Density dependent survival of juveniles can interact with extrinsic mortality (e.g., predation) to produce the counter-intuitive result of the same (compensation) or greater (overcompensation) numbers of surviving adults with added mortality. These results occur when density reduction via mortality alleviates negative effects of juvenile density on survival. We have shown that overcompensation often occurs in laboratory studies imposing mortality on aquatic larval mosquitoes. An important unanswered question remains: Is overcompensation likely at natural densities? We designed field studies with three mosquito species at three locations to determine the likelihood of overcompensation in nature. Experimental units were 4 liter black buckets placed in habitats of Aedes species to accumulate detritus resources (for microorganisms that are the food of larvae), and colonization by local Aedes so that larval densities encompass the range of natural densities. After colonization, the natural population of larvae was removed and counted. Containers were then stocked with larvae of one species at densities that span the natural range. We then recorded survival over 14 d in these containers, and analyzed number of survivors via nonlinear regressions to determine if natural densities are sufficient to yield overcompensation if extrinsic mortality occurs.
Results/Conclusions
The function we fit to survival data was: S = aH/[1+(a/b)Hd], where S=survivors, H=initial stocked density, and a, b, and d are parameters to be estimated. The parameter d determines the shape of the curve, with values of d>1.0 indicating overcompensation and d=1.0 indicating compensation. Our results indicate that d was often significantly greater than 1.0, and also frequently was not statistically different from 1.0. Our field studies show that compensation or overcompensation are likely to be common, but not universal, in the upper quartile of densities observed in nature. We observed similar responses of Aedes triseriatus in two years, and differences in the responses of Aedes albopictus at two geographically distant sites. This result indicates compensation and overcompensation may be common for natural populations of larval container mosquitoes. Overcompensation by mosquito larvae likely has important consequences for attempts to control mosquito populations, and the diseases that they transmit, by imposing mortality on larvae.