Habitat characteristics, demography, and movement of Yosemite toad (Anaxyrus [=Bufo] canorus) in the Sierra Nevada, California

Background/Question/Methods: The Yosemite toad (Anaxyrus [= Bufo] canorus) is a high-elevation species endemic to the central Sierra Nevada that is associated predominantly with wet meadows used for breeding habitat. A. canorus has declined in distribution and abundance throughout its historical range; reasons for the declines are unknown but may be related to disease and habitat modification. The existing Kings River Experimental Watersheds (KREW) project in the Sierra National Forest provides an opportunity to study the effects of timber harvest and underburning on this sensitive species. We monitored meadow conditions, A. canorus demographics, and adult A. canorus movement in up to five breeding meadows in 2006-2012 prior to tree thinning and prescribed fire treatments. Meadow conditions were characterized through groundwater well measurements, surface water mensuration, and water chemistry measurements during the breeding period and throughout the ensuing snow-free months. A. canorus demographics in the meadows were calculated using mark-recapture techniques during the breeding period. Movement and upland terrestrial habitat use were analyzed through radio-tracking of adult A. canorus as they left the breeding areas.

Results/Conclusions: A. canorus breeding populations were small, with less than an estimated 20 adult males in each study meadow and similar counts of adult females. Results suggest that abundances in several meadows are declining, although numbers of males in one meadow may be increasing. Study meadows had acidic groundwater with annual averages ranging from pH 5.5 to 6.1. Electrical conductivity, a measure of inorganic dissolved solids in water, was low with averages between 20 and 35 µS/cm. Water depths in breeding areas were very shallow with medians generally less than 2 cm and most maximums less than 10 cm.  Survival of eggs and larvae to metamorphosis was related to surface water duration. After breeding, adult A. canorus moved a mean distance of 270 m from aquatic breeding sites and made extensive use of terrestrial environments in the mixed-conifer forest. Occupied terrestrial sites had less canopy cover and fewer woody species than unoccupied sites. Adult toads showed site fidelity to both aquatic breeding sites and upland terrestrial areas. These findings will be compared to the data being collected during and after the tree thinning and prescribed fire treatments, which were implemented starting in the late summer of 2012.