Parasites can simplify host-population dynamics and reduce extinction risk

Question: Can parasites stabilize host-population dynamics and thereby reduce the risk of host extinction? Model: We analysed a discrete-time model of logistic growth in an annual, asexual host population. Infection was assumed to reduce host fecundity, but not host survival. Using simulations, we determined whether parasites stabilized or destabilized host-population dynamics. We calculated the coefficient of variation and the effective population size (N-e) as measures of host extinction risk. Ranges of key variables: Per capita birth rates in the host population varied from just above replacement (1.1) to 50. Equilibrium infection prevalence ranged from 0.25% to 99.75%. Infection caused a two- or ten-fold disadvantage in fitness parameters, with the fitness cost being density-independent, density-dependent, or both. Conclusions: Parasites stabilized host-population dynamics, thereby reducing the coefficient of variation and increasing host N-e, for a broad range of host birth rates. Thus parasites could, in theory, reduce host extinction probabilities in otherwise unstable populations with high intrinsic birth rates.