Environmental, landscape, and host-related factors associated with parasitism and its effects on larvae and metamorphic frogs of Rana pipiens

The emergence of several diseases affecting amphibian populations worldwide has prompted investigations into factors that determine the distribution and abundance of parasites in frogs. Environmental and land use determinants of the structure of parasite communities of metamorphic frogs of Rana pipiens were examined in Minnesota. The parasite communities uncovered comprised of 25 taxa, and nearly every frog examined (∼98%) was infected with one or more helminth species. Larvae of trematodes, which have complex life cycles and use tadpoles as intermediate hosts, dominated the communities. The distributions and abundances of parasites across populations were related to the availability of forest within 2 km distances, and up to 10 km distances, from wetland perimeters. As more forest was available, mean parasite abundances and richness of parasite communities increased. In contrast, frogs that emerged from wetlands within landscapes dominated by agriculture, where forested habitats were much more limited, tended to be infected with fewer parasite species, at lower abundances. Loss of fragments of natural habitats as a result of intensified agricultural land practices probably accounts for fewer visits to wetlands by the definitive hosts of these parasites to wetlands remaining, and therefore, explains the lower abundances in metamorphic frogs. In addition to this field study, interactions between larvae of R. pipiens and two parasites that occurred in Minnesota, Echinostoma trivolvis (and related trematodes) and Ribeiroia ondatrae, were investigated in the laboratory. Individuals were exposed to cercariae at different stages of development and monitored for survival and, in the case of R. ondatrae, for abnormal limb development. For both parasite species, survival was dependent upon the developmental stage of tadpoles at the time of infection. Individuals infected with either of the parasites at early tadpole stages (Gosner [1960] 24–25) experienced higher mortality than later staged-tadpoles, and mortality at these early life stages often exceeded 90%. The ability of R. ondatrae to induce limb malformations was also stage-specific, with the majority of malformations observed in tadpoles infected during the limb bud phase (Gosner 27–28) of limb development. The importance of other hosts involved in parasite life cycles and temporal aspects of frog-parasite interactions are emphasized.