Saltmarshesbiologicalcontrolsoffoodwebsina

Saltmarshes:biologicalcontrolsoffoodwebsinadiminishingenvironmentIvanValiela*,DeborahRutecki,SophiaFoxBostonUniversityMarineProgram,MarineBiologicalLaboratory,WoodsHoleMA02543,USAReceived1November2003;receivedinrevisedform18November2003;accepted1December2003ThisessayisdedicatedtoJohnM.Teal,aleaderinbasicandappliedstudiesofsaltmarshecosystems,nowretiredfromtheWoodsHoleOceanographicInstitution,butstillremarkablyandadmirablyactiveinsaltmarshrestorationeffortsAbstractThisessayreviewstwoimportanttopicsincoastalecology:theworkontherelativeroleofbottom–upandtop–downcontrolsinnaturalcommunitiesandthelossofwetlandsworldwide.Insaltmarshesandothercoastalwetlands,bottom–upandtop–downmechanismsofcontrolonnaturalcommunitiesarepervasive.Bottom–upeffectsthroughnutrientsupplymaypropagatetouppertrophiclevelsviabetterfoodquality,orindirectlybyalteringwaterandsedimentquality.Top–downcontrolbyconsumersalterslowertrophiclevelsthroughconsumptionofprimaryproducers,andindirectlybytrophiccascadesinwhichhigherpredatorsfeedongrazers.Thecombinedforcingofbottom–upandtop–downcontrolsgovernassemblagesofspeciesinnaturalcommunities,mediatedbyphysicalandbiogeochemicalfactors.Althoughthereismuchinformationaboutbiologicalcontrolsofcoastalfoodwebs,moreinformationisneeded.Evenmoreimportantisthatlargelossesofwetlandareoccurringalongcoastlinesworldwideduetoavarietyofeconomicandsocialactivitiesincludingfilling,wetlandreclamation,andsedimentinterception.Suchlossesareofconcernbecausethesewetlandsprovideimportantfunctions,includingexportofenergy-richmaterialtodeeperwaters,nurseryandstockhabitats,shorelinestabilization,andinterceptland-derivednutrientsandcontaminants.Theseimportantfunctionsjustifyconservationandrestorationefforts;barringsuchefforts,wewillfinditincreasinglydifficulttofindcoastalwetlandswherewecancontinuetogainfurtherunderstandingofecologyandbiogeochemistryandlacktheaestheticpleasurethesewetlandsprovidetosomanyofus.D2004ElsevierB.V.Allrightsreserved.Keywords:Coastalwetlands;Bottom–up;Top–down;Wetlandlosses;Mangroves;Macrophytes0022-0981/$-seefrontmatterD2004ElsevierB.V.Allrightsreserved.doi:10.1016/j.jembe.2003.12.023*Correspondingauthor.Tel.:[phone];fax:[phone].E-mailaddress:[e-mail](I.Valiela).(2004)131–1591.IntroductionResearchdoneinsaltmarsheshasmademanycontributionstoenvironmentalscience,andhasgeneratedanumberoflivelycontroversies.Experimentalaswellascomparativeandcorrelationalapproacheshaveallbeenusedinsaltmarshes,eachcontributingtoadvancesinthefield.Acomprehensivetext,nowinasecondedition(MitschandGosselink,2000),aswellaspublicationofreviewsreportedinamajorconferenceonrecentissuesandadvances(WeinsteinandKreeger,2000)makeitlesspressingtorevisitthemanytopicsilluminatedbyworkinsaltmarshresearchduringthetimethatJEMBEhasbeeninexistence,manyofwhichhavebeenexploredinitspages.Asinalldisciplines,thereisanoverwhelmingplethoraofrecentpublicationsonsaltmarshes,aseeminglyreasonablynarrowandspecializedtopic:asearchinjustonesearchengine(AquaticSciencesandFisheriesAbstracts)yielded991referencesthatappearedjustinthedecade1993–2003.Belowweincludejustafewofthemanyworthyreferencesthatcouldhavebeencited.Drasticcurtailingofbibliographiccitationsseemsanincreasinglyunavoidablefeatureoffuturereviewsofanybutthenarrowestofscientifictopics.Fortunately,theavailabilityofthetimelyWeinsteinandKreeger(2000)andMitschandGosselink(2000)compendiamakesitsomewhateasierforustomorenarrowlyfocusthiscontribution.Thefirstpartofthisessayreviewsworkinsaltmarshenvironmentsontherelativeroleoftop–downandbottom–upcontrolsinnaturalcommunities.Thisisatopicinwhichdifferingviewshaveflaredintherecentecologicalliterature,butwasnotacentralthemeinWeinsteinandKreeger(2000)orMitschandGosselink(2000).Thereshouldbemorethanalittleinterestonthistopic,sinceinfact,humanbeingsareconductingworldwideexperimentsaswefertilizetheworld’snaturalwatersandover-fishmanytoppredators.Progressinknowledgeoftherelativeprimacyofcontrolbyconsumeractivitycascadingdownfoodwebs,orbynutrientsforcingupthroughfoodwebs,orhowthesecontrolsmayinteractinspecificenvironments,seemhighprioritiesforecologicalresearch.Second,itwouldberemissherenottore-emphasizeatrulymajorconservationissue,thewidespreadlossofcoastalwetlandsaroundtheworld.Wetlandlosshasbeenrightlyhighlightedbymanyauthorsinmanysettings,butgiventhatcontinuinglossofwetlandsremainsaglobalissue,itmeritsrevisitinginatributetoamajorinternationaljournalwhosecontenthasfromthebeginningsupportedenvironmentalconservation.2.Bottom–upandtop–downcontrolsoffoodwebsThereisavenerablehistoryofthoughtonthesearchforgeneralitiesabouthownaturalcommunitiesarecontrolled,fromElton(1927),throughthecriticalandprofounddialoguesbetweenSolomon(1949)andAndrewarthaandBirch(1954)andothers,tothestill-seminal‘‘etude’’byHairstonetal.(1960).Perhapsoutoftheneedtosearchforbroadconclusions,inmanyrecentpapers,thereseemstobeatendencytohighlightoneortheothermechanismofcontrol.I.Valielaetal./J.Exp.Mar.Biol.Ec