1,720,988 research outputs found
Intraguild interactions between spiders and ants and top-down control in a grassland food web
No full textSanders D, Platner C. Intraguild interactions between spiders and ants and top-down control in a grassland food web. Oecologia. 2007;150(4):611-624.In most terrestrial ecosystems ants (Formicidae) as eusocial insects and spiders (Araneida) as solitary trappers and hunters are key predators. To study the role of predation by these generalist predators in a dry grassland, we manipulated densities of ants and spiders (natural and low density) in a two-factorial field experiment using fenced plots. The experiment revealed strong intraguild interactions between ants and spiders. Higher densities of ants negatively affected the abundance and biomass of web-building spiders. The density of Linyphiidae was threefold higher in plots without ant colonies. The abundance of Formica cunicularia workers was significantly higher in spider-removal plots. Also, population size of springtails (Collembola) was negatively affected by the presence of wandering spiders. Ants reduced the density of Lepidoptera larvae. In contrast, the abundance of coccids (Ortheziidae) was positively correlated with densities of ants. To gain a better understanding of the position of spiders, ants and other dominant invertebrate groups in the studied food web and important trophic links, we used a stable isotope analysis (N-15 and C-13). Adult wandering spiders were more enriched in N-15 relative to N-14 than juveniles, indicating a shift to predatory prey groups. Juvenile wandering and web-building spiders showed delta N-15 ratios just one trophic level above those of Collembola, and they had similar delta C-13 values, indicating that Collembola are an important prey group for ground living spiders. The effects of spiders demonstrated in the field experiment support this result. We conclude that the food resource of spiders in our study system is largely based on the detrital food web and that their effects on herbivores are weak. The effects of ants are not clear-cut and include predation as well as mutualism with herbivores. Within this diverse predator guild, intraguild interactions are important structuring forces
Habitat structure mediates top-down effects of spiders and ants on herbivores
No full textSanders D, Nickel H, Grutzner T, Platner C. Habitat structure mediates top-down effects of spiders and ants on herbivores. BASIC AND APPLIED ECOLOGY. 2008;9(2):152-160.Differences in structural complexity of habitats have been suggested to modify the extent of top-down forces in terrestrial food webs. In order to test this hypothesis, we manipulated densities of generalist invertebrate predators and the complexity of habitat structure in a two-factorial design. We conducted two field experiments in order to study predation effects of ants and spiders and, in particular, of the wasp spider Argiope bruennichi on herbivorous arthropods such as grasshoppers, plant- and leafhoppers in a grassland. Predator densities were manipulated by removal in habitats of higher and lower structural diversity, and the effects on herbivore densities were assessed by suction sampling. Habitat structure was changed by cutting the vegetation to half its height and removing leaf litter.We found a significant negative effect of this assemblage of generalist predators on plant- and leafhoppers, which were 1.6 times more abundant in predator removal plots. This effect was stronger in low-structured (cut) than in uncut vegetation. Densities of the most abundant planthopper Ribautodelphax pungens (Delphacidae) were 2.2 times higher in predator removal plots. Furthermore, adult plant- and leafboppers responded more strongly than juveniles and epigeic species more strongly than hypergeic species. The presence of predators had a positive effect on plant- and leafhopper species diversity. In a second field experiment, we tested the exclusive impact of Argiope bruennichi on its prey, and found that its effect was also significant, although weaker than the effect of the predator assemblage. This effect was stronger in grass-dominated vegetation compared to structurally more complex mixed vegetation of grasses and herbs. We conclude that habitat structure and in particular vegetation height and architectural complexity strongly modify the strength of top-down forces and indirectly affect the diversity of herbivorous arthropods. (c) 2007 Gesellschaft fur Okologie. Published by Elsevier GmbH. All rights reserved
Potential positive effect of the ant species Lasius niger on linyphiid spiders
No full textSchuch S, Platner C, Sanders D. Potential positive effect of the ant species Lasius niger on linyphiid spiders. JOURNAL OF APPLIED ENTOMOLOGY. 2008;132(5):375-381
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
Trophic diversity in a Mediterranean food web—Stable isotope analysis of an ant community of an organic citrus grove
No full textPlatner C, Piñol J, Sanders D, Espadaler X. Trophic diversity in a Mediterranean food web—Stable isotope analysis of an ant community of an organic citrus grove. Basic and Applied Ecology. 2012;13(7):587-596.Ants as generalist predators and mutualists of herbivores can play an important role in relative stable agroecosystems like plantations. The categorization of the diverse life strategies and traits into ecological groups like trophic levels is essential for a better understanding of food web structures and a better prediction of changes in communities. Stable isotope technology provides simultaneously detection of trophic levels and the ultimate C source of many species.
We studied a highly diverse Mediterranean ant community in an organic citrus grove in Tarragona, NE Spain, and analyzed stable isotope contents of 17 species of ants together with dominating plants and important spider and aphid species to establish trophic guilds and detect seasonal changes. The results revealed significant differences between species spanning over a huge range in δ15N-values of at least 10.67‰ which is only comparable to a Peruvian tropical forest with a much higher species diversity. The trophic levels of ants reflected most of previous knowledge on predaceous vs. plant feeding habits. Messor harvester ants and Camponotus species had the lowest δ15N-values. Aphids, smaller spider species, and most other ant genera, including the dominating species Formica rufibarbis and Lasius grandis, had intermediate δ15N-levels. The large spider Dysdera crocata and the typical Mediterranean ant Pheidole pallidula had higher δ15N-values, but two specialized predatory ants with very tiny workers had the highest trophic level. We found unexpectedly high δ13C-values with a high seasonality for several ground-living ant species. The possible role of soil fauna as a second main food resource besides the most commonly analyzed green food chain is discussed. Our results support the hypothesis that the strong seasonality intrinsic to Mediterranean climate and the high heterogeneity of different plant resources and microclimatic conditions in the organically managed plantation are reflected by a notably high trophic diversity of the ant community.Ameisen können als generalistische Prädatoren und Symbiosepartner von Pflanzenfressern eine wichtige Rolle in relativ stabilen Agrar-Ökosystemen, wie z.B. Plantagen und Obstwiesen, spielen. Die Einteilung von Arten mit diversen Lebensstrategien und Eigenschaften in ökologische Gilden, beispielsweise in trophische Gruppen, ist von zentraler Bedeutung für ein besseres Verständnis von Nahrungsnetzen und bessere Voraussagen von Änderungen in den Lebens-Gemeinschaften. Die Analyse stabiler Isotope bietet die Möglichkeit der gleichzeitigen Aufdeckung trophischer Ebenen und der ursprünglichen Kohlenstoff-Quellen vieler Arten.
Wir haben die hoch diverse mediterrane Ameisengemeinschaft einer ökologisch bewirtschafteten Zitrus-Plantage untersucht und die natürlichen Gehalte stabiler Isotope von 17 Ameisenarten zusammen mit dominanten Pflanzen und wichtigen Spinnen- und Blattlausarten gemessen, um trophische Gilden zu etablieren und saisonale Veränderungen zu entdecken. Die Ergebnisse zeigen mit etwa 11‰ eine enorme Bandbreite der δ15N Werte der Ameisen; so viel wie bisher nur für einen Regenwald in Peru mit einer viel größeren Artenzahl beschrieben wurde. Die trophischen Ebenen der Ameisen decken sich gut mit den für viele Arten bereits beschriebenen zoo- und phytophagen Ernährungsweisen. Ernteameisen der Gattung Messor und Camponotus-Arten hatten die niedrigsten δ15N Werte. Blattläuse, die kleineren Spinnenarten und die meisten omnivoren Ameisen wie die dominanten Arten Formica rufibarbis und Lasius grandis hatten mittlere 15N Gehalte. Die große Spinne Dysdera crocata und die im Mittelmeer-Gebiet häufige Ameise Pheidole pallidula hatten deutlich höhere δ15N Werte, doch die höchste trophische Ebene bildeten zwei spezialisierte räuberische Ameisen mit sehr kleinen Arbeiterinnen. Für einige Ameisenarten der Bodenoberfläche fanden wir unerwartet hohe δ13C Werte mit einer starken jahreszeitlichen Änderung. Wir diskutieren die mögliche Rolle der Bodenfauna als zweite Haupt-Nahrungsquelle neben der bisher hauptsächlich analysierten „grünen” Nahrungskette. Die Ergebnisse unterstützen die Hypothese, dass sich die starke Saisonalität des mediterranen Klimas und die große Heterogenität vielfältiger Pflanzenarten und mikroklimatischer Bedingungen auf der organisch bewirtschafteten Mandarinenbaum-Wiese in einer bemerkenswert hohen trophischen Vielfalt der Ameisengemeinschaft ausdrückt
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
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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