DAMS AND DOWNSTREAM AQUATIC BIODIVERSITY - POTENTIAL FOOD-WEB CONSEQUENCES OF HYDROLOGIC AND GEOMORPHIC CHANGE

Responses of rivers and river ecosystems to dams are complex and varie d, as they depend on local sediment supplies, geomorphic constraints, climate, dam structure and operation, and key attributes of the biota. Therefore, ''one-size-fits-all'' prescriptions cannot substitute for local knowledge in developing prescriptions for dam structure and oper ation to protect local biodiversity. One general principle is self-evi dent: that biodiversity is best protected in rivers where physical reg imes are the most natural. A sufficiently natural regime of flow varia tion is particularly crucial for river biota and food webs. We review our research and that of others to illustrate the ecological importanc e or alternating periods of low and high flow, of periodic bed scour, and of floodplain inundation and dewatering. These fluctuations regula te both the life cycles of river biota and species interactions in the food webs that sustain them. Even if the focus of biodiversity conser vation efforts is on a target species rather than whole ecosystems, a food web perspective is necessary, because populations of any species depend critically on how their resources, prey, and potential predator s also respond to environmental change. In regulated rivers, managers must determine how the frequency, magnitude, and timing of hydrologic events interact to constrain or support species and food webs. Simple ecological modeling, tailored to local systems, may provide a framewor k and some insight into explaining ecosystem response to dams and shou ld give direction to mitigation efforts.