Determining vulnerability of stream communities to climate change at the landscape scale

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.