Masthead: Kaweah Range

Sierra Nature Notes, Volume 2, November 2002

The continuing effort to find and save the mountain yellow-legged frogs of the Sierra Nevada
by Casey Ray
Field Biologist, Sierra Nevada Aquatic Research Lab (SNARL)

In the Sierra Nevada mountains, a large research effort is in progress to understand and ultimately reverse a decline in the numbers of the mountain yellow-legged frog (Rana muscosa). This native frog has existed in the Sierra for over a milion years. It is now found only in the higher elevations of the Sierra and in very small numbers in the transverse ranges of southern California. Their disappearance is consistent with a worldwide downward trend in amphibians, as declines in populations and extinctions of many species are currently being discovered around the globe.

Mountain yellow-legged frog
Photo by Vance Vreedenburg

The mountain yellow-legged frog was once the most numerous amphibian in the higher elevations of the Sierra, where it utilizes lakes, streams, and wetlands to live and reproduce. In recent decades, however, it has disappeared from a significant portion of its original range. This has prompted an extensive survey effort throughout the national parks of the Sierra Nevada, to determine where these frogs still exist and to study what factors are influencing their decline.

Dr. Roland Knapp, of the University of California Santa Barbara, and Dr. Kathleen Matthews, of the U.S. Forest Service, began conducting amphibian surveys in the Sierra Nevada during the summers of 1995 through 1997. During the first two summers, survey crews searched for frogs in the lakes, ponds, and wetlands of selected basins in the John Muir Wilderness. In 1997, surveys covered basins along the eastern crest of Kings Canyon National Park. Dr. Knapp continued the project through the field seasons of 2000, 2001, and 2002, and the number of lakes surveyed and the ground covered by crews are both larger than anyone thought possible. In total, almost 8000 lakes have been visited, including all lakes of Yosemite and Sequoia-Kings Canyon National Parks.

The information collected through the efforts of field biologists who survey lakes creates a broader picture of the current range and numbers of frogs and other aquatic species. It also provides some insight into the overall health of aquatic systems in the Sierra Nevada. Considering limited information from past literature, we can only speculate as to the extent of the mountain yellow-legged frog decline. Dr. Knapp's survey is the most extensive done to date. As such, this project has developed a knowledge base from which future management plans can be drawn.

The fate of the mountain yellow-legged frog has attracted the attention of several other biologists and agencies as well. The resulting collaboration between researchers and managers has been an evolving process. The California Department of Fish and Game and the U.S. Forest Service have both begun a survey effort. The protocol that is used in the national parks is currently being used in areas of the national forests. The range maps and database resulting from this effort will provide insight into the patterns of decline throughout the total range of the frog, regardless of the political boundaries (of which the frog knows nothing, of course…) and opens the door to multi-agency management efforts.

The current survey protocol being used by researchers during initial visits to lakes is rather inclusive, acquiring as much usable data about the water bodies and their inhabitants as possible. Most impressive is the effort to visit every mapped lake, pond, or marsh represented on USGS 7.5 minute topographic maps within the national parks and some areas of the John Muir Wilderness. Unmapped water bodies encountered by biologists while in the field are surveyed as well.

Any cross-country traveler in the Sierra Nevada understands the difficulties and frustrations that this task alone could involve. Water does not limit itself to places with trails. Nor does it limit itself to the soft grassy flats of a basin floor. Water can collect in any curved palm of the mountains' many arms, and in any bowl or cup into which snow melts in spring. To each and every reservoir of a Sierra Nevada winter, be it a puddle or a sea, the field crews go. When we come upon one of these teal alpine pools, a survey begins.

At each lake the entire shoreline is searched for the presence of amphibians, including a walk up each inlet and down each outlet. Descriptions of the lakeshore and underwater substrate are recorded. In certain lakes, benthic invertebrate and zooplankton samples are taken using fine mesh nets. To understand more about the effects of non-native trout on lake ecology in the Sierra, gill nets are set in larger ponds and lakes. From this kind of fish sampling data, information on age structure and density of fish populations can be deduced. Site maps are drawn and the depths of water bodies are determined using a sounding line or the alternative method of swimming and diving, a rather invigorating way to end a lake survey!

Surveys are normally conducted by one person and so, depending on the distance between lakes, several surveys can be done in a day by a typical three person crew. Gill nets are usually set each day and night, so the crews often move daily to camp near netted lakes. We work constantly in the open reaches of the alpine lake basins, be it under rain or relentless sun. Work days are long with no breaks until a survey is finished. We are sometimes helped along by the hospitality of backcountry trail crews and rangers, which is invaluable in boosting morale, but it is more common to see no one at all in the trail-less alpine lake basins.

As a field biologist, the exposure to the seemingly infinite nooks and crannies of the mountain range is as priceless as it is endless. The exposure to the underside of a heavily laden backpack is character building, at best. The exposure to one another, day after day, with only pikas and songbirds to join the conversation provides a unique look at human nature. But through the efforts of a diverse group of seasoned mountain travelers hired to conduct the surveys, the job gets done.


Top: three year old tadpole
Bottom: typical tadpole gathering near shore.
Photos from

It's been said that, "Around here, if you got water, you got frogs." Unfortunately, these easy assumptions may be true elsewhere, but do not hold true in the high basins of the Sierra. The mountain anurans (frogs and toads) here fill a more specific niche in alpine lakes and wetlands. The species most intensively studied now is the mountain yellow-legged frog, but observations of any amphibians are recorded during the surveys. Other species include the Pacific tree frog (Hyla regilla), the Yosemite toad (Bufo canorus), and the western toad (Bufo boreas).

The mountain yellow-legged frog is rather specific in its habitat needs, and the parameters of good frog sites are still being discovered. The larvae of this frog live from 2 to 4 years in the same lake until they metamorphose into subadults, a stage of development before full adulthood. The larvae, along with subadults and adults, must spend the winter in water bodies deep enough or with enough water flow to avoid freezing. These frogs also seem to prefer lakes with grassy shorelines and silty, near-shore habitats. Many habitats that are otherwise suitable, however, are no longer occupied by mountain yellow-legged frogs because of the presence of non-native trout. These trout, introduced into the mountains at the turn of the century, show a trend of replacing frogs in many lake basins. With the frogs forced into the recesses of their original range, good habitat is hard to recognize.

The Pacific tree frog seems to be the healthiest and most successful of the bunch. It is able to reproduce in small, warm pools or the outskirts of lakes, and the larvae grow into subadults within one summer season. These frogs are found in almost any basin and are able to escape the predation pressure of non-native trout by utilizing fish-free ponds and pools or the shallow areas of lakes that fish cannot access. So far, they show very few signs of disease or stresses from weather patterns, which are now becoming apparent in the mountain yellow-legged frog.

The two species of toad are rather rare and less well known in the higher altitudes. They are both seen in the early summer in good marsh habitat as small black larvae. Adults in both species are known to spend the winter underground in rodent holes. The Yosemite toad exists only within the central Sierra Nevada, and it is being considered for listing as an endangered species along with the mountain yellow-legged frog. The western toad exists in the northern and southern Sierra Nevada.


The initial surveying of the lakes, ponds, and marshes of Yosemite and Sequoia-Kings Canyon National Parks was completed in the 2002 summer field season. A much clearer picture of the status of the mountain yellow-legged frog, as well as other Sierra Nevada anurans, has materialized. The bottom line remains that the number of mountain yellow frogs is declining.

Biologists have postulated a number of theories for amphibian declines. Research in the Sierra has shown several key factors to have a negative impact on frog numbers. The most well-known threat to them is the introduction of non-native trout in otherwise fishless, alpine lake basins. Another critical factor in the current decline of the species is the occurrence of chytrid fungus. This disease affects the mouth parts of tadpoles and is fatal. It is now showing up in populations in several parts of their current range.

Other threats being looked at by researchers are the occurrence of chemical contaminants in the air and water, pesticide drift, and climatic fluctuations, including effects of drought. For more information on this research, go to

In the 2002 field season, crews revisited sites reported to have frogs in the surveys of the previous seven years, as well as an equal number of sites initially found to be frogless. The resurveys consist of determining amphibian presence, counting individuals in the age classes of larvae, subadult, and adult for all amphibian and reptile species, as well as looking for the presence of chytrid fungus. With sufficient funding, it will be possible to continue to resurvey the remaining frog basins into the future. Many of the strongholds of the mountain yellow-legged frog remain healthy with the smaller and fringing populations showing the most dramatic signs of decline. One large source of concern about the growing isolation of frog populations is the inability to recruit individuals into areas where a decline or extinction has occurred. If only a small population remains at one or two lakes within a lake basin, populations are at much higher risk: with no neighboring populations of frogs, there can be no recruitment from nearby lakes should that isolated group die off.

The ideal state of connectivity among remaining populations may help to shape future management efforts. Current attention is being focused on future reintroductions of mountain yellow-legged frogs. Basins that show a propensity for successful recolonization by frogs would have to be chosen, and this could help to fill in the gaps and strengthen frog populations against threatening conditions. Unsuccessful reintroductions have been attempted in the past. However, with a growing understanding of the factors affecting frog survival and a current reintroduction effort in a basin of the John Muir Wilderness and Kings Canyon National Park — which are showing success — there is hope that this could be a feasible management alternative in the future.

Many people respond to news of the current plight of the mountain yellow-legged frog with a question, "Why should I care?" As members of a species which seems to have an easy time proliferating on this planet — able to survive on each of the seven continents, in any number of habitats and climate zones — this is a reasonable question. It is argued by some that the sudden extinctions of amphibians will affect only the amphibian's specific niche, one of a balanced aquatic and terrestrial presence on this planet. Some people say that the earth may live on without its frogs — and the human race with it — and that we need not concern ourselves with the matters of life forms so small and different, or animals so cryptic and slimy. True, many frogs can create a nuisance, calling in a pond at night beside a house in a rural woodland or as exotic intruders to otherwise intact natural systems, where they can reach disturbing numbers.

It is, however, more in line with human curiosity and common sense to consider that in the space of shared air or in a drink of water, lies the innate survival behaviors that bind humans to all living things. The human species developed amidst the constraints and freedoms of a system that was not governed by human influence. The diversity that has appeared within the many classifications of life exists for the healthy functioning of that same system. The human race is currently one hundred percent dependent on the resources of this planet, be they organic or inorganic, and as we unfold the secrets of how the earth has come to be, it would be rather foolish to allow any parts of this planetary system to vanish completely. Our place in the world has yet to be understood, and it may be that only in the shadows of the coexisting life that has developed with us on this planet, will we recognize our true niche and the value of all life surrounding us.

The ultimate goal of this project is to halt the extinction of the mountain yellow-legged frog in the Sierra Nevada. Through this research, we will gain insight into the mechanisms and boundaries of the natural systems of this planet as well. Systems which exist just as much for humans as they do for frogs and in which any voids or changes in one part can create an equal or greater effect in any other part.


For Further Reading
Dr. Roland Knapp's Mountain Yellow-Legged Frog site.

The Mountain Yellow-legged Frog:
Can They be Saved?

by Vance Vredenburg, Ph.D

A Summer Spent Saving Frogs:
Applying Research to the Real World

by Ryan Peek
Biology Department, UC Davis

The AmphibiaWeb Watch List (extinct, missing or endangered amphibians of the world, with map).


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