The effect of parasites on host population density and extinction: Experimental epidemiology with Daphnia and six microparasites

Parasites have been shown to reduce host density and to induce host populat ion extinction in some cases but not in others. Epidemiological models sugg est that variable effects of parasites on individual hosts can explain this variability on the population level. Here, we aim to support this hypothes is with a specific epidemiological model using a cross-parasite species app roach. We compared the effect of six parasites on host fecundity and surviv al to their effects on density and risk of extinction of clonal host popula tions. We contrast our empirical results of population density with predict ions from a deterministic model and contrast our empirical results of host and parasite extinction rates with those predicted by a stochastic model. F ive horizontally transmitted microparasites (two bacteria: white bacterial disease, Pasteuria ramosa; two microsporidia: Glugoides intestinalis, Ordos pora colligata; one fungus: Metschnikowiella biscuspidata); and six strains of a vertically transmitted microsporidium (Flabelliforma magnivora) of th e planktonic crustacean Daphnia magna were used. In life table experiments, we quantified fecundity and survival in individual parasitized and healthy hosts and compared these with the effect of the parasites on host populati on density and on the likelihood of host population extinction in microcosm populations. Parasite species varied strongly in their effects on host fec undity, host survival, host density reduction, and the frequency with which they drove host populations to extinction. The fewer offspring an infected host produced, the lower the density of an infected host population. This effect on host density was relatively stronger for the vertically transmitt ed parasite strains than for the horizontally transmitted parasites. As pre dicted by the stochastic simulations, strong effects of a parasite on indiv idual host survival and fecundity increased the risk of host population ext inction. The same was true for parasite extinctions. Our results have impli cations for the use of microparasites in biological control programs and fo r the role parasites play in driving small populations to extinction.