Effects of predation and nutrient enrichment on a food web with edible and inedible prey

1. Trophic heterogeneity, or differences in edibility or resource consumption among species within a single trophic level, is widespread in natural food webs. Here, we test simple food web models that incorporate trophic heterogenity and that make robust predictions regarding food web responses to nutrient enrichment. To test these predictions, we assembled simple food webs consisting of an inedible alga, a mixed assemblage of bacteria, and a protist bacterivore in laboratory microcosms of contrasting nutrient concentrations. 2. Several results were consistent with model predictions. First, increasing nutrient concentration caused an increase in the abundance of the inedible alga, but only in the presence of the bacterivore. Secondly, nutrient enrichment increased the abundance of bacteria, but only in the absence of their bacterivore. Last, nutrient enrichment had no effect on abundance of the bacterivore. 3. Two results were not consistent with model predictions. First, at low nutrient concentrations, the presence of the bacterivore increased the abundance of bacteria. Secondly, although the abundances of the bacterivore and bacteria were positively correlated, some of the lowest abundances of both occurred in the high nutrient treatment. Thus, while our results were generally consistent with several simple food web models, additional explanations are required for selected food web responses.     

Spatial and temporal variation in host–parasitoid interactions: leafcutter ant hosts and their phorid parasitoids

1. Parasitoid–host interactions are important components of ecological communities. Although parasitoid–host interactions are strongly shaped by evolutionary history, the abundance of both the parasitoid and the host may have a role in determining the nature of the interaction once phylogenetic relationships are considered. 2. Leafcutter ants are hosts of phorid parasitoids and represent a well‐defined and specialised module within a larger network of ant–symbiont interactions. A low specificity host taxa and a positive association between host abundance and parasitoid interaction frequency were expected due to the close phylogenetic relatedness of the hosts. 3. The interactions among all species of leafcutter ants and their parasitoids were quantified in two localities with different species richness. This study also characterised the spatial‐temporal variability of these interactions, determined the patterns of parasitoid specificity and host selection, and tested for an association between host abundance and parasitoid interaction frequency. 4. Contrary to expectation, most parasitoid species were highly specialised and interaction frequency for parasitoid species was not related to host abundance. All host ant species were attacked by more than one phorid species. Some phorid species used more than one host species and showed preference for the same species over space and time, suggesting that there are physiological and/or behavioural restrictions on host use. 5. These results show that there is a tendency for specialisation even when hosts are highly similar in their ecology. From a biological control perspective, these parasitoids may be effective candidates, due to the high specificity of some species and little host‐use variation through time.     

Life history strategies in two coexisting agamospecies of dandelion

Populations of Taraxacum , dandelion, on a Northumberland sand dune system contain 27 agamospecies. Demographic parameters for subpopulations containing a preponderance of two closely related agamospecies, Taraxacum lacistophyllum and T. brachyglossum are described. High and low density populations were stable over a 3 year period. The estimated turnover times differed between densities and species. The early growth characteristics of the two species are described. The plastochrons of the two species are distinct, as are the growth patterns of the leaves, the total leaf length per plant, the number of leaves and rate of leaf birth, and the patterns of allocation of dry weight to the root, shoot, and reproductive organs of the plant. Differences between the species were observed in fruit, scape height, the wind speed required to achieve fruit dispersal, and the pattern of fruit dispersal at different wind speeds. The origin and maintenance of agamospecies diversity is discussed in relation to the observed life history variation.     

Dispersion and the size composition of the aerial insect fauna

Abstract. 1. Beetles sampled in Louisiana at higher levels in the atmosphere were significantly smaller than those sampled at lower levels, which suggests that passive buoyancy is important in high altitude dispersal.
2. A model of long-range dispersal is suggested which combines both active flight and passive buoyancy.