6 research outputs found

    The influence of nutrient enrichment on riverine food web function and stability

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    Nutrient enrichment of rivers and lakes has been increasing rapidly over the past few decades, primarily because of agricultural intensification. Although nutrient enrichment is known to drive excessive algal and microbial growth, which can directly and indirectly change the ecological community composition, the resulting changes in food web emergent properties are poorly understood. We used ecological network analysis (ENA) to examine the emergent properties of 12 riverine food webs across a nutrient enrichment gradient in the Manawatu, New Zealand. We also derive Keystone Sensitivity Indices to explore whether nutrients change the trophic importance of species in a way that alters the resilience of the communities to further nutrient enrichment or floods. Nutrient enrichment resulted in communities composed of energy inefficient species with high community (excluding microbes) respiration. Community respiration was several times greater in enriched communities, and this may drive hypoxic conditions even without concomitant changes in microbial respiration. Enriched communities exhibited weaker trophic cascades, which may yield greater robustness to energy flow loss. Interestingly, enriched communities were also more structurally and functionally affected by species sensitive to flow disturbance making these communities more vulnerable to floods

    Qualitative behavior of three species food chain around inner equilibrium point: spectral analysis

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    This work deals with analytical investigation of local qualitative temporal behavior around inner equilibrium point of a model for three species food chain, studied earlier by Hastings and Powel and others. As an initial step towards the spectral analysis of the model, the governing equations have been split into linear and nonlinear parts around arbitrary equilibrium point. The explicit parameter dependence of eigenvalues of Jacobi matrix associated to the linear part have been derived. Analyzing these expressions in conjunction with some pedagogical analysis, a lot of predictions on stable, unstable or chaotic change of species have been highlighted. Agreement of predictions of this work with available numerical or semi-analytical studies suggest the utility of analytical results derived here for further investigation/analysis of the model as desired by earlier works

    Zooplankton dynamics in a lowland river along temporal and spatial gradients

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    Studies on zooplankton ecology have traditionally dealt with lentic systems, while research on lotic assemblages has lagged far behind. It is especially in the last decades that riverine zooplankton has become the focus of an increasing number of investigations, showing that large lowland rivers often host extremely rich and abundant plankton communities. Surveys conducted in rivers all over the world highlighted the major role played by abiotic constraints in shaping the development of riverine zooplankton, while much fewer studies have dealt with biotic factors, which are generally thought to play a minor role in the main current of rivers. Despite this growing amount of research, questions still remain to be answered regarding the origin of river plankton, how these communities are able to persist in the current and the major mechanisms involved in regulating their spatio-temporal dynamics. The present work deals with different aspects of the ecology of zooplankton in a large lowland river (Po River, Northern Italy). A multi-level approach was adopted, combining different surveys carried out at distinct spatial and temporal resolutions, based on the following main research questions: 1. What are the main abiotic and biotic factors controlling the spatio-temporal dynamics of zooplankton abundance and composition in the potamal reach of a lowland river? What is the effect of disturbance events like floods on community structure? Is the community able to exhibit regular successional patterns? These research topics have been addressed by means of a two-year sampling campaign carried out in the potamal reach of the Po River. Results confirmed the role of abiotic constraints related to seasonality and hydrology, together with that of trophic conditions, as the major drivers of zooplankton dynamics. The comparison of two hydrologically different years suggested that the uncoupling between seasonality and hydrology can significantly influence community density and diversity temporal patterns. The relationship between discharge and zooplankton abundance is not univocally negative, as increases in river flow may at times bring about a net increment of the assemblage, when resuspension of organisms from the river bottom and/or littoral zones prevails over advective losses. Discharge fluctuations strongly affect zooplankton diversity too, both taxonomical and functional, with higher diversity associated with increases in river flow, up to a threshold over which destructive effects dominate. However, the impact of hydrodynamic forcings in shaping lotic zooplankton appears to lose importance in favour of that of seasonality and trophic state when moving down the longitudinal dimension of the river system, and a clear downstream shift towards a truly planktonic community occurs, especially during summer low-flow conditions. 2. Can biotic interactions (predation/competition) within the zooplankton become crucial drivers of community structure under advective conditions? A short-term, high-frequency sampling campaign was carried out in summer 2005 in the potamal stretch of the Po River. To test for the presence of association patterns among zooplankters, which might be suggestive of potential trophic interactions (predation/competition), taxa were aggregated into functional groups according to their feeding ecology, and time series of their abundances were analysed by means of a variance ratio test and multivariate autoregressive models, which revealed the occurrence of compensatory dynamics among functional groups under low and stable, although truly advective, discharge conditions. Evidence on the importance of predator-prey interactions and intra-population regulation mechanisms also came from further analyses on the dominant predator's gut contents and on its main prey's population dynamics. Results showed that, under certain conditions, zooplankton is able to exhibit internal, self-regulatory mechanisms also in the main current of a large river and the view of riverine zooplankton as a mere assemblage of taxa exclusively abiotically controlled is therefore oversimplified. 3. What are the main changes within the plankton community during its downstream transport and how can the observed longitudinal dynamics be explained? What is the contribution of the major tributaries to the main channel zooplankton assemblage? These questions have been addressed by means of a Lagrangian sampling experiment carried out in May 2010 on a 330-km river stretch. Results highlighted how water residence time under spring conditions is too short to allow longitudinal development of zooplankton, which is merely transported downstream without significant changes in abundance and composition. One of the tributaries, the Mincio River, hosted an exceptionally abundant zooplankton community, which resulted in a significant increase in total zooplankton densities in the Po River downstream of its inflow. The Mincio lower course is highly regulated and artificially modified. The result is a system where current is extremely impaired and slowed down, creating favourable conditions for phyto- and zooplankton massive development. As a consequence, the influence of this tributary on the Po River zooplankton during the survey was still detectable as far as 100 km downstream of its mouth and the community composition recorded downstream of the Mincio inflow was more similar to the assemblage found in the tributary than to that found at the upstream stations of the Po River itself. 4. Does the presence of an hydrological discontinuity along the river course, such as a man-made reservoir, induce relevant changes in zooplankton composition and abundance? Are there significant differences in community structure along cross-channel transects (middle channel versus river banks) or is the river a completely well-mixed environment with homogeneously distributed plankton assemblages? A sampling campaign was carried out in summer 2009 at four stations respectively upstream, inside (mid-channel vs. shore) and downstream of the artificial impoundment of Isola Serafini, located in the middle reach of the Po River. The study revealed that longitudinal and lateral hydrogeomorphic heterogeneity may affect community abundance and diversity, so that the assemblage found at one site along the river may also be related to local hydroecological features and not only to processes taking place further upstream. However, in a rectified and channelized river like the Po, this appears to be the case only at extremely low discharge rates and high water residence time. Results of the surveys carried out along extremely artificialized traits of both the Po River (Isola Serafini dam) and its tributaries (lower section of the Mincio River) showed how anthropogenic modifications of the river's natural hydrogeomorphic features may have strong impacts also on biotic components that are usually overlooked by standard monitoring programs, even though their role in the functioning of lotic systems is still not completely understood

    Evaluating ecosystem interventions for improved health outcomes - The case of the Volta Estuary mangroves and malaria

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    Degradative alteration of ecological systems worldwide is progressing at a time when their influence on human wellbeing is becoming more evident. For some ecosystems and aspects of wellbeing, more concrete knowledge exists. Insights into the science of mangrove-health relationships are however limited and fragmented, with no assessments of human perspectives around these phenomena. This study investigated the nature of the mangrove-human health nexus by assessing the impacts of mangrove ecosystem interventions on health-related ecosystem goods and services and self-reported malaria experiences. Using a mix of methods comprising a systematic literature review, key informant interviews, health questionnaires and Qualitative Comparative Analysis (QCA), this study merges three bodies of work. Research participant viewpoints were synthesised regarding the evolution of mangrove characteristics and use patterns over time, and how these are affected by ecosystem restoration. Survey respondents were also engaged in a recall exercise of malaria experiences over the same period, to provide a basis for causal inference analysis using QCA methodology. Results show that mangrove dependence is declining with ecosystem degradation in Ghana, but ecosystem restoration can modulate some negative health impacts of mangrove degradation, such as infectious disease risk and threats to protein nutrition. Further, specific ecological conditions elicited by ecosystem interventions work together diversely to decrease malaria incidence, but mainly to amplify benefits of current malaria vector control interventions. The causal relationships reveal that certain aspects of wetland restoration can be strengthened to deliver conditions that improve consequences of current malaria management strategies. Environment and health managers must collaborate in policy reorientation, monitoring, evaluation, and capacity building to realise more tangible scientific evidence and sustainable cross-sector outcomes. Ecosystem interventions could plug the shortfalls arising from resource constraints in health policy implementation, towards more uniform outcomes especially in marginal communities

    SHIFTING GROUNDS: SCIENTIFIC AND TECHNOLOGICAL CHANGE AND INTERNATIONAL REGIMES FOR THE OCEAN AND OUTER SPACE

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    Emerging planetary-scale environmental problems, such as climate change and space debris, indicate a growing need for effective governance regimes for domains beyond the borders of territorial nation-states. This dissertation addresses the basic question: what explains patterns of success and dysfunction in regimes for non-terrestrial spaces? Under what conditions can global commons regimes function to achieve their goals? The answer depends in a fundamental way on scientific knowledge and technological capability, which create, define, and describe the problems, interests, and practices that shape the formation and features of governance regimes, and thus create the conditions for their effective functioning. This project employs and extends recent revivalist geopolitical approaches examining the influences of material factors (geography, ecology, and technology), and applies them to explain important features of regimes for the ocean and orbital space. This approach claims that geography, ecology, and technology together constitute an influencing context, which creates specific problem structures and constrains possible solution sets, and thereby sets conditions for regime performance. In contrast, recent post-modernist and constructivist approaches discount the importance and influence of material contexts in shaping politics, and are incapable of explaining important aspects of regimes. Rationalist (interest-centered) approaches to theorizing regimes employ thin treatments of the material context, limiting their ability to explain regime content and effectiveness. The explanatory traction of material-contextual factors is demonstrated by a detailed examination of regime formation, content and effectiveness over four periods of ocean governance across five centuries, and orbital space over the last sixty years. These cases demonstrate that successful regime formation must foreground scientific uncertainty, ecological dynamics, and the balance of technological capability. To the extent that global commons regimes ignore the existence and dynamism of these material structures, they are more likely to fail to achieve their goals. Greater consideration of material contexts produces a strengthened International Relations theory of regimes. These findings also suggest ways to improve regime design, outlined in the concluding chapter
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