After publishing my last post, some of you inquired about what happened to the amphibians at that pond in Canyon. I was sad to report on the demise of this cohort; however, I was pleased to hear of the concern for these wonderful yet vulnerable animals. They often do not get the attention or the funding that they deserve. In this post, I’ll tell you about my amphibian work with the Wildlife Health Program and present my hypothesis on what caused the amphibian die-off.
The Wildlife Health Program is tasked with monitoring amphibian populations in Yellowstone. Specifically, we sample animals and collect water samples to detect the presence of pathogens. We are primarily interested in Chytrid fungus and Ranavirus, two deleterious causes of amphibian disease. Therefore, we select sites based on several factors and survey them multiple times during the field season. Each time, we collect samples and environmental data that will help us monitor and understand amphibian disease ecology in Yellowstone.
In brief, both of these pathogens are known to cause massive die-offs and are of great ecological interest. They are found worldwide, including certain places in Yellowstone. Dependent upon many variables, some amphibian populations are more sensitive than others and may experience greater mortality. Consequently, the presence and impact of amphibian disease varies greatly in Yellowstone.
During a typical week, members of our team will visit predetermined sites to survey amphibian populations. At the office, we load our packs with the surveying gear and personal necessities that we will need for the day. These sites are scattered around the park. Some of them are close to roads while others require some hiking. Once at the site, we deploy our dip nets in an effort to catch these slimy critters. Depending on the location, weather, and time of year, we may find adults, tadpoles, metamorphs, or any of these life stages. Speed and skill are vital to our capture success! Once caught, we place the individual in a zip lock plastic bag with plenty of water. The amphibians have plenty of oxygen and moisture for the short time they are detained. When we are finished collecting animals, we begin to process them.
For adult amphibians, we select techniques that sample for disease and identify individuals. First, we measure the snout to vent length and weigh the animal. Next, we use a sterile cotton swab to collect skin cells by rubbing the swab over specific places on the skin. The swabs are then placed in small tubes of ethanol. Lastly, we determine if the animal is a recapture by scanning for a previously implanted PIT tag. A PIT tag is a passive integrated transponder that is injected under the skin. These little cylinders are approximately two by twelve millimeters. They use low frequency radio waves to transmit a unique identification number when read by a scanner (RFID). This is how we can identify individuals. If the animal is above the minimum size and weight and has not been captured before, we will implant a PIT tag with a syringe. After implantation, we dress the wound with a tissue adhesive. When the adhesive dries, we place them back into their specific bag and return them to the pond.
Tadpoles and metamorphs are processed differently. First, we compare the physical characteristics of the animal to illustrations of developmental stages (these are called Gosner stages). We record the Gosner stage on a data sheet and note any visual abnormalities. Young amphibians are often too small to effectively swab for skin cells. Instead, we clip a small portion of the tail for analysis. When we are finished processing all of our young amphibians, we return them to the pond.
Besides the direct sampling of individuals, we also collect water samples to investigate whether we can use environmental DNA (eDNA) to detect Ranavirus and Chytrid fungus. Currently, we are comparing the results from water samples with those of swabs and tail clips. If perfected, we hope to be able to detect and monitor disease without the need to handle amphibians or employ invasive sampling methods.
In an effort to understand the dynamics of amphibian disease ecology in Yellowstone, we also collect environmental data such as water chemistry and temperature, pool perimeters, and weather. We are interested in what variables influence amphibian disease. Specifically, we want to model disease load (the amount of a pathogen on an individual over time) and environmental factors to understand the correlation.
So, what happened to those amphibians at Canyon?
Upon arriving at the amphibian site known as Canyon 3, we first noticed that the water level had drastically receded. Perhaps, part of that smell was not only of the decomposing bison but also the fresh mud that was exposed. Through my observation, I estimate that the water level dropped nearly a foot in two weeks. This increased pathogen concentration and stress in the pond. Adults and metamorphs that developed more quickly migrated away from the site. However, less developed individuals were more dependent on their watery home and could not leave. This explains whey we did not find any adults. Instead, we only observed two unhealthy and mostly deceased metamorphs and tadpoles. Examining two living individuals and the multitude of tiny corpses, we discovered hemorrhage and lesions that provide additional evidence of Ranavirus. Our lab results will support or refute this hypothesis. I’m very interested in understanding how disease affects the population at Canyon 3 and the other amphibian sites throughout Yellowstone.