Aedes aegypti and Ae. albopictus are vectors of pathogens such as zika and dengue, making it imperative that the ecology of both species is well understood. Where they come into contact, populations of Ae. aegypti are often displaced by Ae. albopictus. Theory suggests that coexistence or displacement is determined by the balance of inter- and intra-specific competition. As the larval stages of both species cohabit pools of standing water, it has been proposed that this is the forum for intra and inter-specific competition.
Numerous studies have assessed the strength and direction of larval competition by manipulating heterospecific and conspecific densities. However, the effects of density are often confounded with the effects of resource availability. Our study controls for this by maintaining a constant per-capita resource availability across all densities, allowing us to isolate the components of competition that are independent of resource availability. Heterospecific and conspecific densities of lab-reared Ae. aegypti and Ae. albopictus larvae were manipulated in a five-level fully factorial study design, with the survivorship and development time recorded. Data were analysed by a stepwise-down model selection process of hierarchical Bayesian generalised additive and generalised linear models.
Results/Conclusions
Ae. aegypti had a marginally higher baseline survival probability than Ae. albopictus (0.75 versus 0.73). At the highest level of conspecific density, the survival probability of Ae. aegypti was reduced from the baseline to 0.62. Ae. albopictus suffered greater intraspecific competition, with a reduction to 0.42. The highest level of heterospecific density reduced the survival probability of Ae. aegypti to 0.46 and Ae. albopictus to 0.33. Of the surviving larvae, Ae. aegypti took on average 2 fewer days to develop than Ae. albopictus. Survivors of both species developed faster when reared in higher densities of conspecifics and heterospecifics. However, when both conspecific and heterospecific densities were high, Ae. albopictus developed more slowly.
In terms of survival, Ae. aegypti suffered notably less intraspecific competition than Ae. albopictus, while the latter experienced a greater degree of interspecific competition. These findings are the opposite of what would be expected given the displacement of Ae. aegypti in the wild, and suggest that other competitive mechanisms could be driving this process, such as reproductive interference. By controlling per-capita feeding regimes, we have shown that there are components of larval competition independent of resource availability and that these effects act more strongly on Ae. albopictus.