Why aggregate?.

By promoting coexistence, aggregation has been identified as a major source of biodiversity in insects. This study into the evolution of aggregation in Drosophila produced novel insights into the mechanisms and explanations underlying aggregation behaviour. 1. The conclusions of a literature review together with experimental results suggest that mammals are unlikely to affect insect densities in resources while birds have the potential to do so. 2. Allee effects occur in D. simulans but they are highly dependent on the precise properties of the resource. Mould proved unlikely to mediate Allee effects The relationship between yeast and larvae is more complex than hitherto assumed; competitive interactions may be responsible for the occurrence of Allee effects. 3. Oviposition in D. simulans is non-random and dependent on environmental properties (light) and characteristics of the resource (accessibility or detectabitliy, size of oviposition surface). Females do not respond to the size of resource units. 4. Individual oviposition patterns are highly variable and difficult to select for. Egg distributions generated by isolated flies are the products of different clutches. 5. Male presence and pre-experimental adult density have little effect on D. simulans oviposition behaviour. Females lay more eggs that are more aggregated on high quality substrates compared to those of lower quality. Females avoid using sites already containing eggs on natural substrates but still generate aggregated egg distributions. Resource use overlap can be increased by reducing the number of high quality patches. Egg distributions of D. simulans and D. melanogaster are randomly associated. 6. Females lay fewer, more scattered eggs on grapes with high compared to low sugar concentrations but only if yeast is present. Higher sugar content increases survivorship and adult body size. Female oviposition site choice reflects the quality of the substrate in terms of offspring survival and size. Combined with density-dependent effects this indicates that oviposition choice is a problem of optimal foraging strategy