School of Geography, Earth and Atmospheric Sciences - Research Publications
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ItemInter-specific variation in caddisfly fecundity depends on body size and larval feeding habits(Australian Freshwater Sciences Society, 2022-12-01)Reproductive output (fecundity) is one of the core life history (LH) parameters, because it determines an individual’s fitness and, ultimately, the potential for populations to grow or recover from catastrophic disturbance. Intra-specifically, phenotypic plasticity in LH parameters such as fecundity can co-vary with environmental factors (e.g. temperature) and other LH parameters (e.g. biomass), often in “ecological trade-offs”. Inter-specifically, a species’ fecundity is fixed (genetically determined), but can differ between species as a result of evolutionary processes and “intrinsic trade-offs” or covariance with other LH traits. It is well established that fecundity increases with body size within species; this study tested whether a similar relationship occurs between species of caddisfly. Caddisflies are capital breeders (resources for reproduction are acquired during larval life) so I also tested whether a fecundity–body size relationship co-varied with feeding habit, another LH trait. Using data from the literature and my own, empirically determined estimates of caddisfly fecundity, I demonstrate a positive relationship between fecundity and body size between species. The variance explained however was low. Model fit improved markedly when feeding group was included as a co-variate. Slopes of the fecundity–body size relationships differed with feeding group: the rank order according to decreasing slope was predators > filter-feeders > detritivores > algivores. Although some caddisfly families are associated with particular feeding habits, phylogenetic relationships did not underpin these relationships. The mechanism by which diet influences fecundity in species with different alimentary systems is unclear, but may reflect different strategies of resource allocation to food acquisition vs reproduction. Overall, this evidence suggests that fecundity, and therefore potential rates of population growth, vary between taxa in unexpected ways that have not been considered previously.
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ItemIndelible field evidence of extensive dispersal by caddisflies up river valleys to dry catchment boundaries(Australian Freshwater Sciences Society, 2022-12-01)We know little about the routes travelled by adult caddisflies when flying between streams, nor what proportion of the local species pool will disperse. For movement between major catchments (e.g. rivers with separate outflows into the marine environment), the shortest route may be to fly up one stream valley to the catchment boundary, and then down a valley into the adjacent catchment. If caddisflies do fly along these topographic features created by stream valleys, then we expect to find adults on the ridge between catchments and, if this dispersal is successful, high community similarity between streams on opposite sides of the boundary. In five pairs of streams that each share a catchment boundary, we sampled adult caddisflies at the streams margins (“resident spp.”) and on the boundary ridge (“boundary spp.”). From a species pool of >120 spp., approximately 50% were boundary species, and this group was taxonomically and morphologically diverse. Species composition did not differ between resident assemblages within stream pairs. Boundary species dominated resident assemblages in terms of species numbers and overall abundance of individuals. However, boundary species were not just those that were the most common within assemblages. Dispersal was sex-biased for only a few species, but not of a direction or magnitude likely to constrain demographic connectivity. Overall, this evidence suggests that there is extensive movement of diverse caddisflies over catchment boundaries, and to our knowledge has not been shown before for such a wide range of different species. This dispersal could ensure connectivity between communities and facilitate recovery of populations after local extinctions.
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ItemExperimental tests of eggs-tinction! Shortages of adults or egg-laying habitat cause oviposition failure for aquatic insects in dammed and undammed rivers.(Australian Freshwater Sciences Society, 2022-12-01)For organisms with complex life cycles, shortages of females or egg-laying (oviposition) habitat can limit egg densities, recruitment of offspring and abundances of later life stages. Hence, the determinants of oviposition success may be critical for understanding how human activity impacts stream ecosystems. Many aquatic insects require clean, emergent rocks that project from streams as places to land and lay eggs. In undammed rivers, high nutrient loads and low summer flows can result in dense growths of algae on rocks (algal encrustment) which may inhibit oviposition. Downstream of dams, water impoundment may intensify algal encrustment or strand rocks above the waterline, potentially limiting oviposition and subsequent larval recruitment. Regular surveys (2018-2021, five dammed and undammed rivers) in the Murrumbidgee Catchment, NSW, recorded widespread oviposition failure during summer, despite presence of adult insects (Hydrobiosidae) in all rivers. Algal encrustment was observed at all sites, so this mechanism was identified as a potential inhibitor of oviposition. We tested whether oviposition was limited by insufficient adults or habitat via field experiments that removed algae from rocks, with concurrent trapping of adults. We predicted that eggs would be laid exclusively on clean habitat. In undammed rivers, eggs were laid exclusively on clean habitat, thereby showing that algae inhibited oviposition. In the dammed river, zero eggs were found despite significant habitat supplementation (~2000 clean, emergent rocks across eight sites). Captured adults in the dammed river were scarce relative to the undammed rivers, hence oviposition failure may have been caused by shortages of adults, habitat or both. These results illustrate that oviposition failure can occur when egg-laying habitat is lost due to human activity. However, successful oviposition may also require periods when habitat and adults are concurrently abundant.
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ItemAquatic-terrestrial transitions: When, where and how are populations of aquatic insects regulated?(Global Freshwater Science, 2023-06-05)Population sizes of aquatic insects are hugely variable in time and space. Why is that? Questions like that one have often been put into the too-hard basket, yet answering big, difficult questions is fundamental to scientific progress. We cannot hope to advance our understanding of what promotes or maintains biological diversity without tackling such thorny questions – nor can we hope to understand how to arrest the current extinction crisis. In this talk, I set out the challenges presented by animals that have complex life cycles (i.e. many different life stages) and - in the case of aquatic insects - live and disperse in both the aquatic and terrestrial environments. Populations of such species may be regulated or limited at any life stage and in either or both environments; that’s the challenge. I set out some of the theory that has been erected to address this kind of complexity and review briefly its success in progressing knowledge. I will then highlight some progress that has been made for riverine fauna but also the many, significant knowledge gaps that remain. Filling some of those gaps is fundamental - both for understanding what drives variability in numbers in space and time, but also the steps we might take to halt or reverse declines in biological diversity. Many of these gaps can be addressed by projects of a length that make them highly suitable as topics for PhD students or grant applications, all of which can be warranted by standing on a solid theoretical foundation for why such research matters. Along the way, I will note some things I’ve learned that have come from asking scientific questions for some 40+ years, which I hope may be of particular assistance to those who are just starting or still early in their careers.
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ItemRobust tests of dispersal in aquatic insect communities: snakes and ladders(Global Freshwater Science, 2023-06-05)Integral to many models of ecological communities is the notion that dispersal connects communities in disparate locations. Significant dispersal entails permanent movement of individuals from natal to reproductive locations, and on ecologically relevant scales. Robust tests of these models for aquatic insect communities are scarce. Some approaches are flawed, "snakes", whereas others show promise, "ladders". Snakes include inferring dispersal from spatial patterns of juveniles, using traits that lack empirical evidence, or ignoring critical life history traits. Ladders typically involve examining dispersal stages directly (winged adults), combined with elegant sampling designs that provide unambiguous evidence of dispersal and meaningful insights. We applied a novel survey design to test whether caddisflies disperse between major catchments (rivers with separate outflows into the sea), where the shortest route is to fly up one stream valley to the catchment boundary, and then down a valley into the adjacent catchment. If caddisflies fly along the topographic features of stream valleys, then we expect to find adults on the ridge between catchments and, if dispersal is successful, high similarity between stream communities in adjacent catchments. In five pairs of streams that each share a catchment boundary, we trapped adult caddisflies at the streams margins ("residents") and up on the boundary ridge, where there was no running water. Thus, caddisflies on the boundary must have dispersed. From a pool of >130 species, approximately half occurred on the boundary. Similarity in species composition was significantly higher between resident assemblages within stream pairs than between streams in the same catchment. This suggests that movement of diverse caddisflies over catchment boundaries is extensive and potentially greater than between streams within the same catchment, and therefore, the branching structure of streams may not dominate metacommunity structure. Importantly, such trans-catchment dispersal could ensure community connectivity and facilitate recovery after catchment-wide extinctions.
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ItemField-based evidence that wing metrics can be used to identify good dispersers(Global Freshwater Science, 2023-06-04)Dispersal is a key aspect of population dynamics, but, for winged insects, we know little about the characteristics that make some better dispersers than others. Insects with relatively large and long, pointed wings are hypothetically better at powered flight and hence dispersal than those with relatively small, rounded wings. However, tests have been largely impossible given the difficulty of tracking dispersing individuals. To solve this, we used a novel sampling design. In five pairs of hydrologically isolated streams that each share a catchment boundary, we sampled adult caddisflies at stream margins and up on the boundary, where there was no running water, i.e. caddisflies trapped on a boundary had to have dispersed. Of >130 spp., ~ half were trapped on a boundary (“boundary species”), while the other half were never found on a boundary (“lowland species”). We removed a single pair of wings from 10-30 males of multiple boundary and lowland species. Wings were mounted on microscope slides and photographed. Using images of coupled wings, we measured four wing metrics - wing span and area, aspect ratio and the second moment of wing area - to characterise wing sizes and shapes. Boundary species typically had greater wing areas and wing spans and higher aspect ratios than lowland species. Species with the greatest wing areas (e.g. Triplectides similis, Leptoceridae) or highest aspect ratios (e.g. Hellyethira simplex, Hydroptilidae) were all boundary species. However, values of both those metrics spanned a wide range, and similar values were shared by some boundary and lowland species. Our results show that wing area and span and aspect ratio can be used to identify good dispersers, and hence these metrics show promise for use as evidence-based, dispersal traits. However, overlap between some boundary and lowland species show that behaviour (i.e. choosing to disperse) must also be considered.
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ItemRules of reengagement: integrating geomorphology into river diversion designs(River Basin Management Society, 2020)There are many river diversions around mining pits in Australia. Poor performance of these diversions has led to stricter guidelines for their design, including better appreciation of geomorphic context. The Pilbara region in Western Australia is an area with many open-pit mines and river diversions. There is a poor understanding of the regional watercourses and limited guidelines for the incorporation of geomorphic and environmental elements into river diversion designs. We developed a series of hydrogeomorphic guidelines for headwater channels in the Eastern Pilbara, Western Australia. We undertook a large-scale regional geomorphic analysis of headwater streams, before focusing on the variability in river reach form. Using Structure-from-Motion photogrammetry (SfM), the presence and distribution of channel features were mapped. The result was a high resolution ‘recipe’ or classification of features for a river addressing the natural morphology, roughness contribution and character of natural rivers within the Pilbara. Direct rainfall modelling provided appropriate rainfall flood frequency estimation for these small ungauged catchments. This knowledge was integrated to produce a series of guideline hydraulic criteria for the various headwater channels. These guideline hydraulic criteria can be used to design river diversions, in addition to helping us understand more about the complexity and variability of headwater channels within the Eastern Pilbara. These criteria can also help to set ‘closure criteria’ which specify the conditions that mine owners have to meet to return the mine to public ownership.
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ItemThe Evolution of Corryong/Nariel Creek since European Settlement: Implications for On-going Management Prioritisation(River Basin Management Society, 2014)