Variation in amphibian maturation rates influences population vulnerability to disease-induced declines

Understanding factors that influence population-level responses to emerging threats in declining species is crucial for informed conservation action. In amphibian species impacted by the chytrid fungus (Batrachochytrium dendrobatidis), a pathogen that has caused amphibian declines globally, a commonly reported pattern is that more severe population declines tend to occur at higher elevations. Previous research has suggested that this pattern could be driven by reduced environmental suitability for chytrid fungus at lower elevations. However, delayed amphibian maturation, which is common in cold, high elevation populations, could also increase vulnerability to population decline. Here, we tackle this key knowledge gap, focusing on the critically endangered corroboree frogs (Pseudophryne corroboree and P. pengilleyi), which have experienced a pattern of extirpation at higher elevations, with remnant populations persisting at lower elevations. First, we quantify the age structure of two extant low elevation P. pengilleyi populations and museum specimens (both species) collected before the emergence of chytrid fungus in Australia. Male age to maturation varied from 1 to 3 years, with the extant population with higher chytrid prevalence displaying severe age structure truncation. Second, we use population simulations to calculate elasticity values under a range of scenarios with varying ages to maturation and chytrid-associated mortality. When the population growth rate was fixed at 1, adult survival became increasingly important as age to maturation increases, particularly under a scenario of high chytrid-associated mortality. Our simulation results indicate that delayed maturation could be a previously underappreciated factor associated with an increased risk of amphibian population decline and that earlier maturation could contribute to population persistence. Our study highlights the importance of examining variation in life history traits to better understand population-level responses to novel threats and guide the development of appropriate conservation actions.