Density dependent host mortality in the interaction between the solitary endoparasitoid Aphidius colemani (Aphidiidae), and its host, the green peach aphid Myzus persicae, was examined on greenhouse peppers. The experimental approach attempted to eliminate spatial interdependence in the relationship between host density and host mortality by using different plants to measure parasitism at different spatial scales. Increasing host density at the plant scale caused a significant increase in the proportion of host mortality. However, at the shoot and leaf scale, increasing host density caused a significant decrease in host mortality. This may be one of the first experimental demonstrations of a switch from inverse to direct density dependence. The pattern is assumed to be a result of searching parasitoids using different cues at different spatial scales.
Winter weather limits populations of resident bird species. Although many small-scale or speciesspecific studies illustrated this fact, our knowledge of interspecific differences in population responses to winter temperatures is incomplete due to lack of community-level studies. For this purpose, we have used long-term monitoring data on breeding bird populations of 37 common bird species wintering in the Czech Republic. We predicted that species will differ in their relationship between winter temperature and abundance with respect to their body mass and dietary niche. Smaller species having relatively higher energy expenditure should show closer relationship between breeding abundance and winter temperature than larger species. Concerning dietary niche, abundance of species feeding on animals should be more affected by temperature than abundance of species feeding on plants or omnivorous species. Our results confirmed the second prediction: populations of species preying on animals followed winter temperatures more closely than populations of species feeding on both animals and plants. Food-mediated mortality is probably more important than direct effects of low temperatures. In general, relationships between abundance and temperature were relatively weak in most species and we suggest that possible changes in winter temperatures may not seriously affect populations of common breeding birds in the Czech Republic.