Geometric scaling of microhabitat patches and their efficacy as refugia during disturbance

Summary 1.  A field experiment was carried out to determine whether the accumulation of stream invertebrates in refugium patches during high flow disturbances was influenced by the geometric scaling of the patches, i.e. edge to size relationships. 2.  Flow was reduced experimentally by creating artificial flow refugia and comparing invertebrate colonization of refugium and control patches. There were eight sizes of colonization cage (1·5 cm mesh) with surface area/volume quotients (SA/V) which ranged from 1·2 to 0·5. Control cages were open to the flow; artificial refugia were baffled upstream by 0·3 cm mesh to reduce flow. Four, week‐long experimental trials were carried out during the winter months in an upland Scottish stream. On two occasions, discharge was high, on two it was low. 3.  The results are consistent with the hypothesis that stream invertebrates accumulate in refugia during high flow disturbances. The density of colonists was higher in refugia than control cages after high discharge for some taxa. There was no difference between treatments for any taxon at low discharge. 4.  The shape of soft‐edged microhabitat patches appears to be important in mediating the impact of disturbance in streams. Small refugium patches (high edge to size ratio) appeared to be more efficacious than large patches for taxa that accumulate in refugia during disturbances. Density increased with SA/V in all trials and for all taxa, but the slopes of the lines relating density to SA/V differed between control and refugium patches after disturbance for some taxa. 5.  According to trait‐based analysis, taxa that have high mobility or that are vulnerable to high hydraulic forces may be especially dependent upon small flow refugia to maintain population size. Whether movement is by active or passive means is less important.