Consequences of hydrological alteration for beta diversity of fish assemblages at multiple spatial scales.

Effects of dam operation and extraction of water from rivers on spatial variation in hydrological regimes, and consequences for freshwater biodiversity, are widely predicted but seldom assessed empirically. Evidence of linkages between hydrology and beta diversity contributes to water-management decisions to support landscape-scale biodiversity and avoid inadvertently contributing to further biodiversity decline. Using six lowland rivers in Australia's Murray - Darling Basin that formed a gradient of hydrological alteration, we examined (1) spatial variation in hydrology under modelled scenarios of low water-resource development and flow modification by dams and extraction, (2) how beta diversity of fish among and within rivers was associated with spatial hydrological variation and whether patterns of overall beta diversity differed between native and non-native species, and (3) the associations of spatial and environmental variables and both recent and long-term hydrology with beta diversity. Spatial variation in hydrology among rivers was higher under the modified scenario than under the low-development scenario yet change in the magnitude of within-river (longitudinal) variation was inconsistent between rivers. Beta diversity among rivers was significantly associated with spatial variation in hydrology only in certain circumstances (native species assemblages in specific years). Within-river beta diversity varied among rivers yet was unrelated to longitudinal variation in modified hydrological regimes. Patterns of beta diversity did not differ appreciably if non-native species were included in or excluded from analyses. These findings contradict predictions adopted in ecohydrological science that water resource development homogenises hydrological regimes, in turn causing biotic homogenisation in lowland rivers.

Scaling biodiversity responses to hydrological regimes.

Of all ecosystems, freshwaters support the most dynamic and highly concentrated biodiversity on Earth. These attributes of freshwater biodiversity along with increasing demand for water mean that these systems serve as significant models to understand drivers of global biodiversity change. Freshwater biodiversity changes are often attributed to hydrological alteration by water-resource development and climate change owing to the role of the hydrological regime of rivers, wetlands and floodplains affecting patterns of biodiversity. However, a major gap remains in conceptualising how the hydrological regime determines patterns in biodiversity's multiple spatial components and facets (taxonomic, functional and phylogenetic). We synthesised primary evidence of freshwater biodiversity responses to natural hydrological regimes to determine how distinct ecohydrological mechanisms affect freshwater biodiversity at local, landscape and regional spatial scales. Hydrological connectivity influences local and landscape biodiversity, yet responses vary depending on spatial scale. Biodiversity at local scales is generally positively associated with increasing connectivity whereas landscape-scale biodiversity is greater with increasing fragmentation among locations. The effects of hydrological disturbance on freshwater biodiversity are variable at separate spatial scales and depend on disturbance frequency and history and organism characteristics. The role of hydrology in determining habitat for freshwater biodiversity also depends on spatial scaling. At local scales, persistence, stability and size of habitat each contribute to patterns of freshwater biodiversity yet the responses are variable across the organism groups that constitute overall freshwater biodiversity. We present a conceptual model to unite the effects of different ecohydrological mechanisms on freshwater biodiversity across spatial scales, and develop four principles for applying a multi-scaled understanding of freshwater biodiversity responses to hydrological regimes. The protection and restoration of freshwater biodiversity is both a fundamental justification and a central goal of environmental water allocation worldwide. Clearer integration of concepts of spatial scaling in the context of understanding impacts of hydrological regimes on biodiversity will increase uptake of evidence into environmental flow implementation, identify suitable biodiversity targets responsive to hydrological change or restoration, and identify and manage risks of environmental flows contributing to biodiversity decline.