©CopyrightJASSS

MehdiSaqalli,CharlesL.Bielders,BrunoGerardandPierreDefourny(2010)

SimulatingRuralEnvironmentallyandSocio-EconomicallyConstrainedMulti-ActivityandMulti-DecisionSocietiesinaLow-DataContext:AChallengeThroughEmpiricalAgent-BasedModeling

JournalofArtificialSocietiesandSocialSimulation13(2)1<http://jasss.soc.surrey.ac.uk/13/2/1.html>

Received:19-Feb-2009Accepted:27-Sep-2009Published:31-Mar-2010

Keywords:

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Abstract

Developmentissuesindevelopingcountriesbelongtocomplexsituationswheresocietyandenvironmentareintricate.However,suchsiteslackthenecessaryamountofreliable,checkabledataandinformation,whiletheseveryconstrainingfactorsdeterminethepopulations'evolutions,suchasvillagerslivinginSahelianenvironments.Beyondagame-theorymodelthatleadstoaprematureselectionoftherelevantvariables,webuildanindividual-centered,empirical,KIDS-oriented(KeepItDescriptive&Simple),andmultidisciplinaryagent-basedmodelfocusingonthevillagers'differentialaccessestoeconomicandproductionactivitiesaccordingtosocialrulesandnorms,mainlydrivenbysocialcriteriafromwhichgenderandrankwithinthefamilyarethemostimportant,astheywereobservedandregisteredduringindividualinterviews.Thepurposeoftheworkistobuildavalidandrobustmodelthatovercomethislackofdatabybuildingaindividualspecificsystemofbehaviourrulesconditioningthesedifferentialaccessesshowingthelong-termcatalyticeffectsofsmallchangesofsocialrules.Themodel-buildingmethodologyistherebycrucial:theinterviewingprocessprovidedthebehaviourrulesandcriteriawhilethecontext,i.e.theeconomic,demographicandagro-ecologicalenvironmentisdescribedfollowingpublishedorunpublishedliterature.Thankstoasensitivityanalysisonseveralselectedparameters,themodelappearsfairlyrobustandsensitiveenough.Theconfidencebuildingsimulationoutputsreasonablyreproducesthedynamicsoflocalsituationsandisconsistentwiththreeauthorshavinginvestigatedinoursite.Thankstoitsempiricalapproachanditsbalancedconceptionbetweensociologyandagro-ecologyattherelevantscale,i.e.theindividualtiedtosocialrelations,limitationsandobligationsandconnectedwithhis/herbiophysicalandeconomicenvironment,themodelcanbeconsideredasanefficient"trendprovider"butnotanabsolute"figureprovider"forsimulatingruralsocietiesoftheNigrienSahelandtestingscenariosonthesamecontext.SuchABMscanbeausefulinterfacetoanalyzesocialstakesindevelopmentprojects.

Rule-BasedModelling,RuralSahel,ConfidenceBuilding,Low-DataContext,SocialCriteria

Introduction

Sincethe1960sandindependence,ruralsocietiesinAfricahavereactedinaverylimitedwaytoagricultureprogramssupportedbydevelopmentagencies.Thepoorrelevanceofdevelopmentaidstrategieswithregardtothesocio-economicenvironmentisoftenpointedoutasamajorcauseofthislowimpact.Manydevelopmentprogramshaveunderestimatedseveralsocio-economicfactors(Biershenketal.2000;Saqalli2008):

1. Facingthelackofenvironmental,agriculturalandsocio-economicaldataandinformation,themajorityofdevelopmentoperatorsandresearchersactinginsuchdifficultcontextstendtorelymoreonexternaldatathatcombinetheeaseofaccessintimeandmoney,thequalityintermsoftemporalityandreliability,meaningmainlyremotesensingdataconcerningenvironmentalvariables.Theseconstraintsimplytherebythatenvironmentalfactorsareover-consideredbythemajorityofresearchersanddevelopmentoperators,underconsideringhumanvariables[1].

2. Asaconsequenceofthelastpoint,villagesareconsideredasalmostclosedsystemsinwhichmilletcroppingconstitutesthequasi-uniqueeconomicactivity,otherlocalactivitiesareviewedasminorcomplementsandoff-farmactivitiesaremerelyindicatorsofanoveralldegradationinlivingconditions.Improvementofgrainharvestsperunitofsurfaceisthereforeoftenconsideredastheonlywayforproductivityincreaseofthesefarmingsystemswhilemanpowerproductivityisneglected(KoningandSmaling2005).However,the20thcenturywasaperiodoffields'extensioninaquiteemptyterritory,meaningthatthemostlimitingresourcewaslaborratherthanland(Yamba2004).Moreover,multi-activityisastructuralcharacteristicofthesesocieties( Pauletal.1994 ).Milleville(1989)noticed:"Inmanycases,agriculturalproductionsystemsconstituteonlycomponentsofabroaderruralorpeasantstrategies;thelatterextendbeyondnotonlyagriculturalactivitiesbutalsothegeographiclocalarea".Therefore,farmingsystemscannotbeunderstoodwithoutmakingreferencetoa"systemofactivities"(Lavigne-Delville1999)whereagriculturalproductionsarepartofapaletteofactivities,spreadoutalongtheyearandgenderspecific(MazzucatoandNiemeijer2001 ).Understandingsuchhumanandenvironmentsystemsmeanstoconsiderthedifferentactivitiesthatarepracticedbythepopulation,inoroutofitsterritory.

3. The"household"isviewedasthemaineconomic,productionandconsumptionunit( GastelluandDubois1997).However,thepointisnotobvious:asfarbackas1980,Gastellu(1980)raisedtheissueofthevalidanalysisunit.SeveralunitswereproposedforAfrica,eachonehavingitsownlimitationswhenitcomestodefiningaholisticunit,basedeitherondomesticcriteria(temporaryor

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permanentresidence,orconsumption,alsocalledthe"potunit"),oreconomiccriteria.Thelatterisconfrontedwiththecomplexityofoverlappingmodesofconditionalownership,uses(production,consumption,accumulation)andtransmissibilityofproductionmeans(LambertandSindzingre1995 ;LazarevandArab2000).Thedefinitionoftheseunitsisalsoconstrainedbythepluralityoftheobjectivesandstrategiesofthedifferentfamilymembers.Whicheverwaythefamilyunitisdefined,therewillbeinconsistenciesandexceptions(Vaugelade1997).Whilemanydevelopmentagentsandresearcherspostulatesthelargehouseholdasthemainlocalhumanunit,inducingtoworkonlywiththeheadofthehousehold,onemayobservelocallytheimportanceoftheyoungsterswithinfamiliesandthelargeproportionofmononuclearfamilies.Inadevelopment-orientedpurpose,weproposetodiscussandredefinethenatureandtheorganizationofthefamilyasthemainunittouseineconomiccalculationsforlocaldevelopmentprojectplanning.Therefore,weassumetheindividualasmorerelevantforananalysisofsuchcomplexsituations,consideringthisindividualaslinkedandtiedtosocialrelations,limitationsandobligations.

Therefore,ourpurposeistodevelopnewinsightsthatgobeyondthesepro-environmental,pro-agricultureandpro-householdbiases.Theobjectiveofthepresentresearchisthereforetoinvestigatethebalanceandthedifferentialweightofthevariablesthatdifferentiallyaffectanyindividualinsuchvillageswheresocialandenvironmentalconstraintsarebothdetermininghis/hersustainability.Asanaction-orientedresearch,wefocusedourresearchontheeconomicandenvironmentalsustainabilityofthepopulation,i.e.werestrainedourresearchonthesocialandeconomicvariableswhichconditionthepopulations'differentialaccessestoeconomicandproductionactivities.NigérienSahelshouldbeconsideredasoneofthemostadequatesituationsregardingthiscombinationofconstraints.

DifferentmodelingapproacheshavebeenusedtoanalyzesocialandeconomicquestionsoftheruralworldasreviewedbyLambinetal.(2000).AgentBasedModels(ABMs)werefoundparticularlyusefulforsimulatingthemulti-disciplinaryapproachofamulti-sectorialreality(Berger2001).Theyreducetheneedforlong-termquantitativedatathatarenecessaryforglobalstatisticalmulti-criteriaoptimizationmodels;theycanintegratequantitativeandqualitativevariablesandsoformalizethecombinationofdisciplines(Requier-DesjardinsandRouchier1998).Theyareparticularlyrelevanttodescribethebehaviorofmanyentitiesandthereforethecomplexnetworkofinteractionsbetweenthemandemergence,withouttheneedtodefinetherationalityofthesebehaviorsuponpredefinedparadigm(Drogouletal.2000).OurchoiceoftheABMmodellingtoolisrelatedtothefollowinghypothesiswemadethatcanbetestedonlythroughsuchatool:"small"individualtemporally-basedsocialconstraintsandassetsmayhavehugeeffectsatthecollectivelevel.

Indevelopment-relatedstudies,ABMshavebeenappliedtoanalyzenaturalresourcemanagementissuessuchassemi-nomadiclivestockkeeping(Rouchier2000),hunting(Bakametal.2003),irrigation(Barreteauetal.2004 ),woodtrade(Bacaëretal.2004 ),diffusionofinnovations(Berger2001;SchmitandRounsevell2006 ),grazinglandmanagement( Matthews2006),wateraccessandlandallocation(Bahetal.2006 ):themainquestioninthesearticlesisthewayaresourceoradynamicevolvestakingintoaccountvariousfactorsofwhichhumaninfluenceiscrucial.Thetoolwasevenusedtomoreaction-orientedpurposesfollowingaCompanion-ModelingApproach(ComMod)(d'Aquinoetal.2003 ;www.commod.org).

Inthepresentstudy,theobjectofourmodelingisthedifferentindividualsandfactorswhichwillaffecttheirlivingconditions.

Fromafield-definedquestiontoanagent-basedmodel

Theoverallmethodologicalapproach

Focusingonthepopulationanditsfutureandconsideringthatthelatterdependsoneachvillager'swealthdifferentialevolutionaccordingtohis/hererelationshipswithactivitiesandothersourcesofincomesuchasrelatives,weneedtolookatallthemajorsourcesofwealthonevillagergetaccessto,includingsocialones.Tounderstandthecomplexityofaruralsystem,ourfirstassumptionisthattakingintoaccountallitscomponentsandsimplifyingthemismorerelevantthanneglectingsomeactivitiesbyfocusingontheonewhichseemstobethemostimportant.Itmeanstherebythatagametheorylikemodelisherepremature,becauseitimpliesasmallamountofvariables:lackingdata,norealdiscriminationproceduremaybebuilttodiscriminatethevariablestoexclude.Oursecondassumptionisthattakingintoaccountthestrategiesofallthemembersofafamilyandsimplifyingtheminducesalowergapthansimplifyingittoaunique"household"strategy.Finally,ourthirdassumptionisthat,asvillagesshouldbeconsideredasopensystems,itmeansthatonemayfoundmoreconsistencyinlocalbutimprecisevalues,figuresandrelationsthanonescomingfromliteraturedescribingequivalentbutnotlocalsituations.Thereby,ourmodelingapproachwassupportedbya1.5yearsfieldinvestigationperiodrequiredtodefinethelocalcontext,themaindifferentiationfactorsbetweenvillagersandavillager-basedtypologyofaccesstoeconomicactivities.

Facingthelackofdataandinformation,thefirstpointistoclearlyseparatethesubjectoftheresearch(i.e.thepopulationtodescribewithonlyindividualmodelrules,meaningthesearetheonestodiscussifonewantstoanalysetheadequacyofthemodelregardingitsconsistencywithrealityanditsrobustness),anditscontext(i.e.theagro-environmental,thedemographicalandtheeconomicalfactorsthatconditionthepopulation,whichcanbedescribedthroughformulasbasedonavailabledata).Thepurposeofthisconceptualseparationistofocusthemodelonsocialrulesthatdeterminetheindividualaccesstoeconomicandproductionactivities.Thesecondpointistodefinethevariablestoworkon,tonotbeinfluencedbythedataavailabilitythatsooftendeterminesthevariables'choice.Buildingthemodelneedtolookafterrulesandcriteriaasthechosenvariablestodescribethemodel(i)andonlyafter,tocharacterizethesevariablesthroughdataandinformation(ii).OuroverallmethodologyisbasedonFigure1:

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Figure1.OverallfieldandmodellingmethodologyoftheSimSahelmodel

i. Bothbiophysicalandsocio-economicvariablesweredefinedthroughvillagers'interviews:Itmeansthatforeachintervieweetalkingabouthis/herproduction/economicactivity(gardening,farming,livestockkeepingandmigration),wecollectedtheeconomicandsocialfactorshedescribedascrucialfortheactivity.Forinstance,theaccesstolandforamanwasrepeatedlydescribedaspossibleonlyifhegetsmarried.Marriagewasdescribedasconditionedbythepaymentofadowrytothebride'sfamily.Ifthefatherdoesnotwantorcannotprovidethedowry,theyoungbachelorshouldgetthismoneybyhimself,whichisdescribedaspossibleonlythroughseasonalmigration,meaningtherebytofindaminimumamountofmoneyforatleastthebusticket.Etc.Conditionalityrulesthatdefinethewaythesecollectedvariablesareactingonpeopleandthecontextoftheirapplicationwerebuiltbasedonthenumberoftimesinformationwasrepeatedbyselectedvillagersduringtheinterviewsandthroughcrosscheckingbetweeninterviews.Ifacontradictionappearedbetweenavailableliteratureandinterviews,weselectedthelatter,consideringthatlocalanswersbelongtoacoherentsystem.Forinstance,anthropologicalliteraturesuchasHarragin(2006)overestimatesthesolidarity,atleastwithinthefamilylevel(moneygiftsorloans)whilethissolidarityseemsduringourinterviewstonotbea"between-equals"practice,followinga"chief/clientele"-orientedrelationship(children&parents,elders&youngsters,men&women).Behaviouralgorithmswerethendesignedandcheckedthroughinformalmeetingswithdifferenttypesofvillagers,finallytranslatedintoUMLdiagramsandimplemented.

ii. Weseparatetheneededdataaccordingtothepurposeandthesubjectofourresearch:a. Thestructureandframeworkarethecontext,whichwedidnotinvestigateasitisnotourresearchquestion:alldata

describingvariablesbelongingtobiophysical,demographic(bothhumanandlivestock)andeconomic(prices,costs)domainscomefromtheliteratureonourstudysite(Gérardetal.2001;TurnerandWilliams2002;LaRovereandHiernaux2005;Gérardetal.2007).Wetherebyassumetheirvalidity.Interviewsresultswereusedifliteraturedoesnotprovidethenecessarydata(Forinstance,nodatawereavailableonmigrationcostsasnoresearchhasbeenassessedonthisactivityinthewholeWesternNiger).

b. Thesecondpartdealswithourverytopic,i.e.themechanismsoftherelationsbetweenvillagersandtheirenvironmentandbetweenthemselvesfortheaccessestoproductionandeconomicactivities.Buildingtheserulesfollowsafieldbasedinductiveprocess(Chalmers,1982)uponrepeatedandpunctualobservationsispartoftheconfidencebuildingprocess:agoodreconstitutionofthedynamicprocessesattheindividualleveltestifyforthegoodunderstandingofthesystem(anindividualagentinthemodeldoesrealizeandexperiencetheeventsthemajorityofthevillagersofthesamecategoryexperience:marriageaftergettingthedowry,accesstolandaccordingtohisfamilyrank,etc.).Becauseofdatauncertainty,theseparametersaredefinedasrelativetocompareandhierarchizeelementsandfactorsforeachvillageindividual,therebyavoidingartificialthresholdeffectsduetogapsbetweenvaluescomingfromdifferentdatasources.

c. Finally,weusedtheoreticalhypothesesfoundinthesocio-anthropologicalliteraturethatdescribesocialandenvironmentaldynamicsassupportsfordevelopinghypothesesandtestingevolutionscenarios.

Village&villagers'sampling

Followingthesetwopoints,weusedseveralcollectiveparticipatorytoolstodiscriminatethevillagesofourstudysite,ofwhichtransectsandthree-dayslongvisitsbutalsoParticipatoryResearchAssessments(Chambers,1994)andonePerception-BasedRegionalMapping(PBRM:Saqallietal.2009 ):thislasttoolhelpstobuildaclassificationofthedifferentareasofthestudysiteinwhichweeventuallyselectedfourrelevantvillagesandhamletsontwenty-one.Theselectioncriteriaweresize(big/small),access/noaccesstodryseasoncroppingandroads:Thesetwofactorsaremixedasgoodgravelroadscrossonlyvillageswithaccesstoshallowwater.Meanwhile,noaccesstoshallowwatermeanstohavenoaccesstogardeningwhilenogravelroadsonlymeanstohaveamoreexpensiveaccesstomigration),andinclusion/exclusionofthedevelopmentprojects'area,toreachthehighestamountofdiversesituations:Aswefocusonproduction&economicactivities,accesstoroadappearsascrucialforthemigrationactivity.Shallowwateraccessinavillageopensthepossibilitytogarden,meaningthatwehadalsotolookaftervillageswithoutthisaccess.Wehypothesizedatthebeginningoftheworkthataccesstodevelopmentactions(accesstoinorganicfertilizers,credit,wagesforworkingonagronomictrials)wasadiscriminativecriterionwhiletheyappearedfurtherthattheirextentwastoosmalltoactuallybedeterminant).

Thevillagers'samplewasdefinedupontwo-criterionstratification:Genderwasrepeatedlydescribedduringinterviewsasthefundamentalcriterionofdiscriminationwithrespecttotheaccesstoactivities'Womenhaveonlyaccesstolivestockkeeping&gardeningundercertainconditionsofageandfamilystatus.Menhaveaccesstofarming,migration,livestockkeepingunderothercertainconditionsofageandfamilystatusaswell).Thesecondcriterionisthelevelofresponsibility,i.e.thenumberofdependents.Weusedsemi-directinterviewsfocusedonthemainactivitycarriedoutbytheinterviewedpersonatthetimeoftheinterview:wehavelookedaftertheadequacybetweenthetimingoftheinterviewsandcriticaldatesforeachactivity.Eachpersonwasinterviewedonceortwicedependingonthenumberofactivitieshe/shemanages.126personswereinterviewed,notincludinginvestigationsonvillagehistorywitheldersandvillagechiefs.Thesefieldinvestigationswerecarriedoutfrommid-2004tilltheendof2005.Individualinterviewsprovidedsocialrulesthatconditionthedifferentialaccessandpowerofeachindividual,whichshouldbeparameterizedbyfieldlocation-relevantfigures,comingfromourowninterviewsandother"grey"sources",i.e.reportsfromNGOsandotherinstitutionsworkingonsite.

Overalldescriptionofthestudyarea

Thearea,calledthe"canton"ofDantiandou(approximately2000km 2andfortyvillages,hamletsandherdercamps),islocated70kmnortheastfromthepopulatedvalleyoftheNigerRiverandNiamey,thecapitalcityofNiger(800000inhab.).Itborderstotheeastwiththealsopopulatedfossilvalley(56.36inhab.km2),called"dallol",oftheBoboye.TheFakaraiscrossedbyanetworkofdryandthintalwegs(lessthan5%ofthetotalsurface)wheredryseasongardeningcanbepracticed.Widesandyplainsextendfromthesetalwegs,representing75%oftheareaandthemajorpartofthearableland.Theremaining20%iscoveredbyhardenedlateriticplateaus,whichcanbeusedonlyforpastureandwoodgathering(Loireau-Delabre1998).ThedominantethnicalgroupistheZarma,mainlyfarmerswithagrowinglivestock-keepingactivityintheirfarmingsystem(OlivierdeSardan2003;Saqallietal.2009 ).Figure2showstheannualcalendarforthedifferentactivitiesandthelivestockmovementsaswellasthemainsocialannualevents:

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Figure2.Annualcalendaroftheeconomicandsocialactivitiesinthestudysite

Rainfedagricultureisthemainactivityintermsoflanduseandmanpowerrequirementduringtherainyseason,fromJunetoOctober(Tahirou2002);itisamaleadultoccupationastheyarealwaysthefarmingmanager,usingtheirdependantsasmanpower.Themaincerealismillet(PennisetumglaucumL.Br. ),generallyassociatedwithcowpeaaslegume( VignaungiculataL.Walp ).Agricultureismanagedinanextensiveandanti-riskway:giventheverylowinherentfertilityofthesoilsandtheuncertainrainfallfrom400to650mmperyear(LaRovereandHiernaux2005 ),farmerstendtoclearandsowmoresurfacethantheycanmanage.Indeed,sowingscanoftenfailandvillagershavetorepeatthemseveraltimesinaparcel,uptofivetimesayear(Loireau-Delabre1998).Duringtheeraofspatialexpansion,nowcomingtoanend,villagersseekedtoscattertheirparcelsinthevillageterritoryaccordingtothedistancetothevillageandsoilqualitymainly(Seybou1993,Tahirou2002).Thelocallandinheritancesystemlargelyremainsontheso-called"traditional"mode:theeldestmaleinheritsalltheland,whichforcestheotherbrotherstofindlandsomewhereelse.Thus,landremainswithinmalelineages(Vanderlinden2000).

Seasonalmigrationoccursduringthedryseason,fromOctobertoMay:maleadultsmovetotheGulfofGuineabasintofindjobs(Mounkaïla2003).ThemoneytheybringbackisabigsourceofincomeforthewholeSahelianzone( Reardon1994).Accordingtoourinvestigationsin10villages,thepercentageofadultmenwhomigratevariesfrom70to90%dependingontheopportunitiesthattheirvillageoffersforextra-agriculturalincomegeneration,whichreducetheneedformigration:presenceofamarket,ofdryseasonactivitiesprovideopportunities.Seasonalmigrantsareratheryoung,between20and40yearsold:thissliceconstitutesaveryimportantproportionofthepopulation:in1999,itconstitutes36.7%ofadultmen(lessthan70yearsold)inthedepartmentofKollo(formerlyan«Arrondissement»)thatincludesthe"canton"ofDantiandou(NationalCensus2001).Seasonalmigrationallowsreducingthenumberofpeoplefeedingonthemilletstock,leavingmorefoodforthosewhostayhome.Moreover,themoneytheybringbackhome,includingclothingforthewholefamily,isanimportantcontributionfortheremainingfamilymembers,290€inaverageperfamilyperyear,orroughly30€perinhabitantperyear,quitesimilartothehouseholdincomefrommigrationinthecontiguousOuallamregion(AdaandRockström1993).Thesefundsseemnottohaveresultedintransformationsofthelocaleconomicstructure,accordingtoourobservationsoranalysesbyotherscientists,assignificantincreaseofasedentarykeptlivestock(Loireau-Delabre1998).Basedonourinterviews,mentrytogetmarriedasmuchaspossibletoreachtheMuslimlimitoffourwives,asthemainlocalwealthsymbol,whiletheyactuallyareallfarfromreachingthislimit.Eachmarriagecostinaverage1150€includingthefeasts,meals,giftsandtheweddingchest,sharedwithinthecommunity.Inaddition,thedowryshouldbegiventothebride'sfamily(asumtraditionallyequivalenttotwobulls,i.e.almost300€butvaryingaccordingtothelineagesofthetwofamilies).Tabaski,themainMuslimfeast,whereasheepshouldbesacrificed,isatimewhensheeppricesreach5to6timestheusualprice(from22.5€upto135€).Asalocalpriceindicator,a80kgbagofmillet,whichisenoughtofeedfiveadultpersonsforamonth,costfrom13to35€.

Migrantsalsobuysmallruminantsasameanstosavemoneyduringtherainyseason,therebyprotectingsomemoneyfromtherequestsoftheenlargedfamily(Pierret1999).Thesesavingsaretobeusedmainlyforpayingthenextmigrationtravelexpenses.Departuretimeisnormallyaftercropharvestbutmanyyoungbachelorsfrequentlyleaveearlier,arguingthatthereisstillamarriedmaleinchargeremainingathometohandlecropharvest.

Dryseasonvegetablegardeningcanbepracticedonlyinsomevillages,whichhaveaccessthroughwellstoshallowgroundwater(less

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than6mdeep)intalwegfields.Socialaccesstogardeningisactuallylimitedtomarriedandrespectablewomenwithdaughter-in-lawsorunmarrieddaughters:byowingobedience,theyfreetheelderfromheavydomesticchoresandgivetimeforgardening.Thesewomencanborrowsomelandinthetalwegsforgardening.Thisactivitycanprovideanincomequiteequivalenttothemalemigrationincomes:52,50€perwomancultivatingperyearonaverage.

Finally,socialactivitiesandparticularlymarriageareessentialtoinclude:thesearetheonesthatshowthelong-termefficiencyofone'sstrategy:forinstance,gettingmarriedwithhighlineageandstatuspersonsopensmorepossibilitiestobecomerelativewithwealthierand"well-bred"families;insuchplaceswhereincertitudefacinghazardsisthebiggestthreat,thereisnobetterinsurancesystemwhichisfarmoreimportantthanmoreindividualwealth,asouncertainandweakguaranteefacingcropping,migrationorevenrelatives'harassmenthazards.Thismarriage-builtinsuranceisevenmoreimportantastimegoesby.Fulfillingsocialdutieswhenneeded,suchasfuneralsor"baptisms",meansone'scapacitytoovercomethelifeincertitudeandtherebyisagreatindicatorofbothhis/herwealthandsocialcapitalaccordingtovillagers'perception,whichincreasethechancestobeconnectedtomoresecurenetworksthroughmarriageofhimselfandhis/herrelatives.Etc.

Agentbasedmodelingmethodology

TheselectedABMplatformisCORMAS(CommonResourcesManagementAgent-basedSystem)developedbytheGREENteamoftheCIRAD-TERA(Bousquetetal.2001 ).Themodelissplitintoanagro-ecologicalpartandasocio-economicpart(Figure3):

Figure3.UMLrepresentationoftheSimSahelmodel

Climateandlanddimensionsweresimulatedthroughthecellularautomatamatrixofthemodel,whereeachpixelisaparcelofthevillageterritory.LivestockagentbehaviourrulesandbiophysicalconstrainthierarchyaredefinedforeachpixelbasedontheUMLdiagramswebuiltfromcriteriaidentifiedbythevillagersandfromliterature:forinstance,thelistofthemainfactorstodescribeapieceofland(fertility,gardeningcapacity,reactiontorainfall,weeding,grazingandweedgrowth)weredefinedthroughinterviews,butthedescriptionofthesefactorsaredefinedthroughliteratureandunpublishedresearchdata.

Forthesocio-economicpart,wedevelopedfurthertheentity-centeredABMconceptbychoosingindividualsasthemainmodelagententityandnothouseholdsorfamilies.Themainprinciplewefollowedtoimplementfieldsocialobservationsistotranslatethemintosequencesofbehaviourwithoutintroducingexternalpostulatedrationality,asanempiricalcasestudyapproachfollowingJanssenandOstrom(2006).Noultimategoalisdefined,asallagentsarereactive.Nocognitivestrategicaspectsaredevelopedontomarriageorinheritanceprocedures.Individualsweresplitintoaproductivepart,named"Villager",andasocialone,named"State".Agentsaredefinedasequivalent,whichmeansthattheyhavethesameattributes:onlythevaluesoftheseattributesvary,definingeachagentanditscharacteristics.

TheSimSahelmodelwasvoluntarilyconceivedasaregroupingofnumerousprocedures.ItmeansthatintermsofUMLrepresentation,wemayhavetoshowalotofsmallsequentialdiagrams,eachoneverysimplebutwithalegendasbigasthediagramitself!Therefore,themajorityoftheproceduresarerepresentedintheirsimplestshape,i.e.mathematicalfunctionswithseveralparameters,herewiththeexceptionofthecomplexvillagerrationality-basedfieldextension.

Buildinganagroecologicalmodule

Fertility.Wedecidedtocreateavastenvironmentsoasnottolimitvillageragentsintheirquestfornewfields.Therefore,onecannotseeatransitionofactivitysystems,whichcouldhaveappearedifspacewerealimitingfactor(Boserup1980).Thetemporalscaleisdefinedasoneweekforeachtimestep.Table1showstheparametersusedforthebiophysicalpartofthemodel:

Table1:Implementingthebiophysicalpartofthemodel

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RainfallWeek: 0-21 21-29 29-33 33-37 37-41 41-45 45-52

Weeklyprobabilityofrain

0 25 75 25 0

Weeklymmequivalentofrainperpixelr(t)

0 20 40 65 40 20 0

Annualrainfalldroughtfactor

From67%to125%oftheweekrain

FertilityaffectingeventsInitialfertilityfactorFp

ValleyFp=2;plainFp=1

Rain Rainyseason(afterthefirstrainoccurringintheyear)f(t+1)=f(t)×(1+r(t)/(meanrainfall×10)(meanrainfall=550mm)

Sowingorweeding f(t+1)=f(t)-(1/1000)Harvesting f(t+1)=f(t)-(parcelyield/1000×Fp)Weedsandgrassgrowth(adaptedfordiscretesimulationsfromLassina(1992),Ahonon(1994)andDelabre(1998)

(weedsvalue(t+1)/weedsvalue(t))>1IfTrue:f(t+1)=f(t)-(weedsvalue(t+1)/weedsvalue(t)-1IfFalse:f(t+1)=f(t)

Shrubsgrowth(adaptedfordiscretesimulationsfromd'Herbèsetal.(1997)MontagneandHousseini(1998)andWeseletal.(2000)

(Shrubsvalue(t+1)/shrubsvalue(t)>1IfTrue:f(t+1)=f(t)-(shrubsvalue(t+1)/shrubsvalue(t)-1IfFalse:f(t+1)=f(t)

Manure(adaptedfordiscretesimulationsfromPierret(1999)

f(t+1)=f(t)+(1/1000×(1-ratioManure(t))WithratioManureeffectonyieldsadaptedfromGérard(2005)

Animalsmanuringandgrazingimpact

SurfaceconsideredS

"Dry"cattle Dryseason:transhumance Rainyseason:S=noncultivatedplainandvalley

Calvesandmilkingcows

Bothseasons:S=noncultivatedplainandvalley

Goats Bothseasons:S=Plateau,plaindistantfromatleasttwopixelsfromcultivatedfieldsorgarden

GoatsSheep Bothseasons:S=Plateau,plaindistantfromatleasttwopixelsfromcultivatedfieldsorgardenBothseasons:S=Valleyandvillageonly

Themodelisspatializedthroughadynamic100*100pixelsmapasthemaininterfaceforthevisualizationofinteractions.Thevillageterritoryisspatiallydescribedthroughalayerwhereeachcellhasaninitialagro-pedologicalfertilitypotentialP:theplain(P=1),thevalley(P=2),thevillageandtheplateau(P=0),asimplificationfromthesoilsqualitativedescriptionofGlÄttli(2005).A"parcel"entityissymbolizedbyacellofthemapandcanbeownedbyavillageragentforfarming.Eachpixelfertilityvalueisinitiallyequaltotheparcel'spotentialanditthenvariesaccordingtothevegetativegrowthofgrassandshrubsinthepixel,definedbythelocalpixelrainfallandtheinitialfertility,thecroppingimpactifthelandisownedandcultivated(sowingandpreparingtheland,weeding/notweeding,harvesting)andthemanureleftbylivestockorbyvillagers.

Thefields'manuringandgrazingimpactisdefinedthroughanestimationofthesurface,whichcanbegrazedbythedifferentspeciesasdescribedinTable1.Ateachstep,thissurfaceisgrazeddependingonthespecies(i.e.preferentiallyshrubsforgoats,preferentiallygrassfortheothers)alongaratiobetweenthesurfaceandthenumberofanimalsofeachspecies.Cattlearedifferentiatedbetweenacalfandmilkingcowgroup,stayingonthevillageterritoryallalongtheyear,anda"dry"group,leavingfortranshumanceduringdryseasons.Themanurefromtheremnantherdisthendefinedforeachanimalspecieforeachpixel:halfofoneanimalmanureisspreadingrazedareas

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(atdaytimeandtheotherhalfincorralsatnight(Ayantundeetal.2000 ;Ayantundeetal.2002 ):thislastpartofthemanureisthenspreadsolelyonthefieldsoftheanimalowner,whichmeansthatthisactionleadstoapotentialdifferentiationbetweenfieldsaccordingtothefieldowner'sherd.

Rainfall.Thisfactorcanbeconsideredastheuniqueclimatefactortoimpactonlocalsystems.Asithasahighspatialandtemporalvariability,theobjectivewastosimulatetheseasonalraindistributionandvariabilityasobservedfromICRISAT20yearslongrainfalldata,aswellasthespatialcorrelationofraineventsatthevillagelevel.Climatesimulationisdonethrougha5×5pixel"ClimateBlocks"matrix.Foreachtimestep,i.e.eachweek,eachblockreceivesarainfallprobability,andifitrains,aprobabilityforthequantityofrainaccordingtothetimeofrainyseason.TheprobabilitiesandtheweeklyrainquantityaredeterminedfromtheweathermeansdatabasefromICRISAT(LaRovereandHiernaux2005 ,Gérardetal.2007 ),allowingeachpixeltoreceivetheannualaveragerainfallvalueandtheintraannualvariabilityascollectedatICRISAT.Bothprobabilitiesarethenaffectedbyadroughtfactor,randomlydefinedforthewholeterritoryatthebeginningofeachyear;thatfactordeterminestherainfallvariationfortheyeartocomeandvariesfrom80%to130%ofthe500mmaveragerainfall,therebyreconstitutingthesiteinterannualrainfallvariability.Figure4showsthemapofthevillageterritoryandtherainfallforoneyear.

Figure4.Mapofthevillageterritoryandtherainfallforoneyear

ImplementingsocialrelationshipswithinfamiliesinaFakaravillage

Atthemodelinitialization,fiftyvillageragentsarecreatedinthevirtualenvironment.Villageragentsareage-defined(between1and55yearsold)andgender-defined.Amortalityprobabilityisalsodefined,growingalongtheageoftheagent,eventuallyallowingitanaveragelifeexpectancyof48,5years-oldforbothgendersasdescribedforthearrondissementintheUNDPreport(UNDP2005).Eachmaleadultagentischaracterizedbyarandomlydefinedlineagevaluefrom1(thebest)to4(thelowest),asameasureofthepowerofeachbigkinshipgroupinthevillage.Thisvalueistransmittedtoallhisprogenyandhiswives.Itcanhappenthattherearemore(orless)malethanfemaleadultagentsinonesimulation,orlineagescanhavedifferentsizes.Eachmaleadultagenttakesonepieceoflandasanewproperty,containingseveralparcels(Accordingtoobservations,womendonotownlandbutcanonlyborrowitforgardening).Thenumberofthelatterisdefinedaccordingtothelineagevalue,between3and7parcels,i.e.3to7mapcells.Thesizeofthelandpropertycanextendthereafteraccordingtotheavailablefamilymanpowerandthelineagevalue;thelandisthenchosenthroughacomparisonwithinthenotappropriatedparcelsavailableofaformulacombiningthedistancetovillageandthefertilityoftheparcel(Loireau-Delabre1998p.170-186).Maleadultagentscanthengetmarriedandcreateafamilywiththemselvesasthehead.

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Afterthemodelinitialization,genderandfamilyrankforavillageragentarethemostimportantfactorswithinthevillagecontext( OlivierdeSardan2003).Theydeterminetheaccesstoallthesocialand/oreconomicactivities,i.e.marriage,property,foodandmoneyredistribution,milletfarmingandmigrationformen,gardeningforwomen.

Ranksaredefinedforallchildagentsbutvaryforalladultagentsineachfamily.Theyareupdatedatthebeginningofeachtimestep,accordingtonewborns,newmarriages,reachingadulthoodanddeaths.Bydefault,therankingindexisequalto1,thehighestrank,i.e.theheadofthefamily;fromthenon,thefollowingrulesareapplied:

1. Foreachmaleadultagent,therankingindexincreasesbyonepointifthefatherisaliveandforasmanypointsasthenumberofelderbrothershehasgot.

2. Atweddingtime,femaleadultagentsmovetotheirnewhusband'sfamily.Ranksofmarriedfemaleadultagentsareequaltotheirhusbands'rank,plusonepoint.

3. Ranksofunmarriedfemaleadultagents,i.e.unmarriedsistersinafamily,areequaltotheyoungestadultmaleofthebrotherhoodplusonepoint.

4. Fromthenon,dependentrelativesaredefinedforeachadultagent:theyaremembersofthesamefamily,withalowerrank,andrelatedtohim/herbymarriageorasprogeny.

Themarriagestatusistherebyaveryimportantcriterionasitistheonlyactthroughwhichavillagerincreaseshis/herfamilyranklevel.Thisistrueforbothmen(marriedmenmayinheritlandsorappropriatenewfreeparcels,getaccesstochildmanpowerandchildren-related-prestige)andwomen(marriedwomenhavebetterranks,havechildrenandtherebygetaccesstotherelatedmanpowerforgardeningandchildren-relatedprestige).Aswedonotfocusonmarriagerationality,wedidnotimplementanegotiationbetweencandidatesastheorizedbySimon(1955)andimplementedforagent-basedsimulationbySmall(1999)orWhite(1999),andweratherusedasimpleassortativematingprinciplefollowingBecker(1974):marriagesarematchedthroughacomparisonbetweenmatingmembersuponasamereferencevaluesystem.Asasequentialprinciple,thismethodhasthelowestemergenceeffect,issimpleandfitsbetterwiththeobservedsituation.Aftermajority(16yearsold),marriagecanoccuriftheyoungmaleadultagentcanpatthedowry,throughthemoneyhebroughtbackfrommigrationforinstance.Muslimpeoplemarryuptofourtimesiftheycanaffordthecosts.Malessortthefemaleadultbachelorsaccordingtotheirfamilycharacteristics(lineage,size,familywealth,livestockandland)andtheirpersonalones(familyrank,livestock).Asastrictconditiontoownlandandtherebytobecomea"citizen"inthevillage,eachmaleadultagenthastomarryatleastonce.Earningenoughmoneytoaffordthedowryisthereforevitalforeachbachelorinhisvillage.Asdescribedabove,thedowryreaches300€butvaryingaccordingtothefamilies'lineages.

Inthesimulation,onlymarriedwomencanhavechildren.Foreachtimestep,thepregnancyprobabilityisdefinedasequalto1/52perweek.Atchildbirth,anewvillageragentiscreated,witharandomgender,asamemberofhisfamily,ofhislineageandofthevillage.Simulatedceremonycostsaretakenfromthefamilybalanceasequivalenttothecostofasheep.

Inheritanceisimportantaswellconsideringthepowerlandhavewithinthevillageandthefamily.Ittherebyhassenseonlyinthecaseofthedeathofalandoralivestockowner.Wehavefolloweda"brutal"versionofthecommonlaw:themarriedelderson(evenheisawidower)receivesthewholeheritage,leavingtheotherstoexploitnewlandssomewhereelse(LuxereauandRoussel1997).Aslongasthereremainadultfamilymembers,theonewhoserankisthebestbecomesthenewheadofthefamilyandreceivestheland.Iftherearenoadultsanymorewithinthefamily,thelandisgiventothechiefofafamilyfromthesamelineage.Theheiradoptstheremainingchildren.Livestockinheritancefollowsadifferentmechanism:itissharedequallyamongthecorrespondinggenderasobservedlocally:femaleownedlivestocktofemaleheirs,maleownedlivestocktomaleheirs.

Eachvillageragenthasanindividualfoodbalance,whichdefineshowmuchithastofeeditself.Foodredistribution,relatedtothenumberofdependents,isthemostimportanthierarchycriterionwithinafamily.Ateachtimestep,eachadultagentclassifiesitsdependentrelativeswithanegativebalancetakingintoaccountthe"socialdistance"betweenthem,i.e.thedifferenceofsocialrankswithingenderandagegroups:Thefoodmanagerfeedsfirstlymaleadultagents,thenhiswife/wives,thenothermarriedfemaleadultagents,thenunmarriedones(hisadultdaughtersorgreat-daughters)andfinallychildren.Whilemenarenotpresentinthefamily(i.e.duringmigrationperiods),thefooddistributionmanageristhewomanwiththehighestrank,usuallythewifeoftheheadofthefamily.Thisisconsistentwiththeobservedcommensalityprocedureinvillagesandparticularlyinthebiggestfamilies:thestrongestproductiveelementsseemtobefavoredinafamily:meneatalonebeforewomenwhofeedbabies,andfinallychildreneattheremainingfood).Ifafamilymemberbalanceisnotfulfilledbyredistributionanditsvalueisnegativeattheendofthestep,thevillagerentityissuppressedandconsideredasdead.Table2presentsthesocio-economicfactorsofthemodel:

Table2:Parametersusedforthesocio-economicpartofthemodel

Marriage AssortativesequentialcriteriafordiscriminatingfemalebachelorsLimitations LimitedtofourwivesLivestockeffect A(i,t)=Bfi(t)/8+Ofi(t)/16+Bi(t)/4+Oi(t)/8with:

Bfi(t):bovinelivestockownedbythefamilyOfi(t):ovineandcaprinelivestockownedbythefamilyBi(t):bovinelivestockownedbyIOi(t):ovineandcaprinelivestockownedbyi

Familysituationeffect

F(t)=(Familysize-Rank(i))-Childrensize(i)

Familylineageeffect

L=(lineageamount(4)+1)-Liwith:Li:familylineageofI

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Total Vi=(A(i,t)+F(t))*LiCeremonycosts

1150€(sameamountforall,independentlyfromlineage,accordingtotheobservedvillage-levelagreementsbetweenfamilies)paidbytheheadofthehousehold

Dowrycosts 300€(sameamountforall,independentlyfromlineage:itisanassumptionasdifferencesaccordingtofamilysituationmayexist

Propertyaccess(afterinitialization)Maximum"allowed"property

Familylineageeffect(i)*Familytotalmanpower(i,t)*Rwith:R:randomizedvalue50%chances:R=1;25%:R=1.5;25%:R=2)

"Semi-randomized"researcheffect:

Researchofnewparcelsstartsfromthevillagepixels.ThenumberoftrialsforavillagerI:N=Familytotalmanpower(I,t)

Parcelscomparisoncriteria:

C(j,t)=Fertility(j,t)-DistancetoVillage(j)

Foodandmoneyredistribution

Individualrankbasedassortativeclassificationofthedependents

Milletfarmingandmigrationformen

Onlythemanwiththehighestrank(orbydefaulthiswidow)canownandbythenmanagethelandbyrulingthedependentmalemanpower

Gardeningforwomen

Onlythewomanwiththehighestrankcanborrowandbythenmanageagardenbyrulingthedependentfemalemanpower

Simulatingthevillagersproductionactivities

Agriculture.Onlymaleadultagentscancropmilletfieldsandonlyfemaleadultagentscangarden(thatvillageragenthasthepowertomanagethatland,butitdoesn'tmeanthatheownstotalrightsontheyield).Onlyhouseholdheadscanextendthefamilypropertyaccordingtotheirfamilyneedsitsmanpower,followingFigure5:

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Figure5.UMLsequencediagramofthefieldexpansionpracticeofanindividualagentintheSimSahelmodel

Theeffectofthispracticecanbeseenalongsixtyyearsofsimulation.Figure6showstheexpansionofthefieldsalongthesixtyyearsofsimulation:

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Figure6.Theexpansionoffieldsduringsixtyyearsofsimulation

Therefore,oneparcelproducesonlyifithaseitherbeenappropriatedorborrowedbyavillageragent.Thediagnosisofavailablemanpowerdependsonthechosenfamilystructure.Eachweek,thevillageragentobserveshisparcels'statusandcalculatestheavailablemanpowerhecanuse(hisownandhisdependentrelativeswhoareavailable,accordingtothefamilystructure).Hethenclassifiesprioritiesandchoosesthefieldsoastominimizethelossofproductionoverallhisfields.Forinstance,ifavillageragentseesseveralofhisparcelsinvadedbyweedsandifhedoesnothavealltherequiredmanpower,hechoosestoweedtheparcelwiththemostimportantpotentialloss,i.e.themostfertileparcelinvadedbyweeds.Cropgrowthisgovernedbytheavailablemanpoweravillageragentcanmobilizetocarryouttherightcroppingactionattherighttime,byrainfallandbythefertilityrateateachtimestep.Cropsyieldsresultfromacombinationofthecropgrowthrate,weedsandfertilityeffectrates.Weedsevolveaccordingtoparcelfertility,rainfallandthefarmeragentweedingactions(Lassina1992;MontagneandHousseini1998 ;Sangaréetal.2001 ;Schlectetal.2006 ).Theyaffectmilletproductiononlyifthemilletgrowthlevelislowerthantheweedgrowthlevel.

Fortheirdryseasonvegetablegardens,femaleadultagentsfollowanequivalentsequenceofactions.Onlyfemaleadultagentswithstepdaughtersorwithunmarrieddaughtersatleast12yearsoldcanhaveaccesstogardening.Thesecanmobilizetheavailablefemalemanpowerfromthenoninasimilarfashionthantheheadofthefamilyformilletfarming.

Migration.Onlymaleadultagentsmorethan16yearsoldhaveaccesstothemigrationactivity.Thepricefortheroundtriptravelisthemajorconstraintforleaving.Thedateofdeparturedependsonthefamilyrank,i.e.thenumberofdependentrelatives.Tocomeback,themigrantshouldovercometwoconstraints:(i)forprestigereasons,amaleadultagentcannotcomebackinthevillagewithoutaminimumamountofmoney,atleasttheroundtripticketforthenextmigration,i.e.2*45€.(ii)Eachmaleadultagenthasa1%probabilitytoberacketedbythedifferentcustomsserviceshemeets.Tosimulatethemigrationactivity,wedefinedaweeklygainaccumulationfunctionasfollows:

Gi (t+1) = Gi (t) + g i (t) * [1+(Ni / 10)]

With:Gi(t+1)andG i(t)gainsoftheindividuali,respectivelyattimet+1andt,N ithenumberofyearsofexperienceoftheindividuali,g i(t),theweeklygain:gi(t)=6fromthebeginningofJulytomid-December(coffeeharvestingperiod),g i(t)=15frommid-Decembertomid-February(cocoaperiod),gi(t)=2frommid-FebruarytotheendofJune(noharvests).

Livestockkeeping.Itisherelimitedtoasavingscheme,exceptitsimpactonthefertilityprocess.Eachyear,individualgains(cropyieldsforfamilyheads,migrationformen,gardeningforwomen)aretransformedintolivestock,withadepreciationfactor.Villageragentsbuyanimalsaftercropharvest,whentheyhavegraintosell;however,itisthetimewhenmilletpriceisthelowest(13,75€inaverageper80kgbag).Whenthegranaryisquiteempty,theysellanimalstobuymillet,butatthatperiod,milletpriceishigh(27,50€perbagonaverage).This50%losscanbeconsideredasalivestockkeepingcost.Forinstance,theceremonycostsandthedowryabachelorshouldpayaresimulatedthroughanequivalentamountofmoney.Comingbackfrommigration,themigrantsbuysmallruminantstheyfeedduringtherainyseason.Atharvestingtimeorsometimesbefore,theyaresoldtopaytheticketforthenextmigration.Animalfatteningisconsideredascompensatingthe50%loss.

Resultsanddiscussion

Sensitivityanalysisonseveralselectedparameters:testingtheSimSahelrobustness

Wehaverealizedasensitivityanalysisonseveralparameters.AcompletesensitivityanalysisisyettobeachievedfollowingthethreestepsofRailsbacketal.(2006),i.e.(1)modelsensitivitytoeachparameter,(2)interactionstoparameterpairs,(3)reactionofmanagementalternativestoparametersperturbations.Hereafirststepisproposedbytestingparametersweconsideredasthematicallycrucialinthemodelonwhichweshowedtheresultsoffouroftheseoutputvariables,astheyhaveinterestingimpactpropertiesonthemodelsystem.Thefirsttwoarerelatedtolocalbiophysicalfactors(livestockfecundity,annualdroughtfactor),thetwootheronesrelatetoparametersthatconditionthevillages'society(migrationcost,marriagecost).Theothertestedparametersarerelatedtootherspecificscenariosandshowahighrobustness,andevenastabilityoftheresultsfacingvariationsoftheconsideredparameters.TwentysimulationsofeachsituationwerecarriedoutandthemeanresultsattheendofthesimulationsareshowninFigure7.Thesignificanceoftheresultsistestedwithavarianceanalysis(ANOVA)at95%ofsignificance,usingMATLAB©(standarderrorsnotshownforclaritypurposes).Thefactors'variationsoccuratthebeginningofeachsimulation:

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Figure7.Resultsofthesensitivityanalysisonseveralselectedparameters

InFigure7a,migrationcostsarevaryingaroundtheparameterisedvalueof30KFCFA(~45€):Thelivestockherdsizeandthenumberoffamiliesdecreasesignificantlywithmigrationcost:theeasiertomigrate,themoremigrantscansavemoneythroughlivestockandthemoreeasilytheycangetmarried.Thepopulationlevelreachesapeakaroundthemeanvalueof30KFCFA.Wecansupposethatitisduetotwoeffects:theeasieritistomigrate,themoremigrantsstayabroad,therebyreducingtheirtimeinthevillagehavingbabies(theirpresenceisnecessaryinthemodelforwomentogetpregnant).Ontheotherhand,themoreitisdifficulttomigrate,themoreitisdifficulttogetmarriedandhavechildrenaswell.Therefore,thecultivatedlandproportionisnotsignificantlyaffected:themoreitisdifficulttomigrate,themorethepopulationcroptherebycompensatingforthereductionofthepopulationandofthenumberoffamilies.Thesevariationshavenosignificantimpactonvegetationandfertilitybecausemostofthisadditionallivestockisbasedoncattleleavingthe"terroir"fortranshumanceduring9monthseachyear.

Figure7bshowstheimpactofmarriagecostvariationsaroundtheparameterizedvalueof160kFCFA(~240€).Apartfromthe40-kFCFAcase,theincreaseofthemarriagecostsimpliesareductionofthepopulationbecauseofthereductionofthenumberofmarriages,andtherebytheherdsize,thefertilityandthevegetationindices.At40kFCFA,thisvalueissolowthatthisisnotanymoreaconstraintformarriageoccurrence.Becauseofpolygamy,mengetmarrieduptofourtimesassoonaspossible,whichinduceahigherdemography.

Livestockreproductionvariationsaroundtheparameterisedvalueof3/52perweek(Figure7c)haveobviouslyalargeeffectonthenumberofanimals.However,increasingthenumberoflivestockhasvirtuallynoimpactonanyoftheotherfactorsexceptalimitedbutsignificantdecreaseofthevegetation.Thislackofeffectisalsoduetotranshumance.

Figure7dshowstheresultsofthetestofdifferentvaluesofthedroughtfactorarounditsvalueof1:livestockandthenumberoffamiliescontinuouslyincrease.Environmentalfactorsseemparadoxicallynottobesignificantlyaffectedaswellasthepopulation:actually,adroughtof0.6reducesagriculturalactivitiestothebenefitofmigrationandlivestockkeepingactivities,therebyreducingtheimpactofhumanactivitiesontheresourcesofthe"terroir".Ontheotherhand,ahighdroughtfactor,meaningawetterenvironment,increasesthetwoenvironmentalindicators(vegetationandfertility).

Thesesensitivitytestresultsprovidesomeconfidenceonthemodel,allowingustoconsideritasquitewellconstrainedbythedifferentparametersofitsstructure.Themodelisnotverysensitivetochangesontheconsideredparameters,meaningthatdifferencesinsimulationoutputsbetweenscenariosaremeaningful.Onemaynotethattheparametersfromtheliteraturethatbelongtobiophysicaldisciplinesconditionsostronglythesystemstructurethattheyactuallycreatenewenvironmentsandcontexts.Otherparametersdohave

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aneffectononlysomeportionsofthesystem.

Modelconfidencebuilding:testingtheSimSahelconsistencywithexternalavailabledata

Comparingsimulationoutputsbasedontwoobservedfamilyorganizationtypeswithexternaldata

Thisconfidence-buildingstepiscarriedoutbycomparingsimulationoutputsoftwoscenarios,basedontwodifferentfamilyorganizations:OurfieldobservationsletustonoticethehugegapbetweentheusualAfricanfamilyarchetypeandtheobserveddominantstructure,whichcorrespondtoamulti-decisionfamilyorganizationwherethereisacollectiveorganizationbutnotbasedonacooperationefficiencyasdefinedbyPareto(Sen1983inMeignel1993).Twofamilystructureswerecompared:

1. Aunitaryconceptoftheenlargedandpatriarchalfamily,stillthemainconceptofAfricanfamiliesasusedbydevelopmentagencies,hereafterreferredtoastheUnitaryFamilyStructure(UFS).

2. Acollectivenon-cooperativefamily,whichisnowadaysthemostcommoninthestudyregion( Tahirou2002p.23),andreferredtoastheNonCooperativeFamilyStructure(NCFS)scenario.Theyconstitute76%ofthefamiliesofoursample.

Foreachscenario,fifteensimulationswererunforaperiodofsixtyyears(i.e.twogenerations)eachof3120timesteps.Thetwoscenarios,eachonewithonefamilyorganization,arecomparedtoeachotherandwithliteraturedata.ResultsarepresentedinFigure8:

Figure8.ComparisonoftheannualgrowthrateofcultivatedlandforthetwoscenariossimulationoutputsandinterpolateddatafromLoireau-Delabre(1998)

Thetwoscenariosleadtodifferentpopulationgrowthrates,from50inhabitantsatthebeginningto472inhabitants(±240)fortheUFS(averagegrowthrateof3,91%peryear),andto176inhabitants(±81)fortheNCFS(2,19%peryear).TheGeneralCensusofNigerreportsgrowthratesof3,44%peryearoverthe1977-1988periodand2,75%peryearoverthe1988-2001periodfortheTillaberyregionbut,withinthisregion,2,3%peryearfortheKollodepartment,whichincludestheFakara.AssumingthatonehasevolvedinrealityfromadominantUFStypetoadominantNCFS,thenwemayconsiderthatthemodelhascorrectlysimulatedthedemographicgrowthandtheimpactofthefamilyorganizationonthisgrowth.

Loireau-Delabre(1998)beingcurrentlythebestreferenceontheFakararecenthistory,wecomparetheevolutionoftheacreageofcultivatedfieldsthatshesimulatedandthemodelsimulationresults.Loireau-Delabreacreageisderivedfromfiveaerialphotointerpolations(1950,1965,1975,1985,1992),combinedwithdemographicdatafromthenationalCensusandvillagedemographictrendsfromGuenguant(theselatterresultswereafterwardspublishedinGuenguantetal.(Guenguant2002).TheLoireau-Delabreresultsforthefirst20-yearperiodshowasimilarcultivatedsurfacegrowthratethantheonefromtheUFSsimulationresults,withavaluehigherthan3%peryear.Forthenext20-yearperiod,theLoireau-DelabreresultsshowadecreaseofthisratethatcorrespondsbetterwiththesimulationresultsoftheNCFS,withvaluesaround2%peryear,whereastheUFSscenarioresultsinanaccelerationofthiscroplandexpansion.Loireau-Delabreexplainstheobserveddecreasewiththelackofpotentialarableland.Thisphenomenonappearedinoursimulationsafter50or80yearsdependingonthechosenscenario.Meanwhile,socialconstraintsweimplementedcanbeconsideredasanticipatorsofthelandtension:moreafamilyhavesocialconstraints(noinheritance,nosupportfromrelatives,lowmanpower),morelandisdifficulttoget.TheNCFSscenarioillustratesmoretheimpactsofsuchsocialconstraintsonthedecreaseinpopulationgrowth,mainlyduetothelackofsupportfornewfamilycreations.Thisconstraintwasimplementedbasedontheresultsofourinvestigations:onecannowadaysobserveaclearlackofsolidaritybetweenrichandsomeparticularlypoorfamilies,withlittlelandand/ormanpowerand/orhavingsufferedlowcropyieldsrecently.Wethereforecansupposethatsuchfamiliesareonlyprecursorsofthefuture,whenlandconstraintswillbestrongerandgeneralized,asYamba(2005)describeditfortheMaradiregion.

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Analysingtheseresults,onemayconsiderthehypothesisofashiftinfamilyorganizations,fromthepatriarchalonetothemononuclearoneduringthe70's,meaningthatfaminesthatoccurinthisdecadehadonlycatalyticeffectsonaphenomenonthatwouldhavebeenrealizedanyhow.

Comparingsimulationoutputsbasedontwoobserveddevelopmentprojectactionswithexternaldata

Wecarriedoutthissecondconfidence-buildingtestbycomparingsimulationoutputsoftwoscenarioswithotherscholars'fieldresults.Foreachscenarioandsite,thirtysimulationswerealsorunforaperiodofsixtyyears(i.e.twogenerations).Thesescenariosarerelatedwithtwo"currency"possibilities:

1. Ascenariowhereeachindividualcurrency(i.e.atooltoevaluatetherelevancyofanactivity)money,whateveritsnature(cash,livestock,milletinthestorehouse),hereafternamedFM(FakaraMoney).

2. Ascenariowhereeachindividualcurrencyisreputation,i.e.elementsthatincludesitswealthbutalsosocialposition(rank,lineage)anditscapacitytofulfillsocialduties(supportingrelatives,sacrificinglivestockforceremonies,etc.),hereafternamedFR(FakaraReputation).

Table3presentstheresultsofthedifferentscholarsandourfieldandsimulationoutputs:

Table3:comparisonofthefieldresultsfromourinvestigationsandotherscholarsandsimulationoutputs

FAKARAwithoutprojects

Familysize

Familymanpower(inFLU)

Averagelivestockownedperfamily

%Areacorralledwithlivestock

HouseholdSamplesize

Cattle Goats SheepLoireau-Delabre(1998)Zarmavillagersonly

9,1 6,8 1,3 2,1 36,9 221

LaRovere(2001)p.47/Zarmavillagersonly

8,6 4,8 0,7 0,1 0,1 10,2 366

Tahirou(2002) NA 7 3 4 NA 100Ownfieldresults 8,2 5,2 2,0 1,41 0,2 NA 67FRC(simulated) 7,2±

0,25,7±0,5 3,4 1,17 0,2 8,9 NR

FEC(simulated) 7,1±0,2

5,4±0,5 3,2 0,93 0,3 15,4 NR

FRFW(simulated) 6,9±0,2

5,6±0,5 2,8 1,37 0,5 10,4 NR

FEFW(simulated) 7,0±0,2

5,6±0,4 2,4 0,80 0,5 8,1 NR

FLU:FarmingLaborUnit;NA:nodataavailable.NR:notrelevant

Datareportedbydifferentauthorsareactuallyhighlyvariableamongeachother,particularlyregardingherds.Ourresultsareconsistentwithdatafromthreeauthorshavinginvestigatedinoursite,i.e.theresultsfallwithintheirrange.Thedifferencebetweenourfieldandsimulationoutputsisthat,thankstoourhypotheses,wehavecorrectedtheprobableinterviewbiasesofoverestimatingtheproportionofenlargedfamiliesandunderestimatingthechildmanpower.Concerningoursimulationoutputs,livestockdistributionandsizefitbetterwithobservationsofLoireau-Delabre(1998)andTahirou(2002)whereastheothersimulatedfactors(familysize,familymanpowerandproportionofthe"terroir"corralledwithlivestock)fitbetterwithLaRovere(LaRovere2001).

Wesupposethatthebiggerlivestockherdinoursimulationoutputsrevealstheimportanceofthe"transhumantoffshorecattlesavings"thatmaybeomittedbyinterviewees.Thelowerfamilysizeinoursimulationoutputsrevealstheproportionofsmallfamiliesthatareusuallylessofteninterviewed.Finally,thehighermanpowerlevelinproportionofthefamilysizeinoursimulationoutputsisduetotheinclusionofthechildmanpowerbutalsotothefactthatweconsideredthatwomenandmenhavethesameFarmingLaborUnit.

Conclusion

Themodelappearsfairlyrobustregardingseveralparameterizedcrucialindicatorsandsensitiveenoughregardingseveralliterature-originatedindicators.Theconfidencebuildingstepsshowthatthemodelreasonablyreproducesthedynamicsoflocalsituations.However,thelatterstepshavealsoshownthedifficultytoobtain"real"figures:thethreeauthorswecitedaboveobtainedverydifferentfiguresforcrucialparameters.Suchdifferencesmaybeduetotheinherenteffectoftheinterviewprocesses,theissuesandthemethodology.Thanks

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toitsempiricalapproachanditsbalancedconceptionbetweensociologyandagro-ecology,aslongasthemodelisusedtosimulateruralsocietiesoftheNigerienSahelandtotestscenariosbasedonreliabletheoriesandconceptionsaboutthesamegeographicalandsociologicalcontext,itisbelievedthatthistoolcanbeusedtoestimatetherelative"weight"andpowerofthevariousconsideredfactors.Therefore,themodelshouldbeconsideredasanefficient"trendprovider"butnotasan"absolutefigureprovider".

Theseveralfieldanddataconstraintsorientatedourresearchonthedecisionprocessesthatoccurwithinvillagepopulations.Inanycase,agent-basedmodelsimulationoutputscannotbeusedtoproveassertions.Acompletemethodologyofintegrationbetweensociology,gametheoryandruraldevelopmentisstilltobebuilt:simulatingasocietyandbuildinganagro-ecologicalmodelhavedifferentscientificallyrelevantcriteria.However,therearenobettertoolsyetthanABMstocombinequalitativeinformationandquantitativedata.Inacontextwherethemajorityofdevelopmentdecision-makershaveatechnicalbackground,ABMsareausefulinterfacetointroducesocialstakesindevelopmentprojects.

Acknowledgements

Authorsarethankfulandindebtedtoallinterviewedvillagersfortheirgreatpatience.WeacknowledgethecontributionofDr.Tahirouforhisvaluableadviceonanthropologicalquestions,Pr.F.Gaspart(UCL),P.BommelandDr.C.LePage(CIRAD-TERA)fortheirhelpfulcommentsonthefirstversionsofthemanuscript.FinancialsupportcomesfromtheBelgianDGCDICRISAT-led"Decision-supportprogram".

ThisarticlehasbeentranslatedintoEstonian.

Notes

1ThemostsurprisingexamplemaybethefactthatthreeresearchanddevelopmentprojectsdealwithclimatechangeeffectsonWestAfrica,whennequivalentactionsconcernthedemographicgrowth,thehighestintheworld(forinstance,7.1childperwomaninNiger)

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