Amphibians are important components of aquatic and terrestrial habitats and can represent an important energetic link between the two, particularly in the tropics where diversity and abundance are greatest. However, amphibian populations are declining dramatically around the world, with catastrophic losses occurring even in remote, undisturbed regions of the tropics. Declines often result in the disappearance of over 75% of amphibians at a site, particularly species that breed in streams, and massive losses are reported from 13 Latin American countries in the last 20 years. This research will assess the ecological consequences of amphibian declines by investigating how these losses affect stream structure and function, including biodiversity, productivity, nutrient cycling, and energy and nutrient exchanges between streams and forests. This will be accomplished through intensive examinations of streams located along a moving disease front in Panama before and after disease-driven declines occur.
This research addresses an urgent, global problem and will provide information critical for understanding the ecological consequences of extinction and the loss of biodiversity, one of the eight ?grand challenges? and four focal areas of research in the environmental sciences recommended by the National Research Council. Understanding the potential large-scale and far-reaching consequences of catastrophic reductions in biodiversity and extinction events such as these will allow for more accurate predicting of ecological function in light of declining global biodiversity.
Our study examined how disease-induced selection shapes variability in genes of the major histocompatibility complex (MHC, a critical gene involved in immunity to disease), and addresses a key epidemiological question: How do genetic and demographic characteristics of populations interact to affect disease outcomes? We investigated the relationship between MHC variability in Central American frogs and host susceptibility to a novel fungal pathogen, Batrachochytrium dendrobatidis (Bd). Bd is a chytrid fungus that causes chytridiomycosis, the most prevalent infectious disease in amphibians to date. Recent molecular phylogenetic studies show that this species evolved from within an ancient group of fungi that feed exclusively on dead or decaying organic matter, suggesting that its ability to infect vertebrate hosts is an evolutionarily derived feature; however, the reasons for its sudden emergence as an amphibian pathogen remain a subject of debate, and numerous interacting factors are likely responsible. We have now completed the final year of this grant, and have achieved the three goals we set out for this project: (1) we characterized MHC class I and II genes in non-model frogs, (2) we quantified the spatial dynamics and movement of the Bd epidemic wave in lower Central America, and made a good start on sampling for focal host taxa for future immunogenetic research, and (3) we analyzed historic mark-recapture data on a few target species and in 2010 we re-established capture-mark-recapture studies of several populations of focal species at our sites. We found that Bd is moving southward through Central America and is now east of the Panama Canal in the Darien region of Panama. We found that in all cases, frog communities decline significantly soon after the arrival of Bd, and the presence and abundance of certain species (‘key’ species) can exacerbate the demographic consequences in affected communities. Finally, we characterized loci of MHC immune genes in Central American species, setting the stage for continued studies of immunogenetic variation and the possibility of evolved resistance to Bd. Combined these analyses help us understand mechanisms of population change at different stages of disease exposure, and have set the stage for the next step in analyzing the immunogenetic and demographic determinants of disease emergence in this region. In addition to the scientific results, our project also contributed to education and outreach, through direct training of undergraduates, graduates, and post-docs, as well as presentations at local schools and universities about amphibian declines. Finally, members of our team have participated in the development of videos about our work and the scientific profession that will be used in middle and high school science curricula.
