Pathogen Reverses Competition between Larval Amphibians

Ecologists have often suggested that the presence of pathogens that differentially affect interacting species may affect the outcome of interactions, yet few experimental studies have documented pathogen-mediated interactions using a natural host–parasite system. We studied the effects of a pathogenic water mold, Saprolegnia ferax, on competitive interactions between the Cascades frog Rana cascadae and the Pacific treefrog Hyla regilla. Previous studies have shown that outbreaks of Saprolegnia infection in the Cascade mountains of Oregon, USA, result in high embryonic mortality for Rana but not for Hyla. Thus, we examined how infections of Saprolegnia during amphibian embryonic development could influence larval recruitment and competitive interactions between larval Rana and Hyla. We manipulated the presence of Saprolegnia and embryonic Hyla and Rana in replicated artificial ponds and determined mean survivorship to hatching per pool from daily observations during embryonic development. Pools were then followed throughout larval development, and we recorded mean mass of tadpoles at metamorphosis and time to metamorphosis per pool. The presence of Saprolegnia differentially affected larval recruitment of the two species; larval recruitment of Rana was reduced by 46.2% in the presence of Saprolegnia, whereas Hyla survival was not affected. However, larval Rana that survived Saprolegnia infection developed faster and were larger at metamorphosis compared to individuals not exposed to Saprolegnia. In the absence of Saprolegnia, Rana had strong negative effects on the growth, development, and survival of Hyla. However, in the presence of Saprolegnia, the outcome of competitive interactions between the two species was reversed. Saprolegnia may have positive indirect effects on both Hyla and Rana by regulating both intraand interspecific competition. These results suggest that pathogens can have strong effects on species interactions and may ultimately influence community structure.