Local diversity reduces infection risk across multiple freshwater hostparasite associations

1. In many host–parasite systems, infection risk can be reduced by high local biodiversity, though the mitigating effects of diversity are context dependent and not universal. 2. In aquatic ecosystems, local fauna can reduce the transmission success of parasite free-swimming infective stages by preying on them, acting as decoy hosts, or physically interfering with transmission. However, most prior research has focused on the effect of a single non-host organism at a time and/or has been performed under simplified and artificial conditions. 3. Here, using data on 11 trematode species sampled in different New Zealand lakes, we test whether local biodiversity affects infection risk in target second intermediate hosts, as well as total parasite population size (number of parasites per m), under natural conditions. We considered four components of local biodiversity: total biomass of non-host fish species, diversity (Simpson index) of non-host benthic invertebrates, total density of zooplankton and macrophyte biomass. Our analyses also accounted for host density, a known determinant of parasite prevalence, intensity of infection and total parasite population density. 4. The only influence of local biodiversity we detected was a negative effect of the diversity of nonhost benthic invertebrates on the prevalence achieved by trematodes in their second intermediate hosts: that is, the proportion of individual hosts that are infected. Interestingly, this effect was discernible in all 11 trematode species considered here, even if very weak within some species. 5. Our findings suggest that higher non-host benthic diversity may generally decrease infection risk for target hosts including snails, arthropods and fish. However, reduced infection success did not automatically mean smaller overall parasite population size, as other factors can maintain the parasite population in the face of high local diversity of non-hosts.