Inventory Status of the Mountain Yellow-Legged Frog Species Complex at Sequoia, Kings Canyon, and Yosemite National Parks to Inform Targeted Recovery Actions

Smith, Thomas; Lindauer, Alexa; Knapp, Roland; Boiano, Danny; Grasso, Robert; Stein, Kristie

doi:10.36967/2319394

The endangered mountain yellow-legged frogs (a species complex of Rana muscosa and R. sierrae) were once ubiquitous and abundant in lakes, ponds, and meadows of the high-elevation Sierra Nevada, in both Sequoia and Kings Canyon National Parks and Yosemite National Park. Both species declined or were extirpated in over 90% of their historic habitats due to the introduction of non-native predatory fish (trout) to high mountain lakes and the emergence of the amphibian chytrid fungus (Batrachochytrium dendrobatidis; Bd). Habitat restoration (fish eradication) by both parks has allowed frogs to recover in many sites. Over several decades, frogs in some parts of the Sierra Nevada have evolved resistance or tolerance to Batrachochytrium dendrobatidis. However, other frog populations remain naïve to the disease, putting them at risk of extirpation when the disease inevitably invades. In 2022–2023, we inventoried approximately six hundred sites in both parks to 1) conduct surveillance for new disease outbreaks, 2) quantify frog recovery since previous surveys, 3) identify large frog populations capable of supporting future recovery actions, and 4) identify sites/habitats capable of supporting future recovery actions. The results of the inventory surveys in Yosemite indicate that Rana sierrae remains widespread, represented in virtually every basin or watershed visited. However, abundances (or detection) can be quite low, and occupancy may be indicated by a single individual of a single frog life stage. Yosemite includes many habitats that are characterized by low frog detectability (low elevation, abundant aquatic and shoreline vegetation, meadows). Generally, frog occupancy continues an upward trend, driven primarily by a small number of habitat restoration and frog population recovery actions like translocations and reintroductions. Survey results in Sequoia and Kings Canyon National Parks indicate that both Rana sierrae and Rana muscosa have limited occupancy but are present across the landscape and even thriving in a small number of sites. Although no disease outbreaks (epizootics) were documented within the time period of this survey, the continued spread of Bd across the landscape is evident given the small number of sites now occupied, and epizootics have occurred after the field surveys described here. However, it is notable that Bd-positive populations that remain extant despite past Bd epizootics exhibit population and disease characteristics indicative of tolerance and adaptation to disease. Increases in occupancy and abundance have occurred where habitat restoration and reintroductions have occurred, although the latter have been more limited in spatial scale than in Yosemite.

Inventory Status of the Mountain Yellow-Legged Frog Species Complex at Sequoia, Kings Canyon, and Yosemite National Parks to Inform Targeted Recovery Actions

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