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High host density in patches increases contact rate between hosts and parasites, increasing parasite transmission success. At the same time, for environmentally transmitted parasites, high host density can decrease infection risk to individual hosts, because infective stages are divided among all hosts in a patch, leading to safety in numbers. We tested these predictions using the California horn snail,<jats:italic>Cerithideopsis californica<\/jats:italic>(=<jats:italic>Cerithidea californica<\/jats:italic>), which is the first intermediate host for at least 19 digenean trematode species in California estuaries. Snails become infected by ingesting trematode eggs or through penetration by free\u2010swimming miracidia that hatch from trematode eggs deposited with final\u2010host (bird or mammal) feces. This complex life cycle decouples infective\u2010stage production from transmission, raising the possibility of an inverse relationship between host density and infection risk at local scales. In a field survey, higher snail density was associated with increased trematode (infected snail) density, but decreased trematode prevalence, consistent with either safety in numbers, parasitic castration, or both. To determine the extent to which safety in numbers drove the negative snail\u2010density\u2013trematode\u2010prevalence association, we manipulated uninfected snail density in 83 cages at eight sites within Carpinteria Salt Marsh (<jats:styled-content style=\"fixed-case\">California<\/jats:styled-content>,<jats:styled-content style=\"fixed-case\">USA<\/jats:styled-content>). At each site, we quantified snail density and used data on final\u2010host (bird and raccoon) distributions to control for between\u2010site variation in infective\u2010stage supply. After three months, overall trematode infections per cage increased with snail biomass density. For egg\u2010transmitted trematodes, per\u2010snail infection risk decreased with snail biomass density in the cage and surrounding area, whereas per\u2010snail infection risk did not decrease for miracidium\u2010transmitted trematodes. Furthermore, both trematode recruitment and infection risk increased with infective\u2010stage input, but this was significant only for miracidium\u2010transmitted species. A model parameterized with our experimental results and snail densities from 524 field transects estimated that safety in numbers, when combined with patchy host density, halved per capita infection risk in this snail population. We conclude that, depending on transmission mode, host density can enhance parasite recruitment and reduce per capita infection risk.<\/jats:p>","DOI":"10.1002\/ecy.1905","type":"journal-article","created":{"date-parts":[[2017,5,18]],"date-time":"2017-05-18T15:57:07Z","timestamp":1495123027000},"page":"2029-2038","source":"Crossref","is-referenced-by-count":47,"title":["Host density increases parasite recruitment but decreases host risk in a snail\u2013trematode system"],"prefix":"10.1002","volume":"98","author":[{"given":"J. C.","family":"Buck","sequence":"first","affiliation":[{"name":"Marine Science Institute University of California, Santa Barbara Santa Barbara California 93106 USA"},{"name":"Ecology, Evolution and Marine Biology University of California, Santa Barbara Santa Barbara California 93106 USA"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"R. 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