Compensatory recruitment unlikely in high‐elevation amphibian populations challenged with disease

Abstract

Understanding the causes of population variation in host response to disease, and the mechanisms of persistence, can serve as vital information for species conservation. One such mechanism of population persistence that has gained support is the demographic process of compensatory recruitment. Host populations may persist by increasing recruitment to compensate for reduced survival due to infection, thus limiting the negative effects of the disease on population trajectories. However, high‐elevation populations are inherently vulnerable to stochastic processes and may be limited in their ability to exhibit compensatory recruitment relative to lower elevation populations. We use long‐term mark–recapture data from five populations of boreal toads Anaxyrus boreas boreas, across an elevational gradient in Colorado, before and after pathogen arrival to assess whether populations can persist with Batrachochytrium dendrobatidis (Bd) via compensatory recruitment. Prior to pathogen arrival, we found a life‐history trade‐off between survival and recruitment across elevations, where high‐elevation toads have high survival but lower recruitment and vice versa at lower elevations. Pathogen arrival had a strong negative effect on apparent annual survival and recruitment leading to negative population growth rates and dramatically reduced host abundances. The data did not support the occurrence of compensatory recruitment. Synthesis and applications. Our unique dataset indicates that demographic responses to pathogens may be environmentally (i.e. elevationally) context dependent and highlights the value of long‐term monitoring. We recommend that practitioners verify that potential persistence mechanisms occur across multiple populations and relevant environmental gradients to counter any assumptions of the mechanism existing species‐wide. Quantifying variation in population responses to disease will aid in understanding the bounds of such persistence mechanisms and identify particularly vulnerable populations where mechanisms are nonexistent.

Conservation genomics of an endangered montane amphibian reveals low population structure, low genomic diversity and selection pressure from disease

“…Because diversity is low across the SRM, it may become necessary for managers to consider source populations from outside the SRM to provide an influx of standing genetic variation into extant boreal toad populations. This is particularly true if non-SRM boreal toads have higher fitness when challenged with Bd, as field studies suggest (Hardy et al, 2022;Hossack et al, 2020;Pilliod et al, 2010). Future laboratory infection trials using wild sources of SRM and non-SRM boreal toads will add valuable information for conservation decision-making for SRM boreal toads.…”

“…The boreal toad ( Anaxyrus [Bufo] boreas ) in the southern Rocky Mountains (SRM) is an example of a species that has experienced alarming declines over the last four decades, and most of the declines are linked to Bd (Hardy et al., 2022; Mosher et al., 2018; Muths et al., 2003, 2008; Scherer et al., 2005). While the geographic range of the boreal toad species complex is vast, spanning the western USA, Canada and Mexico (Stebbins, 2003), populations in the SRM have experienced the most severe declines, prompting wildlife agencies (Colorado Parks and Wildlife [CPW], New Mexico Department of Game and Fish and Wyoming Game and Fish Department) to designate this species as endangered or of special concern.…”

Quantifying Intraspecific Variation in Host Resistance and Tolerance to a Lethal Pathogen

“…The boreal toad system is ideal for investigating intraspecific variation in host responses to disease and highlights the potential danger in overgeneralizing a species’ response range-wide. Boreal toad populations in the United States have experienced severe declines and extirpations due to Bd in the southern Rocky Mountains region of their range in Colorado and southeastern Wyoming (Hardy et al 2023; Mosher et al 2018; Muths et al 2003). Conversely, populations in western Wyoming and Montana have persisted despite high Bd prevalence within and among populations (Hossack et al 2020; Pilliod et al 2010; Muths et al 2011; Russell et al 2019).…”