Population-level variation in host responses to disease can be observed across a wide array of host-pathogen systems. Some host populations are extirpated, some persist at lower densities or abundances, and others rebound to pre-disease levels. Understanding the causes of this variation, and the mechanisms of successful persistence, can serve as vital information for species conservation. One such mechanism of population persistence that has gained support in the amphibian Batrachochytrium dendrobatidis (Bd) host-pathogen system 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 amphibian 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 Bd via compensatory recruitment.

Prior to Bd arrival, we found a life history tradeoff between survival and recruitment across elevations, where high-elevation toads have high survival but lower recruitment and vice versa at lower elevations.

Bd arrival had a strong negative effect on apparent annual survival and recruitment, and led 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 suggests 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 any variation in population responses to disease will aid in understanding the bounds of such persistence mechanisms, and identify particularly vulnerable populations where mechanisms are non-existent.

See Supporting Information in Hardy et al.

Any program that can open .csv files.