Host size influences the effects of four isolates of an amphibian chytrid fungus

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.

Body size mediates latitudinal population differences in response to Bd infection in two amphibian species

“…Northern individuals were smaller at the time of infection than southern individuals, and our analyses suggest that the survival difference was mainly mediated by body size. This is in accordance with previous studies (Bradley et al 2015, Burrow et al 2017 showing that smaller individuals were more vulnerable to Bd infection. Smaller individuals may have less developed immune system which may render them more vulnerable to disease (Møller et al 1998, Burrow et al 2017.…”

“…This is in accordance with previous studies (Bradley et al 2015, Burrow et al 2017 showing that smaller individuals were more vulnerable to Bd infection. Smaller individuals may have less developed immune system which may render them more vulnerable to disease (Møller et al 1998, Burrow et al 2017. Smaller individuals may also be more vulnerable to Bd-mediated water loss as they have larger surface area to body mass ratio.…”

“…There is considerable variation in virulence among genetic strains of Bd (Farrer et al 2011, Bataille et al 2013, Greenspan et al 2018 and BdGPL, the global panzootic lineage originating in Eastern Asia, has caused most of the chytridiomycosis outbreaks (O'Hanlon et al 2018). While genetic variation within BdGPL is relatively limited (O'Hanlon et al 2018), there is evidence for virulence differences between BdGPL strains (Becker et al 2017;Burrow et al 2017, Dang et al 2017.…”

“…These differences can be due to genetic differences in traits like immune response and behavior (Richards-Zawacki 2010), and are in some cases linked with direct Bd-mediated selection (Savage & Zamudio 2016, Savage et al 2018. Although infection does not cause direct mortality in the resistant and tolerant species, sub-lethal fitness effects such as decreased growth have been detected (Bielby et al 2015, Burrow et al 2017, Campbell et al 2019).…”

A pathogenic skin fungus and sloughing exacerbate cutaneous water loss in amphibians

“…Small frogs have a higher surface area to volume ratio and thus a higher rate of net water loss relative to body mass (Christian, 1978;Tracy and Christian, 2005;Withers et al, 1982). Body size also influences Bd infection outcome, with smaller frogs being more susceptible to fatal chytridiomycosis (Burrow et al, 2017;Carey et al, 2006;Langhammer et al, 2014;Ohmer et al, 2013). Consequently, we hypothesized that the magnitude of sloughing-induced increases in rates of water loss would be mass specific, with smaller frogs being substantially more affected than larger conspecifics.…”