636 research outputs found
Seed set of male-sterile and male-fertile oilseed rape (Brassica napus) in relation to pollinator density
No full textThe effects of pollinator density on the seed set of a male-sterile (MS) and a
male-fertile (MF) line of winter oilseed rape (Brassica napus) were studied in 24
isolation cages (7.5 m). Pollinator treatments comprised of high pollinator density
(a small honeybee colony and 10 mason bees, Osmia rufa), and a gradient of increasing
O. rufa densities from zero (control) up to 100 bees per cage. High pollinator densities
increased the seed weight per plant from 6.5 g to 56.9 g for the MS line. However seed
weight did not differ between high pollinator densities and controls for the MF line.
Increasing densities of O. rufa had a significant effect on almost all yield components
of the MS line, but only marginal effects on the MF line. The number of seeds per pod,
the seed weight per plant and the harvest index (seed weight/plant dry weight)
of the MS line showed a significantly steeper increase with increasing bee density
than that of the MF line. The results suggest that solitary bees could be used
successfully to replace honeybees as pollinators of MS oilseed rape in isolation cages
Effects of landscape context on herbivory and parasitism at different spatial scales
No full textLocal community structure and interactions have been shown to depend partly on landscape context. In this paper we tested the hypothesis that the spatial scale experienced by an organism depends on its trophic level. We analyzed plant-herbivore and herbivore-parasitoid interactions in 15 agricultural landscapes differing in structural complexity using the rape pollen beetle (Meligethes aeneus), an important pest on oilseed rape (Brassica napus), and its parasitoids. In the very center of each landscape a patch of potted rape plants was placed in a grassy field margin strip for standardized measurement. Percent non-crop area of landscapes was negatively related to plant damage caused by herbivory and positively to the herbivores’ larval mortality resulting from parasitism. In a geographic scale analysis, we quantified the structure of the 15 landscapes for eight circular sectors ranging from 0.5 to 6 km diameter. Correlations between parasitism and non-crop areas as well as between herbivory and non-crop area were strongest at a scale of 1.5 km, thereby not supporting the view that higher trophic levels experience the world at a larger spatial scale. However, the predictive power of non-crop area changed only slightly for herbivory, but greatly with respect to parasitism as scales from 0.5 to 1.5 km and from 1.5 to 6 km diameter increased. Furthermore, the effect of non-crop area tended to be stronger in parasitism than herbivory suggesting a greater effect of changes in landscape context on parasitoids. This is in support of the general idea that higher trophic levels should be more susceptible to disturbance.
(Thies, C., Steffan-Dewenter, I. and Tscharntke, T. 2003. Effects of landscape context on herbivory and parasitism at different spatial scales. – Oikos 101: 18–25.
Importance of habitat area and landscape context for species richness of bees and wasps in fragmented orchard meadows
No full textI investigated how habitat area, habitat connectivity, and landscape context affect the species richness and abundance of trap-nesting bees, wasps, and their natural enemies. The research was done on 45 orchard meadows ranging in size from 0.08 to 5.8 ha and differing in habitat connectivity and the surrounding landscape matrix. Effects of the surrounding landscape matrix were restricted to circles with a radius of 250 m. Only the species richness of natural enemies increased with landscape diversity in the surrounding matrix. Population densities of bees increased with habitat connectivity. Total species richness and species richness of bees, eumenid wasps, and natural enemies significantly increased with habitat area. Significant species-area relationships existed for insect groups but not for plants, thereby confirming the hypothesis that higher trophic levels are more affected by habitat fragmentation than lower trophic levels. The slope of species-area relationships was steeper for mutualistic bees than for predatory wasps and natural enemies. In contrast to expectations, the rate of parasitism did not depend on habitat area but only on the local and regional abundance of hosts. My results suggest that the area and connectivity of habitat fragments is most important for the conservation of habitat specialists, whereas generalists may profit from a diverse surrounding landscape matrix
Resource overlap and possible competition between honey bees and wild bees in central Europe
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