Annual surveys since the early 2000s have documented four amphibian species as widely distributed in Yellowstone: boreal chorus frogs, Columbia spotted frogs, western tiger salamanders, and western toads occur in wetlands and ponds throughout Yellowstone. In 2014, the plains spadefoot (Spea bombifrons) was confirmed in Yellowstone through genetic analyses. These toads are rarely seen because they spend most daylight hours underground. Currently, a single breeding population is known to exist within Yellowstone. However, monitoring efforts are underway to locate additional breeding sites because plains spadefoots typically do not disperse far from their natal pond.
In Yellowstone, amphibians depend on limited suitable habitat with shallow, quiet waters needed for egg laying and larval development. Annual differences in snowpack and precipitation change the extent and location of wetland sites, resulting in considerable year-to-year variation in amphibian reproduction. Breeding data collected across the park and since 2006 show that annual variations in breeding are common. Multi-year monitoring data indicate that amphibian populations using small, shallow isolated wetlands are most susceptible to drought or changes in precipitation. In contrast, amphibian populations occupying deeper wetlands and ponds appear to be more stable through time.
Since the 1950s, air temperatures have increased across this region and changes in the flooding patterns or the complete drying of wetlands have been documented. Since 2006, annual visits to approximately 165 wetlands across Yellowstone have further documented annual variation in the availability of wetlands. This data suggests that in hot dry years (e.g., 2007) upwards of 40% of the park’s wetlands dry up. In cool, wet years (e.g., 2011) most wetlands across the park are flooded and available to support amphibian breeding. Further warming is anticipated for this region and could contribute to the drying of wetlands and influence the distribution and abundance of amphibians and other wetland dependent species.
Disease agents, such as ranavirus and chytrid fungus (Batrachochytrium dentrobatidis), could also 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 and been involved with die-offs of all four widely distributed species in the region. Chytrid fungus does not necessarily cause a fatal infection and usually appears in Columbia spotted frogs and western toads following metamorphosis. The DNA of the chytrid fungus has been identified in skin swabs collected from both species in Yellowstone, though the impacts at the population level have not been determined. Similarly, 70% of tissue samples (tail clips) collected from live larval amphibians in 2015 were positive for ranavirus DNA. These findings highlight that several factors, including host susceptibility and environmental conditions, may determine whether an infection is lethal and results in a die-off or a decline in population abundance.
Studying Amphibians in Yellowstone
The Greater Yellowstone Network (GRYN) has led a collaborative monitoring of wetlands and amphibians in Yellowstone since 2006. Long-term monitoring of amphibian populations provides an opportunity to observe trends that may not be apparent at local scales or in areas with more direct human influences on habitat quality. Amphibians are monitored at catchments (or watersheds), which average approximately 500 acres in size. On average, 30 catchments are revisited during annul monitoring visits. All wetlands within the selected catchment are visited each summer, when two independent observers search for amphibians breeding evidence (i.e., eggs, larvae, or recently metamorphosed individuals) and document important habitat characteristics and the presence or absence of surface water.
The objectives of GRYN’s annual monitoring are to estimate the proportion of monitored catchments and wetlands used for breeding by each native amphibian species annually, to consider whether the rate and direction of use may be changing through time, and to document the number of wetlands within catchments that are potentially suitable for amphibian breeding.
This annual monitoring is then combined with local climate data to carefully examine the links between climate, wetlands, and amphibians. Taken together, amphibian and wetland monitoring data from the last decade, coupled with local climate information, will help support predictions of amphibian occurrence under different climate scenarios.