11,742 research outputs found

    Inconsistent relationships between area, heterogeneity and plant species richness in temperate farmed landscapes

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    Relationships between area, heterogeneity and species richness are fundamental concepts in ecology yet questions remain about how area and heterogeneity trade-off (AHTO) to constrain biodiversity. Although there is growing evidence for unimodal heterogeneity diversity relationships (HDR’s) and an AHTO, tests of the concept and consequences for species richness across a landscape-scale gradient of human-modified ecosystems are rare. Using data from a national (Wales) field survey we analysed relationships between environmental heterogeneity and plant species richness (α and γ). We used ordination to produce a composite metric of heterogeneity and compared this to commonly used metrics. We used niche hypervolumes to categorise the breadth of plant species’ ecological preferences and analysed relationships between species richness, niche width and heterogeneity. The HDR was unimodal with α diversity at the smallest scale and positive with α and γ diversity (non-linear) at the 1 km scale although in low intensity landscapes the HDR with γ diversity was unimodal. There was a unimodal relationship between habitat diversity and γ diversity. Land use intensity was unimodally related to diversity. There were significant interactions between niche width and heterogeneity. Richness of broad niche species increased with heterogeneity with flattening of the curve at higher levels. Narrow niche species were rare and mostly unresponsive. The expected decline in narrow niche species with increasing heterogeneity was not found although they did decline with land-use intensity. Using a unique dataset, an analysis of a large-scale mosaic of ecosystems found that the shape of the HDR varies with land use intensity, the heterogeneity metric, spatial scale, diversity type and niche width. Although heterogeneity can increase species richness, there may be trade-offs at higher heterogeneity. A fundamental constraint on realising the benefit of heterogeneity is the low availability of narrower niche species in local species pools in modified landscapes

    Slowest first passage times, redundancy, and menopause timing

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    Biological events are often initiated when a random "searcher" finds a "target," which is called a first passage time (FPT). In some biological systems involving multiple searchers, an important timescale is the time it takes the slowest searcher(s) to find a target. For example, of the hundreds of thousands of primordial follicles in a woman's ovarian reserve, it is the slowest to leave that trigger the onset of menopause. Such slowest FPTs may also contribute to the reliability of cell signaling pathways and influence the ability of a cell to locate an external stimulus. In this paper, we use extreme value theory and asymptotic analysis to obtain rigorous approximations to the full probability distribution and moments of slowest FPTs. Though the results are proven in the limit of many searchers, numerical simulations reveal that the approximations are accurate for any number of searchers in typical scenarios of interest. We apply these general mathematical results to models of ovarian aging and menopause timing, which reveals the role of slowest FPTs for understanding redundancy in biological systems. We also apply the theory to several popular models of stochastic search, including search by diffusive, subdiffusive, and mortal searchers.Comment: 55 pages, 7 figure

    Acoustic activity of bats at power lines correlates with relative humidity: a potential role for corona discharges

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    With the ever-increasing dependency on electric power, electrical grid networks are expanding worldwide. Bats exhibit a wide diversity of foraging and flight behaviours, and their sensitivity to anthropogenic stressors suggests this group is very likely to be affected by power lines in a myriad of ways. Yet the effects of power lines on bats remains unknown. Here we assessed the responses of insectivorous bats to very high voltage power lines (VHVPL; greater than 220 kV). We implemented a paired sampling design and monitored bats acoustically at 25 pairs, one pair consisting of one forest edge near to VHVPL matched with one control forest edge. Relative humidity mediates the effects of power lines on bats: we detected bat attraction to VHVPL at high relative humidity levels and avoidance of VHVPL by bats at low relative humidity levels. We argue that the former could be explained by insect attraction to the light emitted by VHVPL owing to corona discharges while the latter may be owing to the physical presence of pylons/cables at foraging height and/or because of electromagnetic fields. Our work highlights the response of bats to power lines at foraging habitats, providing new insight into the interactions between power lines and biodiversity

    Influence of constituent particles on fracture of aluminum alloys under high-triaxiality loading

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    Single-edge notch bending tests are conducted to study the influence of constituent particles on the fracture resistance of aluminum alloys 6061, 6063, and 6110 under high-constraint loading conditions. The alloys are tested in the as-cast state after homogenization and artificial aging to temper T6. Each alloy type was delivered with two different volume fractions of constituent particles to enable a quantitative assessment of its impact on the toughness of these aluminum alloys. One variant corresponds to the commercial alloy, whereas the other variant is tailor made with an increased amount of constituent particles by adding Fe and Si to the commercial alloy. All alloys exhibit a dendritic structure with particles clustered at grain boundaries and dendrite arm boundaries. The increased content of constituent particles in the tailor-made alloys is shown to be purely detrimental for the toughness and reduces relevant fracture energy parameters by more than 50% in the alloys tested herein. In the plane-strain-dominated regions of the specimens where the stress triaxiality is highest, crack propagation was found to take place on grain boundaries and dendrite arm boundaries due to void nucleation, growth, and coalescence from the constituent particles. Differences in toughness between the alloys are primarily related to variations in the content, size, and spacing of the constituent particles. A comparison between the three different alloy types, i.e. 6061, 6063, and 6110, shows that strength affects the toughness, but it does not follow the commonly reported trade-off between strength and ductility.publishedVersio

    Multiscale structural optimisation with concurrent coupling between scales

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    A robust three-dimensional multiscale topology optimisation framework with concurrent coupling between scales is presented. Concurrent coupling ensures that only the microscale data required to evaluate the macroscale model during each iteration of optimisation is collected and results in considerable computational savings. This represents the principal novelty of the framework and permits a previously intractable number of design variables to be used in the parametrisation of the microscale geometry, which in turn enables accessibility to a greater range of mechanical point properties during optimisation. Additionally, the microscale data collected during optimisation is stored in a re-usable database, further reducing the computational expense of subsequent iterations or entirely new optimisation problems. Application of this methodology enables structures with precise functionally-graded mechanical properties over two-scales to be derived, which satisfy one or multiple functional objectives. For all applications of the framework presented within this thesis, only a small fraction of the microstructure database is required to derive the optimised multiscale solutions, which demonstrates a significant reduction in the computational expense of optimisation in comparison to contemporary sequential frameworks. The derivation and integration of novel additive manufacturing constraints for open-walled microstructures within the concurrently coupled multiscale topology optimisation framework is also presented. Problematic fabrication features are discouraged through the application of an augmented projection filter and two relaxed binary integral constraints, which prohibit the formation of unsupported members, isolated assemblies of overhanging members and slender members during optimisation. Through the application of these constraints, it is possible to derive self-supporting, hierarchical structures with varying topology, suitable for fabrication through additive manufacturing processes.Open Acces

    Limit theorems for non-Markovian and fractional processes

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    This thesis examines various non-Markovian and fractional processes---rough volatility models, stochastic Volterra equations, Wiener chaos expansions---through the prism of asymptotic analysis. Stochastic Volterra systems serve as a conducive framework encompassing most rough volatility models used in mathematical finance. In Chapter 2, we provide a unified treatment of pathwise large and moderate deviations principles for a general class of multidimensional stochastic Volterra equations with singular kernels, not necessarily of convolution form. Our methodology is based on the weak convergence approach by Budhiraja, Dupuis and Ellis. This powerful approach also enables us to investigate the pathwise large deviations of families of white noise functionals characterised by their Wiener chaos expansion as~Xε=∑n=0∞εnIn(fnε).X^\varepsilon = \sum_{n=0}^{\infty} \varepsilon^n I_n \big(f_n^{\varepsilon} \big). In Chapter 3, we provide sufficient conditions for the large deviations principle to hold in path space, thereby refreshing a problem left open By Pérez-Abreu (1993). Hinging on analysis on Wiener space, the proof involves describing, controlling and identifying the limit of perturbed multiple stochastic integrals. In Chapter 4, we come back to mathematical finance via the route of Malliavin calculus. We present explicit small-time formulae for the at-the-money implied volatility, skew and curvature in a large class of models, including rough volatility models and their multi-factor versions. Our general setup encompasses both European options on a stock and VIX options. In particular, we develop a detailed analysis of the two-factor rough Bergomi model. Finally, in Chapter 5, we consider the large-time behaviour of affine stochastic Volterra equations, an under-developed area in the absence of Markovianity. We leverage on a measure-valued Markovian lift introduced by Cuchiero and Teichmann and the associated notion of generalised Feller property. This setting allows us to prove the existence of an invariant measure for the lift and hence of a stationary distribution for the affine Volterra process, featuring in the rough Heston model.Open Acces

    Movers and shakers: exploring the complex dynamics of aquatic biological invasions

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    Human-mediated introductions of non-native species have provoked innumerable biological invasions, which can have a suite of adverse effects on the communities into which they are introduced. Despite extensive research, there remains a need in invasion ecology for simple methods of predicting how an introduced species will spread and become established. While I predicted that spread can be modelled simply using the characteristics of the invading population, establishment should be explained by the characteristics of the receiving ecosystem. Using the brown trout (Salmo trutta) invasion on the Island of Newfoundland as a case study, I fit a reaction-diffusion model to brown trout population data to predict expected spread and test these predictions against extensive occurrence data. I use statistical models to test the influence of a suite of environmental variables on the establishment of brown trout within the invasion range. I find that observed spread in Newfoundland is slow compared to invasions elsewhere and that two landscape environmental variables show evidence of explaining establishment patterns, but their influence is likely moderated by other factors. My study contextualises the mechanisms contributing to slow aquatic invasions, revealing that studies need to integrate a variety of methods to elucidate the processes governing biological invasions

    Forested buffers in agricultural landscapes : mitigation effects on stream–riparian meta-ecosystems

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    Stream–riparian meta-ecosystems are strongly connected through exchanges of energy, material and organisms. Land use can disrupt ecological connectivity by affecting community composition directly and/or indirectly by altering the instream and riparian habitats that support biological structure and function. Although forested riparian buffers are increasingly used as a management intervention, our understanding of their effects on the functioning of stream–riparian metaecosystems is limited. This study assessed patterns in the longitudinal and lateral profiles of streams in modified landscapes across Europe and Sweden using a pairedreach approach, with upstream unbuffered reaches lacking woody riparian vegetation and with downstream reaches having well-developed forested buffers. The presence of buffers was positively associated with stream ecological status as well as important attributes, which included instream shading and the provision of suitable habitats for instream and riparian communities, thus supporting more aquatic insects (especially EPT taxa). Emergence of aquatic insects is particularly important because they mediate reciprocal flows of subsidies into terrestrial systems. Results of fatty acid analysis and prey DNA from spiders further supported the importance of buffers in providing more aquatic-derived quality food (i.e. essential fatty acids) for riparian spiders. Findings presented in this thesis show that buffers contribute to the strengthening of cross-ecosystem connectivity and have the potential to affect a wide range of consumers in modified landscapes

    Development of Loop-Mediated Isothermal Amplification Assay for Rapid Detection and Analysis of the Root-Knot Nematode Meloidogyne hapla in Soil

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    Soil analysis is crucial for estimating the risk of crop damage by the root-knot nematode Meloidogyne hapla. Here, we developed an analysis assay based on Loop-mediated Isothermal Amplification (LAMP). The LAMP primers were verified for specificity against 10 different nematode species. A manual soil DNA extraction, referred to as SKMM, was developed and compared with a FastDNA kit followed by DNA purification. DNA was extracted with both methods from artificially inoculated soils as well as from naturally infested soil collected from farm fields. The primers exclusively amplified DNA from M. hapla with both colorimetric and real-time LAMP. The detection limit was 193 gene copies and 0.0016 juveniles (12 pg mu L-1) per reaction. DNA concentrations and purity (A(260)/A(230)) were significantly higher using the SKMM procedure compared with the kit. From the field samples collected in 2019, DNA was amplified from 16% of samples extracted with SKMM and from 11% of samples using the kit. Occurrence of M. hapla DNA was confirmed in soil samples from two out of six field soils in 2020 using both real-time LAMP and qPCR. In conclusion, the developed real-time LAMP is a fast and specific assay for detection and quantification of M. hapla DNA in soil
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