1,721,157 research outputs found
Meyer (Birgit), Geschiere (Peter), eds. Globalization and Identity: Dialectics of Flow and Closure
No full textBayart Jean-François. Meyer (Birgit), Geschiere (Peter), eds. Globalization and Identity: Dialectics of Flow and Closure . In: Critique internationale, vol. 6. 2000. Rationalités de la violence extrême, sous la direction de Jacques Sémelin. p. 77
Contribution of diurnal and nocturnal pollinators to the reproductive success of the orchid species Gymnadenia conopsea
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Contrasting resource-dependent responses of hoverfly richness and density to landscape structure
No full textHoverflies (Diptera: Syrphidae) in agroecosystems have gained much attention recently because the larvae of some species are efficient control agents of crop aphids, and adult hoverflies provide pollination services to wild flowers and flowering crops. We assessed the density and species richness of hoverflies in 32 calcareous grasslands, which constitute a semi-natural habitat for adult hoverflies, by means of six transect walks from April to September 2004. Our results show that local habitat factors and landscape factors influenced hoverfly communities, and that their effects on hoverfly richness and density were quite contrary. Hoverfly species richness was affected by factors related to resource heterogeneity such as species richness of flowering plants, area of grassland habitat, and landscape diversity, which all imply the availability of diverse micro- and macrohabitats for adults and larvae. Hoverfly density, in contrast, depended on factors related to resource quantity, Such as the amount of pollen and nectar resources for adults and the amount of larval macrohabitats in the surrounding matrix. Therefore, both adult and larval habitat requirements have to be considered when analysing hoverfly communities in agricultural landscapes. Species guilds responded to specific land-use types Such as annual crops and woodland at different spatial scales, indicating variation in species' mobility and in the degree of spillover effects among neighbouring landscape elements. (C) 2008 Gesellschaft fur Okologie. Published by Elsevier GmbH. All rights reserved
Patch size and landscape effects on pollinators and seed set of the horseshoe vetch, hippocrepis comosa, in an agricultural landscape of central Europe
No full textAgricultural land use intensification has caused a considerable decline of once common semi-natural habitats leaving only small and isolated habitat remnants within a homogeneous landscape matrix. Populations of flowering plant species in these remaining habitat fragments are also declining and can not offer sufficient resources for potential pollinators. The loss of pollinators and consequential pollination limitation is especially severe for obligately out-crossing plant species. In this study, pollination and reproductive success of Hippocrepis comosa was examined. Hippocrepis comosa L is a self-incompatible, declining plant species occurring on fragmented remnants in semi-natural calcareous grassland habitats in an agricultural landscape in Southern Lower Saxony, Germany. Plant-pollinator interactions were analysed in small, medium, and large plant patches in 15 differently sized calcareous grassland fragments surrounded by landscape matrices of differing complexity. Flower visitors were observed during three 15-minute observations, pollen supplementation experiments were performed to test for pollination limitation, and fruits from the plant patches were collected to calculate seed set. Hippocrepis comosa showed evidence of pollination limitation because hand-pollinated flowers set more seeds per inflorescence than open-pollinated flowers and seed set was strongly reduced by decreased visitation rates. Pollinators were most abundant in large patches and in diverse landscapes. Visitation rate was not affected by patch size. Hence, in order to maintain plant-pollinator interactions in an agricultural landscape, conservation efforts should include the preservation of calcareous grasslands that can support sufficiently large patches of a plant species and offer ample rewards for pollinators and the establishment of a diverse agricultural landscape around the semi-natural habitats that will further enhance pollinator abundance
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