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

Abstract

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.