Rumschlag, S. L. | Search bioRxiv

bioRxiv2018

Mass Amphibian Mortality during Overwintering Related To Fungal Pathogen But Not Larval Environment: An Explanation for Declines?

AO_SCPLOWBSTRACTC_SCPLOWDevelopment of infectious diseases within hosts may be shaped by environmental conditions that cause tradeoffs in energetic demands for immune responses against demands for host growth and survival. Environmental conditions may influence these tradeoffs by affecting size of hosts, or tradeoffs may change across seasons, thereby altering the impacts of diseases on hosts. In the present study, we exposed northern leopard frog (Lithobates pipiens) tadpoles to varying larval environments (low leaf litter, high density of conspecifics, 40 {micro}g/L atrazine, caged fish, or control) that influenced size at metamorphosis, a measure of host quality. Subsequently, we exposed these metamorphs of to Batrachochytrium dendrobatidis (Bd), a fungal pathogen linked to worldwide host population declines, at metamorphosis and/or 12 weeks later, prior to overwintering. Bd exposure dramatically reduced survival during overwintering and the effect was strongest when frogs were exposed both at metamorphosis and before overwintering. Larval environments, which determined host size, did not influence effects of Bd. Stage-structured models built to assess the impacts of Bd exposure on host populations suggest that Bd exposure at metamorphosis or before overwintering would reduce annual population growth rates by an average of 19% and 41%, respectively. Our study indicates that northern leopard frog hosts suffered little effects of Bd exposures following metamorphosis and that lower host quality did not hamper a frogs ability to respond to Bd. Instead, we provide evidence that Bd exposure can reduce survival and result in population size reductions via reduced recruitment from overwintering mortality, providing a plausible mechanism for enigmatic declines of amphibians in temperate regions.

ecology↗

bioRxiv2018

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

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.

ecology↗