When exposed to extrinsic sources of mortality, negatively density-dependent populations may experience no net change in overall survivorship (compensation) or an increase in overall survivorship (overcompensation) under certain circumstances. This phenomenon, known as compensatory or overcompensatory mortality, respectively, has been demonstrated in both theoretical and empirical studies across taxa, but little is known on the mechanisms behind it. Timing, amount, and heterogeneity of the extrinsic mortality source(s) are all postulated to influence the likelihood and magnitude of overcompensatory mortality. We tested the effects of various amounts of extrinsic mortality on the level of compensatory mortality in four container mosquitoes (Aedes aegypti, Aedes albopictus, Aedes triseriatus, Culex pipiens). We induced controlled mortality by randomly removing 0, 10, 30, 50, or 70% of individuals from artificial containers 2 days after hatching, a timing preliminary studies have demonstrated to cause overcompensatory mortality in this system. Mosquito larvae were also exposed to a natural predator, Mesocyclops longisetus (Crustacea: Copepoda) to compare artificial mortality treatments to a natural form of extrinsic mortality. In addition to recording survivorship to adulthood, we used data on survivorship, development time, and female size to estimate a rate of increase for each cohort.
Results/Conclusions
We found significant overcompensation at the 50% mortality levels in Aedes aegypti, with an average of 24% survival to adulthood compared to 6% in the control treatment without extrinsic mortality. The remaining treatments involving controlled mortality and predator treatments showed a trend toward overcompensation, but survival to adulthood was not significantly different from controls (compensation). Responses to mortality varied among the other species. Estimated rates of increase followed patterns in survivorship for each species, with overcompensation corresponding to extrinsic mortality causing an increase in estimated rate of increase. These results suggest that intermediate levels of extrinsic mortality are most likely to induce overcompensation. Compensation in response to early mortality (controlled or predator-induced) appears to be the most likely outcome. This study provides insight into the mechanisms mediating this counterintuitive effect of mortality on populations.