Parasite communities as indicators of ecosystem stress.

Many parasites have complex life cycles and for transmission depend on the presence of a variety of vertebrate and invertebrate intermediate hosts, including members of the benthos and zooplankton. Thus, food web dynamics and trophic interactions have a powerful influence on parasite community structure. In addition, many parasites possess free-living stages that are also susceptible to environmental conditions. Therefore, the parasite community within a single host species such as a fish is indicative of environmental stress, trophic structure, and biodiversity. We show that parasite communities of American eels (Anguilla rostrata) in Nova Scotia respond to acid conditions in rivers. Parasite species richness was greater and there were more multiple infections in eels from an experimentally limited river compared to those from an adjacent acidified river. Digeneans were absent in eels from the acidified river. The study was expanded to include 28 sites in the Southern Upland and adjacent regions of Nova Scotia, encompassing a pH gradient increasing from southwest to northeast. Survey results support those obtained by experimental manipulation. Parasite diversity in eels as measured by species richness, Shannon-Wiener Index, and Hill's Number decreased when pH < 5.4. Digeneans were absent from the southwest, where pH < 4.7. Parasite distributions among rivers in adjacent watersheds corresponded to fluctuations in pH in those rivers. These results support the hypothesis that parasite communities are good indicators of environmental stress and biodiversity, because they reflect the presence of many different types of organisms based on the variety of complex life cycles displayed by the different parasite taxa.