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Heterodera schachtii nematodes interfere with aphid-plant relations on Brassica oleracea
No full textAboveground and belowground herbivore species modify plant defense responses differently. Simultaneous attack can lead to non-additive effects on primary and secondary metabolite composition in roots and shoots. We previously found that aphid (Brevicoryne brassicae) population growth on Brassica oleracea was reduced on plants that were infested with nematodes (Heterodera schachtii) prior (4 weeks) to aphid infestation. Here, we examined how infection with root-feeding nematodes affected primary and secondary metabolites in the host plant and whether this could explain the increase in aphid doubling time from 3.8 to 6.7 days. We hypothesized that the effects of herbivores on plant metabolites would depend on the presence of the other herbivore and that nematode-induced changes in primary metabolites would correlate with reduced aphid performance. Total glucosinolate concentration in the leaves was not affected by nematode presence, but the composition of glucosinolates shifted, as gluconapin concentrations were reduced, while gluconapoleiferin concentrations increased in plants exposed to nematodes. Aphid presence increased 4-methoxyglucobrassicin concentrations in leaves, which correlated positively with the number of aphids per plant. Nematodes decreased amino acid and sugar concentrations in the phloem. Aphid population doubling time correlated negatively with amino acids and glucosinolate levels in leaves, whereas these correlations were non-significant when nematodes were present. In conclusion, the effects of an herbivore on plant metabolites were independent of the presence of another herbivore. Nematode presence reduced aphid population growth and disturbed feeding relations between plants and aphids.
Elevated CO2 concentrations affect the elemental stoichiometry and species composition of an experimental phytoplankton community
No full text1.Rising atmospheric CO2 concentrations might affect the primary production and community composition of freshwater ecosystems.
2.We investigated these potential effects in laboratory mesocosms (Limnotrons), using monoculture experiments and competition experiments with the green alga Scenedesmus obliquus and the cyanobacterium Synechocystis sp. PCC6803. The Limnotrons were sparged with ambient air (controls, 380 parts per million volume (ppmv) CO2), moderately elevated CO2 levels (3000 ppmv CO2) or highly elevated CO2 levels (18 800 ppmv CO2).
3.Growth at ambient air led to the depletion of dissolved CO2 during algal bloom development and hence a high pH. In contrast, growth at elevated CO2 levels resulted in high concentrations of dissolved CO2 and dissolved inorganic carbon (DIC), lower pH and low concentrations of nitrate and soluble reactive phosphorus. Elevated CO2 levels did not have a significant effect on the biomass of the algal species, but shifted their elemental composition towards higher carbon-to-nutrient ratios.
4.Competition experiments at ambient air were driven mainly by competition for inorganic carbon. In this case, the cyanobacterium Synechocystis was displaced by the green alga Scenedesmus. Elevated CO2 alleviated the community from carbon limitation, which shifted the species interactions towards competition for nitrogen and phosphorus, and resulted in coexistence of the two species. Hence, our findings do not support the hypothesis that cyanobacteria are generally better competitors for inorganic carbon than green algae.
5.In conclusion, our results show that elevated CO2 levels may cause major changes in freshwater ecosystems, including a reduction in pH, shifts in resource limitation patterns, and changes in the ecological stoichiometry and species composition of phytoplankton communities.
Die ehemaligen Klosterplätze im Küstengebiet / De voormalige kloosterplaatsen in het kustgebied
No full textTrends and population dynamics of a Velvet Scoter (Melanitta fusca) population: influence of density dependence and winter climate
No full textAs many seaduck populations around the world have been reported to be in decline, there is an increasing demand for knowledge about intrinsic and extrinsic factors determining population dynamics of these species. In this study, we analyzed long-term dynamics of the summer population of Velvet Scoters (Melanitta fusca) breeding in the Åland archipelago in the Baltic Sea; in particular, we examined the influence of winter weather and density dependence on population change. The studied population exhibited substantial fluctuations but only a weak negative trend during the total period of 58 years (1949–2007), and no significant trend at all during the latter three decades of the study (1977–2007). We tested for density dependence and incorporated the winter North Atlantic Oscillation index into the model to test for effects of winter conditions. Our final model explained 56.3 % of the variance of population growth of the studied population. Delayed density dependence explained 29.7 %, pre-breeding climate 8.3 %, and post-breeding climate 18.3 % of the variance. That breeding success is density dependent in a delayed manner is in accordance with the apprehension that Velvet Scoters breed at the age of 2 years. We conclude that density dependence and winter conditions must be taken into consideration when discussing population changes in seaducks in general and the Velvet Scoter in particular.
