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Vast alteration of the biosphere by humans is causing a sixth mass extinction. Adaptation to modified environments often is the only means for species to persist, but the extent to which such “evolutionary rescue” can prevent or reverse biodiversity loss is largely unknown. Us- ing results from genomic analyses, a 15-year reintroduction effort, and population modeling, we provide a compelling example of evolution reversing the declines of an imperiled amphibian and allowing the reestablishment of extirpated populations. The once-common mountain yellow-legged (MYL) frog is threatened with extinction by the human-facilitated emergence of a lethal fungal pathogen (Batrachochytrium dendrobatidis; “Bd”). Although most MYL frog populations are extirpated following disease outbreaks, some persist and eventually begin to recover. Using an exome capture approach, we show that MYL frogs have undergone substantial evolutionary change following disease outbreaks. Moreover, in naturally recovering populations observed changes in immune function loci are associated with increased resis- tance/tolerance to Bd infection. Large-scale reintroduction of frogs from rescued populations resulted in the establishment of reproducing populations despite ongoing disease. In addition, results from viability modeling suggest that established populations have a low prob- ability of extinction over 50 years. Collectively, these results provide a rare example of how evolutionary rescue and the reintroduction of resistant/tolerant individuals can allow the landscape-scale recovery of disease-impacted species. This example has potentially broad implications for the many taxa worldwide that are threatened with extinction by anthropogenic stressors.

Establishing historical species distributions can assist conservation translocations for threatened species, and yet, ecological changes necessitate developing restoration targets that are not analogous to historical baselines. Despite its recent conservation translocation to Yosemite Valley in Yosemite National Park, Sierra Nevada Mountains, USA, the historical distribution of the federally threatened California Red-legged Frog (Rana draytonii) in the valley remains unclear. Using archival records, interviews, and museum specimens, we examined the historical evidence for California Red-legged Frogs and sympatric amphibian species in the Yosemite region. We found a paucity of reliable amphibian records for Yosemite Valley since the 19th century, one of the most-visited sites in the US National Park System, and conclude that this is the result of historically low collecting and survey effort prior to the introduction of invasive American Bullfrogs (Lithobates catesbeianus; alsoRana catesbeianaafter Yuan and others 2016) in concert with a bird and mammal study bias from largely diurnal collecting that occurred when California Red-legged Frogs were extant regionally. We found previously undocumented records for individuals of the genusRanafor Yosemite Valley, consistent with a dominant historical hydrology more compatible for Foothill Yellow-legged Frogs (Rana boylii), though none could be definitively identified as California Red-legged Frogs. We conclude that extensive anthropogenic impacts, including acute ecosystem alteration and American Bullfrog introduction, contributed to the failure to detect California Red-legged Frogs in many places regionally once amphibians became a research priority in the latter 20th century. The conservation translocation of California Red-legged Frogs to Yosemite Valley illustrates the integration of historical baselines with contemporary realities, allowing for the complexities of change over time rather than focusing on restoration to an imagined, ideal environment in the past.

Invasive alien species are a major threat to freshwater ecosystems, and American bullfrogs are among the world’s 100 most prominent aquatic invasive species causing negative direct and indirect effect on native aquatic fauna worldwide. Bullfrogs were intentionally introduced into Yosemite Valley, Yosemite National Park in the 1950s where they became well established in the subsequent years. Starting in 2005, the National Park Service (NPS) began bullfrog removal, targeting various life stages using hand, net, and spear techniques. Starting in 2015, the NPS conducted environmental DNA (eDNA) surveys and deployed audio recordings devices to ensure adequate detection of bullfrogs. During the first year of cencerted effort in the Valley in 2005, the NPS removed 86% of all recorded bullfrog. The subsequent decade was spent searching for individuals with lower return on effort. In 2012, the NPS removed the last observed signs of bullfrog breeding, and the last observed bullfrog in 2019. Following removal of the breeding bullfrog population, the NPS began restoration projects for species of special concern. The NPS introduced the federally threatened California red-legged frogs (Rana draytonii) into Yosemite Valley beginning in 2016. This is the first published successful eradication of bullfrogs on a landscape level. National Parks and Monuments often provide refuges for imperiled wildlife and should be managed to remove invasive species. Our work highlights effective bullfrog removal is obtainable and can lead to local recovery of endangered species.

Effectively planning conservation introductions involves assessing the suitability of both donor and recipient populations, including the landscape of disease risk. Chytridiomycosis, caused by the fungal pathogen Batrachochytrium dendrobatidis (Bd), has caused extensive amphibian declines globally and may hamper reintroduction attempts. To determine Bd dynamics in potential source populations for conservation translocations of the threatened California red-legged frog (Rana draytonii) to Yosemite National Park, we conducted Bd sampling in two populations in the foothills of the Sierra Nevada Mountains, California, U.S.A. At one of two sites, we observed lethally high Bd loads in early post-metamorphic life stages and confirmed one chytridiomycosis-induced mortality, the first such report for this species. These results informed source population site selection for subsequent R. draytonii conservation translocations. Conservation efforts aimed at establishing new populations of R. draytonii in a landscape where Bd is ubiquitous can benefit from an improved understanding of risk through disease monitoring and ex situ infection studies.

Fine-scale habitat information can provide insight into species occupancy and persistence that is not apparent at the landscape-scale. Such information is particularly important for rare species that are experiencing population declines, such as the threatened Yosemite Toad (Anaxyrus canorus). Our study examined differences in physical characteristics of occupied and unoccupied toad breeding pools within meadows, and then used a logistic regression model to evaluate if occupancy was related to the physical microhabitat variables. We found that occupied pools on average were deeper by 0.7 cm, warmer by 3°C, and had 50% more surface water along the short axis of the pool. Mean water depth, mean water temperature, the amount of surface water in the pools, mean detritus depth, and mean vegetation height were significant predictors of toad occupancy. Despite variation in larger-scale environmental conditions such as yearly winter snow cover and precipitation, occupancy was not related to individual years and m...

In California, River Otters (Lontra canadensis) are most commonly associated with food-rich lowland aquatic habitats where they forage primarily on fish and crustaceans. Their distribution in high-elevation montane regions of the state, areas in which fish and crayfish were absent historically, is largely unknown. We compiled occurrence records of River Otters in California from elevations >1100 m, and evaluated them using evidentiary standards. Based on 126 records, we report the widespread presence of River Otters in the Klamath, southern Cascades, and Sierra Nevada mountain ranges, including at elevations exceeding 3000 m. Sixty-three percent of the records met our definition as “verified”, and the remaining 37% were considered “unverified”. The distribution of observations through time and habitats in which observations were made were similar between verified and unverified records. River Otter records spanned the period from 1900 to 2010, with 50% occurring between 1991 and 2010. Ninety-three percent of the water bodies with records of River Otters contained nonnative prey (fish and crayfish). Those lacking nonnative prey all supported native prey, including amphibians and reptiles. Based on records that contained River Otter foraging observations, nonnative fishes and crayfish were represented in 89% of the total accounts, and native frogs and invertebrates were represented in 22%. It remains unclear whether River Otters occurred in California's high-elevation water bodies prior to the introduction of fish and crayfish, and additional research is needed to understand the possible influence of nonnative prey in allowing River Otters to expand their distribution in these habitats.

In the Sierra Nevada Mountains of California, Yosemite Toads (Anaxyrus canorus) have declined throughout their range where Brook Trout (Salvelinus fontinalis) have been widely introduced. Amphibians that evolved in fishless habitats often lack the necessary chemical defenses and behavioral responses to avoid predation. True Toads (family Bufonidae), however, possess noxious chemicals that often deter predators. Our goal was to test whether eggs, tadpoles, and postmetamorphic toads of A. canorus are a palatable food source for S. fontinalis, assess the antipredator behavior of tadpoles of A. canorus by exposing them to various predator chemical cues, and determine sublethal effects on three lifestages of A. canorus from sampling by S. fontinalis. We found that eggs, tadpoles, and postmetamorphic toads of A. canorus exhibit absolute unpalatability to S. fontinalis. In addition, tadpoles of A. canorus did not exhibit a significant difference in behavior when exposed to chemical stimuli. Moreover, observational experiments revealed that trout would sample, then reject lifestages of A. canorus unharmed, while trout readily consumed tadpoles of Pacific Chorus Frog (Pseudacris regilla). We therefore infer that early lifestages of A. canorus likely possess existing chemical defenses as a result of interactions with native predators that adequately protect them from nonnative trout predation. Although trout removal often leads to the recovery of other Sierra Nevada amphibian populations, trout removal would likely have no effect on populations of A. canorus given our results. Therefore, we suggest that other factors thought to be contributing toward A. canorus decline should be investigated before efforts are concentrated on removing trout from toad habitats.