SCALING THE EFFECTS OF PREDATION AND DISTURBANCE IN A PATCHY ENVIRONMENT

The effects of hydraulic disturbances on the impact of two predatory b enthic invertebrates on their prey were examined in a stream at two di stinct spatial scales. At the scale of small habitat patches (0.0625 m (2)), hydraulic patch type was an important determinant of the microdi stribution of prey and predators. Prey abundances were similar across all patch types at baseflow, but local densities were higher in patche s identified as low-flow refugia after periods of high and fluctuating flow. The microdistribution pattern of predatory larvae of a caddisfl y, Plectrocnemia conspersa, was similar to that of its prey, whereas p redatory larvae of an alderfly, Sialis fuliginosa, did not shift their microdistribution significantly with discharge and were always most a bundant in low-flow refugia. There was little evidence of an aggregati ve response of predators with prey, even though both predators and pre y are mobile. Both predator species showed similar patch-specific patt erns of per capita consumption rates: uniform consumption-rates across hydraulic patch types at low and moderate flows, but highest in flow refugia during high flows. Species-specific patterns, however, were ap parent in the magnitude and direction of differences between consumpti on rates during disturbance events, and in comparable patches at base flow: At high flow, consumption rates for P. conspersa were exaggerate d (3.9 times higher) in flow refugia but ''at par'' in other patches; for S. fuliginosa they were ''at par'' in flow refugia but reduced in other patches (up to 3.3. times lower). These differences may be relat ed to species-specific foraging behaviours (search vs ambush predators ) and the influence of prey movements on feeding success. Using the pa tch-scale results only, it is difficult to predict the effects of phys ical disturbance on predation intensity at the larger scales of whole habitats, populations or communities. At the large scale (>200 m(2)), net predator impacts were estimated over the stream reach, using a spa tially explicit model that accounts, in an additive way, for habitat h eterogeneity and patch-specific responses of predators and prey. The r elationship between predator impact over the whole reach and hydraulic disturbance differed for the two predators. The predator impact of S. fuliginosa decreased with increasing hydraulic disturbance, as predic ted by the harsh-benign hypothesis. There was no directional trend for P. conspersa, however, and maximum predator impact may occur at inter mediate disturbance levels. For the prey community in this stream, pre dation pressure from S. fuliginosa appears to fluctuate directly with the discharge hydrograph, whereas predation from P. conspersa may be m ore persistent. Flow refugia may play a dual role in the sructure of s tream communities by preventing catastrophic mortality of animals (pre dators and prey) from physical forces during disturbances, and by main taining (or perhaps increasing) predation pressure. Summing the effect s of species interactions in small habitat patches to the larger scale of a whole stream reach indicates that the scale of approach influenc es the observed patterns and their implied underlying process.