Aedes triseriatus and treeholes: Trophic interactions and factors influencing larval growth

The eastern treehole mosquito, Aedes triseriatus (Say), and the ecosystem it inhabits provide a valuable model system for studying ecosystem processes and factors affecting growth and development. Studies reported here were directed at two aspects of the biology of this mosquito in treeholes: (1) trophic interactions among larvae and microorganisms in treehole water; and (2) an experimental analyses of interacting factors affecting growth and metamorphosis of larvae. With regard to trophic interactions, the trophic cascade hypothesis, and the “top-down/bottom-up” hypothesis were tested. When a trophic cascade operates in an ecosystem, changes in abundance of organisms in higher trophic levels result in a cascade of changes in abundance or biomass of organisms in lower trophic levels, with alternating directional responses in alternating lower trophic levels. By contrast, top-down/bottom-up forces operate such that abundance or biomass of organisms at any given trophic level is dependent upon the input of nutrients into the lowest trophic level in the ecosystem. Theory predicts that these inputs should have positive but diminishing effects of the same direction with succeeding, higher trophic levels. A series of laboratory and field experiments showed that larvae of A. triseriatus are a keystone predator in treehole ecosystems due to the significant effect of their presence on the abundance of protists and bacteria. There was no clear evidence for a trophic cascade in treeholes. Experiments involving nutrient manipulations showed the primary nutrient affecting higher trophic levels was organic resources (senescent leaf detritus); anionic nitrogen and sulfur were not stimulatory.; Other experiments were directed at growth and metamorphosis of A. triseriatus larvae when temperature and basal food resources were manipulated. Theory on the population reaction norm of ectothermic animals predicts that animals should have decreased development rates and larger body sizes at metamorphosis at low temperatures compared to the converse at high temperatures; but that development rates and body sizes should be negatively correlated when food supply is varied. Experiments showed that these contrasting predictions held true when cohorts of larvae were subjected to experimental combinations of food and temperature; but there were no direct statistical interactions between food supply and temperature. However, when larvae were exposed to variable food and temperature individually, these reaction norm responses differed: body size was similar among larvae reared at different temperatures. Larval cohort mortality may affect these responses by dynamically affecting the ration of food supply for larvae as temperature dependent mortality occurred. Overall, the results indicated that a density-dependent reaction norm response would be predicted from these findings.