Can a naturally depauperate Ephemeroptera, Plectoptera and Trichoptera (EPT) fauna track river degradation in south-western Australia?

Freshwater aquatic ecosystems are threatened globally. Biological monitoring is required to deliver rapid and replicable assessment of changes in habitat quality. The Ephemeroptera, Plectoptera, Trichoptera (EPT) index is a globally recognised rapid bioassessment that measures taxa richness of three insect orders whose larvae are considered sensitive to freshwater habitat degradation. South-western Australia contains threatened freshwater ecosystems but has depauperate EPT fauna and high endemism, potentially reducing the capacity of the EPT index to track degradation. This study investigated if EPT species richness, composition or individual species tracked physical or chemical river degradation in three catchments in south-western Australia. We sampled EPT fauna and measured water chemistry, erosion, sedimentation, riparian vegetation cover and instream habitat at 98 sites in the winters of 2007 and 2023. We found 35 EPT taxa across the study area with a median number of species per site of two. EPT species richness had weak positive associations with a composite water quality index and dissolved oxygen and weak negative associations with electrical conductivity and total nitrogen. No association was found between physical and fringing zone degradation measures and EPT species richness. EPT community structure generally did not distinguish between sites with high or low degradation levels. The presence of the mayfly Nyungara bunni tracked salinity, dissolved oxygen and nitrogen levels, but its usefulness as a bioindicator could be limited by its restricted range. This study suggests that the EPT index would need modification or combination with other indices to be a useful rapid bioassessment in south-western Australia.

Riparian condition influences spider community structure and the contribution of aquatic carbon subsidies to terrestrial habitats.

Degradation of riparian zones can alter aquatic and terrestrial communities of flora and fauna and disrupt their role in assimilating and mobilising carbon between the two ecosystems. Riparian spiders that predate on emergent aquatic invertebrates can contribute to carbon flux and the structure of aquatic and riparian food webs. The impact of riparian degradation on spiders in temperate rivers of Australia and their role in this broader ecosystem function is poorly understood. We surveyed the riparian zone of four rivers of south-western Australia in areas of natural intact vegetation and degraded agricultural land to explore whether riparian spider abundance, and diversity may be affected by changes to riparian condition. We also assessed the impact of the riparian condition on carbon fluxes between aquatic and terrestrial environments, using stable isotope analysis. We found overall abundance of riparian spiders was higher in degraded agricultural sites compared to natural intact sites and the structure of spider assemblages was different. Orb-weaver spiders (Araneidae and Tetragnathidae) were found to be more abundant in agricultural areas where canopy cover and understory are sparse as a result of livestock grazing and trampling. The contribution of carbon from aquatic invertebrates in a natural intact site was 48.5% for Orb-weavers and 41.6% for Cursorial Hunter spiders but reduced to 19.6% and 39.9% respectively in a degraded agricultural site. These results suggest that the position of spiders in riparian food webs and the amount of aquatic subsidy may change according to the condition and complexity of the riparian zone.

Freshwater tributaries provide refuge and recolonization opportunities for mussels following salinity reversal.

Reversing the effects of secondary salinization, and its impacts on aquatic biodiversity, is a growing global challenge, and particularly prevalent in Mediterranean-climate regions. Remnant freshwater tributaries in salinized landscapes provide significant biodiversity values, including discrete areas of refuge, dilution of salinized reaches, and potential source populations for recolonisation. The importance of these areas for aquatic fauna is widely accepted but rarely evaluated in the field. This study explored how spatial distribution of southwestern Australia's only freshwater mussel species, Westralunio carteri, has responded to the ongoing salinity trend in the Kent River catchment. Our results showed that salinity in the river has begun to reverse following improved catchment management, and also detected the first evidence of an associated recovery of the freshwater mussel population. Mussels in the mainstem were limited to sites around and downstream of a permanently flowing freshwater tributary, suggesting that dilution from this source provides a refuge in the lower reach. At two of those sites, all individuals were <15 years of age, indicative of recolonisation coinciding with salinity reversal around the turn of the century. Interestingly, mussels clearly persisted in other parts of the lower reach throughout the peak salinity period, when salinities regularly exceeded laboratory derived toxicity thresholds for the species. Mussels were not found in the majority of the mainstem or in highly acidic parts of the freshwater tributaries. The presence of old shells at those sites shows that the species was once widespread, and that the current distribution probably reflects a contraction due to historical salinization as well as acidification. Overall, our results show that the W. carteri population in the catchment has taken a first step towards recovery, and highlights the importance of freshwater tributaries in providing both refuge from disturbance and a source of new recruits.

Hierarchical multi-taxa models inform riparian vs. hydrologic restoration of urban streams in a permeable landscape.

The degradation of streams caused by urbanization tends to follow predictable patterns; however, there is a growing appreciation for heterogeneity in stream response to urbanization due to the local geoclimatic context. Furthermore, there is building evidence that streams in mildly sloped, permeable landscapes respond uncharacteristically to urban stress calling for a more nuanced approach to restoration. We evaluated the relative influence of local-scale riparian characteristics and catchment-scale imperviousness on the macroinvertebrate assemblages of streams in the flat, permeable urban landscape of Perth, Western Australia. Using a hierarchical multi-taxa model, we predicted the outcomes of stylized stream restoration strategies to increase the riparian integrity at the local scale or decrease the influences of imperviousness at the catchment scale. In the urban streams of Perth, we show that local-scale riparian restoration can influence the structure of macroinvertebrate assemblages to a greater degree than managing the influences of catchment-scale imperviousness. We also observed an interaction between the effect of riparian integrity and imperviousness such that the effect of increased riparian integrity was enhanced at lower levels of catchment imperviousness. This study represents one of few conducted in flat, permeable landscapes and the first aimed at informing urban stream restoration in Perth, adding to the growing appreciation for heterogeneity of the Urban Stream Syndrome and its importance for urban stream restoration.

Predicting the likely response of data-poor ecosystems to climate change using space-for-time substitution across domains.

Predicting ecological response to climate change is often limited by a lack of relevant local data from which directly applicable mechanistic models can be developed. This limits predictions to qualitative assessments or simplistic rules of thumb in data-poor regions, making management of the relevant systems difficult. We demonstrate a method for developing quantitative predictions of ecological response in data-poor ecosystems based on a space-for-time substitution, using distant, well-studied systems across an inherent climatic gradient to predict ecological response. Changes in biophysical data across the spatial gradient are used to generate quantitative hypotheses of temporal ecological responses that are then tested in a target region. Transferability of predictions among distant locations, the novel outcome of this method, is demonstrated via simple quantitative relationships that identify direct and indirect impacts of climate change on physical, chemical and ecological variables using commonly available data sources. Based on a limited subset of data, these relationships were demonstrably plausible in similar yet distant (>2000 km) ecosystems. Quantitative forecasts of ecological change based on climate-ecosystem relationships from distant regions provides a basis for research planning and informed management decisions, especially in the many ecosystems for which there are few data. This application of gradient studies across domains - to investigate ecological response to climate change - allows for the quantification of effects on potentially numerous, interacting and complex ecosystem components and how they may vary, especially over long time periods (e.g. decades). These quantitative and integrated long-term predictions will be of significant value to natural resource practitioners attempting to manage data-poor ecosystems to prevent or limit the loss of ecological value. The method is likely to be applicable to many ecosystem types, providing a robust scientific basis for estimating likely impacts of future climate change in ecosystems where no such method currently exists.