Ecosystem Expansion and Contraction in Streams

t re< l m s are hydrologically diverse and dynamic ecosystems.Flow may vary between extremes, from high-discharge floods to periods when surface water is absent.Although much is known about the role of floods in shaping ecological processes, far less is known about the biological and chemical changes that occu r du ring periods of water loss in stream ecosystems (Boulton and Suter 1986, Stanley and Fisher 1992).Nowhere is this lack of knowledge more apparent than in desert streams; these lotic ecosystems exist in a setting defined by water limitation, and periods of declining or absent flow are common.However, water loss is by no means unique to desert streams, because intermittent streams are found in many different environments.Moreover, escalating demands on a finite water supply arc increasing the likelihood of drying in streams and rivers worldwide.Irrigation, impoundment, diversion, and groundwater abstraction reduce streamflow in mesic and xeric regions alike.In arid and semiarid areas, large rivers that are devoid of water are common, and in more mesic locales, profligate water use decreases the total amount of surface water present