Contact rate between hosts and parasites increases with host density, increasing parasite transmission. At the same time, for a given number of transmission stages in the environment, high host density can decrease infection risk for individual hosts because transmission stages are divided among all hosts in an area. We tested these predictions using the California horn snail, Cerithideopsis californica (=Cerithidea californica), which is the first intermediate host for at least 18 digenean trematode species in California estuaries. Snails become infected by ingesting an egg or through penetration by a free-swimming miracidium that hatches from an egg deposited with bird or mammal feces. This complex life cycle decouples the source of infective stages (vertebrates) from the hosts that they infect (snails), raising the possibility of an inverse relationship between host density and infection risk. To test the effect of host density on parasite success and infection risk, we manipulated uninfected snail density in 83 cages at eight sites within Carpinteria Salt Marsh (CA, USA). At each site, we quantified snail density and used a data set on bird and mammal density to control for the input of trematode eggs. After three months, we determined infection status of all experimental snails.
Results/Conclusions
We recovered 840 (97.7%) experimental snails, and 170 harbored trematode infections. Of these, 132 (77.6%) were infected by at least one egg-transmitted trematode, and 54 (31.8%) were infected by at least one miracidium-transmitted trematode. As expected, parasite success, indicated by new infections in a cage, increased with snail density and bird and mammal density. Snail density also increased infected snail biomass density, but this effect was weaker because crowded snails grew less, providing fewer resources per trematode infection. At a small spatial scale (cage), per-snail infection risk to egg-transmitted trematodes decreased with snail density, whereas incidence of miracidium-transmitted trematodes increased with bird and mammal density. Large snails were least infected, suggesting that size confers resistance. At a larger spatial scale (site), per-snail infection risk for both egg- and miracidium-transmitted trematodes decreased with snail density and increased with bird and mammal density. We conclude that density of first intermediate (snail) and final (bird and mammal) hosts benefits parasite transmission, crowding decreases parasite biomass, and at some spatial scales and for some transmission modes, per-capita infection risk declines with host density. Thus, the effect of host density on parasitism depends on if one takes a host or parasite perspective.