Disease-Induced Hydra Effect
Pauline van den Driessche
The seemingly counterintuitive hydra effect of an increase in population size caused by an increase in mortality has been observed and modeled in several ecological systems. Here theoretical and simulation results for an infectious disease-induced hydra effect are presented. This occurs when the total population size (infectious plus susceptible individuals) at an endemic equilibrium is greater than the population size at the disease-free equilibrium. This disease-induced hydra effect occurs in models with continuous or discrete time when the intra-specific competition is strong and disease infection sufficiently inhibits the reproduction of infected individuals.