Density dependence is believed to be one of the primary ecological processes regulating mosquito population sizes. However, there is no clear mechanistic explanation of this process. In particular, larval rearing conditions in empirical studies have been inconsistent. Some studies have fed larvae a fixed amount of food irrespective of the number of individuals, some have adjusted food quantities per-capita and others do not report rearing conditions. Therefore it is not clear whether observed associations between larval density and larval development are due to (a) increased density or (b) variable nutrient availability due to increased numbers of conspecifics.
We hypothesised that mosquito populations are regulated in a bottom-up manner by their food supply and therefore “density-dependence” refers to the capacity of the environment to support the nutritional requirements of a number of individuals. We compared two empirical study designs to test this in Aedes aegypti. Each monitored larval and adult development over a matrix of resource availability and larval density. The first design fed larvae on a per-capita basis. The second fed a fixed amount of food unadjusted for the number of individuals; therefore at higher larval densities each received less food per-capita.
Results/Conclusions
We observed opposing associations of density with larval development in each experiment using hierarchical Bayesian generalized linear models. Decreased survival, increased development times and decreased adult wing lengths were observed at high densities when fed a fixed amount irrespective of larval numbers; the opposite effect was observed when fed per-capita. The strength of the association between larval density and measures of larval development was also greatly reduced when fed per-capita. These results suggest that reduced resource uptake due to increased numbers of competitors is a main cause of density dependence in Aedes aegypti. Additionally, failure to adjust food for the number of larvae in empirical studies may occlude the effects of other mechanisms that have beneficial effects for larval development at high densities.
Detailed characterization of ecological processes in mosquitoes is crucial to inform vector and disease control models. Previous empirical studies in mosquitoes have not addressed mechanistic explanations of density dependence. This study demonstrates that resource availability is a primary mechanism of density dependence in Aedes aegypti. Therefore, density dependence in mosquitoes is not a one-dimensional association between density and population growth. Mosquitoes inhabit transient environments; consideration of environmental components of density dependence such as resource availability is paramount.