12 Conservation of Small Populations: Effective Population Sizes, Inbreeding, and the 50/500 Rule Luke J. Harmon and Stanton Braude Introduction and Background Populationsizeisextremelyimportantinevaluatingconservationprioritiesforaspecies. Small populations are at risk of going extinct because of demographic stochasticity and geneticdrift.inthisexercise,youwilllearnaboutthreeofthemeaningsof effectivepopulationsize andhowtoestimatetwoofthem.youwillthenlearnhowtoapplythesetechniquestospecificconservationsituations,usingtheconceptsofinbreeding,theminimum viablepopulationsize,andthe50/500rule. Effective Population Size Populationsizehasamajorimpactonthedynamicsofapopulation.Forexample,inchapter11youusedsimulationstoseethatgeneticdriftreducesallelicdiversitymuchfasterin smallpopulationsofwogglesthaninlargeones.populationsizealsoinfluencesthechances of extinction through demographic stochasticity, the random change in population size overtimeduetorandomvariationinindividualsurvivalandreproductivesuccess.such eventshaveaproportionallylargeeffectinsmallpopulations.forexample,inapopulationof10individuals,oneaccidentaldeathwouldreducethepopulationsizeby10%.in contrast,ifthepopulationweremadeupof1000individuals,oneaccidentaldeathwould reducethepopulationsizebyonly0.1%.thus,smallpopulationsaremuchmorelikelyto goextinctduetodemographicstochasticitythanarelargepopulations. Effectivepopulationsize(N e )helpsusquantifyhowaparticularpopulationwillbeaffectedbydriftorinbreeding.effectivesizetakesintoaccountnotonlythecurrentcensus size of a population, but also the history of the population. Effective population size is thesizeofan idealpopulation oforganisms(idealreferstoahypotheticalpopulation inthehardyweinbergsensewithaconstantpopulationsize,equalsexratio,andnoimmigration,emigration,mutation,orselection)thatwouldexperiencetheeffectsofdriftor inbreedingtothesamedegreeasthepopulationwearestudying.forexample,ifouractual populationof50animalsexperiencestheeffectsofdriftatthesamerateasanidealpopulationof20animals,thepopulationhasadrifteffectivesizeof20. Thereisnosuchthingas the effectivesize ofapopulation.differenteffectivepopulationsizeshelpusestimatetheimpactofdifferentforces.theeffectivesizeyouestimatewill dependonthescientificquestionyouaretryingtoaddress(box12.1).estimatingtheappropriateeffectivepopulationsizeiscrucialinconservationbiology;inmost(butnotall)
126 chapter 12 Box 12.1 Different Ways to Measure Effective Population Size Thereareavarietyofpopulationeffectivesizesthathavedifferentmathematicaland biologicalmeanings.thetermsaresometimesconfusedormisunderstoodassynonymous. Suchconfusioncanhaveseriousimplicationsforunderstandingandmanagingpopulationsofendangeredorthreatenedspecies,asweseebelow. Inbreeding effective size, Nef,referstothesizeofanidealpopulationthatwouldallowthe sameaccumulationofpedigreeinbreedingastheactualpopulationofinterest.pedigree inbreedingoccurswhenanoffspringinheritstwocopiesofagenefromitsparentswhich areidenticalbydescent thatis,theyarebothdirectlydescendedfromasingleallelepresentinoneofthefoundersofthatpopulation(perhapstheparentsarecousinsandeach inheritedtheparticularallelefromthesamegrandfather).nef isthemeasureofeffective populationsizethatemphasizestheeffectthatsmallpopulationsizehasonthechancesof relativesmatingwitheachother.suchmatingsleadtoalossofheterozygosityinthepopulation.thus,thiseffectivesizegivesyouanindicationofthelikelylossofheterozygosity acrossallallelesinyourpopulation. Calculation of N ef ideally requires pedigree data. However, you can estimate the inbreedingeffectivepopulationsize(nef)bycalculatingtheharmonic mean ofthepopulation sizeovertimefromthefoundinggenerationtothepenultimategeneration.thesymbolt representsthenumberofgenerationsforwhichwehavepopulationsizedata.n(0) isthe sizeofthefoundingpopulation,n(1) isthesizeofthepopulationafteronegenerationetc. and,n(t 1)isthesizeofthepopulationonegenerationago. t (a)n ef = 1 1 1 +...+ N(0) + N(1) N(t 1) Variance effective size, N ev,referstothesizeofanidealpopulationthatwouldaccumulatethesameamountofvarianceinallelefrequenciesasthepopulationofinterest;thus, thiseffectivepopulationsizeindicateshowrapidlyallelefrequenciesarelikelytochange. Thisisimportantbecauseitalsoaffectshowrapidlyisolatedpopulationsdivergefromone anotherundergeneticdrift.again,thesymbolt representsthenumberofgenerationsfor whichwehavepopulationsizedata.n(1) isthesizeofthepopulationafteronegeneration, etc.,andn(t)isthesizeofthecurrentpopulation. t (b)n ev = 1 1 1 +...+ N(1) + N(2) N(t ) Inaddition,thefollowingcorrectioncanbeusedateachgenerationifoperationalsexratios arenot1:1.thiscorrectedpopulationsizereflectstheincreasedeffectsofbothinbreeding anddriftwhenthesexesarenotcontributingequallytotheallelepool. 4Nm Nf (c)n s = Nm+Nf (continued on following page)
population effective sizes 127 Noticethedifferencebetweenformula(a)andformula(b).Whiletheinbreeding effective size ismoresensitivetothenumber of original founders [N(0)],thevariance effective size ismoresensitivetothenumber of offspring in the current generation [N(t)].Thisis because,asstatedabove,n ef focusesonthelossofheterozygosityduetopedigreeinbreedinginthepopulation;withasmallinitialfoundingpopulation,closerelativesarelikely tomatewitheachother.incontrast,n ev givesanindicationoftheincreaseinvariance ofallelefrequenciesbetweensubpopulationsduetodrift,anddependsonthenumber ofoffspringproducedbythosefoundersandbyeachsubsequentgeneration,uptothe present-dayn(t). Thesedifferencescanleadtolargediscrepanciesbetweenthesetwodifferenteffective populationsizesinrealpopulations.forexample,increasing populations generallyhave alargernev thannef,whiledeclining populations willgenerallyhavelargernef thannev. Hence,apopulationcomingthroughabottleneckmayhavealowinbreedingeffective size,butitcanhavealargervarianceeffectivesizeifthepopulationbouncesbackrapidly (asisthecasewiththesouthernwhiterhinocerospopulation,whichyouwillworkon inclass). Thereareothermeasuresofeffectivepopulationsizethatfocusondifferentpopulation geneticparameters.forexample,eigenvalue effective size, Neλ,focusesontherateatwhich uniqueallelesarelostfromapopulation.forthisexercise,wewillexamineonlythetwo effectivepopulationsizeestimatesdiscussedabove. cases,effectivepopulationsizewillbesmallerthantheactualnumberoforganismsinthe population.thinkforamomentaboutwhythisisso.aconservativeruleofthumbused bysomebiologistsisthatn e isusuallyaboutone-fifthofthetotalpopulationsize(mace andlande,1991).usingsucharoughestimateisriskybecausen e canbelarger thanthe censussizeofthepopulation,dependingonthehistoryofthepopulationandtheparticular N e underconsideration. Demographic stochasticity, genetic drift, and environmental variation can interact todoomasmallpopulationtoextinction.thisiscalledanextinctionvortex,anditis duetoapositivefeedbackloop(figure12.1):thenegativeconsequencesoflowereffective populationsizemakethepopulationsmaller,causingstrongernegativeeffects,leadingto anevensmallerpopulationsize(gilpinandsoule,1986).forexample,arandomenvironmentalchangemightlowerpopulationsize,leadingtoahigherchanceofpopulation reductionduetodemographicstochasticity.thiscouldlowerinbreedingeffectivepopulationsizeevenmore,leadingtosevereinbreedingdepressionandreducedfertility.this furtherreducesthepopulationsize.chainsofeventssuchasthesemeanthattheextinctionprobabilityforasmallpopulationcanbeextremelyhigh.forexample,pimmetal. (1988)showedthattheextinctionriskforbirdsonsmallislandsoffthecoastofBritain rises with decreasing numbers of nesting pairs. Conservation biologists realize that an extinctionvortexcanbeginwhenhumanscausemajorreductionsinthepopulationsize ofaspecies. Calculating Effective Population Sizes Considerthedataintable12.1forapopulationofEasternfencelizards,Sceloporus undulatus,attysonresearchcenterineasternmissouri:youcanestimatetheinbreedingeffective
128 chapter 12 Environmental variation Catastrophic events Global climate change More genetic drift; less ability to adapt More inbreeding; depression Population more subdivided by fragmentation More demographic variation Lower effective population size (N e ) EXTINCTION Habitat destruction Environmental degradation Habitat fragmentation Overharvesting Effects of invasive species FIgure 12.1. Extinctionvortex. Populationsizedecreasesinapositivefeedbackloop,eventually resultingintheextinctionofthepopulation(fromprimack,2000). sizeforthispopulationusingformula(a)frombox12.1.inthiscase,t =4generations;we calculatetheinbreeding effective population size as: Nef t 1 1 1 1 + + + N(0) N(1) N(2) N(3) 4 4 = = =68.9 69 1 1 1 1 0.058 + + + 140 250 110 26 (12.1) Trythiscalculationyourself itcangetconfusingtakingallofthesereciprocals! Thisestimatedeffectivepopulationsize(69)meansthat,intermsofgeneticinbreeding, thispopulation(withameancensussizeof148lizardsover5years)willaccumulatethe Table 12.1. Sceloporus populationattysonfrom1996to2000. Year 1996 1997 1998 1999 2000 Populationcount 140 250 110 26 180
population effective sizes 129 effectsofinbreedingatthesamerateasapopulationthathadaconstantsizeofonly69 individuals. Incontrast,thevariance effective size,estimatedwithformula(b)inbox12.1,is Nev t 1 1 1 1 + + + N(1) N(2) N(3) N(4) 4 4 = = =70.73 71. 1 1 1 1 0.05655 + + + 250 110 26 180 (12.2) Thiseffectivepopulationsize(71)meansthat,intermsofgeneticdrift,thispopulation (withameansizeof148over5years)willaccumulatetheeffectsofdriftatthesamerate asapopulationthathadaconstantsizeof71individuals.notethat,inthisexample,the meansizeofthepopulationovertimereflectsneitherhowitwillaccumulateinbreeding norhowitwillexperiencedrift.althoughthedifferencebetweenvarianceeffectivesizeand inbreedingeffectivesizeappearssmallinthisexample,thepointisthatthesearenottwo waysofestimatingthe effectivesize;thesearetwodifferenteffectivepopulationsizes.inthe examplesyouworkwithinthisexercise,youwillseehowonepopulationcanhavevery differenteffectivesizesthatinformusaboutverydistinctrisksforthepopulation. Effective Population Size, Inbreeding, and Extinction Justasthereareanumberofdifferentmeaningsofeffectivepopulationsize,therearea numberofdifferentmeaningsofinbreeding.becausebreedingwithcloserelativestypically reducesthepoolofgenescontributingtothenextgeneration,onemeasureofinbreedingis F,inbreedingasameasureofdrift.Inadditiontoincreasingtheimpactofdrift,inbreeding canincreasetheproportionofdeleterioushomozygousgenecombinationsinapopulation, whichleadstolowersurvivalofyoungandthuslowerreproductiveoutput;thisiscalled inbreedingdepression.tobegintoevaluatethepotentialimpactsofsmallpopulationsize oninbreeding,wecanusethefollowingestimation(fromsoule,1980): 1 ΔF=1 1 t. (12.3) 2Nef Inthisequation,ΔF istheincreaseintheinbreedingcoefficientovertime;n ef istheinbreeding effective population size; and t is the number of generations. We can use this equationtopredicthowmuchtheinbreedingcoefficientinasmallpopulationwillincrease over time. If ΔF is 0.6 or higher, the fecundity of individuals in the population may be reduced,andthepopulationcouldbeathigherriskforextinction.forexample,wecan calculatetheincreaseintheinbreedingcoefficientafter100generationsforthepopulation describedabove.forthefencelizards,nefwas69.thismeansthatδf =1 (1 1/138) 100 = 0.52.Wewouldconcludethatafter100generationsthefencelizardsareexperiencingsome inbreeding;however,becauseδf <0.60itmaynotthreatenthepopulation.Ofcourse,the inbreedingcoefficientwillcontinuetoincreaseovertime,possiblytodangerouslevels,if thepopulationremainssmallandisolated.notethattheestimateofδf ignoresgeneflow andevenmoderategeneflowcangreatlyreducetheeffectofgeneticdrift,slowingtherate ofincreaseintheinbreedingcoefficientovertime.soevenmoderategeneflowcanhelp maintaingeneticdiversity.thisisonereasonwhymanyconservationbiologistsadvocate themaintenanceofcorridorsconnectingsmall,isolatedpopulationsofaspecies.
130 chapter 12 Minimum Viable Populations and the 50/500 Rule Youknowthatdemographicstochasticityandgeneticdriftcannegativelyaffectsmallpopulations.Demographicstochasticityleadstotherandomextinctionofsmallpopulations, whilegeneticdriftcancauseareductionofgeneticdiversitywithinapopulation.these factorscaninteractinanextinctionvortex(figure12.1discussedabove)thatcaneventually leadtotheextinctionofapopulation. Todecidewhenthesefactorsmightbeimportantforapopulationofanendangered species,shaffer(1981)proposedtheconceptoftheminimumviablepopulation(mvp). HedefinedtheMVPasthesmallestisolatedpopulation(ofagivenspeciesinagivenhabitat)havinga99%chanceofremaininginexistencefor1,000years,despitetheforeseeable effects of demographic stochasticity, genetic drift, environmental stochasticity (random changesintheenvironment),andnaturalcatastrophes(shaffer,1981).shafferchosethe percentage and time scale to represent what most scientists consider a good chance for survivalofaspecies.quantitativeobjectiveslikethemvpprovidespecificguidelinesfor gaugingthesuccessofconservationprograms(fooseetal.1995).populationssmallerthan themvpareconsideredtobeatsignificantriskofenteringintotheextinctionvortexand becomingextinct,soaconservationprogramcanbeconsideredsuccessfulonlyifitraises theeffectivepopulationsizeabovethemvp. Arelatedconceptisthe50/500rule,proposedbyFranklin(1980).The 50 partofthe 50/500rulestatesthatpopulationswithaninbreedingeffectivepopulationsize(Nef )under 50areatimmediateriskofextinction.Thisisbecause,insuchsmallpopulations,inbreedinganddemographicstochasticitycanquicklypushthepopulationintoanextinctionvortex.The 500 partoftherulemeansthatpopulationswithavarianceeffectivesize(nev ) oflessthan500areatlong-termriskofextinction.inthesepopulations,geneticdriftmay beastrongforce,leadingtoeventuallossofgeneticvariation(franklin,1980).aftervariationislost,thepopulationwillnolongerbeabletorespondtoenvironmentalchanges, andmaybereducedinsizeorgoextinctifanysuchchangesoccur.evenwhenproperly understood,franklin sruleisquitecontroversial.someauthorsquestionitsgenerality,and otherssuggestthatthenumbersaretoosmall.forexample,lande(1995)suggestedthat anypopulationwithnev lessthan5,000willbesubjecttostronggeneticdrift,whichwill depletethegeneticvariationinapopulationandcauselong-termextinctionrisk.recent experimentaltestsoftheruleincaptivehouseflypopulationsalsosuggestthatpopulations mustbehigherthanimpliedbythe50/500ruleinordertosurviveandmaintaingenetic diversity(reedandbryant,2000).asaconservationbiologist,youmaynotusethe50/500 rule,butitisessentialthatyouunderstandwherethe50and500comefrom. Homeworkforthisexercisetakesapproximately45minutes. Case Studies and Data European Adders The European adder(vipera berus) is a small, venomous snake(figure 12.2) distributed throughout Europe (Arnold and Burton, 1978). This snake occupies a wide geographic range,butwithinthatrangesnakesareoftenfoundinsmall,isolatedpopulationsseparatedfromeachotherbyhundredstothousandsofmeters.undernaturalconditions,gene flowbetweenthesesubpopulationsishigh,becausethemalesnakesdispersewidelywhen
population effective sizes 131 FIgure 12.2. EuropeanadderVipera berus. searchingforfemalesduringthespring(madsenetal.,1993).however,incertainregions humanshavedisturbedadderhabitatthroughagricultureandurbanencroachment.now weseeextremelyisolatedsmallpopulationsofadders,surroundedbylargeareasofunsuitablehabitat(madsenetal.,1996).considertheisolatedpopulationofaddersatsmygehuk, onthesouthcoastofsweden.theseaddersareseparatedfromthenearestpopulationby 20kmoffarmland,whichisunsuitablehabitatforadders(Madsenetal.,1996).Usethe dataintables12.2and12.3(madsenetal.,1996)toanswerthefollowingquestions. Questions to Work on Individually Outside of Class 1.Table12.2givesdataonthepopulationsizeoftheaddersforeachyearfrom1984to1990. (a)plotthetotalnumberofadultadders(y)overtime(x). (b)usethesedatatocalculatetheinbreedingeffectivesizeandthevarianceeffectivesize ofthispopulationofadders. (c)explainwhytheinbreedingeffectivesizeandthevarianceeffectivesizeofthispopulationdiffer. (d)recalculatethevarianceeffectivesizeofthispopulationwiththenewinformation onthesexratiosofsnakesinthepopulation(table12.3). (e)dr.fernskipehasarguedthatthereisnosuchthingasthe real effectivepopulation size.doyouagreewiththisstatement?whyorwhynot?usetheresultsaboveinyour argument. (f)basedonthesedataandyourcalculations,makearecommendationtotheswedish governmentconcerningthispopulationofvipers.
132 chapter 12 Table 12.2. TotalnumberofadultaddersatSmygehuk Year 1984 1985 1986 1987 1988 1989 1990 Totalnumberofadults 138 40 34 42 37 41 34 Table 12.3. Adultmaleandfemaleaddersineachyear Year No.ofadultfemales No.ofadultmales Correctedpopulationsize 1984 98 40 1985 29 11 1986 24 10 1987 32 10 1988 27 10 1989 29 12 1990 27 7 Small-group/In-Class exercise Pleasebringgraphpaperandacalculatortoclasstocompletethenextpartofthisexercise. First,readthefollowingbackgroundinformationonAfricanrhinoconservation. African Rhino Conservation Background The rhinoceros is an example of a charismatic megavertebrate that has playedacentralroleinpromotingworldwideconservationefforts.thefiveextantspecies ofrhinoarethelastrepresentativesofalargegroupofspeciesthatreachedapeakindiversitybetween25and5millionyearsago(estes,1991).twoofthefiveextantspeciesoccur inafrica:thewhiterhinoceros(ceratotherium simum)andtheblackrhinoceros(diceros bicornis).despitetheirnames,bothspeciesareadullgraycolor,andcanbedistinguished bytheshapeoftheirmouthparts(figure12.3).theblackrhinohasahook-shapedtriangularupperlipthatallowsittoobtainitsfoodbybrowsingleguminousherbsandshrubs.the whiterhino,ontheotherhand,hasaverywide,squaremouth,andisspecializedingrazing areasofdensegrasses.allrhinoshavepooreyesightandrelativelysmallbrains,butex-
population effective sizes 133 FIgure 12.3. Black(left)andwhite(right)rhinoceri. tremelysensitivehearingandsmell(estes,1991).bothafricanspeciesofrhinoshowgeographicvariation.theblackrhinohasbeendividedintofoursubspecies,western(diceros bicornis longipes),eastern(d. b. michaeli),southwestern(d. b. bicornis),andsouthcentral (D. b. minor).thewesternsubspeciesistherarestandmostisolated,withonlyafewindividualslivinginwesternafrica.thewhiterhinohasbeendividedintotwosubspecies,the northern(ceratotherium simum cottoni)andsouthern(c. s. simum).thesetwosubspecies occupyseparaterangesandaremoredistinct,bothmorphologicallyandgenetically,than thesubspeciesofblackrhinos(emslieandbrooks,1999). Bothspeciesofrhinohaveundergonemajorreductionsintheirrangesinthepastseveral hundred years. Early colonial explorers reported that black rhinos were widespread in distribution and fairly common, while white rhinos were more restricted in range. After European colonization, southern white rhinos were very quickly reduced to nearextinction,reachingalowofjust20individualsin1895.sincethen,numbersofthesouthern white rhino have steadily increased; there are over 8,000 alive today. The northern whiterhino,ontheotherhand,hasshownadramaticdecreaseinrecentyears,declining fromover2,000in1960toonly25individualsin1998.numbersofblackrhinoshavealso declinedsincecolonialtimes.declineswereespeciallyseverebetween1970and1992,when black rhinos declined 96%. The species has recently shown some potential for recovery (Emslie and Brooks, 1999). The main reason for the decline of all rhinos is hunting by humans.europeancolonistskilledhundredsofthousandsofrhinosduringthenineteenth century.morerecently,rhinoshavebeenkilledbypoacherssupplyingmarketsinasiaand themiddleeastwithrhinoceroshorns.inasia,rhinohornsareusedintraditionalchinese medicine,whosepractitionersbelievethatthehornslowerfevers,increasemalepotency, andcancureahostofdiseases.inthemiddleeast,theyareusedashandlesforornamental daggerscalledjambiyas(emslieandbrooks,1999).althoughrhinohornshavebeenused forthesepurposesforhundredsofyears,recentincreasesindemandputseriouspressure onwildrhinos,andpoachingforhornsisthemajorthreattoafricanrhinopopulations today(emslieandbrooks,1999). Black Rhinoceros Diceros bicornis Blackrhinoswereformerlythemostwidespread andabundantspeciesofrhino(estes,1991),butarenowlistedintheiucnredbookas criticallyendangered.directcountsofblackrhinoshaveshowndeclinesofover80%in
134 chapter 12 7 6 Population (in ten thousands) 5 4 3 2 1 0 1969 1974 1979 1984 1989 1994 FIgure 12.4. Theblackrhinoceroshasbeeninseveredeclineinrecentyearsandisenteringapopulationbottleneck(datafromEmslieandBrooks,1999). Year thelast50years(emslieandbrooks,1999).thetotalpopulationsizeofblackrhinosin 1970wasestimatedat65,000.Historically,blackrhinosoccupiedalargerangethroughout Africa.Today,blackrhinopopulationsarefragmented,andthespeciesisrapidlydeclining innumbers(figure12.4).thespecieshasbeendividedintofoursubspecies,eachoccupying a separate geographic range: the western, eastern, southwestern, and south-central black rhinos. The ranges of each subspecies have different climates and habitats. They cansometimesbedistinguishedbycharacterssuchasskintextureandhornlengthand shape.theremayalsobegeneticandbehavioraldifferencesacrossthesesubspecies.the westernblackrhinocerosisthemostrange-restrictedandendangeredofallthesubspecies ofblackrhinos:theentiresubspeciesisrepresentedbyonlyafewscatteredindividuals incameroon.thissubspecies,separatedfromtherestoftheblackrhinosbyhundreds ofmiles,mayrepresentageneticallydistinctlineage,butisthreatenedwithextinctionin theimmediatefuture.thelargestpopulationofblackrhinosisinkenya,whilethemain populationofsouthwesternblackrhinosisinnamibia.thesouth-centralblackrhinois themostcommonsubspecies,withlargenumbersinsouthafricaandzimbabwe,and smallerpopulationsinsoutherntanzaniaandmozambique.thegoalofyourgroupisto establishaconservationplanforblackrhinosasawhole.youneedtodecidehowmuch moneyneedstobeallocatedtoeachofthefoursubspecies,andwhichpopulationswill havethehighestconservationpriority. White Rhinoceros Ceratotherium simum Whiterhinoshavealwaysbeenlesscommonandmorelimitedindistributionthanblackrhinos(EmslieandBrooks,1999).Thisis probablyafunctionoftheirspecializedgrazingdiet.thisfeedingstrategymakesthewhite rhinounique,asitisquitedifferentfromthatofblackrhinosand,infact,fromallother rhinosintheworld(estes,1991).thewhiterhinoisoneofthelargestpurelygrazingherbivoresthathaseverlived.southernwhiterhinosareseparatedfromnorthernwhiterhinos by2,000km,andnowhiterhinohaseverbeenrecordedintheinterveningarea.thesetwo
population effective sizes 135 subspeciesaregeneticallydistinct,withmoregeneticvariationbetweenthesetwosubspeciesthanamongthefoursubspeciesofblackrhino(smithetal.,1995). Southern White Rhinoceros Ceratotherium simum simum Southernwhiterhinos wereonthebrinkofextinctionin1895,whenoverhuntinghadreducedthemtojust20 individualsinonepopulationinsouthafrica.theirnumbershavesincerecoveredsubstantially(figure12.5).therecoveryofthesouthernwhiterhinoisoneofthemajorsuccess storiesinmodernconservationbiology.beforetheirdeclineintheninteenthcentury,their rangeincludedmuchofsouthernafrica.afterinitialrecoveryofthesourcepopulation, manytranslocationswerecarriedout;thesehavesuccessfullyreintroducedrhinosintoareaswheretheyhadbeenwipedout.southernwhiterhinosarenowthemostnumerous subspeciesofrhino;populationscanbefoundinsouthafrica,botswana,namibia,swaziland,andzimbabwe.theyhavealsobeenintroducedintokenya,ivorycoast,andzambia, alloutsidetheirnativerange.therhinoshavefunctionedasamajorsourceoffundingin South Africa, where national parks have sold excess rhinos to private game parks for as muchasu.s.$25,000apiece. Population (in thousands) 9 8 7 6 5 4 3 2 1 0 1890 1910 1930 1950 1970 1990 Year FIgure 12.5. Thesouthernwhiterhinoceroshasrecoveredfromnearextinctionattheturnofthe lastcenturyandthecurrentgrowingpopulationisdescendedfromabottleneckpopulationofonly 20animals(datafromEmslieandBrooks,1999). Northern White Rhinoceros Ceratotherium simum cottoni Northernwhiterhinos haveshownastrikingdeclineinrecentyears,andarenowperilouslyclosetoextinction. Table12.4givesdataonnorthernwhiterhinocerospopulationsbycountryfrom1960to 1998(Emslie and Brooks, 1999). Although the range of northern white rhinos once includedpartsofuganda,chad,sudan,thecentralafricanrepublic,andthedemocratic RepublicofCongo,theyarenowrestrictedtoasmallareainthenortheastoftheDemocraticRepublicofCongo(EmslieandBrooks,1999).Thispopulationnumberedonly25 individualsin1998,anddrchasbeensufferingfromtremendouspoliticalinstabilityover thepast20years.conservationbiologistshavenotlosthopeofpreservingthesubspecies,
136 chapter 12 Table 12.4. Northernwhiterhinosbycountry,1960 1998. 1960 1971 1976 1981 1983 1984 1991 1995 1998 CentralAfrican Few Few Few Few Few 0? Republic Chad Few Few?? 0? 0? DemocraticRepublic 1,150 250 490 <50 13 20 15 30 31 25 ofthecongo Sudan 1,000 400? <300 <50 0? 0? 0? 0? Uganda 80 Few Few Few 2 4 0? Total 2,230 650 500+ <350 <70 15 30 31 25 however,andcitetherecoveryofthesouthernwhiterhino,whichhasgrownfromatotal populationofaround20individualsin1895toover8000today.thenorthernwhiterhinos currentlysurvivinginthedemocraticrepublicofcongorepresentthelastsurvivorsofa uniquelineageofrhinos;theirextinctionwouldbeagreatandirreversibletragedy. Your Job: Help Create Species Survival Plans for African Rhinos (Questions 2 6) SpeciesSurvivalPlans(SSPs)coordinatethemanagementofrareandendangeredspeciesto maintainhealthybreedingpopulations,retaingeneticvariation,andminimize inbreeding. SSPsoftenhavetheconflictinggoalsofpreservingspeciesinacaptiveenvironmentwhileat thesametimeminimizingevolutionarychangeinthespeciesandminimizinglossofgenetic diversityfrominbreedingordrift(templeton,1991).thesecanbesignificantforcesaffectingwild(insitu)andcaptivepopulationsthatareenteringoremergingfrompopulationbottlenecks.yourjobwillbetouserealdatatohelptheiucnrhino-rescueteamgeneratean SSPforthetwospeciesofAfricanrhinos.Afteryouanswerquestions2 5,theentireclasswill meetasacommitteeofthewholetoallocatefundsfortheconservationofafricanrhinos. 2. Your instructor will assign you to one of the three rhino species or subspecies on whichwehavedata.first,youneedtoassessthecurrentgeneticsituationforblackrhinos, orsouthernornorthernwhiterhinos.thisassessmentshouldincludetheinbreedingand varianceeffectivesizesforthewildpopulations.youshouldbeabletoprojectaccumulationofinbreedinginwildpopulationsiftheyaremaintainedatcurrentlevels.(assume equalsexratiosandagenerationtimeof8years.forblackrhinosandsouthernwhiterhinosyouwillneedtoestimatecensussizesfromfigures12.4and12.5)yourinstructorwill copytable12.5ontheboardandyoucanshareyourresultswiththeothergroups. 3.Foryourspeciesorsubspecies,discussthelong-rangeplanformaintainingthegenetic healthofthepopulation.addresstherecommendationsandthetheoreticalframeworkof Franklin s50/500ruleinyourplan. 4.Youshouldalsodiscussthesituationforyourspeciesinthewildanddecidewhether you want to use the wild population to supplement the captive zoo population or vice versa. 5.Finally,yourplanmustincludeprioritiesforbothspeciesandfordifferentpopulationswithineachspecies.Therealityisthattherearelimitedfundsavailableforrhinoconservation,andyoumustgenerateguidelinesaboutwhereresourcesshouldbespent.
population effective sizes 137 Table 12.5. PopulationcensusandeffectivesizesofAfricanrhinos. Blackrhinoceros Diceros bicornis Southernwhiterhinoceros Ceratotherium simum simum Northernwhiterhinoceros Ceratotherium simum cottoni Censussize, Inbreedingeffective Varianceeffective 1997 size(nef) size(nev) N =2,600 N =8,440 N =23 6.Eachgroupwillhaveafewminutestodescribethesituationfortheirrhinosandproposeanallocationofthe$500,000whichAfricanRhinoRescuehasraised.Onceeachgroup hasmadetheirbriefpresentationyouwillhaveachancetoconvinceeachother(andyour instructor)totakeyourrecommendation. references Arnold,E.N.,andJ.A.Burton.1978.A Field Guide to the Reptiles and Amphibians of Britain and Europe.London:Collins. Foose,T.J.,L.deBoer,U.S.Seal,andR.Lande.1995.Conservationmanagementstrategiesbased onviablepopulations.pages273 294in:J.D.Ballou,M.E.Gilpin,andT.J.Foose(eds.),Population Management for Survival and Recovery: Analytical Methods and Strategies in Small Population Conservation.NewYork:ColumbiaUniversityPress. Franklin,I.R.1980.Evolutionarychangeinsmallpopulations.Pages135 140in:M.E.Souleand B.A.Wilcox(eds.),Conservation Biology: An Evolutionary Ecological Perspective.Sunderland, Mass.:SinauerAssociates. Gilpin,M.E.,andM.E.Soule.1986.Minimumviablepopulations:processesofspeciesextinction.Pages19 34inM.E.Soule(ed.),Conservation Biology: The Science of Scarcity and Diversity. Sunderland,Mass.:SinauerAssociates. Hendrik,P.W.2000.Genetics of Populations.2nded.,Sudbury,Mass.:JonesandBartlettPublishers. Lande,R.1995.Mutationandconservation.Conservation Biology 9:782 791. Madsen,T.,R.Shine,J.Lomanmm,andT.Håkansson.1993.Determinantsofmatingsuccessin maleadders,vipera berus.animal Behavior45:491 499. Madsen,T.,B.Stille,andR.Shine.1996.Inbreedingdepressioninanisolatedpopulationofadders Vipera berus.biological Conservation75:113 118. Mace,G.M.,andR.Lande.1991.Assessingextinctionthreats:TowardsareevaluationofIUCN threatenedspeciescategories.conservation Biology 5:148 157. Pimm,S.L.,H.L.Jones.,andJ.Diamond.1988.Ontheriskofextinction.American Naturalist 132: 757 785. Primack,R.B.2000.A Primer of Conservation Biology.Sunderland,Mass.:SinauerAssociates. Reed,D.H.,andE.H.Bryant.2000.Experimentaltestsofminimumviablepopulationsize.Animal Conservation3:7 14. Shaffer,M.L.1981.Minimumpopulationsizesforspeciesconservation.Bioscience 31:131 134.
138 chapter 12 Soule,M.E.1980.Thresholdsforsurvival:maintainingfitnessandevolutionarypotential.Pages 151 169inM.E.SouleandB.A.Wilcox(eds.)Conservation Biology: An Evolutionary Ecological Perspective.Sunderland,Mass.:SinauerAssociates. Walpole,M.,M.Morgan-Davies,S.Milledge,P.Bett,andN.Leader-Williams.2001.Population dynamicsandfutureconservationofafree-rangingblackrhinoceros(diceros bicornis)populationinkenya.biological Conservation99:237 243.