1,720,984 research outputs found
Seed size and germination response: a relationship for fire-following plant species exposed to thermal shock
No full textThermal shock is well known to be an important stimulus for the germination of soil-stored seeds in fire-prone plant communities. Nevertheless, while the overall germination response of different species is known to vary, the interaction between seed size and germination to a range of thermal-shock temperatures is poorly understood. This interaction may be important in regulating post-fire plant community establishment, since larger seeds are able to emerge from deeper within the soil profile than smaller seeds, and are therefore likely to be insulated against high above-ground temperatures by a deeper soil covering. In this experiment we examined how germination of eight co-occurring Western Australian fire-followers was influenced by thermal shock, and whether germination was significantly correlated with seed size. We found that small-seeded species not only showed enhanced germination at higher temperatures, but that their ability to germinate at higher temperatures was also greater than that displayed by larger-seeded species. These findings suggest that while seed size may be a useful general predictor of post-fire recruitment success, under different fire regimes the interaction between seed size, maximum seedling emergence depth, and the ability to withstand different thermal-shock temperatures is complex and may confound recent predictive models
Quantifying individual feeding variability: implications for mollusc feeding experiments
No full text1. In order to quantify the level of variability in seedling consumption displayed by individual molluscs, we placed one snail (Helix aspersa) in each of 51 trays containing (7-day-old) Taraxacum officinale seedlings for 7 days.
2. Initially, individual snails displayed considerable variability in their consumption of seedlings; however, this variability declined with time. The consumption of seedlings was not related to individual snail mass.
3. A second grazing experiment, using five different snail densities in similar experimental conditions to the first, showed that increasing snail number reduced variability within treatment groups.
4. A computer simulation, based on data from the first experiment correctly predicted the basic form of the decline in feeding variability with increasing snail density found in the second. Post hoc changes to the model, based on empirical analysis of the second experiment to account for mutual interference, reduced discrepancies between empirical and model results.
5. This study highlights the consequences that individual feeding behaviour has on feeding trials with molluscs, and provides a simple method by which this variability can be quantified and accommodated within experimental design
Species-level effects more important than functional group-level responses to elevated CO2: evidence from simulated turves
No full textUsing mixtures of 14 calcareous grassland plant species drawn from three functional groups, we looked at the effects of elevated atmospheric CO2 on contrasting levels of ecosystem performance (species, functional group and community). Experimental communities were subjected to ambient (?350 µmol mol?1) or elevated CO2 (?600 µmol mol?1) in controlled environments, with grazing simulated by clipping at monthly intervals for 546 days.We assessed the effect of elevated CO2 on plant performance by quantifying the productivity (biomass) and cover of component species. We also examined the effect of elevated CO2 on the vertical structure of the plant canopy. Elevated CO2 resulted in a significant increase in total community biomass only following nutrient addition. Within functional groups, non-leguminous forb species had significantly greater biomass and cover in elevated CO2 both before and after nutrient addition, although the effect was mainly due to the influence of one species (Centaurea nigra). Grasses, in contrast, responded negatively to elevated CO2, although again significant reductions in biomass and cover could mainly be ascribed to a single species (Brachypodium pinnatum). Legumes exhibited increased biomass and cover in elevated CO2 (the effects being particularly marked for Anthyllis vulneraria and Lotus corniculatus), but this response disappeared following nutrient addition. Vertical structure was little affected by CO2 treatment.We conclude that due to the idiosyncratic responses of individual species, the categorization of plants into broad functional groups is of limited use in guiding our understanding of the impacts of elevated atmospheric CO2 on plant communities.<br/
Distribution and forage use of exotic bumblebees in South Island, New Zealand
Full text linkThe rapid decline in bumblebee populations within Europe has been linked to habitat loss through agricultural intensification, and a consequential reduction in the availability of preferred forage plants. The successful introduction of four European Bombus species to the South Island of New Zealand from England (in 1885 and 1906) provides an opportunity to determine how important different forage plants (also introduced from the U.K.) are to two severely threatened European bumblebee species (Bombus ruderatus and B. subterraneus). In January 2003 we conducted a survey of bumblebee populations across 70 sites in the central and southern South Island, recording which plant species were being used as pollen and nectar sources for each Bombus species. All four bumblebee species showed a clear preference for plants of European origin. Only B. terrestris, the most polylectic species, was recorded feeding on native plant species. The longer-tongued bumblebees, B. hortorum, B. ruderatus, and B. subterraneus, foraged predominantly on just two plant species; Trifolium pratense for both nectar and pollen, and Echium vulgare for nectar. These plant species are now declining in abundance in the U.K. Our results provide support for the hypothesis that the loss of flower-rich meadows, particularly those containing populations of Fabaceae species with long corollae, is responsible for the decline of bumblebee species across Europe. Comparison with earlier bumblebee surveys suggests that long-tongued bumblebees may also be in decline in New Zealand, particularly B. subterraneus which is now very localised and scarce
Measuring surface complexity in ecological studies
No full textHabitat complexity is a potential structuring force in benthic communities. Different studies often estimate complexity in different ways, and it is not always clear how precise the separate techniques are. Here we review three methods of estimating surface complexity: stereo photography, profile gauges, and lengths of chain contoured over the substratum. We derived fractal dimensions for the quadrats in the rocky intertidal zone using each technique. Complexity estimates from chains and profile gauges were related, but neither technique was correlated with the results from stereo photographs. Stereo photographs appeared to overestimate complexity on smooth surfaces. The variance of fractal dimension estimates increased nonlinearly with the mean fractal dimension in each quadrat. Recommendations for the number of replicates needed for a reliable estimate of fractal dimension from a quadrat, therefore, vary as a function of surface complexity. Within the range of complexities typically encountered on rocky shores, as few as three profiles or sets of chains can produce relatively reliable estimates of fractal dimension. The most robust and time effective method, however, would be to sample using as many chain profile sets per quadrat as is logistically feasible. Given the changes in precision with surface complexity, comparisons between studies need to take careful note of the number of replicates and the average level of surface complexity. A null result (no relationship between surface complexity and an ecological variable) could be produced by imprecise estimates of surface complexity based on too few replicate measurements per quadrat
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
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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