Pathogens may act as important drivers of community composition and productivity. For example, the amphibian chytrid fungus, Batrachochytrium dendrobatidis (hereafter Bd), may have cascading indirect effects on algal abundance and composition via altered trophic cascades through its amphibian hosts. Bd-infected anuran larvae may experience mouthpart damage, reduced feeding efficiency, and decreased growth and development. Several studies found that Bd-infected larvae fed less than uninfected tadpoles. However, this may not be the only mechanism to explain the effect of Bd on larval feeding behavior because infected larvae may alter their feeding strategy. Scraping larvae that feed primarily on periphyton may switch to feeding on phytoplankton due to mouthpart damage, resulting in cascading effects on aquatic communities. We investigated how larval infection with Bd influenced the abundance of periphyton and phytoplankton by manipulating the presence of Bd and Western Toad (Anaxyrus boreas) larvae in outdoor mesocosms. We measured phytoplankton biomass, chlorophyll-a content and zooplankton composition and abundance bi-weekly for 60 days. Larval mouthparts were extracted to confirm infection and quantify damage. We predicted that if larvae with Bd-damaged mouthparts switched from primarily feeding on periphyton to phytoplankton, then mesocosms containing infected tadpoles would have a higher periphyton biomass and lower chlorophyll-a content.
Whereas larval presence had a significant effect on periphyton biomass but not phytoplankton, larval infection with Bd did not have a significant effect on phytoplankton and periphyton abundance. However noticeable differences in composition and abundance may be biologically relevant.
Clinical signs of mouthpart malformations include depigmentation of the jaw sheaths and swollen labile papillae, however, effects on tooth rows are often negligible. Therefore, damage to these structures may not be severe enough to induce a shift in diet. Alternatively, mouthpart damage and subsequent community-level effects may simply not be pronounced in this species. Investigating the role of Bd, larval mouthpart damage, and their effects on aquatic systems merits further research.