Annual surveys since 2002 have found the same four native amphibian species in Yellowstone: the blotched tiger salamander, boreal chorus frog, boreal toad, and Columbia spotted frog. On July 31, 2013 a juvenile spadefoot toad (plains spadefoot, Spea bombifrons) was identified at a site west of Fountain Flat Drive and east of Fairy Creek. The individual appeared to have recently metamorphosed, indicated by the presence of a slight tail stub. Yellowstone Wildlife Health Program (YWHP) field staff did not collect the animal as it was the only one observed. The toad was photographed and swabbed for genetic identification purposes before being released. Amphibian expert Dr. Charles Peterson has verified that the animal is a spadefoot toad and the YWHP is working to identify the exact species through genetic analysis.
There have been two previous reports of spadefoots from Yellowstone National Park. The observation of a juvenile spadefoot within Yellowstone suggests that an active breeding site is also present nearby. Additionally, this finding increases the diversity of amphibians known to occur within Yellowstone’s boundaries.
In Yellowstone, amphibians depend on limited suitable habitat with shallow, quiet waters needed for egg laying and larval development. Hydrological fluctuations change the extent and location of wetland sites, resulting in considerable year-to-year variation in amphibian reproduction, so longer-term data are needed to identify any significant trends. However, population data collected since 1992 appear to be within the range of natural variability and suggest that these species are resilient to at least short periods of drought.
After five years of below-normal precipitation, the percentage of visited wetland sites that were suitable for amphibian breeding was 59% in 2007, 96% in 2011, and 75% in 2012. Reports from the 1950s suggest that the boreal toad was more widespread and common then, but it continues to be found at most of the major breeding sites identified since the early 1990s, although at low numbers.
Disease agents, such as ranavirus and chytrid fungus (Batrachochytrium dentrobatidis), could affect the survival and reproduction of amphibian populations in Yellowstone. Ranavirus has been found in tiger salamanders and Columbia spotted frogs collected from die-offs since 2008. Chytrid fungus does not necessarily cause a fatal infection and usually appears in Columbia spotted frogs and boreal toads following metamorphosis. It has been found in swabs collected from both species in Yellowstone, though the impacts at the population level have not been determined. Several factors, including host susceptibility and environmental conditions, may determine whether an infection is lethal and results in population decline.
Studying Amphibians in Yellowstone
Annual measures of amphibian occurrence and wetland suitability allow trends in amphibian populations to be considered in context of the available habitat in Yellowstone. This informs our understanding of how amphibians may be affected by different disturbances such as disease or changes in climate. In addition, because many amphibians and reptiles congregate to breed or overwinter, they can be adversely affected by disturbance or loss of key sites.
The Greater Yellowstone Network collects data on the number of wetlands that are occupied by breeding populations of each amphibian species in Yellowstone. If occupancy (the proportion of suitable wetlands occupied) decreases, that species has probably declined. Researchers randomly select watershed units, known as catchments, from within the four main drainage basins of Yellowstone. Field crews search potential habitat within the catchments in June and July to document the presence or absence of amphibians. Researchers study the results to determine occupancy trends and factors that may be driving them.
Amphibian and wetland monitoring indicates that amphibian breeding hotspots (areas with many species) on the northern range are vulnerable because they occur in a region with few wetlands and high susceptibility to wetland drying. Yellowstone provides a valuable study area; information about the amphibians and reptiles here may shed light on population changes in other high-elevation protected areas of the western United States.