Why Was This Indicator Chosen?
Foothill yellow-legged frogs (Rana boylii) are good indicators of the perennial and ephemeral streams they rely on for breeding and post-metamorphic habitat. Early life stages are sensitive to changes in flow and water temperature, and are vulnerable to both recreational use and aquatic invasive species.
Widespread population declines have led to their listing as a Federal Species of Concern, a U.S. Forest Service Sensitive Species, and a California Species of Special Concern. Foothill yellow-legged frog range and numbers have declined dramatically in Marin County and in the One Tam area of focus over the last 75 years, and they are now only found in Big and Little Carson Creeks. Marin Municipal Water District breeding surveys from 2004 and 2016 indicate the remaining breeding populations have been relatively stable (GANDA, 2016), but they are highly vulnerable due to their small size and isolation from other populations.
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What is Healthy?
A healthy population of foothill yellow-legged frogs would have all life stages in currently occupied streams with stable or increasing numbers of egg masses and adults detected each year. Furthermore, breeding populations would be reestablished in historically occupied streams including Cataract and Redwood creeks. Breeding habitat quality would be improved, and the impacts of humans and non-native predators minimized.
What Are the Biggest Threats?
- Predation by invasive bullfrogs (R. catesbiana) and signal crayfish (Pacifastacus leniusculus), as well as native rough-skinned newts (Taricha granulosa)
- In-stream habitat disturbance that affects the cobble and gravel where foothill yellow-legged frogs lay their eggs
- Chytrid fungus (Batrachochytrium dendrobatidis), which has caused amphibian population declines worldwide, although it does not seem to be affecting foothill yellow-legged frogs on Mt. Tam
- Potential inbreeding as a result of living in small, isolated populations
- Water temperature and flow fluctuations, both of which may occur under future climate change scenarios
What is The Current Condition?
The current condition is Poor because only small populations are found at Big Carson and Little Carson creeks in the One Tam area of focus. Egg mass numbers are low; however, the 12-year average for egg mass maturation is estimated to be greater than 94%.
What is the Current Trend?
A trend of No Change is based on annual survey results since 2004 that show that even though the populations are at great risk because they are small and isolated, their numbers appear to be stable, and egg loss levels have remained consistent over the years.
How Sure Are We?
Confidence in this assessment is High because surveys have been conducted by trained biologists using consistent techniques since 2004. However, while there are no other populations known in the One Tam area of focus, we have not conducted a full survey of suitable habitats since 2003.
What is This Assessment Based On?
- A habitat survey in 2003 (GANDA, 2003)
- Egg mass surveys since 2004, which also document other life stages (egg masses, tadpoles, juveniles, and adults) (GANDA, 2013)
What Don’t We Know?
Key information gaps include:
- Population viability analysis based on existing time-series data from Big Carson Creek, its tributaries, and Little Carson Creek
- The potential range for individual frogs (although chin pattern analysis combined with mark and recapture studies suggest there is very little movement between frogs at Big Carson and Little Carson creeks) (Marlow, 2016)
- Habitat requirements for life stages other than eggs and tadpoles, or for movement from one breeding site to another