1,721,046 research outputs found
Improved fitness of aphid parasitoids receiving resource subsidies
No full textThe availability of nonhost resource subsidies affects the ability of parasitoids to overcome egg and time limitation and maximize lifetime reproductive success. We combined field and laboratory experiments to examine the effects of floral resource subsidies on the reproductive fitness of aphid parasitoids. Under controlled laboratory conditions, sugar resources significantly increased longevity and potential fecundity (egg load) in the endoparasitoids Aphidius rhopalosiphi and Diaeretiella rapae (Hymenoptera: Aphidiidae). Laboratory microcosm experiments showed that increased potential fecundity translated into significantly higher realized fecundity (i.e., rates of aphid parasitism) by A. rhopalosiphi receiving resource subsidies. Mechanisms of enhanced realized fecundity also operated under natural field conditions. Replicated field experiments on cultivated wheat, with host density controlled by experimental placement of aphids, showed that, in general, proximity to floral resource patches significantly increased rates of parasitism. Parasitism rates declined exponentially with increasing distance from floral patches, reaching zero beyond 14 m. Increased potential fecundity reduces the chances of parasitoids becoming egg-limited, whereas increased longevity reduces the chances of parasitoids becoming time-limited during host oviposition. Although the plasticity of egg load and longevity observed under varying conditions of resource availability is probably more relevant to parasitoid evolution, the individual fitness benefits from ephemeral resource patches are certainly important in the ecological enhancement of biological control agents
Airborne LiDAR reveals context dependence in the effects of canopy architecture on arthropod diversity
No full textEcologists have long recognized the important role of canopy heterogeneity in structuring the diversity of animal communities. However, studies directly linking variation in the three-dimensional structure of forests to variation in biodiversity are still rare. For canopy arthropods representing a dominant component of forest biodiversity in montane spruce forests of Europe, we used publicly available airborne LiDAR measurements to test the premises of two existing hypotheses that resource concentration and habitat heterogeneity are potential drivers of faunal diversity at both the tree scale and stand scale. We sampled 391 arthropod species from the canopies of 60 trees; coverage-based rarefaction revealed high completeness of faunal sampling (93.7%). When we controlled for elevation and broadleaf tree cover, we found strong (tree and stand scale) context dependence in the response of arthropod diversity to variation in vegetation structure. Arthropod diversity increased with increasing canopy density at the tree scale and was positively associated only with vegetation heterogeneity at the tree scale, but decreased with increasing canopy density at the stand scale. These trends held across all levels of biological response from total richness to diversity measures to richness of different guilds. Our results showed that different components of vegetation structural complexity drive canopy arthropod biodiversity at different spatial scales. Highest canopy arthropod diversity can be expected in spruce forests with relatively open stands containing individual trees with dense and long crowns. Thus, LiDAR opens new avenues for testing ecological hypotheses and for forest-growth models to be linked with the canopy diversity of forests
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Holly leaf-miners on two continents: what makes an outbreak species?
No full text1. Some herbivore species periodically undergo damaging, high-density outbreak phases followed by less damaging low-density phases. Others maintain steady, low to moderate density levels that do little damage to their hosts. 2. Two closely related holly leaf-miner species were compared that share many ecological traits and have very similar life cycles, but only one of which exhibits outbreaks. Phytomyza ilicicola in the eastern U.S.A. varied widely in mortality and infestation levels, reaching local densities of over 10 mines per leaf In contrast, Phytomyza ilicis in the U.K. showed low infestation and high mortality at all sites. Using data from the literature and from field studies, the factors responsible for these contrasting dynamics were sought. 3. Phytomyza ilicicola oviposits into the leaf lamina, and experiences weak larval competition only at high densities. Phytomyza ilicis oviposits into the leaf midrib, which leads to high mortality of young larvae before mine formation. Multiply mined leaves were therefore very common in P. ilicicola but rare in P. ilicis. 4. Differences in the parasitoid complexes of the two systems accounted for further differences in survival to adulthood. The main (larval) parasitoid, which was found to impose high, density-dependent mortality on P. ilicis, is missing on P. ilicicola. It is replaced by an egg-pupal parasitoid, which varies in its impact at different sites. Multiple emergence of adults from multiply mined leaves is therefore widespread in P. ilicicola but does not occur in P. ilicis. 5. The differences in oviposition behaviour and in the parasitoid complexes are likely to allow P. ilicicola to outbreak when habitat conditions are favourable, while P. ilicis is always tightly regulated
Mapping community change in modified landscapes
No full textHabitat loss is not randomly distributed across modified landscapes, yet spatial patterns of habitat cover are not routinely combined with biodiversity data when assessing or predicting the biodiversity impacts of land use change. Here, we convert point observations of more than 28,000 beetles from 851 species into a continuous biodiversity surface representing the similarity of ecological communities relative to that of pristine forest, effectively integrating on-the-ground biodiversity data with remotely sensed land cover data to predict the magnitude of community change in a modified landscape. We generated biodiversity surfaces for both present-day and pre-human landscapes to map spatial patterns of change in a diverse ecological community to calculate the combined biodiversity impacts of habitat loss and fragmentation that accounts for the exact spatial pattern of deforestation. Our spatially-explicit, landscape-scale index of community change shows how the fine-scale configuration of habitat loss sums across a landscape to determine changes in biodiversity at a larger spatial scale. After accounting for naturally occurring within-forest heterogeneity, we estimate that the conversion of 43% of forest to grassland in a 1300 km2 landscape in New Zealand resulted in a 47% change to the beetle community. © 2009 Elsevier Ltd. All rights reserved
Are invasive species the drivers of ecological change?
No full textInvasive species are widely accepted as one of the leading direct causes of biodiversity loss. However, much of the evidence for this contention is based on simple correlations between exotic dominance and native species decline in degraded systems. Although appealing, direct causality is not the only possible interpretation. A plausible alternative hypothesis is that exotic dominance could be the indirect consequence of habitat modification driving native species loss. In a new paper, MacDougall and Turkington now provide the first direct test of whether invasive species are the drivers of community change, or merely I passengers' along for the environmental ride
Variations on the Author
Get PDF“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
Get PDFWe 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
Get PDFWe 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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