School of Geography, Earth and Atmospheric Sciences - Research Publications
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ItemConnected headwaters: Indelible field evidence of dispersal by a diverse caddisfly assemblage up stream valleys to dry catchment boundaries(Wiley, 2023-10-24)1. Integral to many ecological models is the notion that dispersal connects populations and communities in disparate locations. For stream insects, however, there is very little empirical information about which species and what proportion of the species in a community are likely to disperse, in what direction and how far they travel, and whether dispersal is successful in connecting populations and communities. This study examines whether species of caddisfly can disperse between catchments and potentially connect communities. 2. We used a novel sampling design comprising five pairs of streams. Each stream in a pair flowed from the same boundary, but in opposite directions and into isolated catchments. We surveyed abundances of adult caddisflies on the catchment boundary (boundary species) where there was no water (Up location), and on two locations per stream (Middle, Down locations) where there was permanently flowing water (resident species). If caddisflies fly along the topographic features of stream valleys, then we expected to find adults on the ridge between catchments and, if dispersal is successful, stream communities in adjacent catchments should have similar species composition. 3. Over 130 species of caddisfly (from 18 families) were collected. Approximately half of them occurred on at least one catchment boundary, and Up locations averaged almost 20 species. Boundary species included both the smallest- and largest-bodied taxa and they were a near perfect subset of species in resident assemblages. However, boundary species were not simply the most common in resident assemblages. There was no evidence of sex-biased dispersal. Unsurprisingly, assemblage composition varied across the landscape, within and among catchments. However, resident assemblages within stream pairs were significantly more similar to one another than unpaired streams in either the same or in different catchments. 4. Our results suggest that a surprisingly diverse set of caddisflies disperse up to catchment boundaries and potentially connect streams on either side of catchment divides. Nevertheless, many other caddisfly species were never recorded at Up sites and hence may never or only rarely reach catchment boundaries. 5. These outcomes suggest, firstly, that some caddisflies are capable of trans-catchment dispersal but it is incorrect to assume that all caddisflies are good dispersers, as is often stated in the literature. Secondly, caddisfly assemblages in headwater streams may be less isolated than is often considered and the dendritic structure of stream networks may be less important for their distribution patterns than the proximity of tributaries in neighbouring catchments, in many landscapes. Trans-catchment dispersal could ensure community connectivity and facilitate recovery after catchment-wide extinctions.
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ItemExperimental increases in detritus boost abundances of small-bodied fish in a sand-affected stream(WILEY, 2022-04)Abstract Restoration projects often rely on the assumption that a local intervention will restore diminished populations, without fully understanding the constraints that limit the target species in the first place. In rivers, one common restoration technique is to place large structures, such as wood and boulders, on the bed, with the assumption that fish will subsequently arrive and use them. Nonetheless, providing large habitat structure may not overcome demographic or resource constraints on fish populations, and thus may not aid recovery. We aimed to test if resource constraints (food and cover) are limiting local densities of fishes in a degraded stream by experimentally alleviating these constraints. If the abundance of one or more species is constrained by resource availability, then local numbers of these species should increase following an increase in resources. To test our prediction, we increased the availability of food and microhabitat complexity (cover) at sites in Hughes Creek, a degraded stream in south‐east Australia that has extensive accumulations of sand and limited in‐stream structure. At treatment sites, we hammered pairs of wooden stakes (25 cm apart) into the stream bed so that the ends of stakes protruded just above the water surface at moderate flows. Stakes effectively trapped passing sticks and leaves, which increased local detrital densities and, subsequently, invertebrate densities, hence providing food and cover for fish. Over the course of a year, we compared the changes in fish abundances at treatment sites to unmanipulated control sites. Fish responded quickly to enhanced retention of detritus, with assemblage differences observed between treatment and control sites. We caught more river blackfish (Gadopsis marmoratus), southern pygmy perch (Nannoperca australis), Macquarie perch (Macquaria australasica), and mountain galaxias (Galaxias olidus) at treatment sites on some occasions, indicating that these species may be subject to resource constraints in this stream. The magnitude of observed positive fish responses was influenced by the life‐stage of individuals and local stream conditions. Importantly, treatment effects varied through time and were no longer observed after a large flood affected the study stream. Our results show that resource constraints limit local species abundances and demonstrate a novel method of overcoming these constraints in a small, degraded stream. This is a necessary first step but future work is needed to examine whether increases in abundance are due to the provided resources increasing growth rates, survivorship, or reproduction. This work also highlights the importance of understanding species’ life histories, the broader landscape setting, and the disturbance regime when undertaking site scale restoration.
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ItemNo Preview AvailableA large-scale field experiment across six rivers illustrates how the effects of resource enrichment are context dependent(SPRINGER, 2023-05)Resource supplementation can increase species richness and change the faunal composition of communities, but experiments have produced variable outcomes. An often overlooked element is that species richness can only increase if new taxa can disperse to resource-rich locations and invade established, local communities. We experimentally increased a basal resource (detritus) in six rivers in south-eastern Australia by driving wooden stakes into the riverbed to increase retention of detritus. Control sites were left untreated. Sites were located in agricultural sections with mostly cleared vegetation, but with intact (uncleared) reference sites upstream to provide sources of prospective colonists. We measured channel retentiveness and sampled benthic detritus and invertebrates before and after manipulation. We tested whether: greater retentiveness increased detritus densities, species richness and abundances and altered faunal composition; manipulation sites reached bio-equivalence with reference sites; new species arose from upstream reference areas; and whether outcomes were consistent across rivers. Only three rivers gained increases in detritus densities. All had low pre-existing amounts of in-stream wood compared with rivers that did not respond to treatment. Two rivers (Hughes Creek, Seven Creeks) gained higher species richness and invertebrate densities within 12 months and reached bio-equivalence with reference sites. In contrast, Turtons Creek showed species turnover through replacement of individuals. Only in Hughes Creek was there evidence of successful dispersal from the upstream reference area. The outcomes show that the effects of resource supplementation vary between rivers and suggest that pre-existing conditions (e.g. channel retentiveness) may cause these differences, providing clear evidence of context dependence.
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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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ItemThe influence of geomorphology and environmental conditions on stratification in Intermittently Open/Closed Estuaries(Elsevier, 2023-07-05)Intermittently Open/Closed Estuaries (IOCE) have entrances that close during periods of low river flow. A major characteristic of IOCE is stratification of salinity, dissolved oxygen and water temperature. After IOCE open (either naturally or artificially), large changes in stratification occur as water drains from the estuary to the ocean. The rapid change in water level and loss of the top oxygenated layer of the water column during drainage often causes fish kills in IOCE globally and is related to stratification. Despite this, there are a lack of studies that statistically analyse the relationships between environmental variables and stratification and that quantify changes to stratification during the draining period across multiple IOCE. To fill these gaps, we (1) analysed the relationships between environmental variables and stratification using distance-based Linear Models (distLM) and distance-based Redundancy Analysis (dbRDA) in five different IOCE across Victoria, Australia, and (2) measured near-continuous physicochemical depth profiles and changes in entrance morphology, fluvial inflows, and estuary water level following four estuary openings. The distLM results revealed that maximum air temperature, mouth state and fluvial inflows showed statistically significant relationships with stratification in the IOCE studied. The dbRDA suggested that high maximum air temperatures were associated with low values of stratification in small IOCE, more commonly during closed periods. High fluvial inflows were associated with low values of stratification in large IOCE during open periods (except at one site, Curdies River). Field observations of changes in stratification during the draining period revealed two distinct responses. First, a high energy opening with discharge at the mouth between 70 and 182 m3s-1 and fluvial inflows of 0.87–1.85 m3s-1, causing the IOCE water column to mix and become uniform. Second, a low energy opening with discharge from the mouth between 6 and 37 m3s-1 and fluvial inflows of 0.02–0.03 m3s-1, causing the IOCE to remain stratified. These findings were summarised into a conceptual model showing the sequence of changes during openings for different types of IOCE. Over longer timescales (days to years), our results suggest that differences between stratification during open and closed periods are reflected at a shorter time-scale during the draining period (hours to days). These differences further reflect differences in geomorphology and hydrology between IOCE. Our findings will be useful for estuary managers to predict how stratification in different types of IOCE will change during artificial openings and provide a proxy for predicting their response to climate change.