Predicting where and when disease emergences will occur is of significant interest to disease ecologists. Theory suggests that less diverse host communities may suffer higher burdens of disease than more diverse communities of hosts (i.e. the dilution effect hypothesis). Dilution effects are proposed to occur when less competent species of hosts dilute parasites away from more highly susceptible, competent host species. However, findings for the dilution effect have been equivocal and appear to be context-specific, suggesting that environmental factors may be contributing to the differential outcomes of this proposed phenomenon in nature. Exposure to environmental contaminants represents one way by which host competency may be altered across different environmental contexts, potentially altering dilution effect processes and findings across space and time. Using a multi-host amphibian-trematode-NaCl model, we conducted a series of experiments to understand how contaminant exposure may modify the predicted outcome of the dilution effect. We asked: (1) Does increased amphibian diversity buffer the overall community from trematode infection (i.e. dilution effect)? (2) Does NaCl exposure enhance or suppress the dilution effect within multi-host systems?
Results/Conclusions
When not exposed to NaCl, amphibians in single-species communities were similarly susceptible to trematode infection compared to multi-species communities (i.e. no dilution effect). However, in the presence of NaCl, we detected a dilution effect (up to a 40% reduction in infection in multi-species communities). Specifically, we found dilution effects in communities where spring peepers or wood frog hosts were paired with toads. Relative to the control, exposure to NaCl had no effect on toad susceptibility to parasites but increased spring peeper and wood frog susceptibility to parasites by 35% and 65%, respectively. Thus, toads served as ‘decoys’, diluting parasites away from the more highly-competent spring peeper and wood frog hosts but only in environments contaminated with NaCl. Overall, our results suggest that viewing host competency as a static trait may lead to incorrect predictions of disease outcome in multi-host systems particularly regarding predicted outcomes of the dilution effect. As human populations continue to grow, it is critical that we evaluate how anthropogenic changes to the environment may act on host competency, and the consequences of such changes at the community-level. Such insights could ultimately prove useful for making predictions about the emergence of disease in nature.