1,720,965 research outputs found
Spectral Distance Decay: Assessing Species Beta-diversity by Quantile Regression
No full textRemotely sensed data represents key information for characterizing and estimating biodiversity. Spectral distance among sites has proven to be a powerful approach for detecting species composition variability. Regression analysis of species similarity versus spectral distance may allow us to quantitatively estimate how beta-diversity in species changes with respect to spectral and ecological variability. In classical regression analysis, the residual sum of squares is minimized for the mean of the dependent variable distribution. However, many ecological datasets are characterized by a high number of zeroes that can add noise to the regression model. Quantile regression can be used to evaluate trend in the upper quantiles rather than a mean trend across the whole distribution of the dependent variable. In this paper, we used ordinary least square (OLS) and quantile regression to estimate the decay of species similarity versus spectral distance. The achieved decay rates were statistically nonzero (p < 0.05) considering both OLS and quantile regression. Nonetheless, OLS regression estimate of mean decay rate was only half the decay rate indicated by the upper quantiles. Moreover, the intercept value, representing the similarity reached when spectral distance approaches zero, was very low compared with the intercepts of upper quantiles, which detected high species similarity when habitats are more similar. In this paper we demonstrated the power of using quantile regressions applied to spectral distance decay in order to reveal species diversity patterns otherwise lost or underestimated by ordinary least square regression. RI Nagendra, Harini/A-9103-2009; Rocchini, Duccio/B-6742-201
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Distance decay of β-similarity: spatial or ecological barriers?
No full textβ-diversity is defined as the complementarity in species composition between pairs of sites and plays a crucial role in biodiversity management and conservation, since it allows to detect those environmental gradients that functionally act in determining the variation of species composition. From an ecological viewpoint, the decay of β-similarity principally arises from: (i) a decrease in environmental similarity with distance, which inevitably creates a competition among species with different physiological abilities (niche difference model); (ii) a spatial configuration which creates spatial barriers and thus isolation among habitats, that influences species and gene movement across landscapes. To date, efforts were made to demonstrate the effect of distance on species similarity. However, few tests have been performed at local scale, i.e. at the community level. The aim of this study is to test the distance decay of β-similarity hypothesis at local scale (10x10 m plots) considering both (i) spatial distance between pairs of sites; (ii) the ecological distance between pairs of sites, measured as Euclidean distance in vector spaces defined by ecological variables (elevation, aspect, slope, assolation, spectral reflectance considering both the whole multispectral system of a Landsat ETM+ image and the Near Infrared Wavelength, band 4). β-similarity decay rates at different thresholds (τ) have been quantified by means of quantile regressions based on exponential models, focusing on the main factors influencing their trends
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Distance decay of β-similarity: spatial or ecological barriers? [Distance decay della β-similarità: barriere ecologiche o spaziali?]
No full textLa β-diversità è definita come la complementarietà tra siti in termini di composizione specifica e gioca un ruolo fondamentale nella gestione e nella conservazione della biodiversità, dal momento che permette l’individuazione di gradienti ambientali che agiscono funzionalmente nel determinare la variazione della composizione specifica. Da un punto di vista ecologico, il decadimento della β-similarità all’aumentare della distanza (distance decay) deriva da: (i) una diminuzione della similarità ambientale con l’aumentare della distanza, che crea una competizione tra le specie con differenti adattamenti fisiologici (modello di differenziazione di nicchia, niche difference model); (ii) barriere spaziali che creano isolamento tra habitat influenzando il movimento specifico e genico. Ad oggi numerosi sforzi sono stati fatti per dimostrare l’effetto della distanza spaziale sulla similarità in termini di specie (β-similarità).
Tuttavia, pochi studi sono stati effettuati a scala locale, i.e. a livello di comunità vegetale. Lo scopo del presente studio è quello di testare l’ipotesi del decadimento della β-similarità a scala locale (plot di 10x10 m inseriti nell’ambito del progetto MoBiSIC, Monitoraggio della Biodiversità nei SIC della Provincia di Siena) considerando: (i) la distanza spaziale tra siti; (ii) la distanza ecologica, misurata come distanza euclidea in spazi vettoriali definiti da variabili ecologiche (quota, esposizione, pendenza, assolazione, valori spettrali derivati da un’immagine Landsat ETM+, considerando sia l’intero dataset multispettrale che il vicino infrarosso, banda 4). I tassi di decadimento della β-similarità a differenti soglie (τ) sono stati quantificati tramite regressioni quantili basate su modelli esponenziali, focalizzando l’attenzione sui principali fattori che ne influenzano l’andamento
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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