The number of individuals in a competitive environment can affect the growth rate, survival, size, and fecundity of those individuals, all of which are referred to as density-dependent effects. Overcompensation may occur if few juveniles survive to adulthood in a high-density environment. Overcompensation arises when density dependent survival interacts with extrinsic sources of mortality, such that more juveniles survive to adulthood than if no extrinsic mortality had occurred. The synchrony of juvenile cohorts could affect the likelihood of overcompensation by influencing the proportion the population vulnerable to density dependence or extrinsic mortality. I developed an individual-based model to explore the effects of environmentally-induced hatching synchrony on overcompensation in a mosquito population. The initial density of individuals, inter-hatch intervals, the extent of juvenile mortality, and the timing of juvenile mortality are factors that are explored with the model. At small inter-hatch intervals, the cohorts were asynchronous because juveniles entered the environment at multiple time points and coexist at multiple developmental stages. At large inter-hatch intervals, the cohorts were synchronous because large numbers of juveniles entered the environment simultaneously. The number of adults, juveniles, eggs, and total population size are reported by the model.
Results/Conclusions
The model established a fluctuating equilibrium for juvenile and adult populations, with the amplitude of the fluctuations increasing with the inter-hatch interval. Juvenile mortality at most levels resulted in overcompensation for asynchronous cohorts. Over 90% juvenile mortality was required for the population to decline. Alternatively, any level of additional juvenile mortality resulted in extinction with a large inter-hatch interval ( ~50 time steps). At intermediate inter-hatch intervals, low levels of juvenile mortality (< 30%) would result in overcompensation, but higher mortality levels resulted in extinction. The results indicate that overcompensation due to extrinsic mortality is dependent on both the level of mortality and the hatch synchrony of juvenile cohorts. Synchronous cohorts are likely to exhibit overcompensation at low levels of extrinsic mortality when the inter-hatch interval is intermediate. However, populations with a large inter-hatch interval and synchronous cohorts are unlikely to exhibit overcompensation. Instead, any juvenile mortality results in extinction. Asynchronous cohorts are very likely to experience overcompensation at most levels of extrinsic mortality. The effect of environmental periodicity on overcompensation due to extrinsic mortality is relevant to mosquito control because overcompensatory increase in the adult population could result in increased vector-borne disease transmission.