1,721,130 research outputs found
The role of density regulation in extinction processes and population viability analysis
No full textWe review the role of density dependence in the stochastic extinction of populations and the role density dependence has played in population viability analysis (PVA) case studies. In total, 32 approaches have been used to model density regulation in theoretical or applied extinction models, 29 of them are mathematical functions of density dependence, and one approach uses empirical relationships between density and survival, reproduction, or growth rates. In addition, quasi-extinction levels are sometimes applied as a substitute for density dependence at low population size. Density dependence further has been modelled via explicit individual spacing behaviour and/or dispersal. We briefly summarise the features of density dependence available in standard PVA software, provide summary statistics about the use of density dependence in PVA case studies, and discuss the effects of density dependence on extinction probability. The introduction of an upper limit for population size has the effect that the probability of ultimate extinction becomes 1. Mean time to extinction increases with carrying capacity if populations start at high density, but carrying capacity often does not have any effect if populations start at low numbers. In contrast, the Allee effect is usually strong when populations start at low densities but has only a limited influence on persistence when populations start at high numbers. Contrary to previous opinions, other forms of density dependence may lead to increased or decreased persistence, depending on the type and strength of density dependence, the degree of environmental variability, and the growth rate. Furthermore, effects may be reversed for different quasi-extinction levels, making the use of arbitrary quasi-extinction levels problematic. Few systematic comparisons of the effects on persistence between different models of density dependence are available. These effects can be strikingly different among models. Our understanding of the effects of density dependence on extinction of metapopulations is rudimentary, but even opposite effects of density dependence can occur when metapopulations and single populations are contrasted. We argue that spatially explicit models hold particular promise for analysing the effects of density dependence on population viability provided a good knowledge of the biology of the species under consideration exists. Since the results of PVAs may critically depend on the way density dependence is modelled, combined efforts to advance statistical methods, field sampling, and modelling are urgently needed to elucidate the relationships between density, vital rates, and extinction probability
Extinction and Spatial Structure in Simulation Models
No full textAspects of within‐population spatial structure are often neglected in the modeling of population viability. To analyze the relevance of the spatial structure of single populations to population persistence, we compared the results of three models developed for the territorial, arboreal gecko Oedura reticulata: (1) a spatially structured model in which both low and high densities incur mortality costs due to increased movement, (2) a spatially structured model in which the Allee effect is removed, and (3) a spatially unstructured model in which there are no effects of density on mortality. Compared with nonspatial model populations, spatially structured populations exhibited reduced persistence. The Allee effect contributed only a small amount to the reduction in persistence. Increased mortality at high densities caused by difficulties in finding territories markedly reduced persistence in the spatially structured models compared with the density‐independent nonspatial model. We argue that the inclusion of elements of spatial structure may considerably influence the estimation of extinction risk in population viability analyses
Demographic Stochasticity Does Not Predict Persistence of Gecko Populations
No full textWe present a population viability model for an arboreal gecko (Oedura reticulata). This gecko needs a habitat of smooth‐barked Eucalyptus woodlands. In Western Australia its distribution has declined dramatically, largely through clearance of woodlands, but populations persist within woodland remnants. Evidence from extensive field data suggests that the gecko was formerly distributed through much of the original eucalypt woodlands, and that geckos show little movement between patches. The populations in all woodland remnants seem to be isolated. We ask whether the present distribution of the gecko across remnants could have been produced solely by the extinction of populations through demographic stochasticity. To test this possibility, we developed a stochastic, individual‐based model including environmental stochasticity and estimated the percentage of extinct populations of different size from known field characteristics and the time span since the clearing of the woodland. The model predicted a relationship between remnant size and gecko persistence, driven by demographic stochasticity, that is qualitatively similar to the observed pattern. Despite extensive testing, however, we found that the model predicted an incidence function much too optimistic for the observed distribution of populations in small remnants. This discrepancy between field data and our model is due to a series of implicit assumptions. Thus, our modeling exercise sheds light on the procedures commonly applied to population viability analyses of single populations of endangered species. The implicit assumptions involved in such models make many predictions vague. We suggest that for the study of declining species like O. reticulata it is essential to adequately test extinction models and therefore population viability analyses
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
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
Artenschutzforschung und Biotopschutzforschung für Deutschland
No full textDie vorliegende Veröffentlichung beruht auf einer anderthalb jährigen Studie zum Stand der Forschung imBereich Arten- und Biotopschutz, die vom Institut für Landschaftsplanung der Universität Stuttgart, im Auftragdes Bundesministers für Forschung und Technologie (BMFT) durchgeführt wurde. Sie dient sowohl als Hintergrundinformation für den vom BMFT geplanten Förderschwerpunkt Arten- und Biotopschutz als auch als Informationsquellefür alle diejenigen, die sich über den gegenwärtigen Stand der Naturschutzforschung informierenmöchten und einen Einstieg in die relevante Literatur suchen. Das vorliegende Buch wurde nur möglich durch dieBereitschaft zahlreicher in der Arten- und Biotopschutzforschung tätiger Wissenschaftler, uns ihre Erfahrung zurVerfügung zu stellen, sei es in Form intensiver Diskussionen oder in Form von Beiträgen für dieses Buch. Wirbedanken uns bei allen für ihre konstruktive Mitarbeit.Wir sind uns bewußt, daß wir in der uns zur Verfügung stehenden Zeit nicht alle für den Arten- und Biotopschutzwesentlichen Bereiche im gleichen Ausmaß abdecken konnten. Wir glauben, daß es uns dennoch gelungenist, eine gute Synthese der uns vorliegenden Meinungen und Informationen zu bilden. Für die Zusammenstellungder Themen und die Übersicht zur Arten- und Biotopschutz-Forschung zeichnen wir alleine verantwortlich. DieFörderung der Studie durch das BMFT bedeutet nicht, daß die hier vorgebrachten Meinungen der Standpunkt desBMFT darstellen. Verantwortlich zeichnen die Autoren der jeweiligen Kapitel.Unser ganz besonderer Dank gilt unseren Partnern im BMFTund bei der Kernforschungsanlage Jülich fürdie rasche und unkonventionelle Untersützung auch bei Problemen, die sich sonst administrativ oft nicht leichtlösen lassen. Ohne diese reibungslose Zusammenarbeit hätten weder dieses Buch noch die ihm zugrunde liegendeStudie verwirklicht werden können
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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