A bit of sex stabilizes host-parasite dynamics

To date, only a few studies have focused on the effects of sex on populatio n dynamics. Previous models have typically found that sexual reproduction d ampens population fluctuations. Although asexual and sexual reproduction ar e just the two endpoints along a continuum of varying rates of sex, previou s work has ignored the effects of intermediate degrees of sex on population dynamics. Here we study the effects of partial sexual reproduction (i.e. s ex occurs only every few generations or with small probability in each gene ration) on the coupled population dynamics of a Nicholson-Bailey host-paras ite model. We show that complex dynamics are simplified for high host popul ation growth rates if the frequency of sex is sufficiently high in both hos t and parasite: sex decreases fluctuations in population density, and leads to non-chaotic dynamics for population growth rates that would result in c haotic dynamics in the absence of sexual reproduction. However, the simplif ication does not increase gradually with an increasing frequency of sex but appears abruptly at low-to-intermediate frequencies of sex. For some param eter settings, intermediate frequencies of sexual reproduction can simplify the dynamics more than lower or higher frequencies. Thus, in agreement wit h earlier results, sexual reproduction typically stabilizes complex populat ion dynamics in our models. Additionally, our results suggest that low-to-i ntermediate frequencies of sex may often be as (or even more) stabilizing a s high frequencies. (C) 2001 Academic Press.