Collaborative Research: LTREB Renewal: Long-Term Dynamics of Amphibian Populations Following Disease-Driven Declines

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This project will examine the processes of invasion of a novel pathogen into a population for the case of a fungal pathogen that causes the disease chytridiomycosis in frogs and salamanders. Outbreaks of emerging infectious diseases are increasingly recognized as major threats, not just to human populations but also to wildlife, and they increasingly threaten global biodiversity. The initial invasion of a novel pathogen into a susceptible host population can cause a disease outbreak resulting in high levels of mortality and declines in population size. When this happens, natural selection can occur for both the host and pathogen populations resulting in evolutionary changes in the host's susceptibility and tolerance to infection by the pathogen and the pathogen's ability to damage the host (virulence). These changes can in turn determine whether the host population can persist and recover from the disease. Understanding these evolutionary processes is crucial in development of conservation strategies for threatened species. Chytridiomycosis has had catastrophic effects on amphibians worldwide, and has been linked to numerous species extinctions in recent decades and many more species at risk. The researchers will investigate the patterns of evolutionary change in both the pathogen and the host (mountain yellow-legged frogs), following the invasion of the disease into hundreds of high elevation lakes in the California Sierra Nevada. This project will contribute to the understanding of the role of infectious diseases, such as chytridiomycosis, as agents of evolutionary change in natural populations. It will provide critical information to state and federal agencies, facilitating endangered species recovery and will train and educate undergraduates, graduate students, and the general public.This research builds on data from a long-term study of the population dynamics of mountain yellow-legged frogs (Rana sierrae and Rana muscosa) in the complex landscape of the California Sierra Nevada, and the affects of Batrachochytrium dendrobatidis (Bd) as it has invaded and spread through hundreds of frog populations. In most cases, invasion of Bd results in outbreaks of the disease chytridiomycosis, rapid frog population declines, and local extinctions. In some cases, long-term persistence of frog populations occurs with Bd in an enzootic state in which the impact of the pathogen is greatly reduced. The research will extend and leverage 25 years of host population and disease data and archived genetic samples from R. sierrae/R. muscosa and Bd. These resources will provide insight into how populations of both host and pathogen change during the transition from pre-pathogen arrival, to disease outbreak, to enzootic disease, to potential recovery of the pre-disease host population abundances. This dataset will be used to investigate the genetic basis for differences in host resistance/tolerance and pathogen virulence. Cutting-edge genomic analysis of existing frog and fungal samples will complement laboratory experiments on Bd virulence and frog susceptibility using Bd cultures and frog mucosal samples from wild host populations. Continued surveys of wild host populations will identify new disease outbreaks and describe the transition from initial outbreak to persistent enzootic state.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.