Foothill Yellow-Legged Frog
By: Tara Johnson-Kelly, with assistance from Robert Shields, Geoff Cline, and Mark Jennings
An adult foothill yellow-legged frog. Photo By: Tara Johnson-Kelly
If you’ve ever been hiking in the foothills near Carmel Valley or Yosemite, you may have stopped at the edge of a cool, rocky stream to take a break in the dappled shade. If you were to look more carefully at this stream, you may spy a rough-skinned, gray-brown frog perched on a sunlit boulder among the riffles and pools. This unassuming frog may seem like an ordinary and expected occupant of the stream, but there is more to it than meets the eye.
What is a foothill yellow-legged frog?
This demure little frog perched on a rock is known as the foothill yellow-legged frog (Rana boylii). It is native to foothill and mountain streams from the Sierra-Cascade crest in Oregon, south to the San Gabriel Mountains in Los Angeles County at elevations largely below 6,000 feet. It is a medium-sized frog about 3 inches long with rough, gray-brown skin, faint ridges called “dorsal-lateral folds” along the sides of its back, and a triangular, buff-colored patch on its snout. Their inner thighs are typically yellow, as its name suggests, and its chest and throat are white with dark spots. The foothill yellow-legged frog lives in shallow, slow-flowing streams with riffles, boulders, and cobbly bottoms. It is commonly seen perched on boulders within the stream channel where it can bask in the dappled sunlight, watch for insect prey, and quickly jump into the water to escape predators.
The life history of the foothill yellow-legged frog coincides with seasonal patterns in water flow and periods of daylight. They breed in the spring between late April and early July after seasonal flood waters subside and the streams are shallow and slow-moving. During breeding, adults congregate in the wide, shallow, and sunlit reaches of the steam. Males call females mostly from under water (Hayes et al. 2016).
A foothill yellow-legged frog tadpole resting in the streambed. Photo By: Tara Johnson-Kelly
When a female responds to a mating call, the male climbs onto her back and fertilizes the eggs externally as the female lays them. Females lay a single cluster of up to 2,000 eggs attached to the downstream side of a rock or underneath cobbles or boulders near the margin of wide, shallow parts of the stream, usually near tributary confluences. These egg masses are typically found in relatively warmer water temperatures ranging between 63.5℉ (17.5 °C) and 66.2℉ (19℃) and are never found in water colder than 55.4℉ (13℃) (Hayes et al. 2016). In about two to three weeks (depending on the water temperature), the eggs hatch into dark-colored tadpoles. They later change as they grow larger, into olive-colored tadpoles with mottled patterning that matches the streambed. The tadpoles usually remain in the general vicinity of the breeding area, feeding on algae for three to four months until they metamorphose into their subadult form in late summer to early autumn, usually before winter rains flood the stream. Once metamorphosed, subadults move from their hatching site in the main channel to upstream tributaries. Adults also move to tributaries or terrestrial habitat near the stream to pass the winter months and avoid floods after heavy storms. They tend to remain in or near a stream throughout the year, preferring to live near slower-moving, sunny pools under an open tree canopy.
Why is the foothill yellow-legged frog in decline?
Overhead view of the foothill yellow-legged frog habitat. Photo By: Geoff Cline
The foothill yellow-legged frog population has been in rapid decline for decades and has disappeared from 45% of its historic range (Jennings and Hayes 1994), but why is this happening? A number of factors, including human impacts to stream flow patterns, the introduction of pesticides, herbicides, and invasive species, disease, and climate change have all contributed to the decline of the foothill yellow-legged frog population (Thomson et al. 2016).
Infrastructure projects such as dam construction, seasonal bridge installation, and bridge and culvert replacements can take many months to several years and have significant temporary and/or permanent impacts to stream habitat (Thomson et al. 2016). Such impacts could include changes to seasonal water velocity, changes in water temperature, silt build up in the streambed, and chemical pollution, which could affect the egg and larval stages. Eggs and larvae can die from drying out if the stream dries up early, or they can be killed by being swept away by high velocity floods (Thomson et al. 2016). They can also be killed by suffocation if the aquatic environment comes in contact with curing concrete. If eggs get covered in dirt or debris, the eggs will be smothered and fail to hatch. Infrastructure projects in or around streams can temporarily hinder or permanently alter the pattern of water flow in the stream and can even cause temperature changes that make the stream unsuitable for frog breeding.
Pulsed-flow water releases from dams can directly kill frogs and alter habitat suitability (Thomson et al. 2016). Dams can also create barriers that interrupt genetic exchange between populations of frogs within a watershed. One study found that frog populations living in streams regulated by dams had smaller population sizes and faced a greater extinction risk than populations living in unregulated streams (Thomson et al. 2016).
Urban and agricultural development around streams can also degrade stream habitat and lead to the introduction of pesticides, herbicides, exotic diseases, and invasive species. Creation of permanent ponds to irrigate agricultural fields creates habitat for the invasive American bullfrog (originally from the eastern half of the United States), which eats larvae, subadults and adults of foothill yellow-legged frogs. Agricultural practices that divert water from streams, such as marijuana cultivation, can lead to premature drying of the stream and death of frog eggs and larvae. Pesticides and herbicides can enter the stream from agricultural runoff, eliminating frog prey, killing their eggs and larvae, and even disrupting frog sexual development. UC Berkeley scientist Tyrone Hayes found that atrazine, an herbicide primarily used in industrial agriculture, found in the environment can turn male frogs into female frogs.
Additional foothill yellow-legged frog habitat. Photo By: Tara Johnson-Kelly
Development around streams can also contribute to the spread of chytridiomycosis disease, or “Chytrid disease.” Chytrid disease is caused by the introduced Chytrid fungus (originally from the Korean Peninsula), which eats the cartilage and teeth rows of frog larvae, and the delicate skin of subadult and adult frogs, creating open sores and scar tissue. This makes feeding and metamorphosis difficult for infected larvae, and water uptake and discharge of “frog pee” difficult for infected subadult and adult frogs. Chytrid fungus spreads through contact with water droplets containing the fungus spores, which makes it highly transmissible in frog populations. Invasive species like the American bullfrog, which are relatively immune to Chytrid disease, can also carry the disease into habitat occupied by native frogs, decimating the native population while the invasive species thrives.
Since the foothill yellow-legged frog’s life cycle is so closely linked to seasonal changes in water flow and temperature, this species has been negatively affected by past major flood events and may be sensitive to future climate changes. As temperatures warm within the frog’s range, snowpack amounts may decrease. Less snowpack would lead to reduced spring and summer streamflow. In areas where snowpack is lost altogether, streamflow may be further reduced to brief surges of storm runoff from seasonal rains. Warming temperatures can also cause a shift in the seasonal timing of precipitation. Changes in the amount of streamflow from snowmelt and timing of precipitation could prove challenging factors for the foothill yellow-legged frog to adapt to in the future. Less precipitation may also increase conflicts between human needs and frog conservation and could influence how regulated streams are managed.
Why should we care to protect the foothill yellow-legged frog?
Well, the foothill yellow-legged frog can be viewed as a form of natural pest control and an indicator of stream health. They forage for invertebrates like spiders, beetles, and flies, as well as aquatic invertebrates, other amphibians, and small rodents. By feeding on pests like insects and small rodents, foothill yellow-legged frogs help to keep balance in the stream ecosystem. If populations of these pests were left unchecked, they may eventually eat all of the vegetation that filters pollutants and prevents bank erosion. Without this vegetation, the water would become polluted from unfiltered runoff, and the stream banks would become unstable and likely collapse. Foothill yellow-legged frogs in turn provide food for countless other native wildlife in the food chain. If they are absent from suitable stream habitat in their range, it is a sign that something is wrong in the watershed and can be devastating to other wildlife, such as garter snakes, that depend on them for food.
What is being done to save the foothill yellow-legged frog?
Robert Shields holding a foothill yellow-legged frog. Photo By: Geoff Cline
In what ways are people acting to save the foothill yellow-legged frog? The first real steps to protection and recovery are currently underway. In June 2017, the California Department of Fish and Wildlife (CDFW) listed the foothill yellow-legged frog as a Candidate Threatened species under the California Endangered Species Act (CESA). Through this action, CDFW gave the foothill yellow-legged frog the same legal protections that a threatened species has under CESA, making harassing, harming, pursuing, hunting, shooting, wounding, killing, trapping, capturing, and collecting individuals (also known as “take”) illegal without special permits.
On March 20, 2020, CDFW listed certain regional populations of the foothill yellow-legged frog as threatened or endangered, depending on their location. This means that projects that may result in take of the species will likely need to get special take authorization from CDFW in order to proceed. This authorization may take the form of an Incidental Take Permit or a Natural Community Conservation Plan. Take authorization under CESA requires specific avoidance and minimization measures and full mitigation for project impacts to the foothill yellow-legged frog.
In addition to protection under CESA, the U.S. Fish and Wildlife Service is considering listing specific regional populations of the foothill yellow-legged frog under the Endangered Species Act, a decision that may be reached as soon as June 2023 (Jeff Alvarez, personal communication). If the frog is federally listed as threatened or endangered, federal permits such as Biological Opinions and Habitat Conservation Plans will likely be required for projects that have potential to impact foothill yellow-legged frogs and their habitat.
Ultimately, the best tool for conservation of the foothill yellow-legged frog is scientific research. Research helps to guide the creation of laws and policies that effectively protect wildlife in ways that will promote species recovery. Although much has been discovered about the frog’s current abundance, distribution, habitat requirements and life cycle, more research is needed to understand how hydrology affects different life stages (Thomson et al. 2016), how frogs utilize habitats after metamorphosis, how much genetic variance exists between local populations, and survivorship of larvae and juveniles.
Some other conservation steps that have been recommended include:
Timing of water releases from upstream dams to better coincide with the foothill yellow-legged frog egg and larvae seasons to reduce the possibility of washing the eggs and larvae away.
Removing dams to naturalize water flows for salmonids which would have a direct benefit to foothill yellow-legged frogs by creating a natural flow of water in their breeding streams. The water flow would decrease in late spring when the foothill yellow-legged frogs are laying eggs and larvae are developing.
Issuing CDFW Streambed Alteration Agreements to help protect the foothill yellow-legged frog’s habitat from degradation caused by development projects along the beds and banks of streams.
Controlling nonnative fish and invasive species such as American bullfrogs, western mosquitofish, Louisiana red-swamp crayfish, and red-eared sliders.
Tara Johnson-Kelly showing the underside of a foothill yellow-legged frog.
Fun Facts
Some individuals have orange or red coloration on their inner thighs instead of yellow (Thomson et al. 2016).
The last joint of the fingers and toes is curved to aid in climbing slippery, algae-covered rocks (Jeff Alvarez, personal communication).
Males typically perform mating calls underwater but have also been observed calling from above the surface (Thomson et al. 2016).
They can live to be 7.2 years in the wild (Thomson et al. 2016).
Can travel between 500 to 1,386 meters in a single day, using streams as travel corridors. Female frogs tend to move upstream during the spring and downstream during the fall and winter (Thomson et al. 2016).
The scientific name honors a “Real 49er,” Dr. Charles Elisha Boyle, a Columbus, Ohio, physician who came to California to search for gold at Coloma in 1849.
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