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Society for Conservation Biology Biodiversity and Ecological Redundancy Author(s): Brian H. Walker Source: Conservation Biology, Vol. 6, No. 1 (Mar., 1992), pp. 18-23 Published by: Wiley for Society for Conservation Biology Stable URL: http://www.jstor.org/stable/2385847 . Accessed: 29/08/2013 09:57 Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at . http://www.jstor.org/page/info/about/policies/terms.jsp . JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact [email protected]. . Wiley and Society for Conservation Biology are collaborating with JSTOR to digitize, preserve and extend access to Conservation Biology. http://www.jstor.org This content downloaded from 128.206.9.138 on Thu, 29 Aug 2013 09:57:58 AM All use subject to JSTOR Terms and Conditions Essays Biodiversity and EcologicalRedundancy BRIAN H. WALKER CSIRO Divisionof Wildlifeand Ecology P.O. Box 84 Lyneham,A.C.T. 2602 Australia Abstract: Thispaper addressestheproblem ofwhichbiota to choose to bestsatisfytheconservationgoals for a particular region in theface of inadequate resources.Biodiversityis taken to be the integrationof biological variabilityacross all scales,from thegenetic,throughspecies and ecosystems, to landscapes. Conservingbiodiversityis a daunting task, and thepaper assertsthatfocusing on species is not thebest approach. The bestway to minimize species loss is to maintain theintegrity of ecosystemfunction.The importantquestions therefore concernthekinds of biodiversitythatare significantto ecosystemfunctioning.To bestfocus our efforts we need to establishhow much (or how little) redundancy thereis in thebiological compositionof ecosystems.An approach is suggested,based on theuse offunctionalgroups of organismsdefinedaccording to ecosystemprocesses.Functional groups with littleor no redundancywarrantpriority conservation effort.Complementary species-based approaches for maximizing the inclusion of biodiversity withina set of conservationareas are compared to thefunctional-groupapproach. Resumen: Estearticulo trataelproblema de que biota elegir a los efectosde maximizar los objetivos de conservacion para una regionparticular cuando se hace frentea recursos inadecuados. La biodiversidades considerada como la integraci6n de variabilidad biologica a lo largo de todas las escalas, desde genetica;pasando por especiesy ecosistemas, hasta paisajes. La conservacionde la diversidades una empresa intimidatoria,y el articulo afirma que el enfocque a nivel de especiesno es el camino correcto.La mejorformade minimizar la perdida de especies es mantenerla integridad de la funci6n del ecosistema Por lo tanto, las preguntas importantesestan relacionadas con los tipos de biodiversidad que son significativospara el funcionamiento de los ecosistemas.Para enfocarmejornuestrosesfuerzosnecesitamos establecercuanta redundancia hay en la composicion biol6gica del ecosistema Se sugiereun metodopara resolver el problema, basado en el uso de gruposfuncionales de organismos,definidosde acuerdo aprocesos ecosistemicos.Los gruposfuncionales con poca o nula redundanciajustifican un esfuerzode conservaci6nprioritario.Metodos complementarios,basados en especies,para maximizar la inclusion de biodiversidaddentrode una coleccion de areas de conservaci6n,son comparados con el metodode gruposfuncionales. Preface addressthe issue of functionaldiversityand ecological redundancyin communitycomposition.To do thisrequiresdevelopmentofa functionalapproachto describingbiologicalcomposition,ratherthansole relianceon the conventionaltaxonomicapproach.Adherenceto a policy thatplaces equal emphasis on every species is ecologicallyunsoundand tacticallyunachievable. This paper presentsa functionalapproachto analyzing in the beliefthatthisapproachprobiologicaldiversity, vides a more effectivemeans of minimizingthe decline in biodiversity broughtabout by humandisturbance.Its take-homemessageis this:Ifscientistsare to contribute usefullyto the inevitableincrease in managementand theyneed to politicaldecisions relatingto biodiversity, Paper submittedDecember 17, 1990; revisedmanuscriptaccepted April 19, 1991. Introduction The developing concern about human impact on the globe has focusedattentionon the issue of biodiversity and pushed it into prominenceon manyagendas. It is reflectedin a numberofnew developments,such as the 18 Conservation Biology Volume 6, No. 1, March 1992 This content downloaded from 128.206.9.138 on Thu, 29 Aug 2013 09:57:58 AM All use subject to JSTOR Terms and Conditions Walker andEcological Redundancy 19 Biodiversity conventionon biodiversityand the IUCN international most recent programsadopted by the ScientificCommitteeon Problemsof the Environmentand the InternationalUnion of Biological Sciences, to name just a few.It will be a majoragenda itemforthe 1992 United and Development. NationsConferenceon Environment the debate is the rhetoricsurrounding Unfortunately, oftenconfusingand superficialand could divertpolicies and activitiesintodirectionswhere desiredgoals won't ofwhatis be achieved.Thereare variousinterpretations and itsconstantuse and misuse meantby "biodiversity," in themediahas induceda negativereactionto theterm leadingto in some sectionsofthe scientificcommunity, its rejectionas a serious scientifictopic. The popularput it has unfortunately izationofdecliningbiodiversity issue when in in the categoryof a "flavor-of-the-month" factit is a serious and difficult problem thatdeserves long-termscientificconsideration. includesall thosechangesthat Decline in biodiversity biologicalhethave to do withreducingor simplifying erogeneity,fromindividualsto regions. Included are such phenomenaas phenotypicplasticity;geneticvariabilitywithina population(allowingfora wide rangeof genotypic responses to environmentalconditions); between populaecotypicvariation(genetic variability tionswithina species); species richness(the numberof species in a community);species (alpha) diversity(involvingboth the number of species and the relative numbersofindividualsper species); functionaldiversity kinds different (the relativeabundancesof functionally which extends of organisms);gradient(beta) diversity, to diversityresultingfrom speciation of ecological equivalents(gamma or delta diversity-see Cowlinget al. 1989); communitydiversity(the number,sizes,and of communities,sometimesreferred spatialdistribution to as patchiness);and even the diversityofthe scales of patchiness(landscape diversity).Taken together,"ecologicalcomplexity"is a bettertermforall theseaspects of biologicalheterogeneity. and A decline in anyoftheserepresentssimplification The questionwe need thereforeloss of "biodiversity." to answeris "So what?" I divide the reasonsformaintaining biodiversityinto two categories:ecological and "others."Norton(1988) putsthese"other"concernsintothreecategoriesofvalues-commodity, amenity,and moral. I exclude here the moral or ethical arguments,not because theyare There but because theyare nonscientific. unimportant, and argumentsin are grayareas in all such distinctions, favorofconservingpandas or koalas generallyinvolvea mixtureof moral concerns and scientificawarenessof genetic uniqueness and rarity(to which I will return here to the genuinelymoralargulater). I am referring ments-which neverthelessare usuallyassociatedwith "charismaticmegavertebrates"rather than fungi or nematodes.The ethicalimplicationsofspecies loss is an importanttopic,but fora different essay. The amenity issues are generallyrathervague, except for specific examples where values are attachedto particularspecies. The commodityissues mostlyseem to relate to futurepossible benefitsfromas-yet-unknown specific pharmacologicalproperties,etc. Such issues can be grouped under the headingof option foreclosure;and while no one would argue againstit, it is difficultto attachlevels ofprobabilityand potentialbenefitsin the absence of appropriateinformation. The majorreasonsadvanced forconcernsabout biodiversityare ecological. Each of the various aspects of ecological complexitydescribedabove, has been implicated to a greateror lesserdegree in theways in which ecosystemsand communitiesfunctionand (in particular) persist.Initially,therewas uncriticalacceptance of an assumedpositiverelationshipbetween species richness and "stability," but thiswas broughtinto question by May (1972) and others,and the focus switchedto the kinds of diversity.Based on both theoreticaland empiricalevidence it seems that it is diversityat the level (patchiness),ifanything, community thatis important in long-termstability.Nevertheless,it is species richnessthat is most commonlyinvoked in concerns about biodiversity,and the approach to the problem usuallyinvolvesdevisingmeans(includingpoliticaland legal) to preventdecline in biodiversity. In itselfthisis From reasonable. a conservationviewpointefperfectly fortsto preventthe loss of any species are laudable. However,thesurvivalofparticulartarget(favored) species can usuallybe assuredby correcthabitatmanagement,controlofexploitation,or both.This is not to say thatcorrecthabitatmanagementis simple,and it should notbe dismissedin a facileway.Its mostdifficult aspect is probablydealingwith habitator landscape fragmentationand its consequent denial of access to refugiaat criticaltimes(which reflectsthe importanceofchanges in landscapediversity).Resolvingtheoverallproblemof decline in biodiversityrequiresmore thanfocusingon particularcases of species conservation.It requiresunthe relationsbetweenbiodiversity and ecoderstanding systemfunctionand applyingthisunderstanding. and EcosystemFunction Biodiversity Given that the objective is to minimizereduction in biodiversity(includingthe loss of species), the importantquestionsin thisdebate concern the kinds of bioto the ways ecosysdiversitythatare most significant tems function,because this is how to best focus our conservationefforts. Whichkinds,and whatamounts,of lead mostreadilyto significant biologicalsimplification or irreversiblechanges in the inherentstructureand functionof an ecosystem(i.e., to an unsustainabledecline in its resilience)?Put anotherway,which aspects ConservationBiology Volume 6, No. 1, March 1992 This content downloaded from 128.206.9.138 on Thu, 29 Aug 2013 09:57:58 AM All use subject to JSTOR Terms and Conditions 20 Redundancy andEcological Biodiversity Walker and which kindsof species, are most imof diversity, portantto ecosystemfunction? Ecologically,all species are not createdequal. Atone or "drivers,"of the extreme,some are determinants, ecosystemof which theyforma part.At the otherextreme are those that are "passengers."Removingthe butloss ofthepassengers formercauses a cascade effect, leads to littlechange in the rest of the ecosystem.Apparentpassengers,at one timescale, mayofcourse turn and thisdistinction out to be infrequentdeterminants, needs to be treatedcarefully.It raises the major legitimate argumentagainstclassifyingspecies in termsof theircontributionto ecosystemfunction.Nevertheless, attemptingto deal with each species individuallybecomes impossible-consider,forexample,justthenumber of invertebratespecies in a hectareof tropicalrain reexamforest.Providedwe deliberatelyand iteratively ine theguildcomposition,theadvantagesofa functional approacharguein itsfavor.This does not mean thatwe should ignore what we know about particular"keystone" or otherwiseimportantspecies, but ratherthat we shouldinclude thisknowledgein a more systematic and thoroughanalysisof ecosystemfunction. Puttingtheproblemanotherway,we need to askhow much,or rather,how little,redundancythereis in the biological compositionof ecosystems.This question is at the heart of ecological science-the relationsbetween structureand functionof ecological communities-and as such should excite anyone who is interested in ecology. Withoutknowinganythingabout ecological redundancy,how can we value a decline in biological diversity? Contrast,forexample,an ecosystemwhere a single, withthe situaant predatoris prominent, wide-ranging Australian jack-jumper tion involvingthe southeastern ants (nominally Myrmecia pilosula), which were thoughtuntilrecentlyto constitutesuch a species. "M. pilosula" is now knownto include at least fivesibling southeasterntaxonomic species. They differdramaticallyin chromosomenumbers,but onlyslightlyin morphology(Imai et al. 1988). Theircollectivedistribution in suitable habitatsruns fromthe Blue Mountainsto and sevTasmaniaand partsofsouthernSouthAustralia, eral species are verywidespread.In some places only one species is found,but in othersup to threemaybe associated.None occupy the whole area, sympatrically but the threemostcommonspecies togethercover virtuallythe fullgeographicalrange of the group (R. W. Taylor,personal communication).There are no apparent ecological distinctionsbetween these entities,so that,although they are reproductivelydistinctgene pools, and clearly"good" species, theyconstitutein effecta singlefunctionalecological unit.Althoughit is of great scientificinterestto understandthe nature and few would disevolutionaryhistoryof theirdiversity, pute thataccordingall ofthesespecies thesame priority (and thereforeconservationeffortand expenditure)as thesingle-speciespredatormightbe misdirected,under some circumstances. Regrettableas it mightbe, it is mostlikelythatglobal biodiversityconcernswill ultimatelyreduce to a costbenefitanalysis.Withouta knowledgeofredundancy,or more broadly,the relationshipsbetween levels of biodiversityand ecosystemfunction,we cannot estimate eitherthe costs or the benefits. One adversebut absurdresponseto thisassertionis thatacknowledgingthatthe loss of some species may not be as ecologicallycriticalas the loss ofsome others is tantamountto advocatingtheirremoval.CriticsinvokeErhlichand Erhlich's( 1981 ) fableabout rivetpoppers on an aeroplane.What I am suggestingin no way supportsanyactionsor policies thatdeliberatelylead to WhatI do advocate is thatthe a decline in biodiversity. in to reduce the decline bestway to succeed our efforts in biodiversity is to focusinitialattentionon the aspects the resilthatare criticalformaintaining ofbiodiversity in this Resilience of concerned. the ecosystems ience to is of the ecosystem to be the capacity context taken maintainits characteristicpatternsand rates of proallocationofphocesses (such as primaryproductivity, surfacehydrology,energyexchange,nutritosynthate, ent cycling, herbivory, etc.) in response to the inherentin its climaticregime.By maintainvariability ofecosystemfunctionwe minimizethe ingtheintegrity chances of losing the many species we have not yet describedand those ofwhose veryexistencewe are as yet unaware. If we consider the case of a decline in numbersof individualswithina species,the analogous issue is populationviabilityanalysis:What are the required conditions,and what is the criticalnumberof interactingindividuals, to maintain a population? Or, in a more generalsense,what is the relationshipbetween density and populationviability,and what determinesit? The (e.g., genetic variproblem involvesboth longer-term abilityloss) and more immediateprocesses (effectsof effectsof competiextremeevents,density-dependent tion,minimumbreedinglevels,dispersaland reinvasion rates,and so forth).At the multispecieslevel, the same issues remaina concern,but perhaps more important are the issues relatingto species interactionsand the ecosystemprocesses describedabove. A SuggestedApproach How do we addressthe problem? As impliedearlier,a necessarystep in analyzingthe functionalrelationshipsbetween biological diversity and persistencein an ecosystemis to get away froma purely species-centeredview of biodiversity,and to ConservationBiology Volume 6, No. 1, March 1992 This content downloaded from 128.206.9.138 on Thu, 29 Aug 2013 09:57:58 AM All use subject to JSTOR Terms and Conditions Walker andEcological Redundancy 21 Biodiversity consider it instead in terms of functionallydifferent kindsof organisms.The appropriatebasis fordefining thespecies functionaltypes(guilds) is theway thebiota regulatesecosystemprocesses. Definingthem in this way focuses attentionon the processes that maintain ecosystem and community function, and on how changes in the relativeor absolute abundances of the functionalgroups concerned,and in theirpatternsof will influencethese processes. distribution, is an iterativeprocedureinvolvStep one, therefore, inganalysesofecosystemfunction(identifying theratelimitingor otherwiserelativelyimportantprocesses in the systemof concern) coupled to the developmentof appropriatecorrespondingfunctionalclassificationsof thebiota,throughguildanalysesof one sortor another. The objective should be to tryto further subdividethe in the of nontrivial species a guild on basis functional attributes(nontrivialin the sense thattheyare related to limitingor dominantecosystemprocesses for that ecosystem).If this cannot be done and there are still severaldifferent species in the group,thenon the basis of currentknowledge,thereis some ecological redundancywithinthe guildconcerned.An obvious problem in this regard is the time scale on which functionis ofa particularspeconsidered.The separatesignificance cies may only become apparentunder particularenvironmentalconditions,and such time-dependent, epithat sodic featuresofguildanalysisconstitutea difficulty mustbe consideredfromthe perspectiveof long-term ecosystemfunction. Steptwo is to determinethenumberofspecieswithin each guild.Those representedby only a few or even a singlespecies are clearlyunable to withstandanyloss of species and constitutean obvious,immediateconservationfocus. Step three is to furtherexamine the interactions among the species in each guild. Complete functional redundancyonlyoccurs if,followingtheremovalofone species, there is densitycompensationamong the remainingspecies. A complicatingfactoris thatthe different species in a guild,while all performingthe same to different environfunction,may respond differently mentalconditions.Withthecompleteset ofspecies,net guild abundance (or function)may remain relatively environment. Loss of some constantundera fluctuating species may well lead to an increase in abundance of others(i.e., densitycompensationoccurs), but because the diversityof response to environmentalconditions has been reduced,net guild abundance may thenfluctuate more in response to environmental fluctuations. Once again,in the absence of adequate information, we need to adopt a successive approximationapproach based on what we do know. The finalstepis to considertherelativeimportanceof the functionalgroups (the analogue to the question about species importance).The logical progressionin ecological studies,fromstructureto functionand then to the relationsbetween them,indicates that the approachto thisstep is to examinehow a changein abundance of a functionalgroup directlyaffectsecosystem and communityprocesses,and how such a change influencesthe net effectof the biota (throughchangesin the timingand overallratesofpredation,dispersal,herbivory,decomposition,nutrientretentionand uptake, biomassaccumulation,etc.). In otherwords, all the issues involvedin ecological stabilityanalysisare considered,but usingfunctionalgroupsinsteadof taxonomic species. Achievingthis step involves developing conceptual and analyticalmodels using a combinationof existingknowledge(see step one above) and a rangeof experimentsspecificallydesignedto examine these relations.It is a fruitful area forexperimentalecologists and will most likelyinvolvereciprocalexperimentsin which,on the one hand,functionalgroupsare removed froman ecosystemand the effectson functionare measured,and on the other,functionis altered(changes in nutrientcycling,hydrologicalregime,etc.) and changes in functional groupabundanceor performanceare measured. Functionalgroups considered (on the basis of presentknowledge) to be the major driversof the systemwarrantinitialattentionin thisapproach. Complementary Species-based Approaches Given the complexities of definingand establishing functionalgroups,particularly the extentof lumpingor thereis a dangerthat(as one reviewerof an splitting, earlydraftput it) we may replace one taxonomicapproachwithanother,more confusingone. However,as indicatedearlier,I do not advocate a complete switch. Too oftenin thedevelopmentofecologytherehas been a swingfromone extremeto another(the "association" vs. the "continuum,"equilibrium vs. disequilibrium, etc.) withthe eventualrealizationthatboth approaches were valid and thatthe extentto which each was important depended on the nature of the system.No doubt,in the ensuingdebate on how to maintainbiodiversity,the use of both species and functionalgroups will turnout to be appropriate,at different scales. One species-based approach, which perhaps best complementsthe functionalapproach, is concerned with weightingspecies (or other taxa) according to theirtaxonomicdistinctness, and in thisway identifying prioritychoices for conservingbiodiversity.The approachhas so farbeen mostcomprehensively dealtwith be Vane-Wrightand colleagues (Vane-Wrightet al. 1991) and is encapsulatedin May's (1990) account of theproblem.The approachinvolvesderivingsome measureoftaxonomicdistinctness based on the topologyof a hierarchical taxonomic classification.Using these weightings,Vane-Wrightand colleagues have shown ConservationBiology Volume 6, No. 1, March 1992 This content downloaded from 128.206.9.138 on Thu, 29 Aug 2013 09:57:58 AM All use subject to JSTOR Terms and Conditions 22 andEcological Biodiversity Redundancy how the priorityorderforthe minimumset of reserve in a particular areasneeded to conservethebiodiversity markedlyfromthe set derived taxonomicgroupdiffers by givingall species equal weight.May (1990) has indicated thatthe methodused to derive the weightings work,and Faith(in press) has developed needs further a measureof phylogeneticdiversitythatresolvessome The significanceofthemeasurein the ofthedifficulties. context of this paper is that the value of a species is based on its contributionto overall featurediversity. Functionalgroupsare also characterizedby theirdifferso thatsimilarmeasuresoffeaturediversity entfeatures, may be useful in placing relative importancevalues (functionalas opposed to phylogeneticimportance)on functionalgroups. different The main differencebetween the functionalgroup approach I have suggested and the taxonomicet al. has to do distinctnessapproach of Vane-Wright with the scale of concern.Taxonomic distinctnessis a valuable tool forhelpingto choose amongmanydifferent areas to ensure that maximumbiodiversityis included (for example,in a reservenetwork).The functional group approach focuses attention on which species are ofmajorconcernin managing,or identifying appropriateboundariesfor,a particulararea or regionto minimizethe loss ofbiodiversity. The two are therefore complementaryin devising an overall conservation strategy. Conclusion thatwill deIn futurepoliticaland economic tradeoffs cide how much nationsare preparedto pay to maintain biodiversity (in termsofforegoneproductionor direct restorationcosts), ethical and commodityarguments will certainlyplaya role,but theweightofevidencewill most likelycome fromthe ecological side. The worrito polsome cost ofdecline in biodiversity, particularly iticianswho maybe held accountable,is the threatof a collapse in the "stability"of ecosystems(whateverthat means). This threat,however, will become progressivelyless an issue as the passage of timerevealsscientists'inabilityto demonstrateit. There will always be highlymotivatedconservation organizationsthat collectively take on the plight of some hundredsor even thousandsof visible,identified species, but this is an immeasurablysmall part of the problem.Ifthese species act as "umbrella"species,and helps the plightof others, enhancingthemunwittingly ofsuch organizationsarepositivelymagthentheefforts nified.Such claims,however,are statementsoffaithand to considerwhetherthe acthereis generallyno effort tions takento promotethe welfareof elephantsor lemurs,for example, are having a positive or negative effecton thewelfareoflooselyconnectedspecies,such Walker as butterfliesor soil-surfacelichens. Identifying these focalspecies in the contextof a functionalanalysiswill permitan evaluationoftheirumbrellarole and will also highlightappropriate(and inappropriate)conservation activities. Five categoriesof species have been, and are,used to justifyspecial conservationeffort (Noss 1990). In summarytheycan be labeled as ecological indicator,keystone,umbrella,flagship(charismatic),and vulnerable species. Ifa strategyto enhance one or more such species turnsout to have an overallnegativeeffecton the viabilityof manyotherspecies in the ecosystem,then theargumentsinfavorofsuch a strategy obviouslyneed to be questioned.Althoughmanyreserveswere originallyestablishedaroundfocalspecies and some are still intentionally managed to conserve just those species, conservationorganizationsresponsibleformanagingan area generallyavoid such a stance, and their actions constitutea genuineeffort to enhance thewelfareof all species. They are, unfortunately, confrontedwith the problemofnot knowingenoughabout all thesespecies to be confidentof achievingtheirobjective. Changing the focus fromparticularspecies to functionalgroups, and couplingthiswith an analysisof ecological redundancy,particularlyin those functionalgroups thatare on the "driver"side ofthe continuum,is a good startto improvingthe situation. In termsof an overallapproach to conservingbiodiversitywe need to resolve two issues: (1) how to choose theoptimalset ofbitsofa regionor oftheworld to maximizethebiodiversity theyinclude,and (2) how to manage any area or regionto ensure the long-term persistenceof all its biota (includingspecies we don't yet know exist). The functionalgroup approach addressesthe latterissue. WhatI have suggestedhere clearlyraisesmoreproblems thanit solves.But thisis largelydue to its stage of development.Thusfar,theidea offunctionalgroupshas been restrictedto verygeneralor global classifications. Functionalaspects of biodiversityhave been discussed in general terms(e.g., Noss 1990), but the notion of functionalgroupshas yetto be applied in a detailedway to particularecosystems.Giventhe disappointingprogress in achievingprogramsusingindividualspecies apthe analysisof ecological reproaches to biodiversity, serious attention. deserves dundancy Acknowledgments Thispaperwas greatlyimprovedby commentsand conFrancis fromAlanAndersen,Dick Braithwaite, tributions Dan Chris Graham Crome, Faith, Harrington, Margules, Steve Morton,and JohnWoinarski(who suggestedthe "drivers"and "passengers"analogy). I also thankBob May and an anonymousrefereefortheirvaluable suggestions. ConservationBiology Volume 6, No. 1, March 1992 This content downloaded from 128.206.9.138 on Thu, 29 Aug 2013 09:57:58 AM All use subject to JSTOR Terms and Conditions andEcological Redundancy 23 Biodiversity Walker LiteratureCited interactionhypothesis.JapaneseJournalof Genetics 63:159185. and C. HilCowling,R. M.,G. E. 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I., C. J.Humphreys,and P. H. Williams.1991. Vane-Wright, What to protect-systematicsand the agony of choice. Biological Conservation55:235-254. May,R. M. 1990. Taxonomyas destiny.Nature347:129-130. ConservationBiology Volume 6, No. 1, March 1992 This content downloaded from 128.206.9.138 on Thu, 29 Aug 2013 09:57:58 AM All use subject to JSTOR Terms and Conditions