| Understanding the reciprocal interactions between ecosystems and disease is an emerging frontier in ecology. Here I explored the causes and consequences of disease in two model systems: malformations in amphibians and chytridiomycosis in Daphnia. Using voucher specimens (1946--1988), interviews, and resurvey data, I found that infection by a trematode (Ribeiroia ondatrae) likely explained deformity patterns in six of eight historical accounts. Amphibian population extirpations occurred at four of five sites associated with Ribeiroia. I conclude that parasite-induced malformations are not new, but qualitative evidence suggests they have increased in prevalence. To evaluate the combined effects of trematode infection and predation, I integrated field surveys and experimental exposures of salamander larvae to Ribeiroia and conspecific attack. Limbs exposed to injury and infection exhibited significantly more abnormalities than those exposed to either factor alone, emphasizing the importance of interactions between parasitism and predation in driving disease.; Analysis of long-term (15 yr) patterns of infection by a chytrid ( Polycaryum laeve) indicated that infection is controlled by Daphnia density and water mixing. Stratification inhibited transmission by separating hosts and parasites into the epi- and hypolimnion, respectively. Infection reduced Daphnia reproduction and population growth rate. By combining experiments, fish diets, and lake surveys, I found that infection increased host predation risk. Humic water eliminated selective predation, suggesting that selectivity results from parasite-induced changes in host appearance. Correspondingly, humic lakes were more likely to support P. laeve, indicating that environmental characteristics can moderate predator-prey-parasite interactions.; Finally, I reviewed the effects of eutrophication on macroparasitic-, macroparasitic- and non-infectious diseases. While low levels of eutrophication can increase parasite richness and abundance, higher levels lead to declines of parasite richness. However, through effects on host stress or intermediate host abundance, eutrophication can cause outbreaks of generalist parasites with simple life cycles. Given that nutrient-loading will increase in the future, I emphasize identification of thresholds and feedbacks between eutrophication and host-pathogen dynamics.; Collectively, these results highlight the importance of an ecosystem-scale perspective on disease. Host-parasite interactions are an embedded component of ecological systems, and feedbacks among environmental change, community structure and disease epidemics are likely to be widespread and influential. |
