Influence of Catchment Condition and water resource development on waterbird assemblages in the Murray-Darling Basin, Australia

River regulation and water resource development have resulted in significant deterioration in aquatic ecosystems worldwide, including reductions in wetland extent, changed flow regimes, and declines in biodiversity. The impacts on the composition and distribution of waterbird assemblages has not been studied previously at the scale of a major river basin. We investigated the relationship between waterbird assemblage composition and ecosystem health across the 21 catchments of the Murray-Darling Basin in south-east Australia, which contains major wetlands that have been adversely affected by river regulation and over-allocation of water for irrigation. We allocated 51,000 surveys of 96 waterbird species, obtained from the New Atlas of Australian Birds database, to 117 one-degree grid squares for multivariate statistical analysis (MVA). Hierarchical clustering showed five main groups of squares reflecting strong biogeographic gradients. Pronounced spatial autocorrelation in the waterbird assemblage data was found. Unequal survey effort across grid squares and varying taxonomic scope also hindered conventional MVA and interpretation. To circumvent these constraints, survey data were recompiled at the half-degree square resolution after removing surveys with few waterbird records, leaving 17,448 surveys of 80 species. A novel sequential approach of multivariate regression of distance matrices, ordination of Bray-Curtis residuals, and post hoc correlation of the independent variable was used to test the hypothesis that assemblage composition varies systematically with Catchment Condition, after controlling for spatial autocorrelation, biogeographic trends and unequal survey effort. Ordination of the residuals of the half-degree square Bray-Curtis association matrix revealed a strong relationship between a nine-point index of Catchment Condition and waterbird assemblage composition. The colonial nesting waterbird guild (egrets, herons, ibis and spoonbills), was uniquely identified as being aligned with catchments in moderate to good condition. Waterbird assemblage composition shows significant spatial variation throughout the Murray-Darling Basin, influenced by the hydrological and ecological condition of catchments as well as by natural biogeographic factors. The least degraded catchments offer the best habitat for the colonially nesting waterbird guild, the group most adversely affected historically by river regulation and water diversions. These catchments require protection from water resource development if such habitat is to be maintained. Our results support the conservation objective of improving wetland health in degraded catchments through delivering environmental flows to ensure breeding and population maintenance of colonial nesting waterbirds.