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Biodiversity and Ecological Redundancy
Author(s): Brian H. Walker
Source: Conservation Biology, Vol. 6, No. 1 (Mar., 1992), pp. 18-23
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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
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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
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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
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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
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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.
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andEcological
Redundancy 23
Biodiversity
Walker
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