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Summary Freshwater ecosystems appear to be sensitive to even minor climatic shifts, and the dendritic nature of rivers as well as patchy distribution of habitats within the terrestrial landscape could limit the ability of species to track suitable climate conditions. Although the importance of dispersal is recognised in theory, there is great uncertainty when quantifying the capacity of species to shift their distributions in response to climate change. The influence of dispersal capacity on species’ vulnerability to climate change was assessed, using the modelled projections of 527 freshwater species in New South Wales (NSW), Australia. Species’ future ranges were calculated by iteratively identifying colonisation of accessible habitats and loss of suitable habitats within network models. The accessibility of new habitats was based on a given dispersal mode (aquatic, semi-terrestrial and aerial). The relative impact of dispersal parameters on projected range were evaluated alongside other known sources of uncertainty (climate and emissions scenarios, modelling algorithm and biological group), analysed collectively in a generalised additive mixed-model, and spatially to locate regions of NSW where projections are associated with the most uncertainty. Our simulations (1.4 million scenario combinations) suggest at least a third of species will lose more than half their range under climate change. Nevertheless, we emphasise the broad uncertainty that any average encapsulates. Dispersal capacity only had a minor impact on projected range shifts relative to other modelling assumptions but the network-pathways and maps of uncertainty have value for conservation planning at large scales. Projected range losses initially decreased rapidly as dispersal rates increased but the benefits are reduced above 2–3 km year−1. Taxa restricted to dispersal within the stream network (aquatic) were more vulnerable to climate change than taxa with semi-terrestrial or aerial dispersal and maps of variation due to dispersal mode and rate indicate where habitat connectivity would be most beneficial. This study demonstrates the breadth of uncertainties that challenge plans for improving ecosystem adaptation under climate change and highlights where in the landscape those differences were consistent. We emphasise the need for freshwater conservation studies to be ecologically representative, to focus on broad-scale connectivity for taxa that can move between catchments, and an accessible network of refugia for taxa with more limited dispersal.

Summary 1. As the climate changes, species are expected to shift to higher latitudes and altitudes where suitable habitat is available if dispersal is not constrained by geographic barriers. We analyse patterns of turnover in freshwater macroinvertebrate assemblages to identify which communities are most likely to be at risk from climate change, and the location of geographic barriers that could impede such adaptive range shifts. 2. We analysed macroinvertebrate data from standard biological assessments at the family level, from surveys of all coastal basins of New South Wales, Australia, covering a latitudinal gradient of more than 1000 km. We used variance partitioning to separate the variation in composition explained by climate, among-site distance, human disturbance and other stream factors. 3. Montane stream assemblages showed high turnover in response to climatic variation. Turnover in coastal-fringe streams was least affected by climate, but strongly correlated with distance and stream variables. Significant shifts in assemblage composition occurred between habitats within catchments and across catchment boundaries. 4. Montane stream assemblages are most vulnerable to climate change because their distribution is most responsive to climatic factors, and elevated sites are isolated from one another, reducing the scope for altitudinal migration. Dispersal limitations in coastal-fringe assemblages will also increase their vulnerability to habitat loss from sea-level rise. For all stream classes, the separation of many neighbouring catchment assemblages, owing to either limited dispersal or the lack of suitable habitat, is likely to constrain adaptive range shifts. This would lead to an overall reduction in beta diversity among reaches and subsequently to a reduction in landscape-level gamma diversity.