Habitat structure and ecological drivers of disease

Habitat can influence disease directly, through effects on hosts and parasites, or indirectly, through effects on ecological drivers of disease. We illustrated direct and indirect connections between habitat and outbreaks using a case study in the plankton. We sampled yeast epidemics in 18 populations of the lake zooplankter Daphnia dentifera. Lake size drove variation in two types of habitat structure, size of predation refuges and strength of stratification. Those habitat factors, in turn, indirectly linked to epidemics through two pathways involving nonhost species. In the first pathway, larger lakes had larger hypolimnetic refuges from vertebrate predation and greater densities of Daphnia pulicaria, a completely resistant species that can reduce disease risk for D. dentifera hosts by removing parasite spores from the environment. In lakes with more D. pulicaria, epidemics started later in autumn and remained smaller. In the second pathway, smaller lakes had shallower penetration of light, which correlated with stronger thermal stratification and higher densities of an invertebrate predator (Chaoborus) that spreads disease by releasing spores from infected hosts. Lakes with weaker stratification had fewer of these predators and smaller epidemics. In the second pathway, deeper light penetration may also decrease disease by imposing direct mortality on spores. Thus, this case study shows how habitat structure could influence epidemics through direct and indirect effects on the host-parasite system. Understanding these multiple mechanisms can enhance prediction of disease outbreaks as habitat modification continues in lakes and other ecosystems worldwide.