Quantifying responses of aquatic insects to environmental change

Aquatic biota inhabiting intermittent waterways exist across a broad spectrum of hydrological variation, where habitat conditions frequently change in availability and quality. In addition to recurrent drying, intermittent waterways of Australia periodically experience extreme dry spells (drought) and extreme heatwaves. Future climate change is predicted to increase climate variability and the frequency and intensity of extreme events across many regions. Although many taxa are adapted to the conditions of intermittent waterways, biodiversity loss is a potential consequence of climate change, where highly sensitive organisms cannot cope with unprecedented changes in their habitat conditions. Aquatic invertebrates are used globally as indicators of ecosystem condition in freshwater. Within the broader group of aquatic invertebrates, the insect orders Ephemeroptera (mayflies), Plecoptera (stoneflies), and Trichoptera (caddisflies) (EPT) are among the most sensitive to organic pollution. It is likely that as EPT are sensitive to pollution, they may also be sensitive to projected environmental changes related to increased climate variability, and hence be vulnerable to localised extinction. However, many EPT taxa occur in intermittent waterways, and are therefore adapted to survive in unstable habitats which may make them more resilient to changes in climate variability. Despite their use in bio-assessment, little is known on the behavioural patterns, habitat requirements, or tolerances to habitat disturbance for most Australian EPT species or even genera, and therefore the vulnerabilities of individual taxa to the effects of climate change are hard to predict. This thesis examines how the EPT of south-east Queensland (SEQ) respond to instream disturbances of drying and increased temperature, to identify taxa most at risk of localised extinction under projected hydrologic and thermal changes. Improved ecological understanding of these taxa was needed to address the thesis aims. This study addresses knowledge gaps of EPT genera including environmental drivers of assemblage composition, recruitment patterns, and traits that infer resistance or resilience to disturbance. Spatial and temporal variations in EPT assemblages were explored over one year through monthly sampling of four headwater streams. These sites represented a gradient in habitat conditions from cleared agricultural pasture to near pristine forest, and intermittent to perennial flow regimes. Sites were chosen to explore how EPT respond to the disturbance of both human alteration (e.g. reduced shade and altered morphology) and hydrologic variation. EPT richness was highest in habitats with the lowest anthropogenic disturbance; however abundances of these taxa were relatively low compared with cleared streams, where a few taxa of high abundance (from the families Caenidae, Baetidae, Leptophlebiidae, and Hydropsychidae) dominated the assemblages. Variation in riparian canopy cover and conductivity between streams, and habitat morphology within streams best explained the differences in EPT assemblages. Temperature and flow also influenced assemblages. Despite episodes of drying and flash flooding throughout the year, most taxa were recorded before and after these extreme events, indicating resistance and resilience to hydrological disturbance. Patters in late instar recruitment and responses to anthropogenic and hydrologic variation were also explored for ten genera using the temporal abundance data. Some taxa were collected in high abundances in the anthropogenically disturbed conditions or displayed high tolerance to conditions of hydrologic variability including low flow and low dissolved oxygen; whereas others have specific requirements, evident through their response to stream degradation, and recovery post hydrological disturbance. Information is presented on timing of peak late instar abundance for sampled taxa, which also revealed differences between genera of the same family, and regional differences when compared to studies from elsewhere in Australia. Some taxa were identified as having flexible recruitment patterns, well adapted to the frequent hydrological disturbance, whereas others were univoltine with slow development and a lagged recovery response to disturbance. The maximum thermal tolerance limits of EPT taxa were tested through laboratory experiments, to determine which species may be most vulnerable to temperature rises associated with climate change. These 50% lethal temperature (LT50) tests gave conservative estimates for lethal tolerances, by assuming that thermal thresholds were the only factor influencing survival of each taxa. However, although not tested within this thesis, increased temperatures may have other chronic affects that may cause localised extinction before temperature thresholds within an environment are reached. The results of this experiment revealed that water temperature conditions within the sampled locations of SEQ are already outside the maximum thermal thresholds of some taxa. However, a few taxa were found to tolerate even the highest projected temperatures for the near future climate change scenarios. The results of this study are regionally specific, as differences in tolerance limits were found for the same family groups when compared to other similar Australian and international studies. How aquatic biota respond to changes in water quality and availability experienced during hydrologic intermittency influences the likelihood of population persistence. The movement responses of caddisflies and mayflies during a drying event were explored using a field experiment, where an enriched 15N tracer was added to a pool in a flowing and a non-flowing stream, to isotopically label resident aquatic insects as a mark-recapture method. Aquatic and emergent insects were subsequently sampled over a three week period post-labelling. This was the first time this method has been used in Australia, and was found to be an effective means of tracking insect movements over short time-frames. The results indicated that mayflies and caddisflies do not prematurely emerge from pools as a rapid response to hydrologic intermittency; rather some taxa appeared to emerge in a seasonally synchronised manner, prior to seasonal drying. Others had less seasonally synchronised emergence and these were the taxa that were tolerant to conditions in the refugial pools. The isotope labelling suggested that no juvenile taxa moved between pool refuges through the hyporheos or across the terrestrial landscape. Movement of adult mayflies away from the emergence area did not occur in either stream, indicating that mayflies were not seeking alternate habitats. Further research is required to fully understand the movements of adult caddisflies. Current information on behavioural patterns, tolerances and survival traits of EPT taxa was then collated based on data obtained in this study and from available literature. Using this information, the EPT taxa of SEQ were assigned tolerance scores based on their responses to hydrological disturbance and thermal alteration, to evaluate potential responses to future climate change. This method identified nine taxa within the study area that may be vulnerable to localised extinction given projected hydrological disturbances. My assessment also highlighted critical knowledge gaps, including lifespans and ecological information on the adult life stage, which is needed to better understand the life histories of these taxa, and would strengthen the confidence of taxon-specific vulnerabilities to environmental changes. This thesis suggests that not all EPT taxa are sensitive, revealing that some taxa are resistant and resilient to both agricultural disturbances and hydrological variability. The EPT genera of SEQ each have diverse habitat requirements, tolerances, and behavioural patterns, developed from exposure to local conditions. However, the projected increases in hydrologic disturbances and thermal stress due to climate changes may directly or indirectly affect all species studied within this thesis, altering the biodiversity within the regions aquatic ecosystems. Taxa with generalist habitat requirements, flexible life histories, and high disturbance tolerances will most likely persist under future conditions. However, almost half of the taxa studied had specific requirements and limited tolerances; and consequently biodiversity loss has already occurred within streams with altered land use. These taxa are vulnerable to the projected changes, and likely to experience localised extinctions.