Abstract
Amphibians, the most threatened group of vertebrates, are seen as indicators of the sixth mass extinction on earth. Thousands of species are threatened with extinction and many have been affected by an emerging infectious disease, chytridiomycosis, caused by the fungal pathogen, Batrachochytrium dendrobatidis (Bd). However, amphibians exhibit different responses to the pathogen, such as survival and population persistence with infection, or mortality of individuals and complete population collapse after pathogen invasion. Multiple factors can affect host pathogen dynamics, yet few studies have provided a temporal view that encompasses both the epizootic phase (i.e. pathogen invasion and host collapse), and the transition to a more stable co-existence (i.e. recovery of infected host populations). In the Sierra Nevada mountains of California, USA, conspecific populations of frogs currently exhibit dramatically different host/ Bd-pathogen dynamics. To provide a temporal context by which present day dynamics may be better understood, we use a Bd qPCR assay to test 1165 amphibian specimens collected between 1900 and 2005. Our historical analyses reveal a pattern of pathogen invasion and eventual spread across the Sierra Nevada over the last century. Although we found a small number of Bd-infections prior to 1970, these showed no sign of spread or increase in infection prevalence over multiple decades. After the late 1970s, when mass die offs were first noted, our data show Bd as much more prevalent and more spatially spread out, suggesting epizootic spread. However, across the ~400km2 area, we found no evidence of a wave-like pattern, but instead discovered multiple, nearly-simultaneous invasions within regions. We found that Bd invaded and spread in the central Sierra Nevada (Yosemite National Park area) about four decades before it invaded and spread in the southern Sierra Nevada (Sequoia and Kings Canyon National Parks area), and suggest that the temporal pattern of pathogen invasion may help explain divergent contemporary host pathogen dynamics.
Highlights
With thousands of amphibian species experiencing population declines around the world [1], amphibians are facing a global biodiversity crisis, and many suggest this is emblematic of a global mass extinction [2]
Despite sampling biases of museum specimens that were collected for reasons unrelated to disease ecology, retrospective studies, like this one, can provide insight regarding pathogen invasion history and disease dynamics
Our survey of museum specimens collected over a 100-year period in the Sierra Nevada found no evidence of Batrachochytrium dendrobatidis (Bd) before 1939
Summary
With thousands of amphibian species experiencing population declines around the world [1], amphibians are facing a global biodiversity crisis, and many suggest this is emblematic of a global mass extinction [2]. Though multiple factors play a role in these declines, the invasion and emergence of the fungal pathogen, Batrachochytrium dendrobatidis (Bd), and the ensuing epizootics (epidemics in wildlife) are implicated as major contributing factors [3]. Genomic studies have revealed that there are multiple lineages of Bd. For example, the Global Panzootic Lineage (Bd-GPL) is associated with Bd-epizootics and host population collapse [12], but other lineages are found in areas where epizootics have not been found (e.g. South Korea) [12,13,14,15] and many species survive infections. In the Americas (North, Central and South America), many of the reported declines of amphibians are attributed to Bd-GPL epizootics, yet most occurred decades before Bd was discovered [17, 18]. Retrospective studies are needed to help create a timeline for Bd emergence and spread
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