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Pacific Southwest Research StationFOREST SERVICE, U.S. DEPARTMENT OF AGRICULTURE

High Sierra Ecosystems

Fall 2003

With a rich diversity of aquatic habitats, including deep lakes, shallow ponds, and rushing streams, Dusy Basinin Sequoia-Kings Canyon National Parks typifies the high Sierra ecosystem where mountain yellow-legged frogsusually thrive. Yet throughout the Sierra, aquatic ecologist Kathleen Matthews found entire water basins emptyof these amphibians. Comprehensive studies showed that exotic trout caused the precipitous decline.

Naturally FishlessPerched in glacial cirques and mountain meadows,thousands of high Sierra Nevada lakes reflectsurrounding snow-covered peaks and cobalt skies ofa seemingly undisturbed environment. Streamsmeander through flats, rush down hillsides, andcascade as waterfalls over hanging valleys. Withsnowmelt their source, these high Sierra lakesand streams are sheltered from many of thepollutants and human activities affecting waters atlower altitudes.

Additionally, most of the high Sierra is administeredas Wilderness by the USDA Forest Service or USDINational Park Service. The mandate for manage-ment of high Sierra lands designated as Wilder-ness is preservation for future generations, with natural forces unconstrained.

Yet high Sierra aquatic ecosystems are by nomeans completely protected; in fact, the effectsof a single human activity, fish stocking, havedrastically changed these ecosystems. The im-pact is so extensive that certain species, such asthe threatened mountain yellow-legged frog, arenow completely absent from entire water basinswhere they formerly occurred in large numbers,says aquatic ecologist Dr. Kathleen Matthews.

While native fishes do inhabit a few high Sierralakes and streams, the majority of high Sierrawater bodies are, in their natural state, fishlessecological communities.

“These high-elevation aquatic ecosystemscontain relatively few species in comparison to many other aquatic ecological systems,”explains Matthews. The food webs are fortifiedby algae and zooplankton at the bottom anddominated by amphibian, reptilian, and avianpredators at the top. Barring exotic introductions,these waters lack fish altogether.

Why are high Sierra lakes and streams naturallywithout fish? From two million to 15,000 yearsago, Pleistocene-Age glaciers scoured the highSierra of its soils and organisms. The steepstream gradients and waterfalls left in the wake of glaciation were difficult for colonizing fish tonavigate, leaving both the waterways andglaciated cirques and basins, now filled withsnowmelt, without fish.

Considering that the mountain yellow-leggedfrog survives the intense cold, heavy snowfall,and ferocious winds typical of harsh Sierrawinters, it is highly adapted to the alpineenvironment. Still, the life history of this

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The Role of Fish Stocking in Amphibian Declines

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An Era of Fish StockingAs early as the 1850s, miners, sheep-herders, and early settlers moved nativespecies of trout into fishless high Sierralakes and streams, seeking to supplementtheir food supply. Private groups and indi-viduals, as well as government agencies,continued stocking fish—often exoticspecies not found in California—and by1915, the eminent field zoologistJoseph Grinnell had observed the effectof these introductions.

Grinnell noted that in lakes where troutwere stocked, tadpoles of mountainyellow-legged frogs were absent. How-ever, at the time of Grinnell’s surveys,trout introductions were still limited, andamphibians remained by far the mostnumerous land-based vertebrates in thehigh Sierra.

During the years that followed, stockingfor recreational fishing accelerated rapidlyand aerial stocking became widely used.The National Park Service limited stockingin Yosemite and Sequoia-Kings CanyonNational Parks in the 1970s, termi-nating it altogether in the 1990s. Butself-sustaining trout populationsremained in many national park lakes,and fish stocking continued on ForestService lands.

Alarming Amphibian DeclineThen, in 1994, a group of scientistssurveyed areas where Grinnell had foundmountain yellow-legged frogs early in the

20th century. The scientists’ findingsrevealed a dramatic range contraction:Only 15 percent of the locations whereGrinnell had recorded sightings still hostedthe species. Certainly, a distinct change inhigh Sierra ecology had taken place. Thenext step was to determine the cause ofthe decline and whether other animals inthe food chain were affected.

Beginning in 1995, Kathleen Matthews,working with Roland Knapp of the SierraNevada Aquatic Research Laboratory,University of California, and a field team ofover 20 research assistants, undertookone of the most extensive field studies ofthe impacts of introduced fish conductedin the United States. The goal of theirthree-year study was to pinpoint whetheror not trout were, in fact, affecting thenative biota including mountain yellow-legged frogs. The scientists designed theirfieldwork so that statistical analysis couldbe used to describe the extent of any effectin unequivocal, quantitative terms.

Trout—A Definitive FactorThe scientists compared 1,000 lakesin Kings Canyon National Park to asimilar number in the adjoining JohnMuir Wilderness where the State hadcontinued stocking practices. TheseNational Park and Forest Service Wilder-ness areas had remarkably similar land-scapes, and their aquatic ecosystemsvaried primarily by the percentage of lakesbearing trout.

The scientists analyzed their data fromthree perspectives—landscape, water-shed, and individual water bodies. At all three scales, an increase in troutdefinitively correlated with a decrease inmountain yellow-legged frogs. When thescientists looked at each lake separately,accounting for habitat effects, the frogswere three times more likely to occurand six times more abundant when fishwere absent.

The scientists were especially interestedin lakes that were deep enough to neitherfreeze solid during winter nor dry up insummer. These lakes were used forstocking, and the scientists hypothesized

Major ThemesNaturally Fishless: Barring exotics, theaquatic ecosystems of high Sierra lakescontain algae, zooplankton, aquaticvegetation, aquatic insects, amphibians,garter snakes, and aquatic birds.

Introduced Trout: 150 years of fishstocking introduced exotic species oftrout into many of the lakes.

Research ResultsUnequivocal Relationship: The presenceof trout is definitively linked to a decline inmountain yellow-legged frog populations.

Effects Throughout the Food Chain:Trout presence also dramatically reducednumbers and distribution of Pacific tree-frogs, mountain garter snakes, and large-bodied zooplankton.

Lakes are Resilient: Once trout areremoved, lakes return to their initialnatural state within 11 to 20 years,assuming there are frogs in the sur-rounding area and viable zooplanktonegg banks.

Pacific Southwest Research Station

Adult frogs emerging fromthawing lake

that the same water bodies were essentialto the highly aquatic mountain yellow-legged frog. Results of the study supportedthis hypothesis, with the authors con-cluding that predatory trout now occupiedthe lakes where mountain yellow-leggedfrog tadpoles were found historically in thegreatest numbers. The frogs, concludedKnapp and Matthews, were increasinglyrestricted to marginal and isolatedhabitats, their fragmented populationsslowly becoming extinct.

Strong Ecosystem EffectIn a separate study of the same 100,000-hectare area, Matthews and colleagueslooked at whether the trout affected Pacifictreefrog populations. The most abundantamphibian in western North America, thisspecies does not have a prolonged tadpolephase that restricts it to deeper lakes andexposes it to predatory trout over anextended period.

Still, the results of the study showed asurprisingly strong effect. There was a majorreduction in Pacific treefrogs where troutwere present. For example, in KingsCanyon National Park, 26.6 percent of all

water bodies contained Pacific treefroglarvae, as compared to 7.2 percent of thosein the John Muir Wilderness.

Even more dramatic were the results of amountain garter snake survey in the samelakes. This snake feeds on amphibians,including both the mountain yellow-leggedfrog and the Pacific treefrog. In KingsCanyon National Park the scientists located62 garter snakes; in the adjacent John MuirWilderness not one garter snake was foundin the 1,044 lakes surveyed.

Additional studies showed that at trophiclevels below the frogs, trout were clearingthe lakes of larger zooplankton, fairy shrimp,and benthic (bottom) invertebrates.

Although the lakes had low resistance tothe introduction of trout, with faunal make-up changing drastically in their presence,one of the studies revealed an exciting at-tribute of high Sierra lakes: they had a highlevel of resilience, reverting to their naturalstate once trout were removed.

Even where trout had drastically altered thefauna, lakes returned to near normal withintwo decades, with some lakes recovering inperiods as brief as eleven years. The findinghas important implications for restoration ofalpine lake ecosystems, implying that simplyby removing trout, at least some lakes canreturn to their original state.

The scientists conducting the study—Matthews and Knapp, along with OrlandoSarnelle of the Department of Fisheries andWildlife, Michigan State University, East

Lansing—attribute the recoveries to lifestages that can travel to and recolonizelakes where trout have been removed.Adult stages of many benthic invertebratesare winged, scientists point out, and nearbypopulations of adult frogs are able to traveloverland. Viable egg banks, a potentialsource of zooplankton, are also important.

Yet the scientists, while optimistic, wave aflag of caution. Conditions are changingrapidly, they emphasize, with frog popu-lations quickly disappearing and zooplank-ton egg banks aging. “As time passes,”Matthews says, “return to natural condi-tions becomes less likely without a higherlevel of intervention.”

Although scientists initially speculated that amphibian declines resulted fromglobal environmental factors, this researchdemonstrates that local causes, such asfish stocking, play an important role insome cases. While the designation“Wilderness” implies protection, fishstocking permitted on Forest ServiceWilderness contributed to amphibiandeclines and major ecosystem changes,indicating that as currently defined,Wilderness does not offer completeprotection to aquatic systems.

Pacific Southwest Research Station3

Searching for emerging adult frogs

Biologist collecting plankton sample

Photo: Rick Ziegler

Garter snake eating a mountain yellow-legged frog

Aerial fish stocking

Photo: Phil Pister

...T...To Manago Managementement

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A Conversation with Kathleen MatthewsQQ

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Your research shows that exotic trout have affected high Sierra lake ecosystemsat several trophic levels. Why is the ecosystem found in high Sierra lakes sovulnerable to introduction of a single species?In comparison to many aquatic ecosystems, the food web typical of high Sierra lakesis relatively simple. Therefore the introduction of even a single species—especially atop predator that consumes members of the food chain ranging from zooplankton toamphibians—has drastic effects.

What are some of the key elements of a restoration plan for mountain yellow-legged frogs and high Sierra aquatic systems?Although the lands are managed by two national agencies, the USDA Forest Serviceand the USDI National Park Service, most responsibility for managing fish and wildlife rests with the California Department of Fish and Game. At present, this includesjurisdiction over fish-stocking practices for lakes situated on high Sierra national forestlands. Coordination between these agencies is absolutely essential to a successfulrestoration plan and management of threatened species.

Halting fish stocking will empty some lakes of trout solely as a result of attrition.In larger lakes, where exotic trout tend to be self sustaining, our studies showthat fish can be removed by netting. These simple measures could reverse thedecline of the mountain yellow-legged frog and gradually restore the naturalecology of high Sierra lakes.

At this point entire basins lack frog populations. How could mountain yellow-legged frogs be restored to these regions?The restoration process has to start with removal of trout from many of the lakes.Introducing frogs into lakes with trout still present certainly would not be successful.

In frogless basins there are many considerations. For example, does it makesense to potentially harm a healthy population of frogs for restoration ofother areas? And with several subspecies present, a nagging ques-tion is which populations are best adapted genetically for intro-duction into frogless basins. It is possible that these basinshave lost subspecies of their own.

What could the restoration starting point be?In some basins, as many as half of the lakes stillretain mountain yellow-legged frogs. By gillnetting trout from some of the frogless lakes,we could pave the way for frogs to returnto more of the lakes in these basins. Thiscould be the quickest way to enlarge andstabilize the populations of thisthreatened species.

Time is of the essence—restorationbecomes more difficult as populationscontinue to erode.

Not Pristine: Due to fish stocking, high SierraWilderness aquatic systems are greatly altered.

Drastic Effects: Introducing even a singlepredatory species into an ecosystem canreverberate up and down the food chain.

Multiple Agencies: A successful recoveryplan for high Sierra aquatic ecosystemsrequires cooperation between multiplestate and federal agencies.

Simple Measures: Terminating fish stockingand netting trout from many of the lakesthey now occupy could be sufficient toreverse the decline of mountain yellow-legged frogs in some areas.

Frogless Basins: Where mountain yellow-legged frogs are absent from entire basins,more complex measures are required torepopulate them.

Starting Points: Restoration in basinsthat still contain frog populations is theplace to start.

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Mountain yellow-legged frogwith transmitter

Pacific Southwest Research Station

Kathleen R. Matthews, Ph.D.Sierra Nevada Research CenterPacific Southwest Research StationUSDA Forest Service800 Buchanan StreetAlbany, CA 94710

Phone: 510-559-6454e-mail: [email protected]://www.fs.fed.us/psw/programs/snrc/aquatic

“Most of my work for the USDA ForestService has been devoted to Wildernessareas,” says aquatic biologist KathleenMatthews. “I have always been fascinatedby natural environments, especially theSierra Nevada and marine systems.”

Studying the effects of cattle grazing ongolden trout in the Golden TroutWilderness, Inyo National Forest, was oneof Matthews’ first projects when she beganworking for the USDA Forest Service as anaquatic ecologist in 1992. Her researchexamined golden trout habitat selection,movement patterns, and the effects ofcattle grazing on stream channelmorphology, spawning, and golden troutpopulation structure.

Since then she has continued to work onimproving the management of aquatic

systems in the wilderness. Matthewsspecializes in finding solutions for betterresource management through quantitativefield research. During the past eight years,she has walked hundreds of trails through-out Kings Canyon National Park and theJohn Muir Wilderness, gathering tens ofthousands of data points and puttingtogether an accurate picture of the status ofaquatic systems in the high Sierra. Herinspiration is the wilderness itself and thedrive to provide the information necessaryto manage it properly.

Matthews’ initial research was on marinefishes. She completed postdoctoral studiesat the Canadian Government Pacific Biolo-gical Station, British Columbia, where shestudied lingcod and rockfishes. During herPh.D. studies at the University of Washing-ton School of Fisheries, she focused onrockfishes in Puget Sound.

For her M.A. degree, Matthews studied atMoss Landing Marine Laboratories, throughSan Jose State University, analyzing themovement of fishes on artificial and naturalreefs in Monterey Bay. Her B.S. degree is inzoology from the University of California,Santa Barbara.

Scientist Profile

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Pacific Southwest Research Station

fascinating animal makes it especiallyvulnerable to changes in the aquaticenvironment. “Unlike many frog species,whose tadpole stages last but a fewmonths, the mountain yellow-leggedfrog spends up to four years as atadpole,” notes Matthews. Egg massesare laid in water, and even the adults arehighly aquatic compared to otheramphibian species, she explains.

During the last few decades, scientistsaround the world have detected aworldwide decline in amphibians. Theglobal nature of the reduction both interms of species and overall numbers ledscientists to postulate that whatever wascausing the decline was global in nature.Factors such as increased ultraviolet lightpenetration as a result of stratosphericozone loss, pesticide contaminants, andhabitat loss appeared to play a role, atleast in some areas.

However, the decline of the mountainyellow-legged frog, whose native range only includes the high Sierra and a fewSouthern California locations, has alocal cause.

Continued from front page

Writer Anne M. Rosenthal holds B.S. and M.S. degrees in Biological Sciences from StanfordUniversity and a Certificate in Technical Writing from San Jose State University. A science writerbased in the San Francisco Bay Area, she served as editor of Jasper Ridge Views, a publicationof the Stanford University Jasper Ridge Biological Preserve, for ten years. Her articles have beenpublished in Scientific American On-line, Astrobiology Magazine (NASA), and California Wild(California Academy of Sciences).

Unless otherwise noted, all photos taken by K. Matthews

Photo: Karen Pope

Yellow-legged Frog Research Study Area

John Muir Wildernessfish stocking

Kings CanyonNational Park

no current fish stocking

High Sierra

For Further Reading

The United States Department of Agriculture (USDA) prohibits discrimination in all its programs andactivities on the basis of race, color, national origin, gender, religion, age, disability, political beliefs, sexualorientation, and marital or familial status. (Not all prohibited bases apply to all programs.) Persons withdisabilities who require alternative means for communication of program information (Braille, large print,audiotape, etc.) should contact USDA’s TARGET Center at: 202-720-2600 (voice and TDD) To file acomplaint of discrimination, write: USDA Director. Office of Civil Rights, Room 326-W, Whitten Building,14th & Independence Avenue, SW, Washington, DC 20250-9410, or call: (202) 720-5964 (voice orTDD), USDA is an equal opportunity provider and employer.

Pacific Southwest Research StationUSDA Forest ServiceP.O. Box 245Berkeley, CA 94701

What’s NextTo restore the mountain yellow-legged frog successfully scientists need to knowmore about the frog’s life history and the habitat requirements for each of its lifestages. With this in mind, Matthews is currently focusing on the basic ecology ofthe mountain yellow-legged frog. Her research team is studying the species innational park areas where trout are less abundant and the natural ecosystem isrelatively undisturbed. The scientists’ goal is to understand how the frog interactswith its environment in a more natural state.

One aspect of Matthews’ work is discovering how far the frogs travel. By taggingfrogs with small electronic chips, the team can find individual frogs repeatedly.This helps the scientists follow the frogs’ movements, define their home ranges,and determine how long the frogs live.

The Draft Conservation Strategy for Mountain Yellow-legged Frogs envisions aSierra-wide plan to manage high-elevation lakes, with national and state agenciesinvolved in returning many lakes to a natural fishless habitat.This approach would help protect many native species,including the mountain yellow-legged frog. Future researchwill include other amphibians in decline throughout theSierra, such as the Yosemite toad, foothill yellow-leggedfrog, and red-legged frog.

BBllaauusstteeiinn,, AA..RR..;; WWaakkee,, DD..BB. 1995. The puzzle of decliningamphibian populations. Scientific American 272: 52-57.

DDrroosstt,, CC.. AA..;; FFeell lleerrss,, GG..MM.. 1996. Collapse of a regional frog faunain the Yosemite area of the California Sierra Nevada, USA.Conservation Biology 10: 414-425.

GGrr iinnnneellll ,, JJ..;; SSttoorreerr,, TT..II . 1924. Animal life in the Yosemite. Berkeley:University of California Press.

KKnnaapppp,, RR..AA..;; MMaatt tthheewwss,, KK..RR.. 1998. Eradication of non-native fish from asmall mountain lake: gill netting as a non-toxic alternative to the use ofrotenone. Restoration Ecology 6: 207-213.

KKnnaapppp,, RR..AA..;; MMaatt tthheewwss,, KK..RR.. 2000. Non-native fish introductions and thedecline of the mountain yellow-legged frog from within protected areas.Conservation Biology 14: 428-438.

KKnnaapppp,, RR..AA..;; MMaatt tthheewwss,, KK..RR..;; PPrreeiisslleerr,, HH..KK..;; JJeell lliissoonn,,RR.. 2003.Developing probabilistic models to predict amphibian site occupancy in apatchy landscape. Ecological Applications 13: 1069-1082.

KKnnaapppp,, RR..AA..;; MMaatttthheewwss,, KK..RR..;; SSaarrnneell llee,, OO.. 2001. Resistance andresilience of alpine lake fauna to fish introductions. EcologicalMonographs 71: 401-421.

MMaatttthheewwss,, KK..RR.. 2003. The response of the mountain yellow-legged frog,Rana muscosa, to short distance translocations. Journal of Herpetology 37:621-626.

MMaatttthheewwss,, KK..RR..;; KKnnaapppp,, RR..AA.. 1999. A study of high mountain lake fishstocking effects in Sierra Nevada Wilderness. International Journal ofWilderness 5: 24-26.

MMaatttthheewwss,, KK..RR..;; KKnnaapppp,, RR..AA..;; PPooppee,, KK..LL.. 2002. Garter snakedistributions in high elevation aquatic ecosystems: Is there a link withdeclining amphibian populations and nonnative trout introductions? Journalof Herpetology 36:16-22.

MMaatttthheewwss,, KK..RR..;; PPooppee,, KK..LL. 1999. A telemetric study of the movementpatterns and habitat use of Rana muscosa, the mountain yellow-legged frog,in a high-elevation basin in Kings Canyon National Park, California. Journal ofHerpetology 33: 615-623.

PPooppee,, KK..LL..;; MMaatttthheewwss,, KK..RR.. 2001. Movement ecology and seasonaldistribution of mountain yellow-legged frogs in a high elevation Sierra Nevadabasin. Copeia 2001: 787-793.

PPooppee,, KK..LL..;; MMaatttthheewwss,, KK..RR.. 2002. Influence of anuran prey on thecondition and distribution of Rana muscosa in the Sierra Nevada.Herpetologica 58: 354-363.

WWaakkee,, DD..BB.. 1991. Declining amphibian populations. Science 253(5022): 860.

Published byPacific Southwest Research Station, USDA Forest Service

800 Buchanan Street, Albany, CA 94710 • 510-559-6300 • http://www.fs.fed.us/psw

0Frog with pit transmitter tag