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©Amphibian &Reptile Conservation Amphibian&Reptile Conservation(Fall1996), 1(1):i-9
Printedin the UnitedStates.Allrightsreserved
The Elucidation of Amphibian Declines
Are Amphibian Populations Disappearing?
Jamie K. Reaser
CenterforConservationBiology, DepartmentofBiologicalSciences, Stanford University, Stanford, Cali-
fornia94-305-1901
Key Words
Population declines, extinction, conservation, research, monitoring, amphibians
Abstract
Information regarding mostam- impactonthestateoftheenviron- amphibians, and at what spatial
phibiandeclines isanecdotaland ment, as well as a decline in our andtemporalscales.Wealsoneed
naturalfluctuations in amphib- culturalheritageandhumanwell- toknowwhatthe loss ofamphib-
ian population size are not un- being. Both biologists and con- iansmeansforthecontinuedfunc-
common. However, biologistscan cernedcitizenshavevitalrolesin tioningofecosystems. Ultimately,
no longerfindamphibians in re- arnphibian conservation. A brief we need toknow whatthe disap-
gions where they were once nu- list ofpossible citizen actions to pearance ofamphibians signifies
merous, and have directly ob- helpprotectamphibiouslife ispre- forhumanwell-being.
served population declines and sentedhere.
speciesextinction. Inventoryand
The paucity of data
monitoring programs are being
establishedworldwidein orderto Amphibian populations world- Unfortunately, information re-
assess the status ofamphibian wide seem to be declining. Even gardingmostamphibiandeclines
populations and to attempt to the causal observer can not find is anecdotal. Forseveral species,
identify causes ofdeclines. Fac- frogs, toads, and other amphib- range reductions are well docu-
tors that may be contributing to ians as numerously as they once mented, but local population de-
localamphibiandeclinesinclude could. Within the few short de- clines are less evident. For most
naturalpopulationfluctuations, cades ofourlifetimes, a wide va- species,studiesonlyprovidefrag-
natural succession and other rietyofamphibians seemto have mentary pictures ofpopulations
changesinvegetation, introduced been disappearing. Population onpopulationtrends.
predatorsandcompetitors,patho- declines and species extinction Researchonamphibianecol-
gens, excessive collecting, toxic dotthepagesofpersonaljournals. ogyhashistoricallylaggedbehind
compounds,andhabitatdestruc- Biologistsnowsearchharderthan that of other vertebrate groups
tion. Climatedisturbance,pollut- ever, asking"why" andhopingto becauseamphibiansareoftendif-
ants, and increases in UV-B ra- find the answers so that future ficult to study and funding is
diation have been implicated in generations don't have tobe told hardertoobtain.Thoseconcerned
some well documented regional whatthey are missing. about the loss ofNeotropical mi-
amphibian losses. Thesefactors Findinganswersisnoeasier gratorybirdscanreferencebroad
maydecreaseamphibianpopula- thanfindingfrogs.Thereisample ranging, standardized datasets
tionsizebycausingmassmortal- evidence that humankind has from numerous monitoring pro-
ity, reducing the ability ofindi- greatlyimpactedthedistribution grams. Several ofthese datasets
viduals to produce viable off- and abundance of animal and spanmultipledecades;onewasini-
spring, and/or by inhibiting dis- plant species worldwide through tiated as earlyas 1900. However,
persal ofindividuals. A loss of extensive habitat alteration and long-term(decadeorlonger)moni-
amphibianswillhaveasignificant degradation. Such observations toring programs exist for only a
can be made daily, and by non- fewamphibianspeciesandonlyat
fCaoxr:rUe1sp5o)nd72e3n-c5e9,20t,el:e-(mbali5l):7j2k5r-9t9o1a5d,® bhioowleovgiesrt,s.isWhhoawtswpeecnifeiecdltaondknuoswe,s tsipoecnidfiycnasimtiecs.sAcmapnhtiypbiicaalnlpyobpeuldae--
leland.Stanford.edu impactthepopulationdynamicsof scribedas"boomorbust;"natural
FEATURE
6 Jamie K. Reaser (Scientific)
fluctuationsmaybetherulerath- spring, the long-term population finitivelyassessed.
erthanthe exception. Thus, only trend may be stable. Long-term Amphibianscannotbefound
very long-term datasets are use- stability may also be attained if inmanyofthelocationswherethey
fulinvalidatingsuspectedtrends amphibians from other locations were once numerous. Yellow-leg-
andelucidatingthemechanismsof recolonize sites where popula- gedfrogs,red-leggedfrogs,spot-
amphibianpopulationdeclines. tions havebeen annihilated. For ted frogs, leopard frogs, western
Thefirstworldwideeffortto example, Norman Weitzel and toads,cricketfrogs,andtigersala-
assimilate data and hypothesize Howard Panik observed Pacific manders are a few ofthe North
the causes and consequences of chorusfrogs(Pseudacrisregilla) American amphibians dwindling
amphibian declines was held in in Nevada and found that in 80% in the number of sites of occur-
Irvine, California in 1990 (Blau- of the years between 1975 and renceandpopulationsize.Theam-
stein and Wake 1990). Since this 1989frogsproducedoffspringthat phibianqueueforlistingunderthe
meeting,aninternationalinvesti- becamemembersofthenextgen- United States' federal Endan-
gatory team, entitled the Declin- eration.Thepopulationwasanni- gered Species Act has become so
ing Amphibian Populations Task hilatedfromthebreedingpondten longthatspeciesruledasjustified
Force (DAPTF) of the Species timesbynaturaldisturbancesthat forprotectionareprecludedfrom
Survival Commission (SSC), In- included flashfloods,streamdry- itforyearsbystacksofpreceding
ternational UnionfortheConser- ups,andsuddenincreasesinwater paperwork.
vationofNature(IUCN),hasbeen temperature. Yet,aftereachlocal Herpetologists have wit-
formed andworkinggroups have extinction event, this population nessedthevanishingofamphibian
beendesignatedtoaddresspoten- wassoonreestablishedbycoloniz- populations,andevenentirespe-
tial causes {e.g., toxins, UV-B ra- ingchorusfrogs. cies.BiologistsStephenCornand
diation, pathogens) and geogra- Extremely long-term data- JamesFoglemanconductedanex-
phicregions.Thefindingsofwork- setsarerequiredtodistinguishbe- ceptionalstudy,documentingsix
ing groups and individual scien- tweennaturalpopulationfluctua- populations of the leopard frog
tists are published quarterly in tions and anthropogenically in- (Ranapipiens)inColoradoforthe
the DAPTF newsletter, Froglog, duceddeclines.JosephPechmann decade 1973-1982. In 1973onlya
makinginformationreadilyavail- and his colleagues monitored singlepopulationfailedtorepro-
able.Conservationorganizations, populations ofone species offrog duce.Forfrogsthismaynotbeun-
naturalist societies, and regional and three species of aquatic- usual,butby1981noleopardfrogs
agencies have been successful in breeding salamanders in the couldbefoundatanyofthesites.
establishing local amphibian in- southeastern U.S.fortwelvecon- Thestudyendedwithatotalab-
ventoryandmonitoringprograms secutiveyears.Theyfoundnoevi- senceofR.pipiensintheregion.
thatofteneffectivelyutilizeamas- dence ofdrastic declines for any The golden toad (Bufo per-
sivevolunteerworkforce.County- species,althoughthepopulations iglenes) ofCosta Rica, so named
and continent-wide initiatives, sizes did fluctuate. However, bi- because of the male's bright or-
suchasthe North AmericanAm- ologists Michael Reed and An- angecolor,neverfailedtoshowup
phibian Monitoring Program drew Blaustein recently reana- foritsannualspringbreedingorgy
(NAAMP),areinvariousstagesof lyzed Pechmann's data as well as (see Figure 1, page 4) from the
design, testing, and implementa- that from four other long-term early 1970sthrough 1987. Martha
tion. studies using a statistical tool Crumpandhercolleaguescounted
called power analysis. All these 1500individualadultgoldentoads
What we do know studies,analyzedbythismethod, in 1987,butnotedthatonly29tad-
indicated no declines. While the poles metamorphosed andjoined
Amphibianpopulationscanfluctu- datasetsfromthese studieswere thepopulation.From1988to 1990
ategreatlybetweenyears;varia- not extensive enough to reveal these biologists located only 11
tionsinmoisture,predation,com- statistical evidence of a decline, toads.Bufoperigleneshasnotbeen
petition,disease,andcatastrophic the lack ofdecline in populations seenatthe studysite since.
eventsmaygreatlyinfluencepopu- ofthese amphibians could not be In Australia, just north of
lationsize.Populationsmaysuffer supported.Thus,evenwithtwelve Brisbane,abizarrelittlefrogwas
greatlosses.Yet,ifthesamepopu- yearsworthofscientificallyrigor- discovered in 1973. The gastric
lations experience "good years" ous data, the status ofthese am- brooding frog (Rheobatrachus
thatresultinmanysurvivingoff- phibianpopulationscannotbede- silus), so named because it swal-
FEATURE (Scientific) The Elucidation ofAmphibian Declines 7
lowedandbroodeditsyounginits in UV-B radiation have been im- 2) Reduction ofFitness
stomach, was animmediate won- plicatedinsomewelldocumented hypothesis
dertoscienceandapotentialboon regionalamphibianlosses (seere-
forphysiologistsinterestedinfind- views by Barinaga 1990; Wyman One or more sublethal factor(s)
ing cures for ulcers and possibly 1990; Blaustein and Wake 1990; reduces the ability ofindividual
amphibianstoproduce viableoff-
othergastricdisordersinhumans. Tyler 1991; Phillips 1994; Blau-
Aloss ofworldwide significance, steinandWake 1995). spring(i.e.,"fitness").Thiseventu-
ally leads to population declines
the frog has not been seen since Unfortunately,theremaybe
andevenpopulation-andspecies-
1979,leavinglittleclueastowhat asignificanttimelagbetweenthe
levelextinction.Geneticvariation,
caused its extinction. Strangely negative influence of a factor on
growthrate,sizeatmaturity,lon-
enough, its natural habitat was amphibians and evidence of a
gevity, and physiological con-
foundinseeminglypristinetropi- populationdecline.Prospectingfor
calforest,farfromroutinehuman cause and effect relationships is, satmrpahiintbsiaalnlsi.nflSuoemnceetahmepfhiitbneisasnosf
disturbance. therefore, exceptionally difficult
have such specific conditions for
evenincontemporarystudies.Di-
breedingthatevensubtleenviron-
agnosisisfurthercomplicatedbe-
mental changes can result in the
Why are amphibians cause factors can act in concert failure ofapopulationtobreed.
andtheirrelationshipisrarelyob-
declining?
vious. For example, frogs have
Couldtheabovementioned frogs been observed to die ofan infec- 3) Failure to Rescue
havesuccumbedtonatural,local- tion caused by a common, wide- hypothesis
izedclimaticdisturbancessuchas spreadmicrobecalledAeromonas
Theobserveddeclinesareprima-
drought ...beenthevictimsofop- hydrophilla that is not normally rilydrivenbythefailuretorees-
pforrotmunaisyteitc, puantdehtoegcetnesd...glsoubfafleraetd- fpaactthoorgsenmiacy. iAnnhiybintufmrbogesr'iofmmotuhneer tablish populations following lo-
cal extinction. Typically, when a
mospheric phenomenon? Could systems,makingthemsusceptible
localpopulationgoesextinct,the
the little known golden toad and toinfection.
habitat is colonized by amphib-
gastric brooding frog merely be Three general hypotheses
ians dispersing from nearby
sittingitoutunderground,await- illustrate the mechanisms by
ing what they consider more fa- which various factors can cause sources (this reestablishment is
termedthe"rescueeffect").Under
vorableconditions? theextinctionofamphibianpopu-
this scenario, changes in the
Itisdifficulttogeneralizeas lations,andeventuallyspecies:
chemical or structural environ-
tothecausesofamphibiandisap-
ment prevent amphibians from
pearances. Noteveryamphibian
dispersingwidely.
population, norevery species, is 1) Mass Mortality hypothesis Severalbiologicalcharacter-
declining.And,thosethatarede-
clining are doing so at varying Afactororcombinationoffactors istics ofamphibians are likely to
impede recolonization following
rates and scales. Figuring out influences amphibians in such a
whatishappeningtoamphibians manner as to induce mortality of local extinction: (1) physiological
andwhyisexasperatinglydifficult individuals, sometimes entire lqiumiirtaetmieonnsts()parmtaickuelariltydwiaftfiecrulrte,-
andexhaustinglytimeconsuming. populations. Different factors
even impossible, for amphibians
Locally,factorsthatmaybe may contribute to mortality at
to persist in or travel through
contributing to amphibian de- varying points in amphibian de-
suboptimal habitat; (2) amphib-
clines include natural population velopment. However, the decline
ians tend to have small home
fluctuations, natural succession ofmanypopulationsisnotmerely ranges,manymoveonlyshortdis-
and other changes in vegetation, aproblemofproducingviableoff-
tances, and rarely "wander"; and
introduced predators and com- spring. It is apparent that some
(3) amphibians, especially the
petitors,pathogens,excessivecol- factors are influencing adult sur- adults of many species are ex-
lecting,toxiccompounds,andhabi- vivorship because many of the
tremelyfaithfultoaspecificloca-
tat destruction. Climate distur- rapid declines are occurring in
tion,orsetoflocations,andareun-
bance, pollutants (particularly periods far shorter than the ani-
likelytoabandonsitesevenifthey
thoseassociatedwithaciddeposi- mals' life span.
cannolongerbreedthere.
tionandpesticides),andincreases
FEATURE
8 Jamie K. Reaser (Scientific)
The specific means {e.g., vital components of the world's You can make a difference
physiologicalprocesses)bywhich ecosystems.Amphibianscomprise
Approximately5,000amphibians
thesegeneralmechanismsoperate one-quarterofallvertebratespe-
have been described by science,
are not well understood and are cies on earth and sometimes con-
withadditionaldescriptionsbeing
rarelyinvestigated.Inpart,thisis stitute the highest percentage of
due to the fact that solving the vertebrate biomass in a given cataloged at a rate of 1 to 2 per-
puzzlerequiresthecooperationof area. This measure may be posi- icemnmteaasuyeraarb.leT;hewreatdeono'ftlkosnsoiws
experts across disciplines as di- tively correlated with a species' howmanyamphibianshavecome
verseasgeologyandgenetics.Re- contribution to ecosystem func-
and gone without recognition.
gretfully,mostbiologistsarehighly tion; i.e.,itisoneindicationofthe Theamphibiandecline"cri-
specialized and rarely trained or organisms' importance to main-
ses" demands that the status of
encouraged to work with col- tainingthesystem'sintegrity.Am-
amphibianpopulationsberapidly
leagues from other fields. When phibiansconsumeaquaticvegeta-
assessedandthatwheredeclines
theyhave,however,piecesofthe tion,invertebratesandotherver-
areapparent,mechanismsbeiden-
puzzlefallintoplace.Forexample, tebrates,andareeatenbynumer-
tified,managed,andrecoverypro-
and interdisciplinaryteaminOr- ouspredators.Therefore,amphib-
egon led by Andrew Blausteinis iansplaymultiple,vitalrolesinthe mgroarmeseeassitlayblsitsahtede.dTthhiasniascmcuocmh-
nowabletoillustratehowstrato- food chainofecosystems.
plished. There are far more am-
sphericozonedepletionmaylead Amphibians are apparently
phibiansthanbiologistsinvestigat-
toamphibianpopulationdecline.It decliningeven in seemingly pris-
ingtheirdeclines.Fundingishard
workslikethis:Ultraviolet-bradia- tine, protected areas worldwide.
to come by, particularly for the
tion penetrates the Earth's thin- Because of these trends, many
long-termstudiesthatarecritical
ning,protective ozone shield and biologistsareponderingwhether
tounderstandingamphibianpopu-
beams its way to earth where it amphibian declines should be in-
lationdynamics.Also,timeisnot
comes in contact with amphibian terpreted as a warning signal;
onthesideoftheamphibianpopu-
eggs. The high-level and/or pro- that is to say that the disappear- lationdynamics—humanpopula-
longedexposuretoradiationcaus- anceofamphibiansindicatesthat
esdamagetoeggs' DNA(thege- somethingisgravelyamissinthe tion and resource consumption
continue to increase, rapidly
netic information template) mol- biosphere. Because amphibians
changingthelandscapethatam-
ecules,whichinturnresultsinthe have permeable gills, skin, and
phibianshavebeenevolvinginfor
deathofcellsandthustadpolesdo eggs; have diverse life histories;
roughly350millionyears.
notdevelop.Asadultfrogsdieand arewidelydistributedandoccupy
are not replaced by new genera- a variety ofhabitats, their popu- Yes,there is hope. Amphib-
ian populations have rebounded
tions,thepopulationdeclinesand lation dynamics may qualify as
and sites have been recolonized
eventuallygoesextinct.Blaustein reliable gauges ofenvironmental
andtheotherinvestigatorsfurther health (ifonly we can learn to in- followingmassive die-offs. Main-
tenance and recovery ofenviron-
learnedthatdifferentamphibian terpretthesignals).
mental quality, and the restora-
species have varyingamounts of Frogsaretotemsofluckfor
tionoffragmentedlandscapeswill
photolase,anenzymethatcanre- numerous native culture; many
DNA enable amphibians to persist.
pair damage.Thedeclining huntingpoisons,ceremonialhallu-
Asacitizenconcernedabout
Cascadesfrogislowinphotolase, cinogens,andmedicinaldrugsare
amphibians, yourrolein amphib-
whilethecoexistingandsuccessful amphibianproducts. Amphibians
DNA ian conservation is as critical as
Pacificchorusfroghasgood are chemical factories and the
repaircapabilities;extracopiesof compounds they produce may thatofanyhighlytrainedbiologist.
Thefollowingisaverybrieflistof
photolasegenessecureprotection. hold cures to all sorts of ills, in- the many actions that you can
cluding AIDS and cancer. If
take to help protect amphibians,
you've had painkillers adminis-
The impact of amphibian tered recently, you may have a and maintain their vital roles in
declines frogortwotothank.Foranexcel- the circle oflife.
A
loss ofamphibians will have a lentreviewofamphibiancontribu-
• Becomeavolunteerassistantfor
significant impact on the state of tions to medicine, see Grenard's
alocalamphibianmonitoringpro-
theenvironment.Amphibiansare (1994)MedicalHei-petology.
FEATURE (Scientific) The Elucidation ofAmphibian Declines 9
gramorresearchproject. Contact References (Pseudacris regilla) in northwestern
your regional wildlife agency for Nevada. Great Basin Naturalist 53:
Barinaga, M. 1990. Where have all the 379-384.
information on studies in your froggies gone? Science 24: 1033-1034.
area. Wyman, R.L. 1990. What's happeningto
Blaustein, A.R. 1994. UV repair and the amphibians? Conservation Biology
• Enlighten other people to the reegsgiss:taAncelitnok stoolapropUuVl-aBtiionnadmepchliibnieasn? 4: 350-352.
wonders andplight ofamphibians Proceedings ofthe NationalAcademy of
by harnessing your enthusiasm Sciences (USA) 91: 1791-1795.
While the Bird. Conservation Specialist
and knowledge. Talk to children, for the Smithsonian Institution's Mi-
Blaustein, A.R. and Wake, D.B. 1990.
vtohteimnegdpiuab,lilco.cal officials, and the pDehcelnionimnegnaomnp?hibTiraennpdospuilnatEiocnosl:oAgyglaobnadl gcTrohaaetuotBrhiyordrBsei:drWdhheCraeftnitreYsrot,ubJoCaoakmn,ieDBroKi.nTRgoeBaSaascveker
Evolution 5: 7. Threatened Species (Stackpole Press
• Support legislation that pro- 1995). In 1993 Dr. Paul Ehrlich, pre-
motes healthy, intact ecosystems. Blaustein,A.R.andWake, D.B. 1995.The sented Jamie with the opportunity to
puzzleofdecliningamphibianpopulations. pursue her interest in amphibians
Scientific American 272(4): 52-57. throughformal investigation as his
• Fight legislation that weakens doctoral student at Stanford Univer-
control of pollution and land de- Corn, PS. and Fogleman, J.C. 1984. Ex- sity. Both Jamie and Dr. PaulEhrlich
velopment. tinction ofmontane populations ofthe are interested in what amphibianpop-
northern leopard from (Rana pipiens)
ulation trends might indicate about the
in Colorado. Journal ofHerpetology 18: health status ofthe biosphere.
• Encourage government agen- 147-152. OneofJamie'sgreatestrewardsof
cies to fund long-term research herworkwithamphibiansistheoppor-
projects on amphibians. Crump, M.L., Hensley, F.R., and Clark, tunity to mentorundergraduate stu-
K.L. 1992. Decline ofthe golden toad: dentsivhoareinterestedinamphibian
Underground or extinct? Copeia 1992: ecologyandconservation.Anopenin-
• Respectyourwetlandsbykeep- 413-420. vitationisextendedtothoseinterested
ing them healthy. Do not pollute tovisitheratStanford University.
them with unnatural refuse such Grenard, S. 1994. Medical Herpetology.
Reptile and Amphibian Magazine, Potts-
as litter and harmful chemicals ville, Pennsylvania. 139 pp.
{e.g.,petroleumproducts andpes-
ticides). Pechmann,J.H.K.andWilbur,H.M. 1994.
Putting declining amphibian populations
in perspective: Natural fluctuation and
• Organize routine cleanup pro- human impacts. Herpetologica 50: 65-
jects. 84.
• Admire amphibians in the wild; Pechmann, J.H.K., Scott, D.E., Sem-
don't keep them as pets (animals laintdschG,ibRb.oDn.s,,CJa.lWd.we1l9l9,1.J.DPe.c,lViintitn,gL.aJm.-,
kept for research, in legitimate phibian populations: The problem ofsep-
conservation breeding projects, arating human impacts from natural
and as educational displays such fluctuations. Science 253: 892-895.
as in zoological parks and aquari- Phillips,K. 1994. TrackingtheVanishing
ums are not considered pets and Amphibians. St. Martin's Press, New
contribute to the conservation of York. 244 pp.
species).
Reed, J.M. and Blaustein, A.R. 1995.
Assessmentof"non-declining"amphibian
Byjoiningforces,biologistsandcon- populations using power analysis. Con-
cerned citizens around the world servation Biology. In Press.
can become avery powerful lobby
Tyler, M.J. 1991. Declining amphibian
fortheconservation ofamphibians. populations—a global phenomenon? An
And amphibians, inventoried and Australian perspective. Altyes 9: 43-50.
monitoredbythesepeople, maybe
Weitzel, N.H. and Panik, H.R. 1993.
a powerful gauge for ensuring the
Long-term fluctuations of an isolated
protection ofall life. population of the Pacific chorus frog
t>«-
