Evaluating pathogen occurrence and coexisting threats across amphibian species distributions
The incidence and frequency of emerging infectious diseases (EIDs) of wildlife have increased in the last 50 years. The spread of EIDs is a major concern because it can cause vulnerable species and even entire taxa to experience population decline and extinction. For example, numerous amphibian species drastically declined or went extinct in Mesoamerica between the 1980s and early 2000s, likely due to the introduction and spread of the pathogenic fungus Batrachochytrium dendrobatidis (hereafter ‘Bd’). While most of the initial declines were documented at high elevations, further studies conducted throughout the region after 2005 revealed that some species also declined in lowland undisturbed ecosystems and that prevalence and intensity of Bd infection vary with host species, geographic location, seasonality, and microhabitat conditions. In Chapter 1, I examined the dynamics of Bd in lowland amphibian communities in the tropic forests of Costa Rica. I found that Bd is widespread and exhibits enzootic dynamics (i.e., well adapted to local climate, exhibits low prevalence, and low to undetectable mortality rates). In Chapter 2, I described the current Bd enzootic dynamics across elevations and ecoregions in Costa Rica. I found that Bd exhibits seasonal dynamics, especially in lowlands. I also identified direct-developing, stream-dwelling amphibians as one of the groups most affected by the introduction and spread of Bd. In Chapter 3, I quantified the spread of Bd from pre-2005, when populations experienced epizootics (i.e., times of high infection prevalence and high disease-associated mortality) to post-2005, when populations experienced enzootics in Costa Rica. I found that 80% of the area of undisturbed ecosystems overlaps with the predicted distribution of enzootic Bd and identified several hotspots for disease. Finally, in Chapter 4, I conducted a threat assessment in different spatial scales for 46 direct-developing, stream-dwelling frog species endemic to Mesoamerica. At both regional and local levels, I found evidence that Bd was the main driver of the decline of most species. Together, my results add to the understanding of host-pathogen dynamics in the Tropics and address actions for regions and species that need immediate conservation management.