The Yosemite toad (Anaxyrus [Bufo] canorus) is endemic to the higher-elevation (> 1,980 m [> 6,500 ft]) aquatic habitats of the Sierra Nevada Mountains of California. Once historically abundant, it is estimated that this toad has been extirpated from significant portions of its historical localities, and many of its remaining populations appear depleted. Depletions and extirpations were first recognized during the 1970s, and on 29 April, 2014, the U.S. Fish and Wildlife Service added the Yosemite toad to the Lists of Endangered and Threatened Wildlife and Plants as a threatened species. In the late 1990s, the realization that these declines could rapidly place the species at risk of extinction led to a U.S. Department of Agriculture (USDA) Forest-Service-initiated multi-agency effort to develop a conservation strategy focused on attenuating the risk factors responsible. This conservation assessment is the first step toward the development of this conservation strategy and consists of three parts: (1) a synopsis of Yosemite toad ecology designed to better understand conditions necessary to provide for viable populations; (2) a review of Yosemite toad distribution and abundance over its historical geographic range to describe the risk; and (3) an evaluation of 16 risk factor categories to identify which may contribute the greatest risk to the Yosemite toad and its habitat.
Yosemite toads occupy both aquatic and terrestrial habitats. They breed and rear primarily in shallow still water habitat; use meadows, springs, and terrestrial upland habitats for foraging, refuge, and movements; and overwinter in underground terrestrial sites. Tadpoles develop rapidly in very shallow, typically ephemeral aquatic habitats. Mortality through metamorphosis can be very high, with abiotic factors (desiccation and/or freezing) sometimes causing total or near loss of a year’s cohort. Mortality of small post-metamorphic toads also appears high, likely because of high overwinter mortality. The long-lived adults may be key to long-term persistence of populations given the low recruitment in some years. Post-metamorphic life stages (juveniles and adults) occupy habitats some distance from breeding sites seasonally. Little is known about seasonal movements, especially for juveniles, but movements that range several hundred meters from breeding sites are recorded for adults. The population structure and dynamics of Yosemite toads are unclear. Yosemite toads are currently recognized as one taxonomic unit, but genetic data imply that more than one discrete lineage may be concealed within what is now called Yosemite toads. Moreover, the relationship between Yosemite toads as a taxonomic unit and its closest relatives is ambiguous and needs clarification.
Yosemite toads occurred on both sides of the Sierra Nevada mountain divide between the southern portion of the Lake Tahoe Basin and the headwaters of the Kings River between 1,980 m (6,500 ft) and 3,414 m (11,200 ft). Based on pre-1980 information, most (> 99 percent) of the historical range is on federal land including six national forests and two national parks; the remainder of the range is on private and state-owned lands. Historical abundance data are mostly anecdotal, but Yosemite toads were described as being common, and at least one population had several hundred individuals; these data also imply that Yosemite toads were most abundant in the elevation region above 2,438 m (8,000 ft) and below areas of permanent snow and ice. Yosemite toad information obtained since 1990 includes quantitative occurrence and abundance data. Recent occurrence data, based on a USDA Forest Service monitoring program for high elevation amphibians and other survey data, reveal patchy extirpations range wide, with Yosemite toad populations still distributed across their original range. The few data that exist on recent abundances suggest populations may be very small (< 20 adult males) compared to historical levels with relatively few large populations remaining across the geographic range. Whether these populations are persisting in small numbers or on a slow trajectory to extirpation is not known.
For the 16 potential risk factors identified during the Conservation Assessment process, definitive data are generally lacking. Risk factors that affect meadow hydrology or impact the long-lived adults including in their upland nonbreeding habitats may be most significant. Further, small populations may be more vulnerable to risk factors that would be of less concern for larger populations. Given these considerations, observational data of Yosemite toad habitat and circumstantial evidence suggest that climate change, livestock grazing, recreational activity, and the amphibian chytrid fungus (Batrachochytrium dendrobatidis, Bd) may be important factors. Livestock grazing and recreational activities were identified as important to address because of their widespread occurrence, high overlap with Yosemite toad habitats, potential effects on breeding habitat (e.g., meadow hydrology) and/or adults, and the ability of participating agencies to make meaningful management changes. A Forest-Service-sponsored research project formally addressed livestock grazing. Those results indicated that when livestock grazing use occurred at levels at or below grazing standards (i.e., 30 through 40 percent use of grass and grass-like plants, depending on meadow seral stage and condition) there were no detectable differences in toad occupancy or density (of various life stages) among areas that were grazed and areas that were not grazed (e.g., fenced meadows). This study occurred over a relatively short time period (5 years) and toad densities were highly variable among meadows. The primary drivers related to toad presence and densities were water year type and meadow wetness. No formal studies have been conducted on recreational activities and the Yosemite toad. Initial studies suggest that Bd may be an important factor in Yosemite toad declines, but the results are not conclusive and this is a major information gap. Several other risk factors (fire management; locally-applied pesticides; roads; vegetation and fuels management) are more prevalent at low to mid-elevations of the species range, may affect adults in their nonbreeding habitat, and may be particularly important where they overlap with small populations. These also have the potential to be effectively addressed by management efforts of agencies participating in this conservation assessment. Habitat loss and fragmentation may result from a variety of these risk factors. Four risk factors (acid deposition, airborne contaminants, climate change, ultraviolet radiation) have effects that originate globally or extra-regionally (from the perspective of the Sierra Nevada), and as such, are largely beyond the jurisdiction of agencies participating in this assessment. Of these, climate change may pose a high risk to Yosemite toads by altering precipitation patterns that may result in significant changes to breeding habitats among other possibilities. Participating agencies may be able to respond indirectly to these global risk factors by instituting land management actions that ameliorate local risk factors and result in higher resiliency of Yosemite toad populations. Experimental and survey data have found no direct effects from introduced fish, acid deposition, and ultraviolet radiation.
Ongoing research address several aspects of the ecology of the Yosemite toad. Studies are examining Yosemite toad habitat relationships, hydrology of breeding meadows, demography, movement ecology, and genetics.
Conservation options for consideration in a conservation strategy for the Yosemite toad include management at multiple scales; identifying and managing within priority basins (watersheds); maintaining and restoring meadow and other habitats; and developing options for effective management of livestock grazing and recreational activities. Further research on Yosemite toad genetics, and the relationships between Yosemite toad populations and habitat and recreational activities, Bd, and climate change are proposed.