Many ecological models have been shown to yield a hydra effect where an interaction between density dependence and human- or predator-caused mortality can have a counter-intuitive effect of increasing annual survival and population size. I explore the existence and behavior of the hydra effect in structured population models in discrete time. Alternative model structures that I consider are driven by empirical patterns. Among most large mammals, for example, density dependence is strongest for juvenile survival, many predators kill predominantly young prey, and hunting is focused on adults.
Results/Conclusions
Sequential timing and stage specificity of mortality in seasonal environments alter the magnitude of response and the dynamical consequences of the hydra effect. Predator control can have counter-productive consequences, and hunting sometimes can enhance annual survival and population size.