Belinda Robson

Physicochemical changes induced by long-term, landscape-scale disturbances such as agricultural clearing may affect stream invertebrate biodiversity and species' trait distributions. However, communities may be buffered from these impacts if generalist strategies (e.g. habitat flexibility and omnivory) are widespread, conferring greater resistance to disturbance. To determine whether such buffering occurs, we sampled sixteen low-order streams in southwestern Australia, assigned a priori to categories of high (> 30%, cleared, n = 8) or low (< 30%, uncleared, n = 8) clearing, collected invertebrates, and tested for physicochemical correlations. Invertebrate species were assigned traits related to streamflow and feeding according to existing literature. Taxonomic composition differed between high and low levels of catchment clearing, associated with differences in nutrient concentrations, conductivity, and depth. Differences in composition were associated with trait responses to land use, but feeding- and flow-related traits did not adequately predict responses to landscape clearing. Although categorical traits predicted responses to agricultural disturbance, they poorly predicted the prevalence of generalism (omnivory and habitat flexibility). Generalist modes of existence may be hidden within a lack of trait responses. Trait databases must find ways to define omnivory and habitat flexibility as fixed traits to obtain meaningful conclusions from trait analysis.

Although environmentally friendly rice farming is expected to contribute to biodiversity conservation in paddy fields, scientific testing of the impact of individual management practices in rice cultivation is still in its early stages. This study used field experiments to quantify the effects of winter flooding and summer, mid-season drainage (a short-term soil drying in the middle of the rice growing season) on animal communities in paddy fields. We used a 2×2 factorial design with the factors winter flooding (±) and mid-season drainage treatment (±), crossed, and three replicate paddy fields in each combination (total n = 12). Our field experiments showed that winter flooding had little effect on the taxonomic richness and abundance of freshwater insects and spiders, but it increased the number of egg masses in amphibians such as the montane brown frog Rana ornativentris and the Japanese black salamander Hynobius nigrescens. In contrast, mid-season drainage negatively affected the taxonomic richness and abundance of freshwater insects, and the abundance of spiders. In particular, mortality usually occurred in species whose larval stage overlapped the period of mid-season drainage. Winter flooding promotes the biodiversity of animal communities in paddy fields. For amphibians that breed in the early spring, winter flooding provides breeding habitat. Mid-season drying had negative effects on animal biodiversity. Delaying or ceasing mid-season drainage would improve reproductive opportunities for freshwater insects and maintain species diversity. Diversification of winter flooding and winter drying across a rice paddy landscape and prolonged summer flooding practice may improve conservation outcomes.

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.

Ecosystems comprise all the life forms (microbes, plants, animals) and the inorganic context (sediment types, water regimes, water quality, etc.) that they live in. Together, the life forms interact with each other. They influence, and are influenced by, their environment, carrying out ecosystem processes such as the transport of nitrogen and carbon. This chapter introduces the terminology used to describe lake and river ecosystems. It describes the function of freshwater ecosystems and the methods scientists use to quantify these functions, including concepts such as ecosystem productivity, metabolism, food webs, populations, and metapopulations. Furthermore, this chapter describes ecological communities, their diversity and assembly, and the concepts of metacommunities and metaecosystems.

1. In many regions, climatic drying is shortening hydroperiods and freshwater biodiversity is declining. Aquatic species that lack a desiccation-resistant life stage are predicted to have the least ability to persist in drying climates, yet such species may occur in intermittent waterbodies. We examined the distribution of two crustacean species that lack desiccation-resistant life stages: the isopod Paramphisopus palustris and amphipod Chiltoniidae sp.nov., before and after the commencement of severe climatic drying in Swan Coastal Plain wetlands, south-western Australia. Historical distribution data for these species were obtained from studies of the same set of wetlands in 1989–1990.

2. We determined whether population or body size differed between hydrological regimes (perennial, semi-perennial, seasonal) between 1989–1990 and 2018–2019 for both species. Amphipods, isopods and environmental variables were sampled from 42 wetlands in 2018–2019. Thirty-three wetlands (22 perennial, 11 seasonal) persisted with the same hydrological regime in both time periods and eight wetlands that were perennial in 1989–1990 had shifted to a drier hydrological regime (four became semi-perennial, four became seasonal) by 2018–2019.

3. Drying wetland hydrological regimes did not correspond to more limited spatial distributions for either species. Hydrological regime had no effect on amphipod counts, although abundances were lower in 2018–2019 than 1989–1990. In 2018–2019, male amphipods were larger than females in seasonal wetlands, but sexes were the same size in perennial wetlands and females were larger than males in semi-perennial wetlands. Furthermore, amphipods of both sexes were considerably smaller in semi-perennial than seasonal or perennial wetlands.

4. Isopod distributions differed between the two sampling periods, with populations primarily lost from wetlands that remained perennial. This unexpected result was likely caused by prolonged periods of high summer–autumn water temperatures and stratification in shallow perennial wetlands that caused anoxia in bottom waters and made surface waters excessively hot for isopods. By contrast, drying seasonal wetlands seemingly offered suitable habitat in summer–autumn. Mean isopod head and body length did not differ between hydrological regimes or sexes, but became smaller between 1989–1990 and 2018–2019 (15 mm average body length in 1989 vs. 8.3 mm in 2018–2019; 17 mm maximum size in 1989–1990 vs. 13.2 mm in 2018–2019).

5. The absence of population losses associated with drying hydrological regimes indicates that the current level of drying is within the tolerance range of both species. However, increased temperatures may be causing body size (and possibly fecundity) to decline for isopods.

6. As global warming continues to shorten wetland hydroperiods in many regions, numerous species will struggle to complete their life cycles, leading to extirpation. Our findings suggest that climate change may also cause conditions in perennial wetlands to exceed the tolerances of species that lack desiccation resistance traits. This emphasises the need to better understand both environmental and habitat changes along the drying trajectory in wetlands and the fundamental life-history and physiological traits that enable the survival of aquatic invertebrates that lack desiccation resistance. This includes not only species responses to drying, but also responses to other stressors (e.g., prolonged stratification) caused by global warming.

Non-perennial rivers are valuable water resources that support millions of humans globally, as well as unique riparian ecosystems. In Australia, the Earth’s driest inhabited continent, over 70% of rivers are non-perennial due to a combination of ancient landscape, dry climates, highly variable rainfall regimes, and human interventions that have altered riverine environments. Here, we review Australian non-perennial river research incorporating geomorphology, hydrology, biogeochemistry, ecology, and Indigenous knowledges. The dominant research themes in Australia were drought, floods, salinity, dryland ecology, and water management. Future research will likely follow these themes but must address emerging threats to river systems due to climate change and other anthropogenic impacts. Four high level opportunities for future research are identified, namely: (1) integrating Indigenous and western scientific knowledge; (2) quantifying climate change impacts on hydrological and biological function; (3) clarifying the meaning and measurement of “restoration” of non-perennial systems; and (4) understanding the role of groundwater. These challenges will require inter- and multi-disciplinary efforts supported by technological advances. The evolving body of knowledge about Australian rivers provides a foundation for comparison with other dryland areas globally where recognition of the importance of non-perennial rivers is expanding.

Little is known about odonate ecology and phenology in warm mediterranean climates where most wetlands are intermittent. We identified variables associated with the presence, relative abundance, assemblage composition and body size of odonates in spring and summer, to understand the influence of intermittency and season. We hypothesized that size at eclosion would be smaller in seasonal wetlands in summer due to time stress caused by drying. Nymphs, exuviae and adults were sampled in spring and summer from 22 intermittent and perennial suburban wetlands in south-western Australia. Spatial and environmental variables within and between wetlands were measured and associated with adult and nymph distributions. Exuviae were collected to quantify size at eclosion. As in temperate perennial wetlands, wetland-scale variables (submerged, emergent and terrestrial vegetation, water temperature) were most strongly associated with assemblage composition of adults and nymphs, but landscape-scale variables (distance to nearest patch of native vegetation, distance to nearest large lake) were also associated with adult assemblages. Two abundant dragonfly (Orthetrum caledonicum Libellulidae, Hemicordulia tau Hemicorduliidae) and one damselfly (Xanthagrion erythroneurum Coenagrionidae) species emerged at smaller body sizes in summer than spring, but from both intermittent and perennial wetlands. Consequently, declining photoperiod and warmer summer temperatures, rather than wetland drying, probably caused reduced size at eclosion. Although the influence of vegetation, temperature and photoperiod on odonate assemblages appears similar in temperate and mediterranean climates, odonate phenology differs markedly. Fitness cost of emerging at a smaller adult size may be outweighed by the increased likelihood of successfully reaching emergence in drying waterbodies in summer. More field data on size at eclosion in warm climate regions, and laboratory experiments manipulating temperature and photoperiod, are needed to confirm the generality of patterns shown here.

Crayfish perform important roles within freshwater ecosystems, including in regions where global warming is causing prolonged drying of waterbodies. However, little is known about responses of crayfish to habitat drying from both a behavioural and physiological perspective. We compared burrowing ability, survival and metabolism of the crayfish Cherax quinquecarinatus from a seasonal stream and a perennial stream. Burrowing ability and crayfish survival were quantified in a mesocosm experiment contrasting sediment type (sand vs. clay/sand mixture) and water regime. Aerobic scope, standard metabolic rate (SMR) and maximum metabolic rate (MMR) were also compared using intermittent flow respirometry. Crayfish from the seasonal stream showed limited burrowing ability but higher survival in the drying treatment, while the perennial stream crayfish burrowed strongly in the clay/sand sediment. Higher survival suggests that crayfish from seasonal streams might be physiologically better adapted to drying. Larger crayfish burrowed more proficiently, reaching the saturated hyporheic zone refuge in the clay/sand sediment treatment. SMR/MMR/aerobic scope did not differ between populations or respirometry runs; however, SMR differed between individuals, perhaps due to personality traits. There was a significant negative relationship between MMR/aerobic scope and weight. Sediment type may limit C. quinquecarinatus burrowing and persistence through drying. Crayfish populations did not differ in terms of metabolism; however, crayfish from seasonal habitats may possess more efficient physiological adaptations to drying. This study highlights the need for greater research attention on the effects of climatic drying on both the behaviour and the physiology of species exposed to climate change.

Climate change is altering hydrological cycles globally, and in mediterranean (med-) climate regions it is causing the drying of river flow regimes, including the loss of perennial flows. Water regime exerts a strong influence over stream assemblages, which have developed over geological timeframes with the extant flow regime. Consequently, sudden drying in formerly perennial streams is expected to have large, negative impacts on stream fauna. We compared contemporary (2016/17) macroinvertebrate assemblages of formerly perennial streams that became intermittently flowing (since the early 2000s) to assemblages recorded in the same streams by a study conducted pre-drying (1981/82) in the med-climate region of southwestern Australia (the Wungong Brook catchment, SWA), using a multiple before-after, control-impact design. Assemblage composition in the stream reaches that remained perennial changed very little between the studies. In contrast, recent intermittency had a profound effect on species composition in streams impacted by drying, including the extirpation of nearly all Gondwanan relictual insect species. New species arriving at intermittent streams tended to be widespread, resilient species including desert-adapted taxa. Intermittent streams also had distinct species assemblages, due in part to differences in their hydroperiods, allowing establishment of distinct winter and summer assemblages in streams with longer-lived pools. The remaining perennial stream is the only refuge for ancient Gondwanan relict species and the only place in the Wungong Brook catchment where many of these species still persist. The fauna of SWA upland streams is becoming homogenised with that of the wider Western Australian landscape, as drought tolerant, widespread species replace local endemics. Flow regime drying caused large in situ alterations to stream assemblage composition and demonstrates the threat posed to relictual stream faunas in regions where climates are drying.

Isopods play many important roles within freshwater ecosystems (including as shredders, prey, and detritivores), yet we know little about their responses to disturbance or whether they vary among populations. In a region undergoing severe climatic drying that is changing hydrological regimes in wetlands, we compared responses to drying (including survival) among populations of an endemic isopod Paramphisopus palustris (Amphisopidae).

The survival of individuals from four isopod populations (two each from seasonal and semi-perennial wetlands) were tested in a drying experiment with three treatments: control (permanently inundated), saturated sediment (water level maintained equal with the sediment surface), and dry (microcosms gradually raised out of the water causing sediments to dry out). Microrefuge use (three levels: no response, sought shelter or sought shelter/burrowed) was also compared among populations.

As expected, isopod mortality was highest in the dry treatment for all populations. Unexpectedly, isopods from the two semi-perennial wetlands showed higher survival in response to drying than those from seasonal wetlands. All isopods showed dormancy in response to the dry treatment and curled their bodies to limit water loss from their ventral gills. Microrefuge use differed among populations but not between the two hydroregimes. However, there was some similarity in population responses according to genetic clade.

Isopods displayed some resistance to drying through dormancy behaviour and seeking microrefuges, which will increase population persistence under a drying climate. However, the reduced survival of individuals from seasonal wetland populations suggests that surviving in these wetlands is more challenging than in semi-perennial or perennial regimes. These challenges are likely to be exacerbated as climate change further decreases hydroperiods and limits the availability of damp microrefuges. Consequently, longer-term population persistence may be challenged as more wetlands become seasonal.

The responses of endemic species are not always correctly predicted from knowledge of more widespread and commonly studied species and our results suggest that there may be variation among populations. Although P. palustris showed some innate responses to drying, their long-term persistence (and that of other endemic species) may be threatened as environmental conditions become more extreme.