Unraveling theimportanceofricefieldsforwaterbird populations inEurope
Gregorio M.Toral • Jordi Figuerola
Abstract Rice fieldsareanalternativehabitatforwaterbirdsandprovidefoodand shelterformany avianspecies,butthereisalackofinformationabouthowtheuseofrice fields translatesintopopulationleveleffects.Theaimofthisstudywastotesttherela- tionshipbetweentheuseofricefields byEuropeanwaterbirdsandtrendsintheirpopu- lations.Wetestedthisrelationshipduringtheautumnmigrationseasonandduringthe breeding season.Basedoncountsconducted overthelast23yearsinnaturalmarshesand areasof ricefieldsinDon˜ana(SWSpain),an indexofricefieldusewasconstructedfor 76 birdspecies,whichwasthencomparedtothesespecies’Europeanpopulation trends obtainedfromtheliterature.Apositiverelationship wasfoundbetweenwaterbirdpopu- lationtrendsand theuse ofricefieldsduringautumnmigrationseason.Ourstudysuggests thatchangesintheCommonAgriculturePolicyinEuropeleadingtoreductionsinareasof ricecultivationmay haveimportanteffectson waterbirds.Therestorationofformermarsh areasandthemaintenanceofricecultivationwould seemtobemore environmentally friendlyapproachesthantheuseoftheseareastogrowalternativecropsorsolarfarms.
Keywords Agriculture Don˜ana Migration Shorebird Wetland
Introduction
Intensehabitattransformationlinkedtohumanactivityhasmoldedthecompositionand abundanceofaviancommunitiesthroughouttheworld(Murphy2003;Lemoineetal.
2007)andagriculturalintensificationhasbeenassociatedwithseriousdeclinesinthe populationsofmanyfarmlandbirdsoverrecentdecadesinEurope(Donaldetal.2001,
2006;Wretenbergetal.2007).Therearealsomany studiesabouttheeffectsof agriculture
G.M.Toral() J.Figuerola
DepartmentofWetlandEcology,Don˜anaBiologicalStation,C.S.I.C.,Avda. Ame´ricoVespucios/n41092,P.O.Box1056,41080Seville,Spain
e-mail:
J.Figuerola
e-mail:
onwaterbirdpopulations.Duncanetal.(1999)showedanegativeeffectofagriculture intensification onwinteringduckspopulations.Longetal.(2007)concludedthatan increaseintheareaofagriculturallandcouldbeassociatedwithdecreasingpopulationsin Anseriformes.However,somecropscouldact asanalternativeor‘buffer’habitat forsome waterbirdspeciesthatfeedorbreedinagriculturalareas(Sa´nchez-Guzma´netal.2007)and therearesomestudiesreportingpositive effectsofagricultureoncertainspeciesof waterbirds(e.g.,Gauthieretal.2005;Fasolaetal.2010).
Wetlandsareoneofthemostproductiveecosystems onEarth.Nevertheless,theyhave beensubmittedtointensivetransformationanddrainageandtheyaretodayindecline throughout theworld(CzechandParsons2002).Itisestimatedthatover50%ofthe world’swetlands havebeenlostsince1900.Overthelastfewcenturies thenaturalwet- landsoftheMediterranean regionhavebeenreducedinareaby80–90%duetopressure fromhumanpopulation growthandtheconversionofwetlandsintoagriculturaland urbanizedareas(Finlaysonetal.1992).InsouthernEuropeandNorthAfricalargeareasof pristinewetlandshavebeentransformedintoricefields, acropthatcovers581,978hain Europe(FerreroandNguyen2004).
Previousworksuggeststhatricefields maybeanimportanthabitatforwaterbirds throughout theworldandinsomeareasmayinfactbetheprimaryforaginghabitat availableto them(CzechandParsons2002).Severalstudieshaveshowntheimportance of ricefieldsasawinteringsiteforwaterbirdsindifferentlocationsaroundtheworld,suchas California(ElphickandOring1998;ElphickandOring2003)or Cuba(Acostaetal.1996) inNorthAmerica;Portugal(Lourenc¸oand Piersma2008)orSpain(Rendo´netal.2008)in Europe;orJapan(Maeda2005)inAsia.Furthermore,ricefields areusedbyavarietyof waterbirdsasbreedingsites(FasolaandRuiz1996),althoughtoalesserextentthanas foraging sites(CzechandParsons 2002). Despite alltheinformationabouttheimportance ofthiscropforwaterbirds,thepossibilitythattheuseofricefieldsactuallytranslates into populationleveleffectshasreceivedlittleattention.Sincericefields differinhabitat structure,watertabledepth,anddisturbance levelsfromnaturalmarshes,andgiventhat thesefactorsareknowntoaffecthabitatselectionbywaterbirds (BolducandAfton2004; Canepucciaetal.2007),itislikelythatnotallspecieswill use this alternativehabitatwith thesameintensity.
HundredsofthousandsofbirdsusetheareaofDon˜ana(SWSpain),whichincludes marshesandricefields,inthecourseofayear.TheDon˜anamarshesarethemostimportant winteringsiteformigratingwaterbirds intheMediterranean(Rendo´netal.2008)andone ofthemain migratorystopoversforwaterbirdsusingtheEastAtlantic flyway.Inthispaper wetestthehypothesisthatthespeciesthatusericefieldsmostintensivelywillbenefitat thepopulationlevelfromthepresenceofthisalternativehabitat.Wetestthishypothesisby analyzingtherelationshipbetweenricefield useandpopulationtrendsinEuropefor76 speciesofwaterbirdspresentinnaturalmarshesand/orricecropsinsouthernSpain.
Methods
InSpain,ricefields cover118,000(FAOSTAT2004)andthelargestsuchareainSpain (36,000ha)issituatednearthemarshesoftheDon˜anaNationalPark, a55,000hawildlife reservenorthoftheGuadalquivir estuaryinAndalucia(SWSpain).Ofthe180,000haof freshandbrackishmarshespresentin1900,36,000 haweretransformedintoricefields between1926and1997(Garc´ıa-NovoandMart´ın-Cabrera2005;Rendo´netal.2008). Additionally,othertypesoftransformationhavereducedtheareaofnaturalmarshesto
30,000haatpresent(Enggass1968;Garc´ıa-NovoandMart´ın-Cabrera 2005)andchanges inthemarshes’hydrologicalstructurehavedramaticallyalteredtheflood patternsofthe area.DuetosummerdroughtsandtheunpredictabilityofrainfallinMediterraneanclimate zones,theperiodsofmaximumfloodlevelsinricefieldsandnaturalmarshesdonot overlapintime,therebyensuringthatricefields canactasanalternativehabitatfor waterbirdsinthearea.
Bird’scountsandpopulationindices
Weusedbirdcountsconductedoverthelast23years(1980–2003)intheDon˜anaNational Parkandsurroundingareathatarepartofthelong-termmonitoringprogramruninthearea ( analyzed terrestrial sur- veysfromthreesectorsofricefields(2,756ha)andsevensectorsofmarsh(3,192ha) sincethehighestnumberofsurveysexistforthesetensectors.Anindexofricefieldusein relationtonaturalmarshusewascalculatedforeachspeciesbasedontheproportion of positivecounts (countsinwhichatleastonebirdofaparticularspecies wasdetected)in rice fieldsin relationtothe numberofpositivecountsin bothhabitats(naturalmarshesand ricefields).Wedidn’tbasetheindexofricefielduseonabundance datatoavoidthebias producedbybigflocksofwaterbirds.WeusedEq.1tocalculatetherice-fielduseindex foreachspeciesandmonthbasedonitspresence/absence.
Rice-fielduseindex¼MPR=ðMPRþMPMÞ:ð1Þ
MPR(MeanPresenceinRicefields)=numberofpositivecountsinricefields/total numberofcountsinricefields.
MPM (Mean Presence in Marshes)=number of positive counts in marshes/total numberofcountsinmarshes.
Indiceswereonlycalculatedforspecieswithatleastfivepositivecountsinthearea (76species).Wecalculatedtwoseparateseasonal-use indices(breedingandautumn migration)adjustedtocropandbirdphenology.Wecalculatedseasonalindicesasthemean valueofthemonthsincludedintheseason.Thebreedingseasonindexwascalculatedusing datafromApriltoJuly,aperiodthatlargelycoincides withriceseeding andgrowth. The autumnmigrationseasonindexwascalculatedwithdatafromOctober toJanuary,months includingautumnmigrationandthewinteringperiodthatcoincidewith cropripening,soil tilling, andthe shallowfloodingoffields,whichdryoutbythe endofDecember–January.
Wedefinepopulationasa‘distinctassemblageofindividualswhichdoesnotexperience significant emigrationorimmigration’(DelanyandScott2006).Europeanpopulation trendsforwaterbirdswereobtainedfrom‘WaterbirdPopulation Estimates’(Delanyand Scott2006),wheretrendsareexpressedasoneofthefollowingcategories:increasing, stable,decreasing,fluctuatingandextinct. Thosecategorieswerescoredforthe analysesas positive(2),notrend(1),ornegative(0),andtheresultingdependentvariablewastreatedas anordinalvariableintheanalyses. Weassigned notrendvalue(1)toMarmaronetta angustirostris,theonlyspecieswithafluctuatingpopulationtrendforEurope.Inadditionto ricefieldusewe alsoestimatedmeanmigrationdistanceasdegreesoflatitudebetweenthe meanbreedingandmeanwinteringranges(seedistribution mapsinCramp1977–1985).
Statisticalmethods
TherelationshipbetweenpopulationtrendsinEurope(asadependentvariable)andthe rice-fielduseindex(asanindependentvariable)wastestedusingGeneralizedLinear
MixedModels(GLMMs).Weconsideredthepossiblelackofindependence ofoursam- ples,sincespeciesfromthesamefamilyshareanumber ofmorphologicalandlifehistory featuresandthereforecouldhavesimilarpreferences intheirhabitatselectionforfeeding and/orbreeding.Toavoidthispossiblebiaswemadeanexploratory analysis,usinga nestedGLMtoexaminehowtrendsinEuropeanbirdpopulations varybetweendifferent taxonomiclevels(order,familyandgenera).Mostofthevarianceinpopulationtrendswas unrelatedto taxonomy(82%).Familyandgeneraexplained12and6%,respectively,ofthe variance;novariancewasexplainedbyorder.Furthermore,weusedanestedGLMto examinethevariationintheuseofricefields betweendifferenttaxonomiclevels(order, family,andgenera).
TheGLMMswerefitusingtheGLIMMIXprocedureinSAS9.2.Thepopulationtrend inEuropewasmodelledbyspecifyingamultinomialerrordistribution andacumulative logitlinkfunction,withtheexplanatoryvariablescodedasfixed effectsandfamilyand generanestedwithinfamilyasrandomeffects(orderwas excludedasrandomeffectbased ontheresultsoftheexploratoryanalysis).Becausethetaxonomywasincorporated inthe modelwith ahierarchicalstructure(i.e., generanestedwithinfamilies),themodelhelps to correctforthephylogeneticeffectsdescribedbetweenthesetaxonomiccategories.
Twodifferentsetsofmodelswerefitted tothedata,oneforeachofthetimeperiods usedtocalculatetherice-field useindex.Theexplanatoryvariablesintheinitialmodels includedrice-fielduseindex,migrationdistance(anditsquadraticterm),aswellasthe two-wayinteractionwiththerice-field useindex.Weincludedmigrationdistanceasan explanatoryvariableandalsoincludeditsquadratictermtoexplorethepossiblediffer- encesin populationtrendsbetweenshort,intermediate, andlongdistancemigrantsbecause previousstudieshavefoundAfro-Palearctic migrantbirdstohavenegativepopulation trends overthelastfewdecades incomparisontoshort-distancemigrantsorresident birds (Sandersonet al.2006).Wealsoaddedtheinteraction ofrice-fielduseindexandmigration distance sincethishabitatisveryimportantasafeedingsiteandcouldhaveadifferent valueforwaterbirdspeciesdependingontheirrequirementsoffood,whichcouldbe relatedtothedistancetheyhavetocoverduringmigration.Wefollowedabackwards regression-model selectionprocedure,andthefactorcontributingleasttothemodelwas removedateachstepbeforethemodelwasrefitted.Variableswereretainedinthemodel untilalltheinteractionsincludingthevariablehadbeenexcludedfromthemodel;alower probabilitylimitof0.10wasusedtoretainafactorinthemodel.Onlyfactorswith P\0.05were interpreted as statistically significant.We usedthis methodof model selectioninsteadofAkaikeInformationCriterion(AIC)becausewehaverandomeffectsin ourmodels.Although therangeofapplicationofAICcanbeexpandedtorandomeffect, thisextensionisinanearlystageofdevelopment(BurnhamandAnderson2002).We intendedtoincludealsobreedinglatitudeasanexplanatory variableinouranalyses. However it was strongly correlated with migration distance (r2 =0.41; F=48.2; DF=1,70;P\0.001),andtheinclusionofbothvariablesintheanalysesmayproduce spuriouscorrelations. Forthesereasonsweconfirmedinseparateanalyses(notpresented) thattherelationships foundwereinfactwithmigrationdistanceandwerenotsignificant whenusingbreedinglatitudeinsteadofmigrationdistance.
Results
Although theautumnmigrationperiodandbreeding seasonindiceswerepositively cor- related,therelationshipwasnotstrong(r2 =0.29;F=27.4;DF=1,69;P\0.001),
Fig.1 Variationin thenumberofwaterbirdsthat usetherice fieldsandmarshesinDon˜ana duringtheyear. Notethatthenumbers referonlytotheareasincluded inourstudyandnottothecompletesurfaceareaof marshesandricefieldspresentinDon˜ana
suggestingthat somedifferencesoccurinspecies’habitatpreferencesbetweenautumnand spring.Birdabundanceinricefields incomparisontonaturalmarsheswashigherduring theautumnmigrationseasonthaninspring(Fig.1).
Therewasimportantinterspecificvariabilityintherice-fielduseindex(seeTable1) thatwasnotexplainedbytaxonomy, withvaluesrangingbetween0(e.g.,Aythyanyroca) and1(e.g.,Sternanilotica).Varianceexplainedattheorderlevelvariedbetween20and
24%dependingonthetimeperiodanalyzed.Atthefamilylevel, thevaluesvariedbetween
0and21%.Novariancewasexplainedatthegeneralevel.
Thefinalmodelfortheautumnmigrationseasonretainedthreevariables:useindex, migrationdistanceanditsquadraticterm(Table2).According tothismodel,population trendsinEuropewerepositivelyrelatedtotheautumnrice-fielduseindex,andhada quadraticrelationship withmigrationdistance,withmorenegativepopulationtrendsfor speciesmigratingovermediumdistancesthanforlongdistanceandresidentspecies.Inthe caseofthebreedingseason,thefinal modelwasverysimilarandretainedfourvariables (Table2).Therelationshipofpopulationtrendswithmigrationdistancewassimilarto whatwefoundduringtheautumn.Rice-fielduseindexduringthebreedingseasontended tobepositivelyrelatedtopopulationtrends(P=0.08)andmorestronglyinresidentthan forspeciesmigratingoverlongerdistances(P=0.06)(Table2).
Discussion
Wefoundimportantinterspecificdifferencesinthefrequencyofuseofricefieldsas opposed to natural marshes in Don˜ana, which were related to population trends in waterbirdpopulationsinEurope.Thissuggeststhatthealternativehabitatsprovidedby ricefieldsduringtheautumnmigrationperiodfavorthepopulationsof certainspecies.We alsofoundthisrelationduringthebreeding seasonalthough itdidn’treachsignificance, suggestingthatwaterbirdsbenefitfromusingricefieldsmainlyasafeeding habitatduring theautumnmigrationperiod.
Table1 Valuesfortherice-fielduseindexforthespeciesanalyzedinthetwodifferenttimeperiodstaken intoaccount
Specie / Migrationseason index / Breedingseason index / Order / FamilyAythyanyroca / 0.00 / 0.00 / Anseriforms / Anatidae
Oxyuraleucocephala / 0.00 / 0.00 / Anseriforms / Anatidae
Aythyaferina / 0.01 / 0.00 / Anseriforms / Anatidae
Nettarufina / 0.00 / 0.00 / Anseriforms / Anatidae
Aythyafuligula / 0.03 / 0.00 / Anseriforms / Anatidae
Tadornatadorna / 0.03 / 0.00 / Anseriforms / Anatidae
Anascrecca / 0.10 / 0.00 / Anseriforms / Anatidae
Anasstrepera / 0.10 / 0.00 / Anseriforms / Anatidae
Anaspenelope / 0.19 / 0.00 / Anseriforms / Anatidae
Anasclypeata / 0.14 / 0.03 / Anseriforms / Anatidae
Anasplatyrhynchos / 0.14 / 0.04 / Anseriforms / Anatidae
Anasacuta / 0.33 / 0.00 / Anseriforms / Anatidae
Marmaronetta angustirostris / 0.34 / 0.03 / Anseriforms / Anatidae
Anasquerquedula / 0.19 / 0.00 / Anseriforms / Anatidae
Anseranser / 0.50 / 0.00 / Anseriforms / Anatidae
Egrettaalba / 0.00 / 0.00 / Ciconiiforms / Ardeidae
Nycticoraxnycticorax / 0.00 / 0.00 / Ciconiiforms / Ardeidae
Ixobrychusminutus / – / 0.18 / Ciconiiforms / Ardeidae
Ardeacinerea / 0.60 / 0.14 / Ciconiiforms / Ardeidae
Ardeolaralloides / 0.93 / 0.13 / Ciconiiforms / Ardeidae
Ardeapurpurea / 0.99 / 0.14 / Ciconiiforms / Ardeidae
Bubulcusibis / 0.94 / 0.08 / Ciconiiforms / Ardeidae
Egrettagarzetta / 0.78 / 0.21 / Ciconiiforms / Ardeidae
Burhinusoedicnemus / 0.50 / 0.00 / Ciconiiforms / Burhinidae
Recurvirostraavosetta / 0.17 / 0.11 / Ciconiiforms / Charadriidae
Charadrius dubius / 0.20 / 0.00 / Ciconiiforms / Charadriidae
Himantopushimantopus / 0.47 / 0.14 / Ciconiiforms / Charadriidae
Pluvialissquatarola / 0.48 / 0.21 / Ciconiiforms / Charadriidae
Vanellusvanellus / 0.59 / 0.08 / Ciconiiforms / Charadriidae
Charadrius alexandrinus / 0.44 / 0.33 / Ciconiiforms / Charadriidae
Charadrius hiaticula / 0.67 / 0.43 / Ciconiiforms / Charadriidae
Pluvialisapricaria / 0.64 / 0.00 / Ciconiiforms / Charadriidae
Ciconiaciconia / 0.81 / 0.53 / Ciconiiforms / Ciconiidae
Ciconianigra / 0.97 / 0.00 / Ciconiiforms / Ciconiidae
Glareolaprantincola / – / 0.42 / Ciconiiforms / Glareolidae
Larusgenei / – / 0.00 / Ciconiiforms / Laridae
Larusminutus / 0.12 / 0.00 / Ciconiiforms / Laridae
Sternaalbifrons / 0.00 / 0.13 / Ciconiiforms / Laridae
Larusfuscus / 0.65 / 0.00 / Ciconiiforms / Laridae
Larusridibundus / 0.56 / 0.13 / Ciconiiforms / Laridae
Chlidoniasniger / 0.51 / 0.21 / Ciconiiforms / Laridae
Table1continued
Specie / Migrationseason index / Breedingseason index / Order / Family
Laruscachinnans / 0.79 / 0.08 / Ciconiiforms / Laridae
Sternanilotica / 1.00 / 0.39 / Ciconiiforms / Laridae
Chlidoniashybridus / 0.94 / 0.37 / Ciconiiforms / Laridae
Phalacrocorax carbo / 0.24 / 0.00 / Ciconiiforms / Phalacrocoracidae
Phoenicopterusruber / 0.17 / 0.00 / Ciconiiforms / Phoenicopteridae
Platalea leucorodia / 0.40 / 0.09 / Ciconiiforms / Phoenicopteridae
Podicepscristatus / 0.00 / 0.00 / Ciconiiforms / Podicipedidae
Podicepsnigricollis / 0.00 / 0.00 / Ciconiiforms / Podicipedidae
Tachybaptusruficollis / 0.02 / 0.00 / Ciconiiforms / Podicipedidae
Arenariainterpres / 0.00 / 0.00 / Ciconiiforms / Scolopacidae
Calidrisalba / 0.00 / 0.00 / Ciconiiforms / Scolopacidae
Numeniusphaeopus / – / 0.00 / Ciconiiforms / Scolopacidae
Limosalapponica / 0.33 / 0.00 / Ciconiiforms / Scolopacidae
Tringahypoleucos / 0.47 / 0.00 / Ciconiiforms / Scolopacidae
Tringaerythropus / 0.52 / 0.00 / Ciconiiforms / Scolopacidae
Numeniusarquata / 0.35 / 0.00 / Ciconiiforms / Scolopacidae
Calidrisferruginea / 1.00 / 0.12 / Ciconiiforms / Scolopacidae
Calidriscanutus / 0.33 / 0.42 / Ciconiiforms / Scolopacidae
Limosalimosa / 0.44 / 0.16 / Ciconiiforms / Scolopacidae
Tringatotanus / 0.36 / 0.25 / Ciconiiforms / Scolopacidae
Calidrisalpina / 0.47 / 0.12 / Ciconiiforms / Scolopacidae
Calidrisminuta / 0.61 / 0.00 / Ciconiiforms / Scolopacidae
Philomachuspugnax / 0.64 / 0.38 / Ciconiiforms / Scolopacidae
Tringanebularia / 0.69 / 0.17 / Ciconiiforms / Scolopacidae
Gallinagogallinago / 0.74 / 0.00 / Ciconiiforms / Scolopacidae
Tringaochropus / 0.88 / 0.12 / Ciconiiforms / Scolopacidae
Tringaglareola / 0.97 / 0.41 / Ciconiiforms / Scolopacidae
Lymnocryptesminimus / 1.00 / – / Ciconiiforms / Scolopacidae
Plegadisfalcinellus / 0.98 / 0.09 / Ciconiiforms / Threskiornithidae
Grusgrus / 0.45 / 0.00 / Gruiforms / Gruidae
Rallusaquaticus / 0.00 / – / Gruiforms / Rallidae
Fulicaatra / 0.11 / 0.03 / Gruiforms / Rallidae
Fulicacristata / 0.16 / 0.00 / Gruiforms / Rallidae
Gallinulachloropus / 0.27 / 0.00 / Gruiforms / Rallidae
Porphyrioporphyrio / 0.43 / 0.10 / Gruiforms / Rallidae
OurresultssupportthehypothesisthatthecreationofricefieldsaftertheSecondWorld WarintheMediterraneanRegionhelpedincreasethepopulationsofbreedingandmigrating waterbirds(CzechandParsons2002).Previousworksuggeststhatincreasesinthenumber oflittleegretsinItalyweremainlydrivenbyanincreasingavailabilityofricefields(Fasola etal.2010).Sa´nchez-Guzma´n etal.(2007)recordedpopulationsizesofinternational importance ([1% ofthe biogeographicalpopulationusingthe EasternAtlantic Flyway)for
Table2 RelationshipbetweenEuropeanpopulationtrendsandthevariablesconsideredintheanalysis
FactorMigrationperiodBreedingperiod
Estimate / F1,27 / P / Estimate / F1,28 / PUseindex / 2.0306 / 5.85 / 0.023 / 7.1779 / 3.2 / 0.085
Migrationdistance / -0.1423 / 7.52 / 0.011 / -0.1153 / 5.93 / 0.022
Migrationdistance9migrationdistance / 0.0018 / 5.06 / 0.033 / 0.001894 / 5.01 / 0.033
Useindex9migrationdistance / -0.2516 / 3.74 / 0.063
Significantvaluesareinbold.Estimates correspondtotheregression parametersofeachfactorincludedin thefinalmodelobtainedbybackwardsregression
severalwaterbirdspeciesinthericefieldsofExtremaduraincontinentalsouthwest Spain, andsuggestthatthecreationofthesericefields couldbemodifyingthewinteringand/or feedingsitesofsomeofthewaterbirdsusingthisflyway.Ackermanetal.(2006)foundthat theincreaseinricecultivatedareainCalifornia’sCentralValleyallowedthewintering PacificGreaterWhite-frontedGeese(Anseralbifronsfrontalis)populationtoconcentrate theirhabitatuse,thusreducingthepopulationrangeandroost-to-feed distancesbetween decades.
Ourresultssuggest thatthepositiveeffectofricefieldsonwaterbird populationscould beoperatingatlargescalesformanyspeciesacrossEurope,andmaybeaglobalpattern.
Weanalyzedtheautumnmigrationperiodseparately toavoidtakingintoaccountthe monthsinwhichricefields arenotavailableashabitatforwaterbirds.Aswell,previous studieshavefoundthatricefields areparticularlyattractivetowaterbirdsduringautumn andearlywinterintheMediterranean region,wheninvertebratebiomasslevelspeakand temporarywetlandsareusuallydry(Gonza´lez-Sol´ıs etal.1996).Duringthisperiod,this habitatisofinternationalimportanceasastopover siteinthewesternMediterranean (Finlaysonetal.1992).IntheDon˜anaricefieldsthepeakinthenumberofbirdsusing thesefieldsoccursinautumn(Fig.1).
PreviousworkfoundAfro-Palearctic migrantbirdstohavenegativepopulationtrends overthelastfewdecadesincomparisontoshort-distance migrantsorresidentbirds (Sandersonetal.2006).Ourresultsshowthattherelationship betweenpopulationtrends andmigrationdistance isquadratic,withsomespecieswithextrememigrationdistances (morethanfifty degreesoflatitudebetweentheirmeanbreedinglatitudeandmeanwin- teringlatitude)havingmorepositivepopulationtrendsthanthosewithmediumvalues.The questionofwhyextremelong-distant migratingspecieshavemorepositivetrendsmerits furtherstudy,butitispossiblethattheybenefitfromhavingaverylargewinterrange (althoughtherelationship betweenextremelong-distant migrationandlargewinterrange shouldalsobeexplored). Thiswouldallowthemtocopewiththealterationsoccurring in somewintering areas,includingchangesinrainfallpatternsorvariations inprimary productivity intheSahelregion,thelatterisafactorwhichhasbeenlinkedwithdeclines andpopulationfluctuationsinanumberofEuropeanmigrantsthatwinteredthereduring thedroughtofthe1980s(DenHeld1981;Kanyamibwaetal.1990).
Shultzetal.(2005)suggestthattheBritishfarmlandbirdswhosepopulations have sufferedmostunderagriculturalintensificationarethosewiththemostspecializedresource andhabitatuseandleastcognitiveabilities.Inthissense,benefitsfromtheexploitationof ricefieldscouldbeduetoacoincidencewiththecharacteristicsofthenaturalhabitats exploitedbythesespecies,butcouldalsobeduetoahighplasticityinhabitatselection.
Inthiscase,analternativeexplanationforthepositiverelationshipbetweenricefielduse andpopulationtrendswouldbethatthemostplasticspeciesarebetteratdealingwith human-transformedenvironments.
Althoughwecan’tdemonstrate thatthereisacausalrelationbetweentheuseofrice fieldsandEuropeanpopulationtrends,ourapproachidentifiespotentialimpactsofchanges onricefieldmanagementonwaterbirdsthat shouldbetaken intoaccountbypolicymakers andfutureresearchprojects.Therelevanceofricefields asanalternativehabitatfor waterbirdsinEuropeshouldnotbeunderestimatedsincethelossofnaturalwetlands meansthattheseartificial habitatsnowrepresentanimportantproportionofthetotal habitatareasuitableforwaterbirdsintheregion.Thesurfaceareaofricecultivatedin southernEurope(Italy,Spain,Greece,andPortugal)represents 30.7%ofsuitablehabitat forwaterbirds.InSpain,thispercentagehasnowrisento49.5%(Table3).
Understandinghow ricefieldmanagementaffectsbirdsisimportanttothedevelopment ofsustainable agriculturalsystemsthatcombineeconomicallyviablefarmingandbird conservation. Infact,previousstudieshaveshownthatthereareagriculturalbenefits derivedfromhavingwaterbirdsinricefields,sincetheyimprovestrawdecomposition (Birdetal.2000)orweedcontrol(VanGroenigen etal.2003).However,changesinthe CommonAgriculturePolicy mayreduce thesurface ofricefieldsinsouthernSpainandin therestofEurope.The2003reformoftheCommonAgriculture Policyintroduced a numberofmodificationssuch asthewithdrawalofsupportforfarm production,whichhas meantareduction inthegrantsreceivedbyricefarmers, whoarenowchanging to alternativecropswiththeconcomitantlossofhabitatforwaterbirds. Inlightofourstudy, theawardingofgrantsbytheEUtoensurethemaintenanceofthesurfaceareaofrice cultivated,togetherwiththeimplementationofecologicalagricultureorthetransformation ofricefields intonaturalmarshlands,wouldseemabetteroption.Furthermore,well- designedenvironmentalagriculturalschemesinsouthernEuropemayalsoincreasethe wintersurvivalofbirdpopulationsbreedingfurthernorth(Wretenbergetal.2007).
It is important to note that we are not advocating the transformationof natural marshlandintoricefields:however, managers andpolicymakersshouldbeawarethatthe transformationofricefieldsintoothertypesofcroplands(forexample,cotton)orintosolar farmswillhaveanimportantnegativeimpactonwildlifeconservationinEurope.
Climatechangeinvolvespotentially negativeeffectsonbiodiversity.Itis important that the developmentofalternative sustainableenergyproductionbefavoured,sincethe impact ofclimatechangeneedtobemitigatedthroughlowcarbonenergyproduction. However, theindiscriminatesitingofwindorsolarfarmswithoutanydetailedassessment oftheir environmentalimpactoralternativesmayhaveimportantnegativeimpactsonwildlife. Solarfarmsareusuallysituatedonformeragriculturelandbecauseaflat surfaceisnec- essary.Nowadays,theenvironmentaldamagecausedbytheimplementationofsolarfarms isnotconsideredtoberelevantinAndalucia(PlanAndaluzdeSostenibilidadEnerge´tica
Table3 Areaofnaturalmarshes andricefields insouthernEuro- peancountries,takenfromthe
Pan-EuropeanWetlandInventory
CountryMarshes(ha)Rice
fields(ha)
%Areaof ricefields
(2004)andFAOSTAT(2004) ( WKBASE/)
Spain120,537118,00049.5
Portugal130,94325,19816.1
Italy450,563218,67632.7
Greece166,79422,41311.8
Total868,837384,28730.7
PASENER2007-2013).However, bearinginmindtheresultspresentedhere,thenegative effect that the placing ofsolarfarmsonformerrice fieldswouldhaveonDon˜ana waterbird communitiesmustbetakenintoaccount.Spainnowhasthethird-largestsolarcapacityin theworld,behindonlytheUnitedStatesandGermany (Abend 2008)andcurrentsolar- energyprojectsintheDon˜anaareawilltransformabout1,000haofricefields:this transformationofricefields intosolarfarmsmayrepresentanimportantandsilentsec- ondarylossofwetlandsinsouthernEurope.
Acknowledgments TheJunta deAndaluc´ıafundedthisstudyviatheprojectcontract‘‘Lasaves acua´ticas deDon˜anayelcultivodelarroz:lainteraccio´nentrelaagriculturaylaconservacio´ndelaszonashu´medas’’. GregorioM.ToralwasfundedbyanI3P-CSICgrantfortheformationofResearchers.The‘‘Equipode SeguimientodeProcesosNaturales’’belongingtotheDon˜anaBiologicalStation(C.S.I.C.)hasbeencol- lectingthecensusdatausedinthisstudyformanyyears.
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