Long-term effects of local disturbance history on mobile stream invertebrates

The patchy distribution of benthic invertebrates in streams and rivers is a n important and widely researched phenomenon Previous studies on reasons fo r this patchiness have neglected the potential role of local disturbance hi story, probably because most lotic invertebrates are mobile and any effect of disturbance history was thought to be short-lived. Here we demonstrate f or a New Zealand gravel-bed stream that local disturbance history can have long-term effects on the distribution of highly mobile stream invertebrates . Buried scour chains (100 at each of three 20-m sites within a 350-m reach ) indicated that a spate with a return period of 5 months caused a mosaic o f bed patches with different stabilities. More than 2 months after the spat e, we took random, quantitative samples at each site from five patches that had experienced 4 cm or more of scour during the spate, from five patches with 4 cm or more of fill, and from five stable patches. Density of the dom inant invertebrate taxon, the highly mobile mayfly Deleatidium spp., and de nsities of another three of the seven most common taxa differed significant ly between patch stability categories. Larvae of Deleatidium, the black fly Austrosimulium spp. and the dipteran Eriopterini were most abundant in fil l patches, whereas Isopoda were most abundant in scour patches. Total inver tebrate densities and densities of six common taxa also differed between si tes, although these were only 95-120 m apart. These results show that local disturbance history can have long-term effects on lotic invertebrates and be an important cause of invertebrate patchiness. The observed effects migh t have been even stronger had we sampled sooner after the spate or after a large flood. Disturbance history may influence invertebrates both directly (through dislodgement or mortality) and indirectly, through effects on the spatial distribution of their resources. Our results suggest that the role of disturbance in structuring animal communities dominated by mobile specie s may be more important than previously thought.