Interacting environmental gradients, trade-offs and reversals in the abundance-environment relationships of stream insects: when flow is unimportant

Flow is often presumed to determine the distribution of stream invertebrates across stream beds. When temperatures are high, however, dissolved oxygen (DO) and its interactions with other environmental gradients may be more important. Field surveys were carried out in summer at two sites in a sand-bed stream in south-east Australia. Using quantile regression, we quantified the abundance-environment relationships of a caenid mayfly and an ecnomid caddisfly, and determined whether DO, fine detritus or velocity was the dominant limiting variable, and to gain insight into the causal mechanisms. Local densities of caenids were driven by food resources (detritus) at a site with a short DO gradient. The relationship was completely reversed where long DO and detritus gradients interacted, and here DO appeared to limit density. Densities of ecnomids were limited by prey-rich detritus patches at both sites. The velocity gradient did not explain the distribution patterns in either species. Ecnomid diet altered with changes in the spatial distribution of caenids between sites; caenids were the dominant prey at one site, but proportionately fewer were consumed where there was a negative spatial overlap of predators and prey. These results show that invertebrate responses to environmental gradients can be complex and that flow may be unimportant.