45 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

    A MEASUREMENT OF ELECTROWEAK EFFECTS IN THE REACTION E+E--]TAU+TAU- AT 35.0 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

    STUDY OF INTERMITTENCY IN ELECTRON POSITRON-ANNIHILATION INTO HADRONS

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    Intermittency effects have been studied directly for the first time in e+e- annihilation, using 37 509 hadronic events at an average CM energy of 〈√s〉=35 GeV. The factorial moments F2, F3 and F4 are given for the rapidity distribution and for the two-dimensional distributions in rapidity and azimuthal angle. The effects of cuts in sphericity and particle momentum are large. Comparison with several fragmentation models are made; some models like the Lund model with O(αs2) matrix element give a qualitative description of the phenomena. The importance of detector effects is demonstrated. The results are discussed in terms of various suggested interpretations of this effect. © 1989

    COMPARISON OF INCLUSIVE FRACTIONAL MOMENTUM DISTRIBUTIONS OF QUARK AND GLUON JETS PRODUCED IN E+E- ANNIHILATION

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    Inclusive charged particle production in e+e- annihilation into hadrons is studied in terms of the particle fractional momentum xp. The xp distribution for gluon jets is extracted by comparing two data samples measured in the TASSO detector: nearly symmetric three jet events at centre-of-mass energy W∼35 GeV and two jet events at W∼22 GeV, yielding quark and gluon jets of similar energies (∼11.5 GeV). No significant difference is observed between quark and gluon jets. Monte Carlo models based on parton showers describe the trend and energy variation of the data better than a model with second order matrix element in αs. © 1989 Springer-Verlag

    EXPERIMENTAL-STUDY OF JET MASSES IN E+E- ANNIHILATION AT CM ENERGIES BETWEEN 12-GEV AND 43.5-GEV

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    Data on jet masses, resulting from the decomposition of e+e- hadronic final states into two hemispheres, are presented at centre of mass energies between 12 and 43.5 GeV. Comparisons are made with bare O(αs2) QCD predictions as well as with QCD based fragmentation models. Values for αs and {Mathematical expression} are determined, both with and without hadronization effects included. Upper and lower limits for {Mathematical expression} independent of fragmentation models have been determined to be 0.480±0.025 GeV and 0.047±0.007 GeV respectively. © 1989 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

    PRODUCTION OF CHARMED MESONS IN gamma gamma INTERACTIONS

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    Production of charmed mesons in γγ-interactions at PETRA energies has been observed in the TASSO detector. Cross sections for inclusive D±D^{*±} and D0Dˉ0D0Dˉ0D^0 \bar D^0 D^0 \bar D^0 production have been measured. Neutral and charged meson pairs are estimated to be produced with comparable cross sections, and their sum seems to account for a sizeable fraction of σtot(γγγγ→hadrons) near the cc¯cc¯cc¯cc¯ threshold

    A MEASUREMENT OF THE TAU-LIFETIME

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    We have reconstructed 695 three-track τ decay vertices using a high resolution drift chamber close to the interaction point. From the distribution of decay lengths we measure the lifetime to be (3.06 ±0.20±0.14)×10-13 s. Using this result we find that the ratio of charged weak coupling constant for the τ to that of the μ, G τ/G μ=0.967±0.040 consistent with the concept of lepton universality. © 1988 Springer-Verlag
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