Abstract
Understanding factors that influence host–pathogen interactions is key to predicting outbreaks in natural systems experiencing environmental change. Many amphibian population declines have been attributed to an amphibian chytrid fungus, Batrachochytrium dendrobatidis (Bd). While this fungus is widespread, not all Bd‐positive populations have been associated with declines, which could be attributed to differences in pathogen virulence or host susceptibility. In a laboratory experiment, we examined the effects of Bd isolate origin, two from areas with Bd‐associated amphibian population declines (El Copé, Panama, and California, USA) and two from areas without Bd‐related population declines (Ohio and Maine, USA), on the terrestrial growth and survival of American toad (Anaxyrus americanus) metamorphs reared in larval environments with low or high intraspecific density. We predicted that (1) Bd isolates from areas experiencing declines would have greater negative effects than Bd isolates from areas without declines, and (2) across all isolates, growth and survival of smaller toads from high‐density larval conditions would be reduced by Bd exposure compared to larger toads from low‐density larval conditions. Our results showed that terrestrial survival was reduced for smaller toads exposed to Bd with variation in the response to different isolates, suggesting that smaller size increased susceptibility to Bd. Toads exposed to Bd gained less mass, which varied by isolate. Bd isolates from areas with population declines, however, did not have more negative effects than isolates from areas without recorded declines. Most strikingly, our study supports that host condition, measured by size, can be indicative of the negative effects of Bd exposure. Further, Bd isolates’ impact may vary in ways not predictable from place of origin or occurrence of disease‐related population declines. This research suggests that amphibian populations outside of areas experiencing Bd‐associated declines could be impacted by this pathogen and that the size of individuals could influence the magnitude of Bd's impact.
Highlights
While species extinctions and extirpations have long been attributed to habitat loss, climate change, invasive species, and overharvesting (Pimm, Russell, Gittleman, & Brooks, 1995; Wilcove, Rothstein, Dubow, Phillips, & Losos, 1998), only lately have we considered infectious diseases, which can interact with these other threats, as a force that can shape biodiversity (Smith, Sax, & Lafferty, 2006; Wilcove et al, 1998)
Rates and severity of infectious disease outbreaks have increased (Daszak, Cunningham, & Hyatt, 2000; Jones et al, 2008; Smith, Acevedo-Whitehouse, & Pedersen, 2009) across taxa (Daszak et al, 1999; Martel et al, 2013; McCallum & Jones, 2006), prompting an urgent need to explore the effects that pathogens can have on biodiversity by evaluating the responses of hosts to pathogen across varying environmental conditions
An amphibian chytrid fungus Batrachochytrium dendrobatidis (Bd) is widespread in its occurrence, but the presence of Bd is not always associated with amphibian declines (Daszak et al, 2005; Olson et al, 2013; Ouellet et al, 2005)
Summary
While species extinctions and extirpations have long been attributed to habitat loss, climate change, invasive species, and overharvesting (Pimm, Russell, Gittleman, & Brooks, 1995; Wilcove, Rothstein, Dubow, Phillips, & Losos, 1998), only lately have we considered infectious diseases, which can interact with these other threats, as a force that can shape biodiversity (Smith, Sax, & Lafferty, 2006; Wilcove et al, 1998). Understanding variation in hosts’ responses to different strains of pathogens across environmental conditions will become essential as the world becomes more connected through global commerce and trade, conservation efforts to relocate organisms, and climate change-induced shifts in distributions of hosts and pathogens. Understanding the links between virulence across isolates and variation in host responses across environmental conditions is critical to understanding how the movement of hosts and their associated pathogens influences distributions of infectious diseases. We examine the effects of different isolates of Bd, two from areas experiencing amphibian declines (El Copé, Panama, and California, USA) and two from areas without declines (Ohio and Maine, USA), on the terrestrial growth and survival of American toad (Anaxyrus americanus) metamorphs reared at low or high densities during the larval stage, which resulted “in a relative difference” in body condition. | 9198 suffer greater effects on host growth and survival compared to toads from low-density larval environments
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