by protecting native biodiversity and the eco-
<br />logical (and evolutionary) processes that create and
<br />maintain that diversity. Faced with the complexity inher-
<br />ent in natural systems, achieving that goal will require
<br />that resource managers explicitly describe desired eco-
<br />system structure, function, and variability; characterize
<br />differences between current and desired conditions; de-
<br />fine ecologically meaningful and measurable indicators
<br />that can mark progress toward ecosystem management
<br />and restoration goals (Keddy et al. 1993); and incorpo-
<br />rate adaptive strategies (Rolling 1978) into resource
<br />management plans.
<br />The biotic composition, structure, and function of
<br />aquatic, wetland, and riparian ecosystems depend
<br />largely on the hydrologic regime (Gorman & Karr 1978;
<br />junk et al. 1989; Poff & Ward 1990; National Research
<br />Council 1992; Sparks 1992; Mitsch & Gosselink 1993)•
<br />Intra-annual variation in hydrologic conditions is essen-
<br />tial to successful life-cycle completion for many aquatic,
<br />riparian, and wetland species; variation in these condi-
<br />tions often plays a major role in the population dynam-
<br />ics of these species through influences on reproductive
<br />success, natural disturbance, and biotic competition
<br />(Poff & Ward 1989). Modifications of hydrologic re-
<br />gimes can indirectly alter the composition, structure, or
<br />function of aquatic, riparian, and wetland ecosystems
<br />through their effects on physical habitat characteristics,
<br />including water temperature, oxygen content, water
<br />chemistry, and substrate particle sizes (Stanford & Ward
<br />1979; Ward & Stanford 1983, 1989; Bain et al. 1988;
<br />Lillehammer & Saltveit 1984; Dynesius & Nilsson 1994).
<br />Collectively, limnology research suggests that the full
<br />range of natural intra- and inter-annual variation of hy-
<br />drologic regimes is necessary to sustain the native biodi-
<br />versity and evolutionary potential of aquatic, riparian,
<br />and wetland ecosystems. This emerging paradigm is ex-
<br />pressed in numerous recent statements about the neces-
<br />sity of protecting or restoring "natural" hydrologic re-
<br />gimes (National Research Council 1992; Sparks 1992;
<br />Doppelt et al. 1993; Dynesius & Nilsson 1994; Noss &
<br />Cooperrider 1994). For example, Sparks (1992) sug-
<br />gested that rather than optimizing water regimes for one
<br />or a few species, "a better approach is to approximate
<br />the natural flow regime that maintained ... the entire
<br />panoply of species."
<br />Despite the importance of natural hydrologic variation
<br />in aquatic, wetland, and riparian ecosystems (Kusler &
<br />Kentula 1989; National Research Council 1992; Noss &
<br />Cooperrider 1994; Allan 1995), most ecosystem manage-
<br />ment and restoration efforts (for example, Hesse & Mestl
<br />1993; Toth et al. 1993) have one or more shortcomings
<br />with respect to hydrology. Management decisions gener-
<br />ally have focused on the known or perceived hydrologic
<br />requirements of only one, or at most a few, target
<br />aquatic species (Reiser et al. 1989), potentially neglect-
<br />ing the needs of other species and ecosystem processes
<br />and functions in general. For instance, the vast majority
<br />of instream flow prescriptions and water rights have
<br />been based solely upon the requirements of selected
<br />species of fish (Beecher 1990; Bishop et aT. 190; Kulik
<br />1990; Zincone & Rulifson 1991). The range of flows
<br />needed to sustain aquatic-riparian ecosystems may be
<br />considerably greater than what would be prescribed for
<br />the aquatic system alone if the hydrologic requirements
<br />of riparian species also are considered (Hill et al. 1991).
<br />Other shortcomings include the failure to consider the
<br />influence of hydrologic processes on geomorphic
<br />changes, or on ecosystem functions such as -material
<br />transport and cycling or food-web support, and the fail-
<br />ure to consider the full range of temporal variability in
<br />hydrologic regimes.
<br />Effective ecosystem management of aquatic, riparian,
<br />and wetland systems requires that existing hydrologic
<br />regimes be characterized using biologically relevant hy-
<br />drologic parameters and that the degree to which hu-
<br />man-altered regimes differ from natural or preferred con-
<br />ditions be related to the status and trends of the biota.
<br />Ecosystem management efforts should be considered ex-
<br />periments that test the need to maintain or restore natu-
<br />ral characteristics of the hydrologic regime in order to
<br />sustain ecosystem integrity. Unfortunately, few limnol-
<br />ogy studies have closely examined hydrologic influences
<br />on ecosystem integrity, in part because commonly used
<br />statistical tools are poorly suited for characterizing hy-
<br />drologic data into biologically relevant attributes. The
<br />lack of -appropriate or robust statistical tools has in turn
<br />constrained knowledge about the effects of hydrologic
<br />alteration on ecosystem integrity. Without such knowl-
<br />edge, ecosystem managers will not be compelled to pro-
<br />tect or restore natural characteristics of the hydrologic
<br />regime.
<br />We present an approach (1) to statistically character-
<br />ize the temporal variability in hydrologic regimes using
<br />biologically relevant statistical attributes, and (2) to
<br />quantify hydrologic alterations associated with pre-
<br />Con_scrvation Biology
<br />Volume 10. No. 4, August 1996
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