Invertebrate responses to land use in tropical streams: Discrimination of impacts enhanced by analysis of discrete areas

We identified influences of land-use disturbances on invertebrate assemblages in streams draining eight areas of the Great Barrier Reef catchment in tropical Australia (∼15.7-22°S), a region of high biodiversity. We used distance-based linear modelling (DistLM) to analyse assemblage data (103 taxa), richness and the SIGNAL2 taxon sensitivity index. DistLM of assemblages explained ∼40% of variation across all samples and 7-54% of variation in individual areas. DistLM of richness and SIGNAL2 explained respectively 19-81 and 26-95% of variation. Explanatory variables were land use (especially cropping and grazing v. forest), riparian width, instream habitat, climate (drier south) and water quality (conductivity greater in south). Local impacts of activities such as mining were evident in models of individual areas. A detailed comparison of streams with contrasting riparian management demonstrated a 25% loss of richness, but no change in SIGNAL2 score. Accounting for local environmental gradients and using measures appropriate to the type of disturbance improved identification of impacts, and could form a framework for future regional monitoring of stream ecological condition. The impacts identified may be mitigated by remediation such as riparian rehabilitation, although management at catchment scales is required to be effective.