40 research outputs found

    Ecological networks: Pursuing the shortest path, however narrow and crooked

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    International audienceRepresenting data as networks cuts across all sub-disciplines in ecology and evolutionary biology. Besides providing a compact representation of the interconnections between agents, network analysis allows the identification of especially important nodes, according to various metrics that often rely on the calculation of the shortest paths connecting any two nodes. While the interpretation of a shortest paths is straightforward in binary, unweighted networks, whenever weights are reported, the calculation could yield unexpected results. We analyzed 129 studies of ecological networks published in the last decade that use shortest paths, and discovered a methodological inaccuracy related to the edge weights used to calculate shortest paths (and related centrality measures), particularly in interaction networks. Specifically, 49% of the studies do not report sufficient information on the calculation to allow their replication, and 61% of the studies on weighted networks may contain errors in how shortest paths are calculated. Using toy models and empirical ecological data, we show how to transform the data prior to calculation and illustrate the pitfalls that need to be avoided. We conclude by proposing a five-point checklist to foster best-practices in the calculation and reporting of centrality measures in ecology and evolution studies. The last two decades have witnessed an exponential increase in the use of graph analysis in ecological and conservation studies (see refs. 1,2 for recent introductions to network theory in ecology and evolution). Networks (graphs) represent agents as nodes linked by edges representing pairwise relationships. For instance, a food web can be represented as a network of species (nodes) and their feeding relationships (edges) 3. Similarly, the spatial dynamics of a metapopulation can be analyzed by connecting the patches of suitable habitat (nodes) with edges measuring dispersal between patches 4. Data might either simply report the presence/absence of an edge (binary, unweighted networks), or provide a strength for each edge (weighted networks). In turn, these weights can represent a variety of ecologically-relevant quantities, depending on the system being described. For instance, edge weights can quantify interaction frequency (e.g., visitation networks 5), interaction strength (e.g., per-capita effect of one species on the growth rate of another 3), carbon-flow between trophic levels 6 , genetic similarity 7 , niche overlap (e.g., number of shared resources between two species 8), affinity 9 , dispersal probabilities (e.g., the rate at which individuals of a population move between patches 10), cost of dispersal between patches (e.g., resistance 11), etc. Despite such large variety of ecological network representations, a common task is the identification of nodes of high importance, such as keystone species in a food web, patches acting as stepping stones in a dispersal network , or genes with pleiotropic effects. The identification of important nodes is typically accomplished through centrality measures 5,12. Many centrality measures has been proposed, each probing complementary aspects of node-to-node relationships 13. For instance, Closeness centrality 14,15 highlights nodes that are "near" to all othe

    Measurement of the two-photon reaction γγ →π+π-π+π- at large values of Q2

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    The process γγ→π+π-π+π- has been investigated in reactions of the type e+e-→e+e-π+π-π+π- in the single tag mode. The range of the four momentum squared of one of the virtual photons was 0.28 GeV2/c2≦Q2≦3.6 GeV2/c2, the average being 〈Q2〉=0.92 GeV2/c2; the other photon was quasi real. The reaction is mainly described by the channels γγ→ρ0ρ0 and γγ→4π (phase space), occuring with about equal probability. The Q2-dependence of the cross section is in agreement with the ρ form factor. © 1988 Springer-Verlag

    A MEASUREMENT OF ELECTROWEAK EFFECTS IN THE REACTION e+ e- ---> tau+ tau- at 35-GeV AND 42.4-GeV

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    We report on total cross section and forward backward charge asymmetry measurements of the reaction e+e- → τ+τ- at centre of mass energies of 35.0 GeV and 42.4 GeV using the TASSO detector. Including previous data an analysis in terms of electroweak parameters of the standard model is presented, and lower limits on mass scale parameters of residual contact interactions are given. A combined analysis of electroweak couplings using all our results on leptonic reactions e+e-→l+l- has been performed. © 1989 Springer-Verlag

    MEASUREMENT OF THE ASYMMETRY OF b QUARK PRODUCTION IN e+ e- ANNIHILATION OF s**(1/2) = 35-GeV

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    The forward-backward asymmetry in the reaction e+ebbe^+e^-→bb has been determined at centre of mass energies near 35 GeV using the TASSO detector. We report results from three different methods. The combined result of the measurements is Aˉb=0.21±0.08Aˉb=0.21±0.08\bar A_b=−0.21±0.08 \bar A_b=−0.21±0.08 ignoring B0Bˉ0B0Bˉ0B^0\bar B^0B^0\bar B^0 mixing. Taking mixing into account leads to a corrected asymmetry of AcA_c or bˉ=0.28±0.11Aˉbcor=0.28±0.11\bar b=−0.28±0.11 \bar A{_b^{cor}}=−0.28±0.11 . The axial vector coupling constant of the b quark as calculated from the corrected asymmetry isab=−1.2±0.5 to be compared with the valueabSM=−1 from the standard model

    PRODUCTION AND DECAY OF CHARMED MESONS IN e+ e- ANNIHILATION AT s**(1/2) > 28-GeV

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    We report on a study of inclusive production of D*± mesons in e+e- annihilation at c.m. energies between 28 and 46.8 GeV using the TASSO detector at the PETRA storage ring. A hard D*± energy spectrum is measured with a maximum near ED*±≃0.6 Ebeam. The measured cross section ratio {Mathematical expression} indicates that D* production accounts for a large fraction of the observed charm production. Two complementary methods have been used to determine the forward-backward asymmetry of charm pair production due to electroweak interference. Combining both measurements the product of the axial vector couplings of the electron and the charm quark to the weak neutral current was determined to be gAegAc=-(0.276±0.073), in agreement with the standard model prediction of -0.25. Using a sample of reconstructed D*± mesons, the relative strength of the strong interaction coupling of the c quark compared to that of an average of all flavours is measured as αs(c)/αs(all)=0.91±0.38±0.15, consistent with the coupling constant being flavour independent. An update of our D0 lifetime measurement is presented, based on a considerable increase in statistics, the final result being {Mathematical expression}. © 1989 Springer-Verlag

    A measurement of muon pair production in e+e- annihilation at centre of mass energies {Mathematical expression} GeV

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    The reaction e+e-→μ+μ- has been studied at centre of mass energies between 35.0 and 46.8 GeV using the TASSO detector at PETRA. We present measurements of the forward-backward charge asymmetry (Aμμ) and cross section σμμ for this reaction at three energies. At 35.0 GeV we obtain a cross section relative to the QED prediction of Rμμ=σμμ/σo=0.932±0.018±0.044 and Aμμ=(-10.6-2.3+2.2±0.5)%. At 38.3 GeV we find Rμμ=0.951±0.072-0.057+0.063 and Aμμ=(+1.7-8.6+8.5±0.5)%. At 43.6 GeV we measure Rμμ=0.921±0.037±0.055 and Aμμ=(-17.6-4.3+4.4±0.5)%. Our results are in good agreement with the predictions of the standard model. Including previous TASSO data we present improved determinations of muonic electroweak parameters. We also report on lower limits of possible contributions from contact interactions. © 1988 Springer-Verlag
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