Dry season refuges, refugia and flow‐regime change in Mediterranean climate streams

Abstract Climate change is causing stream flow regimes to change in many regions globally, including southwestern Australia (SWA) where many perennial streams have switched to intermittent flow regimes. In drier landscapes, ecological refuges and evolutionary refugia will become critical for conserving freshwater biodiversity. This study aimed to determine the contribution of dry season refuges to species persistence and community recovery in a forested headwater catchment where perennial streams have been exposed to severe permanent drying, causing them to become intermittent. That is, in streams where formerly no drought refuges were present. Macroinvertebrate assemblages were sampled in dry season refuges within recently intermittent streams. Refuges included small spring‐fed pools, perched pools and subterranean refuges associated with granite inselbergs. Dry streambeds were searched for organisms aestivating in situ, and sediments were collected from each intermittent stream for rehydration. Mantel tests were used to compare the contribution of refuges to species assemblages in the early wet season when intermittent streams had begun to flow. Analysis of similarities was used to compare patterns between dry and wet season assemblages of intermittent streams and the single remaining perennial stream in the catchment, to infer the contribution of the perennial stream to community recovery in intermittent streams. Refuge types had very different assemblages: spring‐fed refuges supported several locally endemic species, but perched pools were dominated by opportunistic colonists. Several taxa were found aestivating in dry sediments, primarily adult Coleoptera and larval Chironomidae. Inselberg springs supported small populations of endemic Trichoptera and Ephemeroptera and provided subterranean refuge for an endemic amphipod. However, dry season refuges did not significantly contribute to community recovery. Rather, assemblages in early winter flows were similar to those inhabiting the perennial stream, showing that recolonisation from perennial streams is likely the primary process of community recovery in these recently intermittent streams. Newly formed dry season refuges supported some locally endemic species, but also colonising species that were formerly absent (or rare) when all streams were perennial. However, continued loss of perennial streams in SWA will result in landscape‐wide reductions in diversity (as colonisation sources are lost), because there are no known evolutionary refugia in this landscape for relictual species to retreat into. Granite inselbergs may become evolutionary refugia for a few species, as perennial streams are lost. Refuges will likely play a pivotal role in persistence of species able to use them; however, the existence of refuges is also under threat from prolonged climatic drying, including refuges newly formed by stream dry. Thus, identification and protection of future evolutionary refugia is a matter of urgency in regions facing drier climates, as it is ultimately evolutionary refugia that will become critical reservoirs of freshwater biodiversity as perennial streams and refuges are lost.