CURRENT RESEARCH
CROSSLINK: CROSS-HABITAT LINKAGES FOR BIODIVERSITY AND ECOSYSTEM FUNCTIONING
SLU: SWEDISH UNIVERSITY OF AGRICULTURAL SCIENCES, DEPARTMENT OF AQUATIC SCIENCES AND ASSESSMENT
Riparian habitats are ecological boundaries characterized by an intimate link between aquatic and terrestrial ecosystems. These habitats are essential for landscape heterogeneity and maintaining multifunctionality at broad spatial scales, which include ecosystem services such as nutrient cycling, carbon sequestration, and freshwater provision. However, human pressures from activities such as agriculture frequently harm the integrity of these coupled ecosystems. The CROSSLINK project is facilitated through BiodivERsA and involves scientists from Sweden, Norway, Romania, Belgium, and Germany. Led by researchers at SLU in Uppsala, CROSSLINK is an ambitious project aiming to better understand the pivotal role of woody riparian buffers in managed landscapes. We have focused on various aspects of biodiversity and ecosystem functioning, including food-web linkages across the stream-riparian boundary.
The paper below serves to introduce the project and demonstrates how CROSSLINK will help inform riparian management. This publication also helps to provide context for our other papers in the Water special issue Ecosystem Functioning in Rivers and Riparian Zones. I led the analysis and writing, and our paper touches on concepts I explored in my postdoc at Eawag in Switzerland (i.e., "magnitude of local change" vs "environmental context" - see below for more details).
For further information see:
Burdon, F. J., et al. 2020. Assessing the benefits of forested riparian zones: A qualitative index of riparian integrity is positively associated with ecological status in European streams. Water 12: 1178; DOI: 10.3390/w12041178
The paper below serves to introduce the project and demonstrates how CROSSLINK will help inform riparian management. This publication also helps to provide context for our other papers in the Water special issue Ecosystem Functioning in Rivers and Riparian Zones. I led the analysis and writing, and our paper touches on concepts I explored in my postdoc at Eawag in Switzerland (i.e., "magnitude of local change" vs "environmental context" - see below for more details).
For further information see:
Burdon, F. J., et al. 2020. Assessing the benefits of forested riparian zones: A qualitative index of riparian integrity is positively associated with ecological status in European streams. Water 12: 1178; DOI: 10.3390/w12041178
FATTY ACID BIOMARKERS REVEAL LANDSCAPE INFLUENCES ON LINKAGES BETWEEN AQUATIC AND TERRESTRIAL FOOD WEBS |
Stream and riparian habitats are meta-ecosystems that can be strongly connected via the emergence of aquatic insects, which form an important prey subsidy for a wide range of terrestrial consumers. However, human perturbations that impact these habitats may propagate across traditional ecosystem boundaries, thus disrupting aquatic-terrestrial food-web linkages. Consequently, there is an increasing appreciation for measuring cross-ecosystem connectivity as an indicator of anthropogenic disturbance, but it remains poorly understood how this system property responds to mitigation and restoration efforts involving riparian buffers. Here we investigate how buffer extent, composition, and network location influences cross-ecosystem connectivity at stream locations in temperate European mixed-use landscapes. We used polyunsaturated fatty acid (PUFA) biomarkers to measure putative aquatic linkages to terrestrial predators (spiders, beetles). Preliminary variation partitioning analysis indicates that buffer extent and composition explains a small, but significant proportion of the variability in the terrestrial fatty acid profile after accounting for predator identity and network position. Using structural equation modelling, we attempt to show a linkage between aquatic insect communities and the PUFA content of terrestrial predators as mediated by buffer extent and composition.
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Responses may reflect terrestrial predator identity, which can differ considerably with buffer presence. Our preliminary findings suggest that riparian buffers potentially alter aquatic-terrestrial food-web linkages by increasing subsidy quality whilst reducing overall system productivity. Using PUFAs in conjunction with other tools to describe cross-ecosystem connectivity will provide a more comprehensive means to assess environmental change and the efficacy of nature-based solutions (i.e., riparian buffers) intended to offset human impacts.
For more information: A working draft is available on request
For more information: A working draft is available on request
POSTDOC RESEARCH
CONTEXT DEPENDENT IMPACTS OF POLLUTION IN STREAMS
EAWAG: SWISS FEDERAL INSTITUTE OF AQUATIC SCIENCE AND TECHNOLOGY
Ecosystems around the world are increasingly impacted by multiple anthropogenic pressures. One pervasive threat to aquatic ecosystems is pollution through diffuse and point-sources associated with human activities. Runoff and leaching from agricultural lands can contribute sediment, nutrients, and pesticides to receiving environments. Similarly, urban areas with associated human populations lead to increased contaminant loads from runoff and wastewater discharges.
In particular, human land use pressures may lead to excessive inputs of nutrients, causing eutrophication and degraded water quality. However, these ‘macropollutants’ potentially obscure the role of 'micropollutants', which can exert pervasive negative effects on aquatic ecosystems. 'Micropollutants' include mixtures of toxicants (e.g., pesticides and pharmaceuticals) at individually low concentrations. |
Combined, these pollutants potentially have adverse impacts that propagate throughout food webs, exerting negative direct (i.e., acute and chronic toxicity) and indirect effects (e.g., via biotic interactions) that may contribute to the reduction of biodiversity and altered ecosystem functioning. Understanding the effects of these multiple stressors requires innovative analyses of observational data, coupled with experimentation and the development of theoretical models to validate empirical evidence.
For my postdoctoral research at Eawag, I contributed to an ambitious project (Ecoimpact) which aimed to disentangle the causal agents of wastewater impacts in stream ecosystems. The observational component of this project involved sampling 24 wastewater-impacted streams across the Swiss Plateau and the Jura Mountains. Measurements made at these sites include macroinvertebrate communities and functional indicators, including detrital processing rates using litter bags and the cotton-strip assay. I continue to collaborate with my colleagues at Eawag on the research conducted during this phase of my career.
For my postdoctoral research at Eawag, I contributed to an ambitious project (Ecoimpact) which aimed to disentangle the causal agents of wastewater impacts in stream ecosystems. The observational component of this project involved sampling 24 wastewater-impacted streams across the Swiss Plateau and the Jura Mountains. Measurements made at these sites include macroinvertebrate communities and functional indicators, including detrital processing rates using litter bags and the cotton-strip assay. I continue to collaborate with my colleagues at Eawag on the research conducted during this phase of my career.
MULTIPLE STRESSORS DRIVE COMPLEX RESPONSES IN STREAM MICROBIAL COMMUNITIES AND ECOSYSTEM PROCESSESMultiple anthropogenic drivers are changing ecosystems globally, with a disproportionate and intensifying impact on freshwater ecosystems. These effects arise through activities that directly modify the environment (e.g., landscape disturbance), as well as indirectly through the alteration of material cycles (e.g., nutrient pollution) and the creation of human constructs (i.e., novel ecosystems), such as wastewater treatment plants (WWTPs). Initially designed to greatly reduce nutrient loads, WWTPs increasingly release a multitude of micropollutants (i.e., low concentration chemical pollutants) and microbes (including antibiotic‑resistant bacteria) to receiving aquatic environments.
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These pollutants may have pervasive impacts on biodiversity and ecosystem services. One important ecosystem process in aquatic systems is organic-matter decomposition, which can be strongly mediated by the influence of temperature and nutrient availability on microbial communities. We combined field and laboratory experiments to study the effects of treated wastewater on organic-matter processing using standardized cotton-strip assays (CSA).
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Firstly, a large-scale field survey at 60 locations across Switzerland showed that decomposition rates were higher below wastewater discharges. Secondly, next-generation sequencing of microbial communities on cotton strips indicated that community structure below WWTPs differed significantly from communities upstream, with fungal richness greatly reduced. Furthermore, decomposition rates could be associated with variation in microbial community structure. Thirdly, flume experiments with mixtures of river water and treated WWTP effluent confirmed that diluted wastewater had a positive influence on microbially-mediated decomposition rates. Two continuous dosing experiments revealed negative effects of micropollutants on decomposition which were masked by the addition of nutrients. Finally, transplant experiments suggested that wastewater-borne microbes enhance rates of decomposition. Taken together, our results affirm the multiple-stressor paradigm by showing that different components of wastewater (i.e., warming, nutrients, microbes and micropollutants) can jointly influence ecosystem functioning in receiving habitats, sometimes with effects opposing each other. Net positive effects of WW inputs causing increased respiration rates may lead to ecosystem 'disservices' via greater efflux of carbon in streams and rivers. However, toxic MP effects may fundamentally alter ecological scaling relationships, indicating the need for a rapprochement between ecotoxicological and macroecological perspectives.
For more information:
Burdon, F. J., et al. 2020. Stream microbial communities and ecosystem functioning show complex responses to multiple stressors in wastewater. Global Change Biology 26: 6363–6382. DOI: doi.org/10.1111/gcb.15302
For more information:
Burdon, F. J., et al. 2020. Stream microbial communities and ecosystem functioning show complex responses to multiple stressors in wastewater. Global Change Biology 26: 6363–6382. DOI: doi.org/10.1111/gcb.15302
DOES CONTEXT INFLUENCE COMMUNITY RESISTANCE TO DISTURBANCE?Assessments of stream health are often based on the use of macroinvertebrates as indicators. This means they are ideal for testing predictions about what factors influence community resistance (or sensitivity) to anthropogenic perturbations. Resistance is a major component of ecological stability, and reflects the property of communities or populations to remain "essentially unchanged" when subject to disturbance. The inverse of resistance is sensitivity - sensitive species or communities show large changes when exposed to environmental stress or disturbance. One objective of my research is to better understand how much community change in response to wastewater perturbations is context dependent.
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Environmental contingency might reflect community sensitivity, mediated through the upstream context (e.g., catchment and local habitat acting as filters on assemblages). This contrasts with the magnitude of disturbance as the principal driver of change (e.g., wastewater dilution factors). My research will contribute to a better understanding of anthropogenic impacts across spatially heterogeneous landscapes, where influences at the watershed and local scales shape macroinvertebrate community structure, and therefore determine the type of assemblages exposed to spatially-discrete disturbances such as wastewater discharges.
For more information:
Burdon, F. J., M. Reyes, A. Alder, A. Joss, C. Ort, K. Räsänen, J. Jokela, R. Eggen and C. Stamm. 2016. Environmental context and magnitude of disturbance influence trait-mediated community responses to wastewater in streams. Ecology and Evolution 6: 3923–3939. DOI: 10.1002/ece3.2165
Burdon, F. J., N. A. Munz ,M. Reyes, A. Focks, A. Joss, K. Räsänen, F. Altermatt, R.I.L. Eggen, C. Stamm. 2019. Agriculture versus wastewater pollution as drivers of macroinvertebrate community structure in streams. Science of The Total Environment 659: 1256–1265
For more information:
Burdon, F. J., M. Reyes, A. Alder, A. Joss, C. Ort, K. Räsänen, J. Jokela, R. Eggen and C. Stamm. 2016. Environmental context and magnitude of disturbance influence trait-mediated community responses to wastewater in streams. Ecology and Evolution 6: 3923–3939. DOI: 10.1002/ece3.2165
Burdon, F. J., N. A. Munz ,M. Reyes, A. Focks, A. Joss, K. Räsänen, F. Altermatt, R.I.L. Eggen, C. Stamm. 2019. Agriculture versus wastewater pollution as drivers of macroinvertebrate community structure in streams. Science of The Total Environment 659: 1256–1265
POLLUTION IMPACTS ON 'BOTTOM-UP' AND 'TOP-DOWN' MEDIATED STREAM FUNCTIONING
Pollution of aquatic ecosystems is a global problem that threatens biodiversity and ecosystem services. How these ecosystems respond to added pressures in the presence of existing anthropogenic stress, and the best ways to disentangle the underlying mechanisms driving changes remain poorly understood. We used a replicated ‘real-world experiment’ to study ecosystem responses (e.g., decomposition) to an additional pressure (i.e., discharges of treated wastewater) across a land-use intensity gradient in twenty Swiss streams. At a total of 60 sampling locations, we performed detrital processing assays using over 1000 fine and coarse mesh leafpacks.
We predicted that toxicants present in wastewater would damage the role of detritivorous invertebrates (a negative ‘top-down’ effect), but increased nutrient concentrations and other changes associated with the input of treated effluent would enhance the microbial processing of organic matter (a positive ‘bottom-up’ effect). We also predicted that wastewater impacts on ‘top-down’ and ‘bottom-up’ effects would be contingent on upstream abundances of detritivores, which in turn would be mediated by stream trophic status. We found that wastewater perturbs the contribution of shredding invertebrates, but seemingly enhances the role of microbes in detrital processing through localised stream warming. |
We used structural equation modelling (SEM) to demonstrate that different human pressures contribute pesticides and nutrients to stream ecosystems with subsiby-stress effects on a keystone detritivore. This helped explained non-linear responses to nutrients, and provided potential causal mechanisms for negative effects of treated wastewater on stream ecosystem functioning.
Overall, our study is the first to show that over a broad range of sites, wastewater impacts consistently reduce decomposition rates of leaves in streams. However, the strength of this impact is context-dependent, mediated by stream trophic status among other factors. Wastewater pollution causes asymmetry in stress across trophic levels, but the positive influences of localised stream warming on microbial decomposers are insufficient to compensate for the impairment to stream detritivores.
For more information: A working draft is available on request
Overall, our study is the first to show that over a broad range of sites, wastewater impacts consistently reduce decomposition rates of leaves in streams. However, the strength of this impact is context-dependent, mediated by stream trophic status among other factors. Wastewater pollution causes asymmetry in stress across trophic levels, but the positive influences of localised stream warming on microbial decomposers are insufficient to compensate for the impairment to stream detritivores.
For more information: A working draft is available on request
WHAT CAN RANKED ABUNDANCES TELL US ABOUT ANTHROPOGENIC CHANGE?
One fundamental macroecological pattern is the hollow curve of species ranked by abundance, also known as ranked abundance distributions (RADs). These describe that frequently, communities consist of a few abundant and many rare species. Despite their ubiquity, the underlying processes controlling the shape of RADs are still the focus of ongoing discussion.
Moreover, despite many studies having found strong negative effects of anthropogenic disturbances on biodiversity, only a few researchers have considered these impacts with regards to RADs. This is surprising, considering their potential for explaining changes in biodiversity patterns at a meso-level. The aim of this research was to assess how well RADs explain pollution-induced changes in stream invertebrate biodiversity, and to extend the paradigm to consider dynamics of regional distributions. For more information: Burdon, F. J. What can ranked abundance distributions tell us about anthropogenic change in stream ecosystems? Poster presented at BES Aquatic Macroecology Meeting, London, UK, September 2016. |
The 'core-satellite' hypothesis posits that as taxa become regionally more common, their mean local abundance should also increase. Disturbances that occur frequently in stream networks, such as wastewater pollution shown here seemingly disrupts this relationship.
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PHD RESEARCH
SEDIMENT INFLUENCES ON STRUCTURE AND FUNCTION IN AGRICULTURAL STREAMS
UNIVERSITY OF CANTERBURY, CHRISTCHURCH, NEW ZEALAND
1. COMMUNITY-SEDIMENT ASSOCIATIONS IN AGRICULTURAL STREAMS
Land management practices associated with agriculture, plantation forestry, and urbanisation have contributed to the increased movement of diffuse pollutants including fine inorganic sediment (< 2 mm grain size) into streams, rivers, and their receiving environments. Sediment is a pervasive stressor that can dramatically affect the biodiversity and ecology of aquatic habitats through a variety of pathways, including the modification of ecosystem structure (e.g., habitat alteration and loss). As part of my Ph.D research, I investigated the influences of sediment on invertebrate communities by surveying 30 agricultural streams along gradients of deposited sediment and dissolved nutrients across the Canterbury Plains in the South Island of New Zealand.
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Invertebrate community composition and faunal traits changed significantly along the gradient of fine sediment, whereas the effects of dissolved nitrates were weak. In particular, % EPT (mayflies and caddisflies) demonstrated a strong non-linear response to sediment, indicating a threshold of approximately 20% sediment cover. This change in community composition was also accompanied by a decrease in average invertebrate body size, thus indicating potential trait-mediated responses. Path analysis indicated that these changes were strongly driven by reduced habitat availability (e.g., infilling of substrate interstices).
For more information:
Burdon, F. J., A. R. McIntosh, and J. S. Harding. 2013. Habitat loss drives threshold response of benthic invertebrate communities to deposited sediment in agricultural streams. Ecological Applications 23:1036-1047.
For more information:
Burdon, F. J., A. R. McIntosh, and J. S. Harding. 2013. Habitat loss drives threshold response of benthic invertebrate communities to deposited sediment in agricultural streams. Ecological Applications 23:1036-1047.
2. EFFECTS OF ENVIRONMENTAL CONTEXT ON BIOTIC INTERACTIONS
Ecological theory indicates that environmental context can strongly affect the outcome of biotic interactions including predation. Sedimentation can remove interstitial refugia, and may contribute to declines in vulnerable taxa through increased mortality and emigration. Using two large predatory insects and a common mayfly prey as a model system enabled me to investigate this experimentally. Both predators significantly increased emigration rates of prey where sediment simplified habitat. Consumptive effects were variable, but the more active, foraging predator exerted greater predation pressure upon prey where sediment removed refugia. Moreover, this consumption was size-biased on larger individuals, contributing to a decrease in the average size of the surviving prey in sediment treatments.
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Such effects on vulnerable taxa may be prevalent in real ecosystems where large mobile consumers are subsidized by inputs of prey from adjacent riparian zones and unaffected upstream reaches. By modifying habitat, sediment may distort biotic interactions and contribute to the decline in sediment-sensitive taxa. The shift in average invertebrate body size observed across the sediment gradient, coupled with my experimental results, also raises interesting questions about trait evolution and eco-evolutionary feedbacks between predators and prey.
3. INFLUENCE OF SEDIMENT DISTURBANCE on stream food Webs
Natural and anthropogenic disturbances commonly alter patterns of biodiversity and ecosystem functioning. However, how networks of interacting species respond to these changes remains poorly understood. We described aquatic food webs using invertebrate and fish community composition, functional traits, and stable isotopes from twelve agricultural streams along a landscape disturbance gradient. We predicted that excessive deposition of fine inorganic sediment (sedimentation) associated with agricultural activities would negatively influence aquatic trophic diversity (e.g., reduced vertical and horizontal trophic niche breadths). We hypothesized that multiple mechanisms might cause trophic niche ‘compression’, as indicated by changes in realized trophic roles.
Food-web properties based on consumer stable isotope data (δ13C and δ15N) showed that increasing sediment disturbance was associated with reduced trophic diversity. In particular, the aquatic invertebrate community occupied a smaller area in isotopic niche space along the sedimentation gradient that was best explained by a narrowing of the invertebrate community δ13C range. |
Decreased niche partitioning, driven by increasing habitat homogeneity, environmental filtering, and resource scarcity all seemingly lead to greater trophic equivalency caused by the collapse of the autochthonous food-web channel. Bayesian mixing-model analyses supported this contention with invertebrate consumers increasingly reliant on detritus along the sedimentation gradient, and predatory invertebrates relying more on the prey using these basal resources.
The narrowing of the fish community δ13C range along the sedimentation gradient contributed to an apparent ‘trophic shift’ towards terrestrial carbon, further indicating the loss of the autochthonous food-web channel. On the vertical trophic niche axis, fish became increasingly separated from aquatic invertebrates with an increase in their estimated trophic position. In combination, these responses were most likely mediated through reduced fish densities and a diminished reliance on aquatic prey.
Although species losses remain a major threat to ecosystem integrity, the functional roles of biota that persist dictate how food webs and ecosystem functioning respond to environmental change. Sedimentation was associated with non-linear reductions in trophic diversity which could affect the functioning and stability of aquatic ecosystems. Our study helps explain how multiple mechanisms may radically reshape food-web properties in response to this type of disturbance.
Read the paper here:
Burdon, F. J., McIntosh, A.R., and Harding, J.S. 2019. Mechanisms of trophic niche compression: evidence from landscape disturbance.
Journal of Animal Ecology doi: 10.1111/1365-2656.13142
For a blog post summarizing the research:
Up silt creek without a niche - how do stream food-webs respond to sedimentation?
The narrowing of the fish community δ13C range along the sedimentation gradient contributed to an apparent ‘trophic shift’ towards terrestrial carbon, further indicating the loss of the autochthonous food-web channel. On the vertical trophic niche axis, fish became increasingly separated from aquatic invertebrates with an increase in their estimated trophic position. In combination, these responses were most likely mediated through reduced fish densities and a diminished reliance on aquatic prey.
Although species losses remain a major threat to ecosystem integrity, the functional roles of biota that persist dictate how food webs and ecosystem functioning respond to environmental change. Sedimentation was associated with non-linear reductions in trophic diversity which could affect the functioning and stability of aquatic ecosystems. Our study helps explain how multiple mechanisms may radically reshape food-web properties in response to this type of disturbance.
Read the paper here:
Burdon, F. J., McIntosh, A.R., and Harding, J.S. 2019. Mechanisms of trophic niche compression: evidence from landscape disturbance.
Journal of Animal Ecology doi: 10.1111/1365-2656.13142
For a blog post summarizing the research:
Up silt creek without a niche - how do stream food-webs respond to sedimentation?
4. CASCADING EFFECTS OF SEDIMENTATION ON BIODIVERSITY AND ECOSYSTEM FUNCTIONING
To further examine the impacts of sediment disturbance on consumer-resource interactions, I conducted a detrital-processing assay in 9 streams along gradients of deposited sediment and dissolved nutrients. This showed that temperature-corrected litter breakdown rates were negatively associated with deposited fine sediment. A reduction in leaf-pack caddisflies (richness, densities, and % composition) along the sediment gradient mirrored the results of the benthic invertebrate survey, and suggested that the decline of these taxa may have contributed to reduced ecosystem function. Laboratory experiments demonstrated the importance of invertebrate 'shredding' to litter breakdown, where sediment additions weakened consumptive interactions and exerted negative effects on caddisflies Differences in consumption rates after correcting for metabolic biomass indicated that taxa-specific responses might be mediated by body size. Moreover, cannibalistic interactions between individuals of the larger caddisfly taxon suggested that reduced resource availability may lead to greater intraspecific competition. This could also be mediated through changes to habitat (the simplification of habitat through the burial of complex elements and infilling of interstitial space) facilitating greater encounter rates.
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For more information: A working draft is available on request
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MASTERS RESEARCH
CROSS-HABITAT INTERACTIONS: RIPARIAN PREDATORS AND STREAM INSECTS
UNIVERSITY OF CANTERBURY, CHRISTCHURCH, NEW ZEALAND
THE LINKAGE BETWEEN RIPARIAN PREDATORS AND AQUATIC INSECTS ACROSS A STREAM-RESOURCE SPECTRUM |
Stream ecosystems are intimately linked to their adjacent riparian habitat by spatial subsidies. Emergent adult aquatic insects form an important food-web linkage to riparian insectivores such as birds, lizards and spiders. As part of my M.Sc research, I surveyed 37 forest streams across differing environmental gradients in the central South Island, New Zealand. This survey aimed to elucidate a positive relationship between standing aquatic prey and predatory riparian invertebrates, with a putative link facilitated by the emergence of adult aquatic insects as prey subsidies. Positive associations were confirmed between stream insect biomass and riparian arachnids after adjusting for the effects of confounding environmental variables. Hierarchical partitioning confirmed the importance of stream insect biomass as a statistically significant contributor to the total explained variance in arachnid biomass, abundance and web density. A survey of web-building spiders along 20-m transects from the stream edge into the forest indicated a strong decline in densities moving away from the stream, with stream insect biomass as a significant co-variate. Riparian spiders appeared to be positively associated with stream insect biomass, suggesting a strong linkage with emergent adult aquatic insect subsidies. This indicates that productivity gradients present in the donor system may affect the magnitude of the interaction between adjacent habitats.
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For more information:
Burdon, F. J., and J. S. Harding. 2008. The linkage between riparian predators and aquatic insects across a stream-resource spectrum. Freshwater Biology 53:330-346.
Burdon, F. J., and J. S. Harding. 2008. The linkage between riparian predators and aquatic insects across a stream-resource spectrum. Freshwater Biology 53:330-346.
HUMAN IMPACTS CONTRIBUTE TO A PRODUCTIVITY GRADIENT DRIVING CROSS-ECOSYSTEM ASSOCIATIONS BETWEEN STREAM INSECTS AND RIPARIAN ARACHNIDS
Stream and riparian ecosystems can be strongly connected via the emergence of adult aquatic insects, which form an important prey subsidy for a wide range of terrestrial consumers. Consequently, human perturbations impacting stream ecosystems may propagate across habitat boundaries. Here I present a case-study demonstrating how human activities may influence stream and riparian invertebrates. Streams impacted by mining supported less benthic insect and riparian arachnid biomass than reference sites, whereas streams in forest fragments surrounded by agricultural land-uses were seemingly more productive, with greater standing crops of stream and riparian invertebrates. Structural equation modelling (SEM) indicated negative indirect effects of mining on riparian arachnids mediated through reduced subsidies of stream insects. In contrast, the SEM suggested both direct and indirect positive effects of agriculture on arachnids. These findings highlight how multiple anthropogenic pressures affecting stream habitats may contribute to productivity gradients determining the biotic connectivity between aquatic and terrestrial ecosystems.
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