The ecology, conservation, and management of Australia’s ephemeral freshwater granite rock-holes

Abstract

Ephemeral freshwater ecosystems are ecologically distinct from their permanent counterparts and are of significant conservation value, yet they are relatively understudied and often overlooked in key policy documents. Furthermore, they are at risk due to the impacts of a series of key threatening processes. I sought to improve understanding of the ecological role and threats to one such ecosystem, the freshwater granite rock-holes found in arid Australia. I used wildlife camera trapping alongside environmental DNA (eDNA) metabarcoding to record vertebrate visitation at rock-holes to determine the extent of their use as a resource to native and invasive vertebrates, as well as test the validity of eDNA metabarcoding as a biomonitoring tool for recording vertebrate visitation to this ephemeral freshwater ecosystem. Environmental DNA metabarcoding was also used to characterise invertebrate communities in rock-holes, and assess variability at spatial and temporal scales. Rock-hole hydrology was modelled and forecast under a series of emissions scenarios to predict the effect of climate change on this ecosystem. Finally, I applied these scenarios to rock-hole invertebrates to assess the potential impact of climate change on rock-hole inhabitants. Wildlife camera traps recorded six native mammal, four reptile, and 18 bird species plus four invasive species known to cause ecological harm. The most common native taxa were macropods, emus, crows and ravens. Visitation increased with prolonged periods without local rainfall, and invasive species visitation (primarily by goats) increased with elevation. Twenty-one vertebrate species were identified with freshwater eDNA metabarcoding, and the method was deemed an effective alternative to wildlife camera trapping at detecting mammal visitations. However, it was less well suited for detecting birds. Rock-hole invertebrate communities mostly comprised crustaceans and insects and varied spatially and temporally, with a peak in species richness in July. Freshwater eDNA metabarcoding was effective for detecting rock-hole invertebrate communities, but an underrepresentation of arid lands species and sequences in public genetic databases limited the taxonomic resolution that could be achieved (i.e. up to Order level). In the arid region of Australia examined here, future changes in climate will result in increases in water temperatures and decreases in hydroperiod under all emissions scenarios. Under the most severe scenarios, modelling showed a 42% decrease in hydroperiod. Experiments that duplicated conditions likely caused by climate change impacted test species Sarscypridopsis sp. and Daphnia clarinata unevenly. These results indicate that climate change will adversely affect some rock-hole invertebrates, although others will likely have capacity to adapt and compensate by increasing their rates of growth and reproduction. My research shines a light on often-overlooked freshwater granite rock-holes, and contributes to understanding of the importance of ephemeral freshwater ecosystems more generally. I have demonstrated that the rock-holes provide potable water to native vertebrate communities and are a habitat for invertebrates. However, unmanaged, these habitats face two key threats: fouling and degradation by invasive species and climate change. In the absence of significant local and global efforts to mitigate the impacts of these threats, degradation of these high value ecosystems is certain. Future research should further explore the impacts of climate change on ephemeral freshwater ecosystems more broadly and involve specific experimental studies on target taxa, as well as the establishment of long-term biomonitoring and custom barcode reference libraries, and the application of practical strategies to mitigate degradation. Abbreviations eDNA: Environmental DNA EP: Eyre Peninsula EWB(s): Ephemeral water body/bodies GRAC: Gawler Ranges Aboriginal Corporation GB: Gawler bioregion GRNP: Gawler Ranges National Park HNR: Hiltaba Nature Reserve