Host Diet Influences Lethal and Sublethal Responses of Hosts to Amphibian Pathogen Exposure

Abstract

AO_SCPLOWBSTRACTC_SCPLOWThe severity of the impacts of pathogens on hosts may be driven by environmental factors like resource availability that create tradeoffs on energetic demands for immune responses and basal metabolic activity within the host. These responses can vary among species from sublethal to lethal effects, which can have consequences for the host population trajectories within a community. Chytridiomycosis, caused by the pathogen Batrachochytrium dendrobatidis (or Bd), has been associated with global amphibian population declines. However, it also occurs in populations without appearing to cause mass mortality; the effect of Bd in these situations is not well understood and environmental factors like food abundance that impact host conditions could play an important role in the magnitude of the pathogens impact. In the present study, we exposed American toad (Anaxyrus americanus), northern leopard frog (Lithobates pipiens), and Blanchards cricket frog (Acris blanchardi) metamorphs to Bd and then reared them in the terrestrial habitat under low or high food environments. We found additive effects of Bd and reduced food abundance on host growth and survival that varied according to species. For instance, Bd-induced reductions in American toad survival were greater under low food conditions compared to high food conditions but survival of northern leopard frogs and Blanchards cricket frogs was not affected by Bd. For northern leopard frogs and Blanchards cricket frogs, low food abundance resulted in the lower growth rates under Bd exposure compared to high food abundance. Additionally, we developed stage-structured population models for American toads to assess if reduced survival of metamorphs exposed to Bd under conditions of low and high food abundance could influence population trajectories; models indicated that Bd exposure would reduce annual population growth rates by 14% under conditions of high food abundance and 21% under conditions of low food abundance. Our results suggest that environmental conditions that influence resource availability for species that are sensitive or tolerant to Bd may increase the negative effects of Bd on host growth and survival, which could have important implications for how populations and communities with infected members respond over time.