| Human alterations of the landscape may change environmental conditions which influence the transmission of parasites, depending on the life histories of parasites and their hosts, as well as on the nature of alterations. Three species of amphibians, Rana vaillanti, Eleutherodactylus fitzingeri, and Smilisca puma were examined from Costa Rica. Frogs were collected and examined for parasites during 2001-2003 from sites in forest or clear-cut cattle pastures. Parasites recovered included nematodes, digeneans, cestodes, acanthocephalans, oligochaetes, mites, opalinids, apicomplexans, hemoflagellates, and rickettsiae. Blood parasites were detected with greater success from heart samples versus toe-clip samples. Microfilarial densities in frog blood samples were significantly correlated with adult female worm intensities. A general linear model was constructed to examine relationships between land use (forest versus pasture), water body type (swamp versus stream), and host attributes, such as sex and size in predicting the abundance and richness of parasites. Parasite abundance was related positively to host size for two parasites of E. fitzingeri, six parasites of R. vaillanti, and two parasites of S. puma. Land use was related to parasite richness in R. vaillanti and to the abundances of one parasite of E. fitzingeri, seven parasites of R. vaillanti, and one parasite of S. puma. The abundances of several parasites of R. vaillanti were associated with the type of water body, often exhibiting significant interactions with land use type (e.g., some parasites were found primarily in pasture swamps and one species was associated with forest streams). Changes in the densities of intermediate hosts, vectors, and/or definitive hosts were likely owing to changes in water quality related to landscape alterations. This study suggests that human impacts on the environment contributes to the distribution and abundance of parasites in amphibian hosts. A Rosenzweig-MacArthur resource consumer model was developed to further examine the relationship between eutrophication and the density of algae, snails, and digenean cercariae. The model results support the hypothesis that eutrophication provides a plausible bottom-up effect leading to increased parasitism. |
