Ed Chester

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.

Climate change is altering hydrologic regimes globally. In the Mediterranean climate region of south-western Australia (SWA), climate drying has caused many perennial streams to switch to intermittent flow regimes. Shifts in flow regime are expected to alter physical and biological processes in streams, including litter decomposition, which is the basis of detrital food webs.

Decomposition of jarrah (Eucalyptus marginata) leaves and associated macroinvertebrates, were measured over 320 days in 2018–19 using leaf bags in four headwater streams in SWA. Two streams retained perennial reaches and two were formerly perennial streams that are now intermittent. Pre-planned comparisons that formed a partial multiple before–after, control–impact design were used to compare the results to an experiment conducted in 1982–83 in some of the same streams when all were perennially flowing. Both experiments used coarse and fine-mesh bags containing 10 g of dry leaves.

In one perennial stream, coarse bags lost more weight than fine bags at the last sampling time only, when shredding caddisflies arrived on the leaf bags. In the other perennial stream, leaf-mining chironomids entered fine-mesh bags and accelerated decomposition so that they lost more weight than the coarse-mesh bags. There was no difference in weight loss between fine and coarse-mesh leaf bags in the two intermittent streams.

In 2018–19, decomposition was slower in dry reaches of intermittent streams than in perennial reaches. Leaf weight loss increased with the resumption of flow in intermittent streams, so that by the end of the experiment, similar amounts of leaf weight had been lost in intermittent and perennial reaches. Thus, although the temporal pattern of decomposition differed between intermittent and perennial reaches, after 320 days, they had reached a similar endpoint.

Over similar experimental duration, mean leaf weight remaining in perennial reaches at the end of the experiment did not differ between the 1982–83 study and 2018–19, showing that leaf decomposition had not changed in reaches that retained perennial flow. As mean leaf weight remaining also did not differ between intermittent and perennial streams in 2018–19, leaf decomposition was robust to flow regime change. However, since 1982–83, these streams have lost populations of shredding caddisflies and stoneflies, which were replaced by other shredders (e.g. leaf mining chironomids Stenochironomus sp.), showing that there was some redundancy amongst shredder species.

As climate change progresses, drying flow regimes will become commonplace in Mediterranean (and other) climate regions globally. This study indicates that litter decomposition may be maintained as streams transition to intermittency although shredder species may change. However, the impact of shredding species on leaf decomposition varies amongst studies, so effects of the loss of shredder species sensitive to drying may also vary, and in some cases their loss may substantially alter ecosystem processes.

Recent climate change is altering the timing, duration and volume of river and stream flows globally, and in many regions, perennially flowing rivers and streams are drying and switching to intermittent flows. Profound impacts on aquatic biota are becoming apparent, due in part to the strong influence of flow regime on the evolution of life history. We made predictions of life-history responses for 13 common aquatic invertebrate species (four caddisflies, five mayflies, two stoneflies, a dragonfly and an amphipod), to recent flow regime change in Australian mediterranean climate streams, based on historic studies in the same streams. Size distributions, phenology, voltinism and synchrony were compared, revealing five main responses. More than half of the species were restricted to perennially flowing streams and were absent from those that had switched to intermittent flows (including all four caddisfly species). These formerly common species are at risk of extinction as climate change progresses. Two mayfly species had divergent responses in voltinism and synchrony, and one relied on drought micro-refuges to persist. One stonefly species changed development timing to suit the new flow regime, and the amphipod species retreated to subterranean refuges. Two formerly common species were not detected at all during 2016–2017. In addition, a new mayfly species and a caddisfly species proliferated under new flow regimes, because they had life histories suited to brief hydroperiods. Importantly, previous life history rarely predicted species’ actual responses to climate-driven flow regime change, raising doubts about the veracity of predictions based on species traits. This is because a species’ potential for flexible phenology or growth rate is not necessarily indicated by life-history traits.

As climate change progresses, large (> 400 km2) fires are becoming more frequent across many biomes, often in association with intense drought. We analysed 5 years of stream macroinvertebrate data, collected before and after a wildfire that burnt > 750 km2 of the Grampians National Park, Australia. The wildfire occurred in 2006, during a 12-year drought (1997–2009). We tested the hypotheses that wildfire alters macroinvertebrate assemblage composition, and reduces taxon richness and among-stream variation. Five burnt and five unburnt headwater stream reaches were compared before and after the fire; a larger number of reaches were used to examine temporal trends in taxon richness. Wildfire altered macroinvertebrate assemblage composition and reduced among-stream variation in assemblages, but was not associated with low reach-scale taxon richness. Fire was associated with increased abundances of predators specialised for soft-sediments, and with reduced abundances of shredding and algal grazing caddisflies. In the short term, suspension feeder abundances increased, overwhelming the negative effects of drought on their abundance. Within 2 years post-fire, assemblages in burnt streams were similar to unburnt streams; within 3 years, among-reach variability in assemblage composition among burnt streams resembled that in unburnt streams. Invertebrate assemblages recovered rapidly in these streams despite the large areal extent of the fire. However, the frequency of wildfires is increasing, potentially permanently altering riparian vegetation structure and composition. As headwater streams depend on riparian vegetation for shading, woody debris and leaf litter, such permanent changes will likely affect biodiversity in headwater streams.

Intermittent rivers and ephemeral streams (IRES) are temporally and spatially dynamic ecosystems, experiencing alternating wet and dry phases and supporting both aquatic and terrestrial habitats. For aquatic species to persist in these variable habitats, they must be resistant or resilient to disturbances such as flow cessation and drying. Resistance mechanisms include tolerance of extreme physicochemical conditions and possession of desiccation-resistant dormant stages, whereas resilience mechanisms require the ability to disperse instream or overland. Similarly, terrestrial species inhabiting IRES during dry phases must be resistant or resilient to periodic inundation of their habitat. Resistance and resilience processes interact to facilitate community recovery after unfavorable periods (e.g., drying for aquatic taxa, flooding for terrestrial taxa). Human disturbances such as flow diversions and streambed gravel mining alter recovery pathways and affect the long-term persistence of aquatic and terrestrial species in IRES.

Climate change is modifying the timing of the onset of winter rainfall in southern Australia, at times creating brief inundation events in seasonal wetlands, termed 'false starts'. False starts may cause abortive hatchings of fauna emerging from sediment egg banks because wetlands dry out before invertebrates can complete their life cycle or reach a drying-resistant life stage. A laboratory emergence experiment was used to determine whether the abortive hatching caused by false starts alters assemblage composition in the subsequent hydroperiod and whether the length of the dry period following a false start alters subsequent assemblage composition. Sediment for the experiment was collected from Lake Joondalup South, Swan Coastal Plain (SCP), Western Australia, because it has a relatively diverse assemblage of desiccation-resistant invertebrate propagules. Most wetlands on the SCP are seasonal and groundwater fed and the region has a mediterranean-type climate Two different habitat types, open water (OW) and fringing trees (FT), with distinct freshwater invertebrate assemblages are commonly found in SCP wetlands. We repeated the experiment in sediment from both habitats to determine whether false starts had the same effect on the two assemblages. Replicate sediment samples from both habitats were placed in microcosms randomly allocated to treatments or controls. To simulate false starts of differing dry-period duration, treatments were inundated for 5 days, then allowed to dry out for different time periods (10, 20 and 30 days) and then inundated for 5 days. Controls were inundated for time periods equivalent to the total duration of each false-start treatment (20, 30 and 40 days). FT sediment had higher organic matter and moisture content than OW sediment. The composition of the emerging assemblage differed between habitats, and emergence was slower from OW than FT sediment. Abortive hatching followed the false start in OW sediment, but subsequently the same assemblage emerged, showing reliance on the egg bank to resupply lost populations. Abortive hatching was not observed in FT sediment, where invertebrates survived drying during the false start, continuing to develop for up to 30 days without surface water, because those sediments retained moisture. Provided that winter-spring hydroperiods continue to inundate OW for several months, these results indicate that invertebrates will be able to complete their life cycles and replenish egg banks following abortive hatching, demonstrating resilience to false starts. False starts to winter inundation rarely occur more than a month prior to the start of 'true' winter rains, so FT assemblages are resistant to false starts, showing the ability to survive dry periods of up to 30 days. Furthermore, survival in damp FT sediment also gave these populations a 'head start', because the surviving assemblage was identical to the continuously inundated control. Assemblages emerging from beneath FT may potentially recolonise inundated OW habitat. So far, changes to SCP seasonal-wetland hydroperiods will be within the coping range of their invertebrate assemblages. As FT assemblages were more resistant to false starts, restoration schemes that increase shading by fringing vegetation should be encouraged.

Climate change is causing prolonged drying in many seasonal wetlands, including urban wetlands, potentially affecting aquatic invertebrates that take refuge in wetland sediment during dry periods and thereby threatening wetland biodiversity. We collected sediment from two habitats: open water (OW) and fringing trees (FT), in eight urban wetlands after seasonal inundation had ended. Both habitats are inundated during winter–spring and dry in summer–autumn. Each sediment sample was divided into subsamples. One set of subsamples were inundated in the laboratory to test the hypothesis that emerging invertebrate assemblages would differ between OW and FT sediments. Another set of subsamples was dried, stored for a year, and inundated to test the hypothesis that prolonged drying would reduce the abundance and taxa richness of emerging invertebrates. The composition of emerging invertebrate assemblages differed between habitats, with more amphibious species found in FT sediment. Invertebrate responses to prolonged drying and storage varied among species: for some, effects depended on habitat type, while others delayed emergence or showed no response. Microcrustacean abundance was unaffected by drying, suggesting that their productivity during refilling may resist drier water regimes. Surface temperatures of dry sediment are cooler beneath FT, and this sediment has higher organic matter, holds more water and is less dense than OW sediment; and FT sediment remained cooler than OW sediment in the laboratory, despite the absence of shading. Fringing trees may therefore provide a refuge for some freshwater invertebrates relying on dormant stages in the sediment to survive drying in urban wetlands.

Intermittent stream systems create a mosaic of aquatic habitat that changes through time, potentially challenging freshwater invertebrate dispersal. Invertebrates inhabiting these mosaics may show stronger dispersal capacity than those in perennial stream systems. To relate different combinations of dispersal and drought survival strategies to species persistence, we compared the distribution and dispersal potential of six invertebrate species across all streams in a montane landscape where drying is becoming increasingly frequent and prolonged. Invertebrates were collected from seventeen streams in the Victoria Range, Grampians National Park, Victoria, Australia. The species analysed were as follows: the caddisflies Lectrides varians Moseley (Leptoceridae) and Agapetus sp. (Glossosomatidae); the mayflies Nousia AV1 and Koorrnonga AV3 (Leptophlebiidae); the water penny beetle Sclerocyphon sp. (Psephenidae); and a freshwater crayfish Geocharax sp. nov. 1 (Parastacidae). These species were widespread in the streams and varied in their dispersal and drought survival strategies. The distribution of each species across the Victoria Range, their drought responses and within-stream habitat associations were determined. Hypotheses of the dispersal capacity and population structure for each species were developed and compared to four models of gene flow: Death Valley Model (DVM), Stream Hierarchy Model (SHM), Headwater Model (HM) or panmixia (PAN). Molecular genetic methods were then used to infer population structure and dispersal capacity for each species. The large caddisfly Lectrides resisted drought through aestivation and was panmictic (PAN) indicating strong dispersal capacity. Conversely, the small caddisfly Agapetus relied on perennially flowing reaches and gene flow was limited to short distances among stream headwaters, resembling the HM. Both mayflies depended on perennial surface water during drying and showed evidence of gene flow among streams: Koorrnonga mainly dispersed along stream channels within catchments, resembling the SHM, whereas Nousia appeared to disperse across land by adult flight. Sclerocyphon relied on perennial water to survive drying and showed an unusual pattern of genetic structure that indicated limited dispersal but did not resemble any of the models. Geocharax survived drought through aestivation or residence in perennial pools, and high levels of genetic structure indicated limited dispersal among streams, resembling the DVM. Despite good knowledge of species' drought survival strategies, the population structure of four species differed from predictions. Dispersal capacity varied strongly among species; most species were poor dispersers and only one species showed panmixia. Therefore, intermittent stream species may not necessarily be better dispersers than those in perennial streams. Species showing strong drought resistance strategies differed in dispersal capacity. Knowledge of life-history characteristics, distribution and refuge use does not necessarily enable successful prediction of invertebrate dispersal pathways or population structure. Dispersal among intermittent streams may be restricted to relatively short distances (km) for most invertebrate species. Thus, frequent drought refuges (perennial water) that provide strong connectivity to subpopulations through stream flow (hydrological dispersal), or continuous terrestrial vegetation (flight dispersal), will be critical to maintain genetic diversity, adaptability and population persistence.