Batrachochytrium dendrobatidis (Bd, or chytrid) is a fungal pathogen that causes chytridomycosis in some susceptible amphibians. Not all amphibians are plagued by this infection, but those that are tend to show drastic population declines when exposed to chytrid. This leaves a collection of individuals that can serve as carriers or vectors of disease, spreading it from one pond to another. One of the interesting characteristics with chytrid is its specificity to tissues that is attacks. Chytrid fungus only attacks keratinized tissues, thos with a certain chemical- keratin- that gives skin its waterproofing.
Since amphibians use cutaneous breathing, or getting oxygen through their skin, it should come as no surprise that amphibians’ skin is less keratinized than human skin and other organisms. In fact, the areas of skin that are keratinized vary throughout an amphibian’s life, with tadpoles having little to no keratin outside of the mouthparts in most species. Where we have teeth for chewing, tadpoles have rough fingernail-like ridges to scrape off their food. Chytrid attacks on tadpoles often are not directly fatal but can lead to lower tadpole condition, smaller size, and longer time to metamorphosis. In adults, chytrid infects far more of the skin, as there’s more keratin for chytrid to feed off of. This leads to a more extensive infection, and more often than in tadpoles leads to death, and the skin can even slough off as the keratin is broken down so much.
For amphibians, their dual life cycle using both aquatic and terrestrial habitats can help to divide resources and lessen intra-specific competition, but it also exposes them to predators, pathogens, and pollutants in both habitats as well. It also means that as researchers, we need to consider two functionally different groups- aquatic larvae that eat phytoplankton and detritus, and terrestrial adults who are carnivores. Two sets of predators, two sets of prey, two sets of environmental conditions, and two sets of competitors. We might talk about “amphibians,” but that’s not a homogeneous group by any means.
In ecology in general, organisms might be lumped into groups in different ways, either based on relationships or functional groups usually. Our ability to construct those groups accurately plays a big role in our ability to best study the world around us. Sometimes, we can put organisms in more than one group based on the type of study that’s being conducted and the questions that we’re asking. With living things, there’s rarely simplicity, and making sense of that complexity helps us to find the answers as best we can. For amphibians, that might mean treating a species as two different groups instead of one.