Parasites impact host dynamics in complex ways that may improve or diminish the capacity of the host population to persist through time. A previous model has shown that parasites can stabilize otherwise chaotic dynamics in their host populations by imposing density-dependent fitness costs (Lively 2006). Further exploration of that model revealed that parasites can also reduce the severity of fluctuations in host density without stabilizing dynamics. Populations experiencing large fluctuations in size are more prone to extinction and loss of genetic diversity over time than populations with mild fluctuations. We analyzed the 2006 model, in which both the probability of infection and host fitness depend on host density, to examine the persistence of the host population in two ways, effective population size (N_e, harmonic mean of population density through time) and extinction risk (CV, coefficient of variation of population size over time). We simulated the model to determine the N_e and CV for four cases: (1) avirulent parasites, which do not alter host fitness or dynamics in any way; (2) virulent (harmful) parasites causing a reduction in fecundity independent of host density; (3) virulent parasites causing density-dependent fecundity reduction; (4) highly virulent parasites causing density-dependent and independent effects.

Results/Conclusions

Our simulations show that harmful parasites can substantially increase host N_e by reducing the severity of population bottlenecks, with the most marked increases for parasites that alter host fitness in a density-dependent way. A similar pattern emerges from the CV results, where parasites imposing density-dependent fecundity costs substantially dampen unstable fluctuations in the host population in comparison with avirulent parasites. In contrast, parasites with only density-independent virulence (i.e., that only reduce the intrinsic birth rate of infected hosts) do not substantially change dynamics compared with avirulent parasites. These results suggest that parasites may positively impact the persistence of their host populations when virulence is intensified by host density.