Print PageHabitat structure can affect epidemic size by influencing the physical environment in which hosts and parasites interact and the distribution of other species that influence disease. In lakes, habitat is often structured vertically due to thermal stratification. We addressed how stratification shapes epidemics of a fungal parasite in natural populations of Daphnia hosts. Daphnia become infected by consuming parasite spores released from dead hosts. Depending on where spores are released, stratification inhibits or promotes contact between parasites and hosts. Stratification inhibits host-parasite contact when dead hosts sink to the bottom of stratified lakes, where their spores cannot mix into host habitat. However, stratification might promote epidemics if Chaoborus eat infected hosts and release spores in surface waters, because the density gradient prevents spores from sinking out. Stratification also influences densities of non-host zooplankton that consume spores without becoming infected; these species can reduce disease through a dilution effect. Finally, stratification segregates nutrient-rich waters near sediments from algae growing in surface waters. This separation typically reduces nutritional quality of algal resources. Poor resource quality, in turn, increases host susceptibility. To evaluate links between stratification, ecological interactions, and disease, we surveyed limnological variables and plankton populations weekly in 18 lakes.
Results/Conclusions
Stratification strength was strongly positively correlated with peak prevalence of infection. This pattern is consistent with the hypothesized mechanisms outlined above. First, Chaoborus densities increased with stratification strength and infection prevalence. These predators release spores in the epilimnion. Second, density of an important “diluter”, Daphnia pulicaria, decreased with stratification strength. Lakes with lower density of this species had larger epidemics. Finally, stronger stratification was associated with smaller increases (even decreases) in resource quality for hosts during epidemics; lakes with such muted response of resource quality through time then had larger epidemics. This pattern suggests that stratification inhibited mixing of nutrients from deep waters to surface waters. That segregation resulted in lower-quality algal resources which then increases susceptibility of hosts to infection. Our results suggest that habitat structure can affect epidemic size through interactions among several physical and ecological mechanisms. Here, through its influence on multiple ecological players, stratification strength promoted disease. More generally, indirect and direct ecological effects of habitat structure may profoundly shape epidemics of infectious disease in a variety of systems.
