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FOLLOWING THE FROGS
The continuing effort to find and save the mountain yellow legged frogs of the Sierra
Nevada
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 is 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.
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 National 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
(photo by Vance Vredenburg)
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.
AT A LAKE
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 within 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 nonnative 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, and 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.
THE AMPHIBIANS
It’s been said that, “Around here,
if you got water….you got frogs.”
Unfortunately, these easy assumptions,
maybe true elsewhere, 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 intensely 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
(photos from mylfrog.com)
enough or with enough water flow to avoid
freezing. These frogs also seem to prefer lakes with grassy shorelines and silty, nearshore 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.
AT THE MOMENT
The initial
surveying of the lakes,
ponds, and marshes of
Yosemite and SequoiaKings 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 wellknown 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 as 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 mylfrog.com.
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. It will be possible with sufficient
funding 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.
WHY CARE?
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, there 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, we will 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.