Defying decline: Very low chytrid prevalence in tadpoles, yet high infection in adults in a naturally recovering frog species

Amphibian chytrid fungus, Batrachochytrium dendrobatidis (Bd), is associated with declines in ~500 amphibian species globally. Decades after initial disease outbreaks, the trajectory of impacted species varies substantially; while some species continue to decline, there are instances of natural recovery, such as the whistling tree frog, Litoria verreauxii, in south-eastern Australia. The decline and subsequent recovery of this species have been quantified through repeated surveys of historically occupied sites over the past 30 years; however, the underlying mechanisms driving this recovery remain unknown. In this study, we investigate the potential factors facilitating the recovery of L. verreauxii by examining Bd prevalence and intensity in both adults and tadpoles. Specifically, we addressed the following hypotheses: (1) Bd prevalence in tadpoles would be lower compared to adults at the same breeding sites, (2) Bd prevalence in tadpoles would decrease over the spring breeding season due to the increasing availability of warm water microhabitats where tadpoles could potentially avoid or clear Bd infections and (3) there would be a negative correlation between Bd prevalence in tadpoles and the abundance and diversity of microfauna, which may consume Bd zoospores. Our findings indicate that tadpole infection prevalence remained consistently low at 1.36% (95% CI: 0.6–2.47%) throughout our spring sampling period, across different developmental stages. Adults had moderate to high prevalence within the same ponds at 50.53% (95% CI: 43.19–57.84%). No effect of temperature or microfauna diversity and abundance was apparent. While the mechanisms driving the recovery of this species remain unknown, low infection prevalence in tadpoles is likely a key component to the species' recovery. Our results emphasize the need for comprehensive investigations in Bd dynamics across all life history stages within recovering and declining amphibian species.