Food webs in phytotelmata: "bottom-up" and "top-down" explanations for community structure

Competitive Outcomes of Aquatic Container Diptera Depend on Predation and Resource Levels

Resources and predation are both known to be important in structuring communities; however the strength of one factor may be affected by the intensity of the other. This study used a fully crossed factorial experiment in laboratory microcosms to examine the ability of a predator, Corethrella appendiculata (Grabham), and basal resources (leaf litter) to differentially affect two competing species of mosquito prey. Increased resources resulted in shorter developmental time and increased survivorship, mass, and population performance for both prey species, except when predation levels were high. Increased levels of predation and resources reduced the negative competitive effects of Aedes albopictus (Skuse) on Ochlerotatus triseriatus (Say). At low levels of resources and predation, the superior competitor, A. albopictus had the higher survivorship, and at high levels of resources and predation, the inferior competitor's survival was greater. Predators in high-resource treatments emerged larger than those in low resources, suggesting the occurrence of a bottom-up cascade or alternative feeding method. This study suggests that survival and coexistence of the two prey species may depend on the interaction of resources and predation, in that high levels of predation are important for the coexistence of both species.

Do local processes scale to global patterns? The role of drought and the species pool in determining treehole insect diversity

Global patterns in community species richness may represent limitations operating at the local scale, such as competitive exclusion and habitat suitability, or, in the case of unsaturated communities, limitations to the species pool of biogeographic regions. Separating the effects of local and regional processes on community richness requires combining small-scale experiments with broad surveys. I examine limitations to the number of aquatic insect species per treehole at the scale of a single woodland, and between eight biogeographic regions. Variation in species richness at the woodland scale can largely be explained by small-scale differences between treeholes in drought disturbance, as shown for British treehole insects for 2 years. At the global scale, drought disturbance is a relatively poor predictor of patterns in the local richness of treehole mosquitoes. Instead, regional differences in the species pool explain most of the variation in local mosquito richness. Treeholes in at least the most species-poor regions appear to be unsaturated with mosquito species. In this system, therefore, local processes do not necessarily scale to global patterns.

Species-energy relationships at the macroecological scale: a review of the mechanisms

Correlations between the amount of energy received by an assemblage and the number of species that it contains are very general, and at the macro-scale such species-energy relationships typically follow a monotonically increasing curve. Whilst the ecological literature contains frequent reports of such relationships, debate on their causal mechanisms is limited and typically focuses on the role of energy availability in controlling the number of individuals in an assemblage. Assemblages from high-energy areas may contain more individuals enabling species to maintain larger, more viable populations, whose lower extinction risk elevates species richness. Other mechanisms have, however, also been suggested. Here we identify and clarify nine principal mechanisms that may generate positive species-energy relationships at the macro-scale. We critically assess their assumptions and applicability over a range of spatial scales, derive predictions for each and assess the evidence that supports or refutes them. Our synthesis demonstrates that all mechanisms share at least one of their predictions with an alternative mechanism. Some previous studies of species-energy relationships appear not to have recognised the extent of shared predictions, and this may detract from their contribution to the debate on causal mechanisms. The combination of predictions and assumptions made by each mechanism is, however, unique, suggesting that, in principle, conclusive tests are possible. Sufficient testing of all mechanisms has yet to be conducted, and no single mechanism currently has unequivocal support. Each may contribute to species-energy relationships in some circumstances, but some mechanisms are unlikely to act simultaneously. Moreover, a limited number appear particularly likely to contribute frequently to species-energy relationships at the macro-scale. The increased population size, niche position and diversification rate mechanisms are particularly noteworthy in this context.