DOI: /j.aquabot Published: 01/11/2016. Peer reviewed version. Cyswllt i'r cyhoeddiad / Link to publication

PRIFYSGOL BANGOR / BANGOR UNIVERSITY Herbivory on freshwater and marine macrophytes Bakker, Elisabeth S.; Wood, Kevin A.; Pages Fauria, Jordi; (Ciska) Veen, G.F. ; Christianen, Marjolin J.A.; Santamaria, Luis; Nolet, Bart A.; Hilt, Sabine Aquatic Botany DOI: 10.1016/j.aquabot.2016.04.008 Published: 01/11/2016 Peer reviewed version Cyswllt i'r cyhoeddiad / Link to publication Dyfyniad o'r fersiwn a gyhoeddwyd / Citation for published version (APA): Bakker, E. S., Wood, K. A., Pages Fauria, J., (Ciska) Veen, G. F., Christianen, M. J. A., Santamaria, L.,... Hilt, S. (2016). Herbivory on freshwater and marine macrophytes: A review and perspective. Aquatic Botany, 135, 18-36. DOI: 10.1016/j.aquabot.2016.04.008 Hawliau Cyffredinol / General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights. Users may download and print one copy of any publication from the public portal for the purpose of private study or research. You may not further distribute the material or use it for any profit-making activity or commercial gain You may freely distribute the URL identifying the publication in the public portal? Take down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. 09. Mar. 2019

1 Herbivory)on)freshwater)and)marine)macrophytes:)a)review)and)perspective) 2 3 4 5 ElisabethS.Bakker a,kevina.wood b,jordif.pagès c,g.f.(ciska)veen d,marjolijnj.a.christianen e, LuisSantamaría f,barta.nolet g,andsabinehilt h 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 a DepartmentofAquaticEcology,NetherlandsInstituteofEcology(NIOOYKNAW), Droevendaalsesteeg10,6708PBWageningen,TheNetherlands,l.bakker@nioo.knaw.nl b Wildfowl&WetlandsTrust,Slimbridge,Gloucestershire,GL27BT,UnitedKingdom, kevin.wood@wwt.org.uk c SchoolofOceanSciences,BangorUniversity,MenaiBridge,Anglesey,LL595AB,UnitedKingdom, j.pages@bangor.ac.uk d DepartmentofTerrestrialEcology,NetherlandsInstituteofEcology(NIOOYKNAW), Droevendaalsesteeg10,6708PBWageningen,TheNetherlands,c.veen@nioo.knaw.nl e GroningenInstituteforEvolutionaryLifeSciences,UniversityofGroningen,P.O.Box11103,9700CC Groningen,TheNetherlands,Marjolijn.Christianen@gmail.com f SpatialEcologyGroup,DepartmentofWetlandEcology,DoñanaBiologicalStation(EBDYCSIC),C/ Américovespucios/n,IsladelaCartuja,E41092Sevilla,Spain,luis.santamaria@ebd.csic.es 1

25 26 27 28 29 30 31 32 g DepartmentofAnimalEcology,NetherlandsInstituteofEcology(NIOOYKNAW),Droevendaalsesteeg 10,6708PBWageningen,TheNetherlands,b.nolet@nioo.knaw.nl h LeibnizYInstituteofFreshwaterEcologyandInlandFisheries,Müggelseedamm301,12587Berlin, Germany,hilt@igbYberlin.de 33 34 35 36 Correspondingauthor:ElisabethS.Bakker,DepartmentofAquaticEcology,NetherlandsInstituteof Ecology(NIOOYKNAW),Droevendaalsesteeg10,6708PBWageningen,TheNetherlands, l.bakker@nioo.knaw.nl 37 38 39 40 Authorcontributions:ESB,KAW,SHandBAN:developingideaandstructureofthepaper;ESB,KAW, GFVandJFP:dataanalysisandgraphics;ESB,KAW,JFP,GFV,MJAC,LS,BANandSHcontributedto writingofthemanuscript 41 2

3 Abstract 42 43 Untilthe1990s,herbivoryonaquaticvascularplantswasconsideredtobeofminorimportance,and 44 thepredominantviewwasthatfreshwaterandmarinemacrophytesdidnottakepartinthefood 45 web:theirprimaryfatewasthedetritivorouspathway.inthelast25years,asubstantialbodyof 46 evidencehasdevelopedthatshowsthatherbivoryisanimportantfactorintheecologyofvascular 47 macrophytesacrossfreshwaterandmarinehabitats.herbivoresremoveonaverage40y48%ofplant 48 biomassinfreshwaterandmarineecosystems,whichistypically5y10timesgreaterthanreportedfor 49 terrestrialecosystems.thismaybeexplainedbythelowerc:nstoichiometryfoundinsubmerged 50 plants.herbivoresaffectplantabundanceandspeciescompositionbygrazingandbioturbationand 51 therewithalterthefunctioningofaquaticecosystems,includingbiogeochemicalcycling,carbon 52 stocksandprimaryproduction,transportofnutrientsandpropagulesacrossecosystemboundaries, 53 habitatforotherorganismsandthelevelofshorelineprotectionbymacrophytebeds. 54 Withongoingglobalenvironmentalchange,herbivoreimpactsarepredictedtoincrease.Thereare 55 pressingneedstoimproveourmanagementofundesirableherbivoreimpactsonmacrophytes(e.g. 56 leadingtoanecosystemcollapse),andtheconflictsbetweenpeopleassociatedwiththeimpactsof 57 charismaticmegayherbivores.whilesimultaneously,thelongytermfutureofmaintainingbothviable 58 herbivorepopulationsandplantbedsshouldbeaddressed,asbothbelongincompleteecosystems 59 andhavecoyevolvedintheselongbeforetheincreasinginfluenceofman.betterintegrationofthe 60 freshwater,marine,andterrestrialherbivoryliteratureswouldgreatlybenefitfutureresearch 61 efforts. 62 63 64 Keywords:*climate*change,*conservation,*ecosystem*functions,*grazing,*seagrass,*stoichiometry* 65

66 1.! Introduction:)25)years)of)research)on)herbivory)on)macrophytes 67 68 1.1Setting*the*scene* 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 In the 1990s two seminal papers appeared in Aquatic Botany that urged for a complete changeintheparadigmthathadbeendominatingmacrophyteecology.despitesomeearlyworkon theimpactofwaterbirdsonfreshwaterandmarineangiosperms(juppandspence,1977;jacobset al.,1981),untilthen,herbivoryonaquaticvascularplantswasconsideredtobeofminorimportance, and the predominant view was that freshwater and marine macrophytes did not take part in the foodweb(e.g.shelford(1918)andtheirprimaryfatewasthedetritivorouspathway(polunin,1984; DuarteandCebrian,1996).Butin1991,Lodgearguedthat,contrarytoconventionalwisdom,live freshwatermacrophytesareengagedinaquaticfoodwebs.in1998,cebrianandduartehighlighted that,whileseagrassessufferedmodestherbivoryratesonaverage,suchrateswerehighlyvariable, andtheimportanceofseagrassyherbivoreinteractionsshouldnotbediscounted.followingonfrom these two papers, Lodge (1998) provided further evidence for the important role of herbivores in freshwater habitats, as compared to other biomes; and Valentine and Heck (1999) demonstrated thatgrazingonseagrassesiswidespreadintheworld'soceans. Together,theselandmarkpapersputmacrophyteherbivoryonthemap.Sincethen,there has been a strong increase in the amount of studies that investigated herbivory on freshwater macrophytes and seagrasses. In this study, we review what we have learned in the 25 years that followedtheappearanceoflodge(1991).furthermore,weidentifynewtopicsthathaveemerged overthistime.thesenewtopicsincludethefastchangesthatmayoccurinmacrophyteyherbivore relationships with the ongoing global environmental change, as well as the potential conflicts betweenherbivoreconservationandherbivoreimpactsonaquaticecosystems.finally,wediscuss how we can improve our understanding of herbivore impacts and what tools may help us in 4

91 92 93 94 achieving this. Following the approach of the seminal papers listed above, we focus primarily on aquaticangiosperms(submerged,floatingandemergent)andaddressbothfreshwaterandmarine ecosystems. 95 1.2Why*thinking*about*herbivory*on*macrophytes*has*changed*over*the*last*25*years 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 The paradigm shift in our perception of macrophyte herbivory, from being considered negligibletobeingacknowledgedasakeyfactorshapingbenthicecosystems,isnotonlycausedby anincreaseinscientificinterestfosteredbytheselandmarkpapers:theeffectofherbivorybecame alsomoreconspicuousoverthelast25years.thereasonsforthisaremethodological,anthropogenic andecological. Methodologicalimprovementsforestimatingherbivoryincludedobservationmethods,such as bite mark counts (Cebrian and Duarte, 1998), experimental approaches, such as herbivore exclusions(seepooreetal.(2012)andwoodetal.(2016)forsynthesesofmarineandfreshwater habitats, including macroyalgae) and direct methods, including video bite counts or isotopic signatures(seetable4fordetails). Anthropogeniceffectsincludedincreasesinthedensitiesofaquaticandmarineherbivoresas a result of increased protection, predator removal, food subsidies from agriculture, and the introductionofexoticherbivores(estesetal.,2011).forexample,steepincreasesinherbivoryrates have been reported for sea turtles in the Arabian Sea and Indonesia (Kelkar et al., 2013b, a), (Christianenetal.,2014),forherbivorousfishintheMediterranean(Pagesetal.,2012)andforgeese in Northwestern Europe and North America (Jefferies et al., 2003; Van Eerden et al., 2005). However,itshouldbenotedthatdespiterecentlocalincreasesinherbivory,whichhaveattracted attention to the role of herbivores in benthic ecosystems, over longer time frames in particularly speciesoflargeherbivoreshaveexperiencedstrongglobaldeclines(jackson,1997;mccauleyetal., 2015;Bakkeretal.2016b). 5

116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 Furthermore, the recent spread of exotic herbivores had major consequences for macrophyteestablishmentandsurvivalin many areas worldwide.for example,tropical lessepsian rabbitfishes (Siganus* spp.) cause overgrazing of macroalgae and seagrasses at the Eastern Mediterranean (Verges et al., 2014b), chubs and rabbit fishes (Kyphosus* spp. and Siganus* spp., Siganidae)overgrazeAustralianandJapanesekelpforests(Vergesetal.,2014a),NorthYAmericanredY swamp crayfish (Procambarus* clarkii) have depleted submerged plant meadows in shallow lakes acrosseurope(rodriguezetal.,2003;gherardiandacquistapace,2007;vanderwaletal.,2013), andintentionalintroductionsofgrasscarp(ctenopharyngodon*idella)havebeenconsideredathreat tonativemacrophytes(wittmannetal.,2014). EcologicaleffectsarerelatedtotheoligotrophicationofmanyEuropeanfreshwatersystems, whichresultedintheirreycolonizationwithsubmergedmacrophytes(jeppesenetal.,2005).inmany systems,however,theimpactofaquaticherbivoressufficedtohaltorreversesuchrecolonization (KörnerandDugdale,2003;Hilt,2006;Bakkeretal.,2013;Hiltetal.,2013;Eigemannetal.,2016). 2.! Quantitative)impacts)of)herbivores)in)aquatic)systems 132 2.1.!Quantitative*impact*of*herbivores*on*plant*biomass*across*ecosystems* 133 134 135 136 137 138 139 140 AgrowingbodyofprimaryresearchhasdemonstratedherbivoreYinducedchangesinoneor moremeasuresofmacrophyteabundance,includingbiomass,twoydimensionalcover,volume,and individualdensity(kirschetal.,2002;marklundetal.,2002;tomasetal.,2005;pradoetal.,2007; Christianenetal.,2012;Pagesetal.,2012;Woodetal.,2012a;Kelkaretal.,2013b,a;Christianenet al.,2014;bakkeretal.,2016b).thesestudies,synthesizedinseveralreviews(cyrandpace,1993; ValentineandDuffy,2006;Gruneretal.,2008;Pooreetal.,2012),confirmedherbivoresaskey driversofbenthicecosystemsaroundtheworld.theoverwhelmingmajorityofstudiesreporteda reductioninmacrophyteabundanceasaresultofherbivory.indeed,arecentmetayanalysisof326 6

7 experimentsinwhichfreshwaterherbivoreswereexcludedfoundthatherbivoryreduced 141 macrophytebiomassby47.2±3.4%(average±ci)(woodetal.,2016).ofthese,300experiments 142 reportedareductioninmacrophytebiomass,while26experimentsreportedpositiveeffectsorno 143 changes.similarly,ametayanalysisongrazingimpactsonmarinemacrophytesfoundthatherbivores 144 reducemacrophyteabundance(bothsubmergedangiospermsandmacroyalgae)by68%onaverage 145 (Pooreetal.,2012). 146 Despitetheirhistoricaldisregard,theremovalofvascularplantbiomassbyherbivoresis,onaverage, 147 muchlargerinaquaticthaninterrestrialecosystems.themostrecentmetayanalysesavailablefor 148 terrestrial,freshwaterandmarinehabitats(turcotteetal.,2014;woodetal.,2016)showthat 149 medianbiomassremovalbyherbivoresis4y8%interrestrialecosystems,whileitis44y48%in 150 freshwaterand40y44%inmarineecosystems(fig1ayc).thus,herbivoresremoveonaverage5y10 151 timesmorevascularplantbiomassinaquaticecosystemsthaninterrestrialones. 152 Yet,theimpactofherbivoresonvascularplantbiomassremovalismuchmorevariableinaquatic 153 thaninterrestrialecosystems,anditrangesasbroadlyasbetween0and100%ofbiomassremoval 154 (Fig1aYc).Underlyingexplanationsforthelargerangeofherbivoreeffectsinaquaticecosystemsare 155 stillunknown.potentialmechanismsinvolvebottomyupeffects,suchasvariationinplant 156 productivity,nutritionalquality,stoichiometry,resistanceandtolerancetograzing(cebrianetal., 157 2009);andtopYdowneffects,suchasvariationinherbivoreabundance,feedingefficiency,size, 158 taxonomy,mobility,metabolismandpredatoreffects(boreretal.,2005). 159 160 2.2.!BottomEup*effects:*the*plant s*perspective* 161 2.2.1.! Primary*productivity*and*herbivory*rates) 162 Studiesinaquaticsystemstraditionallyfocussedonprimaryproductionofphytoplanktoninpelagic 163 habitats,andonlyrecentlylittoralareasreceivedmoreattention(vadeboncoeuretal.,2002; 164

165 166 167 168 169 170 171 172 173 174 175 176 Brothersetal.,2013).TheoreticalpredictionsbasedonLotkaYVolterramodelssuggestthatgrazing shouldincreasewithprimaryproductivity(gruneretal.,2008),becauseasplantsproducemore tissuesordosoatfasterrates,herbivorescanincreasetheirratesofconsumption.thisprediction reliesintheassumptionthatherbivoryratesarebottomyupregulatedbytheavailabilityofplant tissues.empiricalevidenceishoweverconflicting.ametayanalysisbycyrandpace(1993)concluded thatherbivoryincreaseswithprimaryproductioninbothterrestrialandaquaticsystems.however,a morerecentmetayanalysisfoundnosignificanteffectofproductivityonherbivory(orinteraction strength)inaquaticsystems(e.g.(boreretal.,2005)).infact,itiswidelyrecognizedthatfoodquality anddefencesalsohavemajoreffectsonherbivoreperformancethatmightmaskanyherbivoryy productivityrelationship.usingaplantgrowthmodel,hiddingetal.(2016)suggestedthatherbivore grazingeffectsonmacrophytesbecomeimportantabovecertainthresholdsinperiphytonshading andthusreducedproductionofplants. 177 ) 178 2.2.2.! Plant*stoichiometry* 179 180 181 182 183 184 185 186 187 188 189 Differencesinherbivoryratesacrossplanttaxahavealsobeenattributedtodifferencesin plant quality, as perceived by herbivores (Lodge, 1991; Cronin et al., 2002). Generally, there is a positive relationship between the nitrogen content in the plant s tissue and its consumption by herbivores(cebrianandlartigue,2004).thispatternholdsbothwithinandacrossecosystems(elser et al., 2000; Cebrian and Lartigue, 2004; Cebrian et al., 2009). Hence, it has been suggested that higherherbivoreconsumptionratesinaquaticplantsmightbeexplainedbytheirhigherqualityas food arisingfromthelackofcarbonyrichstructuralcompoundsthatstrengthencellwalls,increase resistance to herbivores, and reduce digestibility in terrestrial plants (Gruner et al., 2008). Unfortunately, most comparisons between aquatic and terrestrial systems undertaken to date restrictedtheirestimatesofaquaticherbivorytophytoplanktonconsumption,thusgivinglimitedor noinformationontheconsumptionofaquaticmacrophytes(elseretal.,2000).comparingherbivory 8

190 191 192 rateson,andfoodquality(e.g.,nitrogencontent)of,aquaticandterrestrialvascularplantswouldbe more informative, because they have a close phylogenetic affinity and most differences could be attributedtotheecosystemstheyinhabit(hay,1991;grossandbakker,2012;burkepile,2013). 193 194 195 196 197 198 199 200 201 202 203 204 205 AcompilationofdataonC:Nratiosofvascularplantsinterrestrial,freshwaterandmarine ecosystems reveals that they differ strongly between these systems (Fig 1dYf). Median C:N ratio decreased strongly from terrestrial vascular plants (25Y30) and marine macrophytes (24Y28) to freshwater macrophytes (12Y16) (Fig 1dYf). The high N content (thus low C:N ratio) of freshwater macrophytesmayindicateahigherqualitytoherbivores,potentiallyexplainingthehighherbivory ratesfoundinfreshwatersystems.takingintoaccountthevariationingrowthformsthatoccursin aquaticecosystemsmakesthispatternstronger(fig2ayd).herbivoryratesareloweronemergent macrophytes (median freshwater = 36Y48%; median marine = 24Y36%) than on submerged macrophytes(medianfreshwaterandmarine=48y60%)inbothfreshwaterandmarineecosystems (Fig2a,b).ThesedifferencesfitcloselytheC:Nratioofthedifferentplantgrowthforms,asemergent macrophytes have higher C:N ratios (median freshwater = 28Y32; median marine = 24Y28) than submerged macrophytes (median freshwater = 8Y12; median marine = 20Y24), particularly in freshwaterecosystems(fig2c,d). 206 207 208 209 210 211 212 213 214 Our data compilation indicates that differences in plant quality between terrestrial plants and macrophytes show close links to variation in herbivory rates. Submerged plants need less structural components, resulting in higher N content and higher rates of herbivory. In contrast, herbivory rates and N content of emergent macrophytes are similar to those found in terrestrial plants(fig1a,1d,2ayd).althoughncontentisacknowledgedasanimportantdeterminantofplant qualitytoherbivores,othernutrientsandsecondarycompoundsmayalsoinfluenceit.forexample, freshwater macrophytes and seagrassess have antiyherbivore defenses (Verges et al., 2008; Gross andbakker,2012),whichmaycauselow palatabilityy eveninhighlynutritiousspecies.phenolics, terpenoidsandnitrogenatedcompoundscanbeefficientdeterrentsofaquaticherbivores,although 9

215 216 217 218 219 220 221 222 223 deterrence of generalist herbivores often comes at the cost of higher preference by specialised consumers(vergesetal.,2007).interestingly,infreshwatersystems,theconcentrationsofphenolic compounds decrease from emergent to floating to submerged plants (Smolders et al., 2000), suggestingnotonlyhighernutritionalquality,butalsolessdefendedtissues,resultingingenerally higherpalatabilityofsubmergedplants.however,thisissuestilldeservesmoreresearch,sincemost work has focused on macroyalgae (e.g. (Hay, 1996)). Besides the differences between ecosystems discussedabove,spatialandtemporalvariationinherbivoryatpopulationandindividualyplantlevel has also been linked to plant quality (C:N, % N) and to the presence of antiyherbivore defences (HackerandBertness,1995;Preen,1995;Vergesetal.,2007;Pradoetal.,2010). 224 225 2.3.!TopEdown*effects:*the*herbivore s*perspective* 226 2.3.1.! Herbivore*density* 227 228 229 230 231 232 233 234 235 236 237 238 Withinindividualherbivorespecies,herbivoreimpactonaquaticplantabundanceispositively relatedtoherbivoredensity(stottandrobson,1970;valentineandheck,1991;woodetal.,2012a; Kelkaretal.,2013b).However,analysesofherbivoreimpactsacrossmultipleherbivorespecieshave foundnorelationshipbetweenherbivoredensityandmacrophyteabundance,probablyduetothe confoundingeffectsofinterspecificdifferencesinherbivoreecology(marklundetal.,2002;woodet al.,2012a).forexample,amongplantyeatingwaterfowl,substantialdifferencesexistinmeanadult bodymass(fromthe24gofocellatedcrake,micropygia*schomburgkii,tothe12,000goftrumpeter swan,cygnus*buccinator),whichisknowntoinfluencethespecies dietandtheabsolutequantityof vegetationconsumed(woodetal.,2012a).consequently,whenherbivoredensitieswereestimated asbiomassdensities(thusaccountingforinterspecificdifferencesinindividualbodymass),a significantnegativerelationshipbetweenherbivorebiomassdensityandmacrophyteabundancewas detected(woodetal.,2012a;woodetal.,2016).onlyatrelativelylowherbivoredensitieswere 10

11 positivechangesinplantabundancereported,suggestingthatinaquaticsystemsgreaterherbivore 239 densitiesoverwhelmplantcompensatorygrowthresponses(woodetal.,2016). 240 * 241 2.3.2.! Size,*mobility*and*taxonomy*of*herbivores* 242 Byvirtueoftheirsize,largeterrestrialherbivoresarecriticalagentsofchangeandmaintenanceof 243 theecosystemstheyinhabit(owenysmith,1988;bakkeretal.,2016a).inaquaticenvironments,large 244 herbivoreshavealsobeenidentifiedaskeyspecies(mccauleyetal.,2015),oftenconsidered 245 ecosystemengineers(e.g.(bakkeretal.,2016b)).itisnotsurprising,thus,thatarecentmetayanalysis 246 foundsignificantlystrongerimpactsofmacrograzers(fishes,urchinsandlargemolluscs)than 247 mesograzers(amphipods,isopodsandsmallmolluscs)onmarinemacrophytes(algae,seagrassesand 248 saltmarshes)(pooreetal.,2012). 249 AnotherrecentmetaYanalysisbyWoodetal(2016)foundsubstantialbetweenYtaxadifferencesin 250 effectsofherbivoresontheabundanceoffreshwaterandmarinemacrophytes.echinoderms, 251 molluscs,andfishhadrelativelylargeimpactsonplants,whileinsectsandbirdshadrelativelylow 252 impacts.thereasonforthesedifferencesmaybethemobilityandhabitatpreferencesofeachof 253 thesegroups.fullyaquaticspeciesthatliveunderwaterpermanently,havebeenshowntoproduce 254 thegreatestimpactsonaquaticplants(bakkeretal.,2016b),whilefacultativeaquaticgrazers,such 255 asinsectsandbirds,spreadtheiractivitymorebetweendifferentecosystems(thankstotheirhigh 256 mobility),thusspreadingalsotheirimpactbetweensuchecosystems.woodetal.(2016)also 257 pointedoutthefactthatsomeherbivores(suchasechinoderms,crayfishandmolluscs)have 258 restrictedmobilityandproduceintenseimpactsduetotheirbulkgrazingstrategies,sincethey 259 consumemultipletissuestypesandspecies,thusaffectingagreaterproportionofaplantcommunity 260 (Lodgeetal.,1998). 261 262

12 2.3.3.! Herbivory*and*omnivory* 263 Mostanimalsthatconsumeaquaticplantsareomnivores,veryfewarestrictherbivores. 264 Thereisapositiverelationshipbetweenbodysizeanddegreeofherbivoryinaquaticomnivores:the 265 largertheconsumer,themoreimportantplantconsumption(granivoryandfolivory,particularlythe 266 latter)isintheirdiet(clementsetal.,2009;woodetal.,2012a).hence,percapita,largeherbivores 267 havethelargestimpactonplantabundancethroughgrazing.thehighlevelofgeneralisticfeeding 268 andomnivoryofaquaticplantconsumersmayberelatedtothehigherimpactofherbivoryon 269 aquaticplants,becauseitmayrelaxtheeffectofdirectdensitydependenceonconsumers:whenever 270 consumersovergrazeplants,theycanswitchtoalternativefoodsources(algae,detritusoranimal 271 prey)withinornearbythewaterbody(greyandjackson,2012).furthermore,inaquaticsystems, 272 largeherbivoresandomnivoresoftenfeedonbothabovegroundandbelowgroundplantmaterial, 273 whichmaymultiplytheirimpactonplants,duetothedepletionofundergroundplantresourcesfor 274 regrowth. 275 HerbivoreimpactmayalsobeenhancedbynonYconsumptiveeffects,whicharedocumented 276 tobesevereinfreshwaterandmarinemacrophytes,e.g.bioturbation(lodge,1991).byinitiating 277 barepatches,herbivorescancreatefocalpointsforfurthererosionofmacrophytemeadowsby 278 wavesinshallowareas(christianenetal.,2013).trampling,fecaldepositionandincreasingnutrient 279 concentrationsmayalsoplayarole.whilstpreviousauthorshavecautionedagainsttheassumption 280 thatherbivoreeffectsonplantsrepresentexclusivelygrazinglossesduetoconsumption(mitchell 281 andwass,1996b),manystudiescontinuetoignoretherolesofnonyconsumptiveeffects. 282 283 2.4.!Latitude* 284 Potentialvarianceinherbivoreimpactsonplantabundanceacrossdifferentlatitudeshasbeena 285 topicofgrowinginterestamongecologists.however,theevidenceforaroleoflatitudinaleffectsis 286

13 limited.forexample,inmacrophytefeedingassayscarriedoutbymorrisonandhay(2012),onlyone 287 outofthreecrayfishspeciesshowedapreferenceassociatedwithlatitude(apreferenceforhigher 288 latitudeplants).schemskeetal.(2009)arguedthat,acrossallecosystems,herbivoreimpactsare 289 greateratlowerlatitudes.incontrast,ametayanalysisbygruneretal.(2008)reportedanincreasein 290 herbivoreeffectsathigherlatitudesinfreshwaterecosystems,butnotinmarineecosystems.three 291 morerecentmetayanalyseswithlargersamplesizesfoundnoevidenceoflatitudinalgradientsin 292 herbivoreimpactsonaquaticmacrophyteabundances,someincludingbothvascularplantsand 293 macroyalgae(molesetal.,2011;pooreetal.,2012;woodetal.,2016). 294 295 3.! Ecosystem)consequences)of)herbivory 296 Withtheestablishmentofherbivoryasaanimportantfactorregulatingplantabundance,Lodge 297 (1991)endedhisreviewwiththeconclusionthat thefunctionalimportanceofgrazingremains 298 largelyuntested.indeed,thishasbeenanemergingfieldofresearchoverthelast25years,with 299 particularprogressoverthelast5years.bytheirpresence,herbivoresmayinducedirectchangesin 300 plantdynamicsandindirecteffectsonecosystemfunctioning(fig.3).theeffectsofmarine 301 herbivoresmayincludestimulatedproductionofseagrass(valentineetal.,1997;moranand 302 Bjorndal,2005;Vonketal.,2008b;Christianenetal.,2012),changesinseagrassmeadowstructure 303 (Laletal.,2010),andthereductionofthefluxoforganicmatterandnutrientstosedimentsand 304 plants(byshortcircuitingthedetritalcycle;(thayeretal.,1982;vonketal.,2008a)ortheirexportto 305 nearbyhabitats(christianenetal.,2012).insaltmarshesandaquaticecosystems,anadditional 306 effectofherbivoresisthereturnofnutrientsthroughfaecesandurine(bazelyandjefferies,1985; 307 Hiketal.,1991;Franketal.,2000),thoughinseagrassgrazedbyturtlesthiseffectisreducedby 308 nutrienttransporttoturtlerestingareas(christianenetal.,2012). 309 310

14 3.1Plant*abundance*and*species*composition*and*diversity 311 Whilethelargelynegativeimpactsofherbivoresonmacrophyteabundancehavebeenwell 312 documentedthroughshortytermexclosurestudies,thepersistenceofsucheffectsitisoftenless 313 clear.inparticular,highlyymobileherbivoressuchaswaterfowlcancauselargereductionsinplant 314 abundancebeforeswitchingtoungrazedsites(woodetal.,2012b).ecosystemresponsesto 315 fluctuatinggrazingpressurehavereceivedlittleattentiontodate.however,thereissomeevidence 316 thatrepeatedepisodesofgrazingovertime,suchasbybreedingcoloniesofsnowgeese(chen* 317 caerulescens)inwetlands,cancausesustainedlongytermshiftsinspeciescompositionanddeclines 318 inplantabundance(kerbesetal.,1990).wherenonyselective,generalistherbivoresfeedonmixed 319 assemblagesofmacrophyte,herbivorescanincreasespeciesevennessbyreducingtheabundanceof 320 dominantcompetitorsrelativetosubydominantmacrophytespecies(hiddingetal.,2010b;woodet 321 al.,2012b).however,whentheyfavorcertainsubordinateplantspeciesoverthedominant,theycan 322 reduceeveness(hiddingetal.2010a). 323 Herbivoryintropicalseagrasses(e.g.byseaurchinsandgreenturtles)caninfluencespecies 324 composition(vonketal.,2008b;kelkaretal.,2013b;hernandezandvantussenbroek,2014),but 325 oftenincontrastingways.intropicalmultispeciesmeadows,slowgrowingclimaxseagrassspecies 326 (Thalassia*hemprichii)werepromotedbysmallherbivores(urchins),whileinmoreintensivelygrazed 327 meadowsspeciesdifferenceinherbivore sgrazingpreferencesresultedinthedominanceoffast 328 growingpioneerspecies(kelkaretal.,2013b).largergrazersthatconsumebelowgroundplantparts 329 andcreatebaresedimentpatches,suchasgreenturtlesanddugongs,alsointroducespecies 330 heterogeneitybysettingbackspeciessuccessioningrazedplots(aragonesetal.,2006;christianenet 331 al.,2013). 332 333 3.2Ecosystem*functions*and*services*of*aquatic*herbivores 334

15 3.2.1.*Seed*and*propagule*dispersal 335 Besidestheirdirecteffectsontheirfoodplants,aquaticherbivoresprovideakeyserviceforaquatic 336 ecosystems:thepassivedispersalofabroadvarietyofaquaticorganisms,includingthe 337 aforementionedfoodplants,aswellasmanyothertaxaattachedorassociatedtothem(figuerola 338 andgreen,2002a;brochetetal.,2010a;vanleeuwenetal.,2012).dispersalbymostinlandywater 339 herbivorescontributestotheredistributionofindividualswithinsinglewetlandsandamongnearby 340 ones;whilewaterbirds(mostnotably,migratoryspecies)arethemainvectoroflongydistance 341 dispersalamongwetlandssituatedatseparatedwatersheds,fromregionaltocontinentalscales 342 (Vianaetal.,2013b;Vianaetal.,inpress).Amongwaterbirds,thefrequencyandscaleoflongY 343 distancedispersaleventsisknowntovarywiththevector smorphology,todependonthemigratory 344 strategyandtoscalenegativewithbodymass(greenandfiguerola,2005;vianaetal.,2013b;viana 345 etal.,2013a).inthemarineenvironment,bioticdispersalbyherbivores(seaturtles,ducksandfish) 346 hasbeenreportedforseagrasses(sumoskiandorth,2012)andpassivetransportofotherorganisms 347 isalsoknowntooccur(e.g.sessileinvertebratestransportedbyseaturtlesandcrabs(winston, 348 2012).Theoppositeprocessmayalsotakeplace e.g.anecdotalevidencesuggeststhatgreenturtle 349 hatchlingsusekelpraftsforpassivedispersal(carrandmeylan,1980). 350 Interrestrialsystems,animalYmediateddispersalisoftenfacilitatedbyrewardsencasingthe 351 propagulesorattachedtotheseeds(suchasfruits,elaiosomesandsomepods).incontrast,aquatic 352 plantseedstypicallylackrewards;hence,theirdispersalismediatedbytheingestionofplant 353 vegetativepartsbyherbivores( foliageinthefruit,sensujanzen(1984)),theoccasionalsurvivalof 354 seedsingestedbygranivores(e.g.teals(brochetetal.,2010b))ortheaccidentalingestionofseeds 355 bycarnivores/omnivores(notablyfilteryfeeders,suchasflamingoesandshovelers(verhoeven,1980; 356 Figuerolaetal.,2003).Thisdifferenceresultsinmajordifferencesinselectionpressures,whichcan 357 betracedtodifferencesinpredominantseedtraits(table1).aquaticplantseedsaretypicallysmall, 358 whichfacilitatestheiringestionmixedwiththefoliage,theirsurvivaltogutpassage,andaprolonged 359

16 gutypassagetimesresultinginlongerdispersaldistances(muellerandvandervalk,2002; 360 CharalambidouandSantamaria,2005;Soonsetal.,2008;Figuerolaetal.,2010).Largerseedsoften 361 havethickandimpermeablecoats,necessarytowithstandtheirseverescarificationinwaterbirdguts 362 (whichtendtoretainselectivelysuchlargerseeds(kleyheeg,2015)andtheseverephysicoychemical 363 treatmentexertedbytheirguts(figuerolaetal.,2002;santamariaetal.,2002).suchcoattypes 364 oftenresultinstrongphysicaldormancies,whichmaypostponeseedgerminationuntilingestionbya 365 potentialdispersalvectorhastakenplace(e.g.inseedscollectedfromthesedimentbankby 366 granivoresandfilterfeeders(figuerolaetal.,2003).invertebratepropagulesdispersedbywaterfowl 367 typicallyshowsimilartraits:smallsize,onetoseveralprotectivecoats,anddelayedorstochastic 368 hatching(charalambidouandsantamaria,2002).insomecases,theencasementofrestingeggsin 369 themother sbodymayprovidebothprotectionagainsttheirdigestionandarewardtopotential 370 dispersers havingthusafunctionanalogoustothatofplantfruits,whosefunctionalandadaptive 371 valueofsuchatraitremainstobestudied. 372 Externaldispersalmaytakeplaceattachedtotheanimal sfurorplumage,oradheredtomudy 373 stainedsurfacesinthebodyorfeet(figuerolaandgreen,2002b;frischetal.,2007).available 374 evidencesuggests,however,thatitislessfrequentthaninternaldispersal(brochetetal.,2010b). 375 Onceagain,smallpropagulesaremuchmorelikelytobecomeattachedoradhered,andremainin 376 suchsituationlongenoughforlongydistancedispersaltooccur.othertraitsthathavebeengenerally 377 assumedtofacilitateexternaldispersal,suchasflatshapesandsuitablesurfacestructures(hooks, 378 thorns,hairs),areknowntooccurinplantandinvertebratepropagules(e.g.vivianysmithandstiles 379 (1994)).Asabove,theirfunctionalandadaptivevalueremainstobestudied. 380 Propaguledispersal,particularlyoverlongdistances,influencesthepopulation,geneticand 381 communitystructureofaquaticorganisms(vianaetal.,2014).hence,itisbroadlyregardedasakey 382 ecosystemserviceprovidedbyaquaticherbivores.itseffectis,however,moreevidentwhensuch 383 immigrantsencounteremptyniches(e.g.colonizationofdisturbed,neworrestoredwetlands)than 384

17 whentheyfaceresidentgenotypesorspecies,establishedbeforetheirarrival(louetteandde 385 Meester,2005).Insuchcases,theimmigrant sestablishmentmaybeprecludedbybioticresistance 386 arisingfromintrayandinteryspecificcompetition,mortalitycausedbynaturalenemiesand 387 environmentalfiltering(e.g.habitatchangescausedbyotherspecies).eventhen,theprocessof 388 propaguledispersalrepresentsakeyelementofecosystemresilience particularlyininlandwaters, 389 whicharefragmentedandisolatedbynature.suchresiliencemayprovevital,inthenearfuture,for 390 ecosystemadaptationtoglobalchange byfacilitatingrapidrangeshiftsandthereadjustmentof 391 geneticstructure(e.g.locallyyadaptedgenotypes)causedbytheacceleratingchangesin 392 environmentalconditionsassociatedtoglobalwarming,landusechangesandtheperturbationof 393 globalnutrientcycles(amezagaetal.,2002;röckstrometal.,2009;robledoyarnuncioetal.,2014). 394 Propaguledispersalbyaquaticherbivoresmayalsoentailnegativeeffectsfornativespeciesand 395 ecosystemswhenitmediatesthearrivalandspreadofalienspecies(reynoldsetal.,2015).examples 396 includebothplantandinvertebratespecies(charalambidouetal.,2003;brochetetal.,2009;munoz 397 etal.,2013),althoughtheeffectofdispersalonhumanymediateddispersalisoftenpredominantor 398 difficulttodisentangle(weiszandyan,2010;vanleeuwenetal.,2013).itisalsoworthnotingthat, 399 despitetheirpotentialroleasseeddispersers,aquaticherbivoresmayalsoreducethefrequencyof 400 propaguledispersalbyreducingpropaguleproduction duetotheconsumptionofplantvegetative 401 parts(e.g.(woodetal.,2012a;darnellanddunton,2015))andtheinvertebratesattachedor 402 associatedtothem. 403 404 3.2.2*Biogeochemical*cycling* 405 Grazingandbioturbationbyaquaticherbivorescanhavedirectandindirecteffectson 406 biogeochemicalcycling.inoligotrophicsystems,grazingbysmallerherbivorescanhavepositive 407 effectsbyconservingnutrientswithinthemeadowandclosingthecyclingofnutrientsfromleaf 408 material.leafmaterialcanbeshredded(seaurchins,(vonketal.,2008b)),burrowed(alpheid 409

18 shrimp,(vonketal.,2008a))orexcretedaftergrazing(fish,(kirschetal.,2002)),thusstimulating 410 nutrientretention.largerherbivoressuchasgreenturtlesanddugongsmaytravelbetweendifferent 411 habitatsandstimulateexportofnutrientsbetweenforaging(seagrass)andresting(coralreefs)areas 412 (Christianenetal.,2012). 413 Byforagingandrestinginterrestrialandaquaticenvironments,herbivoresprovideaquaticYterrestrial 414 linkages,transportingcarbon,nutrientsandcontaminantsfromlandtowater(forinstance 415 hippopotamushippopotamus*amphibius*(subaluskyetal.,2015),waterbirds(hahnetal.,2008; 416 Chaichanaetal.,2010)orlargesavannaherbivores(Moss,2015)orvice*versa(forinstancebymoose 417 Alces*sp.(Bumpetal.,2009)).Ithasrecentlybeensuggestedthatthedeclineinlargeherbivore 418 densitiesandtheextinctionoflatepleistocenemegafaunacausedastrongreductioninthecapacity 419 oftransportofphosphorusfromnutrienthotspots,suchasstreamsorfloodplains,towardsless 420 fertileinlandareas(doughtyetal.,2016).longdistancetravelofmigratoryherbivoresalso 421 contributestotransportofnutrientsacrosssitesofvaryingfertility(bauerandhoye,2014). 422 Inseagrassmeadows,grazingbymesoherbivorescanincreaseproductivityandpossiblycarbon 423 sequestration.howeverbelowygroundgrazing(e.g.bydensepopulationsofgreenturtles)orother 424 factorsofdisturbanceintheseagrassrootmatcancausereleaseofancientcarbon,whichmay 425 contributetoincreasedglobalwarming(macreadieetal.,2015).similarly,earlyseasonbelowy 426 groundforagingbypinkyfootedgeese(anser*brachyrhynchus)issufficienttostronglyreducecsink 427 strengthandsoilcstocksofarctictundra(vanderwaletal.,2007).recentstudiesaretherefore 428 stressingthatitiscriticaltomaintainintactpredatorpopulationsthatcontrollargeherbivore 429 densitiestopreventgrazeraggregation,protectcarbonstocksandavoidseagrassmeadowcollapse 430 (Atwoodetal.,2015).Inasubtropicalseagrassecosystem,largepredators(e.g.tigersharks)induce 431 plantspeciesshiftsbychangingtheforagingtacticsoflargegrazers,suchasturtleanddugongs 432 (Heithausetal.,2007).Underlowpredationrisk,dugongsandseaturtlesforagedbyexcavating 433

434 435 nutrientyrichrhizomesofseagrasses.underhighpredationrisks,theychangedtheirforagingtactics, whichstimulatedslowygrowingpioneerspeciesandenhancedcarbonstocks. 436 437 438 439 440 441 Aquaticherbivorescanalsoenhancemethaneemissionthroughthedamageofemergentplant stems(greylaggeese(anser*anser)(dingemansetal.,2011);grasshoppers(petruzzellaetal.,2015)). ThestemsofemergentmacrophyteshavewellYdevelopedlacunarsystemsforgastransport;hence, brokenstemsmayactlike chimneys,providinganopenconnectionbetweenthesedimentandthe atmospherethatbypassesthewaterlayer.ontheotherhand,herbivorescanreducemethane emissionthroughbioturbationandremovalofsubmergedplantbeds(bodelieretal.,2006). 442 443 3.2.3*Coastal*protection 444 445 446 447 448 449 450 451 Seagrasses,mangrovesandsaltmarshesofferimportantcoastalprotectionandsediment stabilizationservices.forseagrassesthisfunctionisgenerallyattributedtoseagrasscanopy properties(hendriksetal.,2010)andcouldbealteredbyherbivory.althoughintensivelygrazed seagrassmeadowshavebeenshowntomaintaintheircapacityforeffectivesedimentstabilization, thisfunctiondegradeswhenherbivoresswitchtobelowgroundgrazing,whichcausesdecreasedbed elevation,erosionandreducedcoastalprotection(christianenetal.,2013).similarly,livestock grazingcanlowersaltmarshes accretionrates(nolteetal.,2015),weakeningtheresilienceand coastalprotectionfunctionofthesesystems. 452 453 3.2.4*Habitat*for*other*organisms* 454 455 456 Macrophytesplayanimportantroleinstructuringaquaticcommunitiesbecausetheyprovide physicalstructure,increasehabitatcomplexityandheterogeneity,affectoxygenandnutrient concentrationsandproviderefugefrompredation(carpenterandlodge,1986;jeppesenetal., 19

20 1998).Macrophytesalsoreleasedissolvedorganiccarbon(DOC)whichcanbeusedbymicrobesin 457 theperiphytonorplankton(findlayetal.,1986).therefore,byconsumingmacrophytes,herbivores 458 maydeterioratethehabitatforotherorganisms.fishproductivitywasfoundtobelowerinseagrass 459 meadowsgrazedbygreenturtles(arthuretal.,2013)andaftermeadowcollapsecausedby 460 overgrazing(reportedforturtlesin(christianenetal.,2013).similarly,herbivorousfishes,dugongs, 461 geeseandotherwaterbirdshavebeenfoundtodrasticallyreduceinvertebratebiomassinseagrass 462 meadowsandsaltmarshes(marklundetal.,2002;sherfyandkirkpatrick,2003;skilleteretal.,2007; 463 Pagesetal.,2012).Impactsoninvertebratesmayoccurevenwheretheproportionalreductionof 464 vegetationislow(bortolusetal.,1998). 465 466 3.2.5.Primary*production* 467 Submergedandemergentmacrophytescansignificantlycontributetotheprimaryproductionof 468 aquaticecosystems(blindowetal.,2006;brothersetal.,2013).thisholdsespeciallyforsmalllakes 469 whichrepresentapproximately99%ofalllakes(downingetal.,2006;verpoorteretal.,2014).direct 470 studiesontheeffectofherbivoryonaquaticplantgrowthandproductionarescarcebecause 471 quantificationoftheimpactofgrazingratesonplantproductionrequirescoupledmeasurementsof 472 ageydependentgrazinglossandturnoverrateofplanttissue(sandyjensenetal.,1994).ingeneral, 473 fastturnoveroftheplanttissue(i.e.highspecificgrowthrate)cancompensateforintenseherbivory 474 undernonylimitingresourceconditions(sandyjensenandjacobsen,2002).cherry&gough(2009) 475 foundthatnymphaea*odoratamaytoleratemoderatelevelsofherbivorybyreallocatingbiomass 476 andresourcesaboveground.onthecontrary,standsofmyriophyllum*spicatumretracttheir 477 resourcestobelowgroundpartsafterdefoliationbyaquaticcaterpillars(milerandstraile,2010). 478 Waterhyacinths(Eichhornia*crassipes)werealsofoundtofullycompensateforlowlevelsof 479 continuousdefoliation,regardlessofnutrientavailability(sotiandvolin,2010).inseagrass 480 ecosystems,reportedeffectsoflargeherbivoreswerepositiveforintermediatedensitiesofgreen 481

21 turtles(e.g.anincreasedtolerancetoeutrophication),andnegativeforhighdensitiesofgreen 482 turtles(e.g.switchtobelowgroundgrazingcausingmeadowcollapse)(christianenetal.,2012; 483 Christianenetal.,2013).Similarly,moderatelevelsofsimulatedfishherbivorystimulatedseagrassin 484 primaryproduction(i.e.compensatorygrowth),whileveryhighlevelsofherbivorydecreasedit 485 (Vergesetal.,2008). 486 AtwholeYecosystemlevel,herbivoryonsubmergedmacrophytesmayhavedifferenteffectsongross 487 primaryproduction(gpp).ifherbivoryresultsinashiftfromclearywatertoturbidconditions(see 488 below),gppcanbeexpectedtodecline,atleastunderintermediateconcentrationsoftotal 489 phosphorus(brothersetal.,2013).ontheotherhand,largegrazersthatremoveoldseagrassleaves 490 coveredinephiphytesandhavebeenreportedtoincreaseprimaryproduction(valentineetal., 491 1997;MoranandBjorndal,2005;Christianenetal.,2012). 492 493 3.2.6.Regime*shifts 494 Shalloweutrophiclakesandlowlandriversmayexistintwoalternativestablesteadystates,aclearY 495 waterstatedominatedbysubmergedmacrophytesandaturbid,phytoplanktonydominatedstate 496 (Schefferetal.,1993;Hiltetal.,2011;Hilt,2015).Indeeperlakes,submergedmacrophytesmayalso 497 contributetothestabilisationofclearywaterconditions(hiltetal.,2010;sachseetal.,2014).shifts 498 betweenclearywaterandturbidstateshavebeenattributedtochangesinnutrientloading,inthe 499 abundanceofzooplanktivorousfish(e.g.bybiomanipulation)and/orinmacrophytecover(scheffer 500 etal.,1993;sondergaardetal.,2007;bakkeretal.,2010).herbivoryonmacrophytesmayalsoplaya 501 significantroleforshiftingmacrophyteydominatedsystemsintotheturbidstate,orpreventingthe 502 shiftfromturbidyintoclearywaterconditions.mitchell&wass(1996a)concludedthatthe 503 cumulativeeffectofwaterfowlgrazingconsumptionwassmallbutmightbecomecriticalwhenother 504 conditionsformacrophytegrowthbecomelimiting(duee.g.tolightlimitationcausedbyhighwater 505 turbidity).arecentmodelingstudyindeedshowedthatherbivoryonmacrophytesoftenbecomes 506

22 importantincombinationwithadditionalstressbyperiphytonshading(hiddingetal.,2016). 507 Herbivorybybirdsandfishmaythustriggerthelossofsubmergedvegetationunderhighnutrient 508 loading(vandonkandotte,1996;paiceetal.,inpress),possiblyincombinationwithotherstress 509 factors.similarly,afterreductionsinnutrientloading,herbivorousbirds)mayinhibittheexpected 510 recoveryofmacrophytes(lauridsenetal.,1993;søndergaardetal.,1996;hilt,2006). 511 512 513 4.! Perspectives:)historical)and)future)changes)in)herbivore)grazing)pressure) 514 Herbivoremanagementandglobalenvironmentalchange,includingwaterlevelfluctuations, 515 eutrophication,temperatureriseandinvasivespecies,feedbackonherbivorenumbers,herbivore 516 distributionandgrazingpressure. 517 518 4.1*Changes*in*herbivore*assemblages*over*time* 519 Thereisagrowingbodyofevidencethatherbivoreassemblageshavevariedovertimeconsiderably 520 intheirdiversityandabundance,andarelikelytocontinuetovaryinthefuture.throughouthuman 521 history,peoplehaveexploitedmanyaquaticherbivorespecies,includingwaterfowl,sirenians, 522 beavers,andmuskrats,forfood,recreation,andanimalproductssuchasskins,furs,feathers,and 523 oils(domning,1982;kitchenerandconroy,1997).humanoverexploitationhashadcatastrophic 524 effectsonmanyherbivorepopulations,withawiderangeofspeciesexperiencingreduced 525 populationsizesandgeographicranges,andevenextinction(jessen,1970;turveyandrisley,2006). 526 Profoundhistoricalchangesinherbivoreassemblageswereparticularlyevidentinshallowseasand 527 coastalhabitats,wherethediversityanddistributionsofmammalianmegayherbivores(sirenians 528 suchasmanateesanddugongs)werereducedheavilyduetohuntingbyhumans(whitehead,1978; 529 Jackson,1997;TurveyandRisley,2006;McCauleyetal.,2015).Freshwatersystemshavealsoseen 530

23 thelossofmanylargeybodiedherbivorespecies,inparticularmammals(moss,2015;bakkeretal., 531 2016b).Additionally,duringthetwentiethcenturyarangeofsemiYaquaticherbivorespecies 532 switchedfromaquatictoterrestrialfeeding,particularlyduringwinter(laubek,1995),further 533 reducingherbivoreabundanceanddiversitywithinaquaticsystems.suchhabitatshiftshavebeen 534 mostwidelydocumentedforavianherbivoressuchasspeciesofswans,geese,ducksandrails 535 (Jefferiesetal.,2003;VanEerdenetal.,2005).Thesubstantialeffectsofhumansonaquatic 536 herbivoresmeantthatthetwentiethcenturyrepresentedalowpointforherbivoreabundanceand 537 diversityacrossaquaticsystems.indeed,wefinditinterestingtonotethatearlierauthorsdrewtheir 538 conclusionsontheapparentunimportanceofplantyherbivoreinteractionsinaquaticecosystems 539 basedonresearchconductedduringaperiodinwhichaquaticherbivoreswererelativelyscarce.for 540 Carribeancoastalecosystemsithasevenbeendocumentedthatthesewereseverelydegradedlong 541 beforeecologistsbegantostudythem,throughthedecimationoflargevertebratesincludinggreen 542 turtlesandmanateesbyabouttheyear1800(jackson1997). 543 Stricterhuntingregulationsandconservationeffortsinthesecondhalfofthetwentiethcenturyhave 544 facilitatedrecoveriesintherangeandpopulationsizesofmanykeyaquaticherbivorespecies(nolet 545 androsell,1998).anexampleistheeurasianbeaver(castor*fiber),reducedbyoverhuntingatthe 546 beginningofthetwentiethcenturytoc.1200individualsineightisolatedpopulationsacrosseurope 547 (NoletandRosell,1998;HalleyandRosell,2002).Followinggreaterlegalprotectionsfromhunting, 548 theeurasianbeaverunderwentsustainedpopulationrecoveryandhasreyestablishedpopulationsin 549 allareaswithinitsformernaturalrange(withtheexceptionofportugal,italy,andthesouthern 550 Balkans),withatotalpopulationofatleast1.04millionindividuals(Halleyetal.,2012).Manyspecies 551 ofherbivorouswaterfowlintemperateregionshavemadesimilarrecoveries(bellrose,1976;ankney, 552 1996).Ofthe21goosespecies(Anserspp.andBrantaspp.)forwhoselongYtermpopulationtrendsin 553 Europeareknown,16speciesarecurrentlyincreasing(Foxetal.,2010).Recentchangesin 554 agriculturalpracticeshaveresultedingreaterterrestrialfoodavailabilityforoverwintering 555 waterfowl,withlargerspeciesbenefitingmoreintermsofpopulationgrowth(jefferiesetal.,2003; 556

24 VanEerdenetal.,2005).Althoughmanyreptilianherbivoresremainendangered,certainspecies 557 suchasthegreenturtle(chelonia*mydas)haverecentlyshownsignsofpopulationrecoveryin 558 responsetothreedecadesofconservationefforts(chaloupkaetal.,2008). 559 Therecoveryofspeciesofaquaticherbivoreshasbeenaidedbytherecentinterestinrewilding 560 ecosystems(donlanetal.,2006).thekeyrolesthatextinctorextirpatedlargeherbivoresplayedin 561 thestructureandfunctioningofterrestrialecosystemshasreceivedgrowingrecognitionfrom 562 researchers(donlanetal.,2006;sandometal.,2014;bakkeretal.,2016a;doughtyetal.,2016). 563 Recently,ithasbeenproposedthatlargeherbivoresmayhaveplayedequallyYimportantrolesin 564 regulatingthestructureandfunctioningofaquaticecosystems(moss,2015;bakkeretal.,2016b). 565 Speciesofaquaticherbivorewhichcanactasecosystemengineers,suchasthebeaver,aretypically 566 primecandidatesforrewildingprojectsduetothewiderecosystembenefitsthatresultfromsuch 567 engineering(collenandgibson,2001).therecentfindingthatbeaver screationofpondsincreased 568 thediversityofherbivorouswaterfowlwithinthelandscapeindicatesthatnaturalrecolonizationand 569 rewildingmayresultinwiderchangestoherbivoreassemblagesthanthetargetspeciesalone, 570 throughthefacilitationofdifferentherbivoretaxa(nummiandholopainen,2014). 571 Therecoveryofpredatorpopulations,vianaturalrecoveryandconservationefforts,canalsoaffect 572 plantyherbivoreinteractions(estesetal.,2011).evidencefromterrestrialandmarinesystemsshows 573 thatherbivoreimpactsonplantscanbereducedaspredatornumbersrecover,becausepredators 574 notonlylowerherbivoreabundancethroughdirectconsumption,butalsoalterherbivore 575 distributionsandreducegrazingintensitythroughindirecteffectsofpredatoravoidancebehavior 576 ( landscapeoffear sensu(madinetal.,2011;kuijperetal.,2013).similarresultscouldbefoundfor 577 interactionsbetweenpredators,herbivores,andmacrophytesinaquaticsystems,butlittleresearch 578 hasbeencarriedouttodate.forexample,arecentstudyfoundthatgreenturtle(chelonia*mydas) 579 habitatusereflectedtradeyoffsbetweenfoodresources,bodycondition,andriskofpredationby 580 tigersharks(galeocerdo*cuvier)inseagrassbeds(heithausetal.,2007).adeclineintigershark 581

25 numbersmaythusresultinastrongincreaseofgreenturtlegrazingonseagrassbeds,potentially 582 resultinginaseagrassbedcollapse(heithausetal.,2014). 583 584 4.2*Exotic*herbivore*species* 585 ThespreadofnonYnativespecieshasbeenakeydriveroftemporalchangesinaquaticherbivore 586 assemblagesinrecentdecades.awiderangeofherbivoretaxahaveestablishedinvasivepopulations 587 knowntoimpactonnativemacrophytes.wellydocumentedexamplesincludebirdssuchasthemute 588 swaninnorthamerica(tatuetal.,2007),mammalssuchasthemuskratineurope(danell,1979; 589 Sarneeletal.,2014),fishessuchasthelessepsianrabbitfishesintheMediterranean(Vergesetal., 590 2014a),molluscssuchasthegoldenapplesnailinAsia(Carlssonetal.,2004),andcrustaceanssuch 591 asredswampcrayfishineurope(gherardiandacquistapace,2007;vanderwaletal.,2013).despite 592 attemptstopreventspeciestransferandestablishmentthroughimprovedbiosecurity,ratesof 593 invasionremainatahistorichigh(cohenandcarlton,1998;jacksonandgrey,2013). 594 595 4.3*Climate*change*and*temperature*rise* 596 Futureenvironmentalchangeisalsopredictedtoalterherbivoreassemblages.Inparticular,climateY 597 drivenfactorssuchasseaylevelriseandchangesinvegetationphenologyandabundancehavethe 598 potentialtoalterwhere,whenandhowmuchherbivoresfeed,andthushavethepotentialtoalter 599 plantyherbivoreinteractionsacrossaquaticsystems(stillmanetal.,2015).forexample,northward 600 shiftsinwinteringrangeinresponsetowarminghavebeenrecordedforseveralwaterfowlspecies, 601 includingsemiyaquaticherbivoreslikegreylaggeese(ramoetal.,2015).furthermore,food 602 requirementschangewithtemperature.withincreasingtemperatures,theenergyrequirementsof 603 temperateectothermanimalsincreaseandtheyconsumemorefood.furthermore,theymaychange 604 theirdietinresponsetotemperature,whichhasconsequencesforthedegreeofplantconsumption. 605

26 Omnivorousfishincreasetheirrelativeconsumptionofplantmaterialwithincreasingtemperatures 606 (Prejs,1984;BehrensandLafferty,2007,2012),apatternthatisrecentlyalsofoundincopepods 607 feedingonseston(boersmaetal.,2016).forendothermsthispatternmaybetheopposite,astheir 608 energyrequirementsdecreasewithincreasingtemperatures,butthishypothesisawaitsempirical 609 testing.warmertemperaturesmayalsoinducehigherperiphytonshadingofmacrophytes(mahdyet 610 al.,2015),makingthemmorepronetoherbivory(hiddingetal.,2016). 611 612 4.4*Herbivore*impacts*under*human*control 613 Whereherbivoreimpactsonmacrophytesaffecthumanactivities,suchasconservation,recreation, 614 andaquaculture,theymaybeviewedasundesirable.inmostcasesitshouldbenotedthatproblems 615 causedbyherbivoresareoftenthedirectorindirectconsequenceofearlierhumanactions.for 616 instance,ongoingeutrophicationoflakesreducestheresilienceofsubmergedplantbedstograzing 617 duetoincreasedshadingbyperiphytonwhichprofitsfromthenutrientloading(hiddingetal.,2016), 618 andreyestablishmentofsubmergedvegetationcanbeinhibitedbygrazersaswell(hauxwelletal., 619 2004).Thecombinedhuntingoflargepredatorsandthecreationofmarineprotectedreserves,as 620 theplacewherelargeherbivoresaresafe,haslocallyresultedinverystronggrazingpressureon 621 seagrassbedsbygreenseaturtles(christianenetal.2014). 622 Suchherbivoreimpactscanleadtoconflictsbetweenpeopleinterestedinherbivorewelfareand 623 conservation,andthoseinterestedintheactivitybeingaffectedbytheherbivore(redpathetal., 624 2015).Todate,conflictshavearisenduetoovergrazingbyreptilian,mammalian,andavian 625 herbivores(table2).incontrast,wecouldfindnoevidenceofconflictsassociatedwithinvertebrate 626 herbivory,whichmayreflectdifferencesinhumanvaluesratherthanecologicalimpact.indeed, 627 evidencefromarecentmetayanalysisshowedthatvertebrateherbivoresdonothaveconsistently 628 greaterimpactsonmacrophytesthaninvertebrateherbivores(woodetal.,2016).vertebrate 629 herbivoressuchaswaterfowl,turtles,andsireniansareoftenconsideredcharismatic,andattempts 630

27 tomanagetheirnumbers,behaviourordistributionsarelikelytoattractmoreoppositionfrom 631 conservationandwelfaregroupsthanthemanagementofinvertebrateherbivores(bremnerand 632 Park,2007;Small,2012).Consequently,itcanbedifficulttoimplementmanagementtoalleviatethe 633 effectsofovergrazing(coluccyetal.,2001). 634 Despitetheseconflicts,notallherbivoreimpactsonmacrophytesareviewedasnegativebypeople. 635 Herbivoreshaveproventobeeffectivebiocontrolagentstohelpreduceanderadicateundesirable 636 macrophytes,suchasoverabundantorinvasivespecies(newman,2004;cudaetal.,2008).awide 637 rangeofherbivorebiocontrolagentshavebeenusedglobally(table3).biocontrolofmacrophytes 638 throughherbivoryhasreceivedgrowinginterestfromresearchersandmanagers,particularlyas 639 somebiotypesofinvasivemacrophytes(e.g.hydrilla*verticillata)havedevelopedresistanceto 640 commonlyyusedherbicides(cudaetal.,2008).moretargetedbiocontrolcanbeachievedby 641 invertebrateherbivores,whichtypicallyshowgreaterspecificityformacrophytespeciesrelativeto 642 vertebrateherbivores(lodge,1991;newman,1991).sincewilson(1964)arguedthat noinsects 643 haveyetbeenusedforthebiologicalcontrolofaquaticweeds,awiderangeofspeciesof 644 coleopteran,lepidopteran,anddipteranbiocontrolagentshavebeenusedsuccessfully(newman, 645 2004;Cudaetal.,2008).Boththeconflictsrelatedtoovergrazing,andtheuseofherbivoresas 646 biocontrolagents,showtheimportanceofimprovingourunderstandingplantyherbivore 647 interactions.forexample,understandingthedensityydependenceofherbivoreimpactson 648 macrophyteabundancecaninformthedensitiesofbiocontrolagentsrequiredtoreduce 649 overabundantmacrophytes(cudaetal.,2008),orallowlakemanagerstopredicttheresponseof 650 macrophytestochangesinwildherbivoredensity(woodetal.,2012a). 651 652 5.! How)to)improve)understanding)of)herbivore)impacts 653 5.1*BottomEup*versus*topEdown*control*across*environmental*gradients* 654

28 Lodge(1991)endedhisreviewwiththeconclusionthat Tounderstandtheinfluenceofherbivory 655 (relativetootherbioticandabioticfactors)onmacrophytepopulationsandassemblages,extensive 656 comparisonsofgrazingdamageacrossenvironmentalgradientsandacrossmacrophyteandgrazer 657 speciesmustbemade. WhereasmetaYanalyseshaveprovidedvaluableinsightintheimpactof 658 differentherbivoresbothonangiospermsandmacroyalgae(pooreetal.,2012;woodetal.,2016), 659 andtherelationshipbetweenmacrophytepalatabilityandherbivoreimpactreceivedincreasing 660 attentionduringthelastdecade,thevariationintheintensityandimpactsofherbivoryalong 661 environmentalgradientshasbeenscantlyexplored.bakkerandnolet(2014)suggestedthat 662 herbivoreimpactmayincreaseatnutrientyrichconditions,duetoacombinationofhigherplant 663 palatabilityandlowertolerancetograzingdamagecausedbyadditionalstressfactors(suchas 664 reducedlightavailabilityduetoperiphytonshading(hiddingetal.,2016).inanexperimentalpond 665 system,herbivorybymallardsprovedtohavestrongerimpactonsubmergedmacrophytesunder 666 eutrophicthanunderoligotrophicconditions.wearenotaware,however,ofanyfieldtestsforthis 667 theory.alternatively,herbivoresmayfacilitatesubmergedmacrophyteswithincreasing 668 eutrophication,forexamplewhenmoderatedensitiesofsmallgrazerssuchasfreshwatersnailsclean 669 submergedplantsfromepiphytes,whichbecomesmoreimportantundereutrophicconditions 670 (Bakkeretal.,2013).Also,whenintensegrazingstimulatestheformationofnewshoots,whichare 671 notyetcolonizedbyepiphytes,largegrazerssuchasgreenseaturtles,cancompensatethenegative 672 effectofeutrophicationforseagrassgrowthtoacertainextent(christianenetal.,2012). 673 674 5.2*Integrating*marine*and*freshwater*studies* 675 Traditionally,freshwatermacrophytesandseagrasseshavelargelybeenstudiedseparately.Todate, 676 theliteratureonfreshwaterandmarineherbivoreimpactshavenotbeenintegrated,andhave 677 largelydevelopedseparatelydespitetheobviousareasofoverlap.ifcrossysystemcomparisonswere 678 beingmade,thesewereoftenmarineyterrestrialorfreshwateryterrestrialcomparisonsandcan 679

29 includebothvascularplantsandalgae(hay,1991;elseretal.,2000;burkepile,2013).thisis 680 unfortunatebecause,leavingafewobviousdifferencesapart(suchasthehighersalinityand 681 connectivityofmarinesystems),theecologyofmarineandinlandywatervascularmacrophytes 682 showsveryfewdifferences.yet,integrativeworkonfreshwaterandmarinevascularmacrophytes 683 stillawaitsitsmoment. 684 685 5.2*Herbivore*assemblages:*towards*functional*groups* 686 687 Forabetterunderstandingofherbivoreimpact,researchersshouldconsiderthewholecommunity 688 ofherbivores,whichcompeteandfacilitateeachother.thishasalreadybeendoneforterrestrial 689 systemslongago(mcnaughton,1985),butexamplesforaquaticsystemsarerare.onesuchexample 690 istheherbivorecommunityinashallowfreshwaterlakethatforagesonpondweedbeds,consisting 691 mainlyofpotamogeton*pectinatus.thesproutingplantsarebeinggrazedbyresidentwaterfowl 692 specieslikemuteswans(cygnus*olor),mallards(anas*platyrhynchos),gadwall(anas*strepera)and 693 coots(fulica*atra).theintensityandtimingofgrazingdeterminehowmuchaboveygroundplant 694 materialisremainingandgrowing(hootsmans,1999).attheendofthesummer,belowyground 695 tubersarebeingformeddependingontheamountofaboveygroundplantmaterial(vanwijk,1988). 696 Thus,moreand,notably,earlierwaterfowlgrazinginsummerresultsinlesstuberbiomassinautumn 697 (Klaassenetal.,2006;Gyimesietal.,2011).Inautumn,tubersarebeingdepletedbymigratoryswans 698 (Bewick sswanscygnus*columbianus)anddivingducks(mainlytuftedducksaythya*fuligulaand 699 pochardsaythya*ferina)untilagrazingthreshold(noletetal.,2001).thedivingducksbenefitfrom 700 thetramplingactivityoftheswans,withoutnegativelyaffectingtheswans intakerate;thistherefore 701 classifiesascommensalism(gyimesietal.,2012).inaccordancewithasequentialpopulationmodel 702 (Jonzénetal.,2002),highesttuberbiomassandinparticulartuberproductionwasgenerallyfoundat 703 sitesforageddowntointermediatethresholdsinthepreviousautumn.inthissystemapositive 704

30 feedbackbetweentubergrazingandtuberproductionresultedfromareductioninselfyshadingora 705 decreaseinneighbourcompetition(nolet,2004). 706 707 Together,theinteractionswithinherbivorecommunitieswilldeterminetheeffectofherbivore 708 diversity,atopicrarelytoucheduponinaquaticvascularplantbeds(butseemarinemacroyalgaeand 709 seagrasssystems,e.g(duffyetal.,2003;burkepile,2013).whereasallinteractionsatthespecies 710 levelareinteresting,awayforwardcanbetogeneralizebeyondspeciesbygroupingherbivoresin 711 guildsorfunctionalgroupsandworkingoutwhichtraitsbestexplaintheirrelativeeffects.suchtraits 712 couldincludebodysize,diet(herbivoryyomnivory),habitat(terrestrialyaquaticlinkages),migratory 713 strategy(sedentaryversusmigratory)andmovementecology(foragingranges).recently,grouping 714 oflargesavannaherbivoresprovedtobeusefultounderstandtheirecosystemimpacts(hempsonet 715 al.,2015).similarly,theecosystemfunctionsofaquaticlargeherbivoresmaybeunderstoodfrom 716 theirhabitatuse,inparticularhowdependentontheaquaticsystemstheyareincombinationwith 717 theirmovementecology(bakkeretal.,2016b). 718 719 5.3Tools*to*study*herbivore*impacts 720 Practically,thehighvarianceinestimatesofmacrophyteabundance,evenwithinasinglestudy 721 system,canmakeherbivoreeffectsdifficulttodetectinaquaticsystemswithoutlargesamplesizes 722 (Woodetal.,2012b).However,numerousmethodshavebeendevelopedandemployedtodetect 723 and/orquantifyherbivoryonmacrophytesandseagrasses(table4).themostdirectapproachfor 724 detectionandquantificationofeffectsisatechniqueknownastethering(kirschetal.,2002;tomas 725 etal.,2005;pradoetal.,2007;pagesetal.,2014).withthistechniqueitispossibletoestimatethe 726 biomass(orcmofleaf)eatendaily,i.e.directherbivoryrates(seetable4).anotherdirectapproach 727 todetectandquantifyherbivoreeffectsareinsituexclosurecageswithsubsequentbiomass 728 measurementsinyandoutside. 729

31 Otherlessdirectherbivorydetectionandquantificationmethodsfocusingonmacrophytesinclude 730 visualestimationsofleafdamageandmeasurementsofmacrophyteperformanceacrossnaturallyy 731 occuringspatialortemporalgradientsinherbivoreassemblageproperties(e.g.density)(table4). 732 Aerialphotographstakenfromdrones(Brandtetal.,2015),echoYsounding(Jägeretal.,2004)and 733 remotesensing(silvaetal.,2008)mayincreasinglybeingusedinthefutureforlargerscale 734 quantificationsofmacrophyteconsumptionbyherbivores.attheherbivoreside,underwatervideos, 735 aquariumfeedingexperiments,molecularmarkers,stableisotopesignatures,gutandfaeces 736 analyses,andwholeylakefishtelemetryhavebeenapplied(table4). 737 Finally,mechanisticmodelsallowassessmentsofherbivoryoverlargerspatialandtemporalscales 738 thanfieldybasedmethods,e.g.longytermpredictionsoffutureherbivory.theyinitiallyneedtobe 739 testedagainstfielddatatodemonstrateaccuracyandhavebeendevelopedforboth,theherbivores 740 andtheplants(table4).recently,aplantgrowthmodelsforaspecificmacrophytespecieshasbeen 741 usedtodetectasynergybetweenherbivoryandshadingbyperiphytonasadditionalstressor 742 (Hiddingetal.,2016).Simulationmodelsofherbivoreforagingcanbeusefultoolstopredictforaging 743 impacts,andteststrategiesforgrazingmanagement(woodetal.,2014a;noletetal.,2016). 744 745 6.! Conclusions) 746 Overthelast25years,asubstantialbodyofevidencehasdevelopedthatshowsthatherbivoryisan 747 importantfactorintheecologyofmacrophytesacrossfreshwaterandmarinehabitats.compilingthe 748 mostrecentdata,weconcludethatherbivoreimpactsinfreshwaterandmarineecosystemsare 749 typically5y10timesgreaterthanthosereportedforterrestrialecosystems.thiscorrespondswith 750 lowerc:nstoichiometryofsubmergedaquaticplants.furthermore,aquatichabitatsare 751 characterizedbylargevariationingrazingpressure.considerablechangeshaveoccurred,andare 752 predictedtooccur,inherbivorediversityandabundance,withwideimplicationsforthecomposition 753 anddynamicsofmacrophytecommunities,aswellasforthestructureandfunctioningofaquatic 754

755 756 757 758 759 ecosystems.therearepressingneedstoimproveourmanagementofundesirableherbivoreimpacts onmacrophytes(e.g.leadingtoanecosystemcollapse),andtheconflictsbetweenpeopleassociated withtheimpactsofcharismaticmegayherbivores.whilesimultaneously,thelongytermfutureof maintainingbothviableherbivorepopulationsandplantbedsshouldbeaddressed,asbothbelongin completeecosystemsandhavecoyevolvedintheselongbeforetheincreasinginfluenceofman. 760 761 762 763 764 MostresearchtodatehasfocusedontheshortYtermimpactsofherbivoresonmacrophyte abundanceandcommunitycomposition.tounderstandtherolesofherbivoresmorefullyweneed toconsidertheirlongerytermimpactsandtheroleofherbivoryinthe(coy)evolutionofboth macrophyteandherbivorespecies.furthermore,abetterintegrationofthefreshwater,marine,and terrestrialherbivoryliteratureswouldgreatlybenefitfutureresearchefforts. 765 766 767 Acknowledgements) 768 769 770 JFPacknowledgesfinancialsupportfromtheWelshGovernmentandHigherEducationFunding CouncilforWalesthroughtheSêrCymruNationalResearchNetworkforLowCarbon,Energyand Environment. 771 772 773 774 775 32

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60 Captions)to)figures) 1460 1461 Figure1.Frequencydiagramofpercentageofherbivory(%herbivory)onvascularplants(a:mean 1462 percentdamage;b,c:netprimaryproductionremoval)andbiomassc:nratio(g/g)across(a,d) 1463 terrestrial,(b,e)freshwaterand(c,f)marineecosystems.freshwaterandmarinedataincludeboth 1464 submergedandemergentplants.themedianvaluesineachpanelareindicatedwithanarrow 1465 accompaniedby M. 1466 Datasources:terrestrial(a)herbivory:percentageofleafareadamaged(Turcotteetal.2014),(d) 1467 biomassc:nratio:infoliage(elseretal.2000);freshwater(b)herbivory:percentageofemergentand 1468 submergedvascularplantbiomassremovedbyherbivoresinherbivoreexclosure/enclosureor 1469 addition/removalexperiments(woodetal.2016),(e)biomassc:nratio:insubmergedandemergent 1470 vascularplants(cloernetal.2002;bakkerunpublisheddata);marine,(c)herbivory:percentageof 1471 emergentandsubmergedvascularplantbiomassremovedbyherbivoresinherbivore 1472 exclosure/enclosureoraddition/removalexperiments(woodetal.2016),percentageofleafarea 1473 damagedinseagrasses(cebrianandduarte1998)(f)biomassc:nratio:inseagrassleaves(atkinson 1474 &Smith1983,Duarte1990,Fourqureanetal.1993,CebrianandDuarte1998,Fourqureanetal. 1475 2010,OlsenenValiela2010),insaltmarshplants(Cloernetal.2002).Thedurationoftheherbivroy 1476 studiesvariedfrominstantaneousmeasurementsofpercentageleafdamageinterrestrialplantsand 1477 seagrassestoexclosuresstudiesinfreshwaterandmarineecosystemsrangingfromaboutaweekto 1478 multiplestudyyears(studydurationsreportedinwoodetal.2016whofoundnoeffectofstudy 1479 durationonpercentageherbivoreplantbiomassremoval). 1480 1481 Figure2.Frequencydiagramofnetprimaryproductionremoval(%herbivory)andbiomassC:N(g/g) 1482 forvascularfreshwaterandmarinemacrophytesofdifferentgrowthforms(emergentand 1483

1484 1485 submerged).emergentplantsincludefloatingplantsandwetlandorsaltmarshplants.themedian valuesineachpanelareindicatedwithanarrowaccompaniedby M.DatasourcesasinFig.1. 1486 1487 1488 1489 1490 1491 1492 1493 Figure3.Synthesizingschemeindicatingtheeffectsofherbivoresonmacrophytebedsandthe functioningofshallowfreshwater(a)andmarine(b)aquaticecosystems.herbivoresaffectplant abundanceandspeciescompositionbygrazingandbioturbation.theirpresencealters biogeochemicalcyclingandprimaryproduction,theytransportnutrientsandpropagulesacross ecosystemboundaries,modifyhabitatforotherorganismsandaffectthelevelofshoreline protectionbymacrophytebeds.symbolsinthefigurearecourtesyoftheintegrationandapplication Network,Univ.ofMarylandCenterforEnvironmentalScience(ian.umces.edu/symbols/). 61

1494# 1495# Table#1.#Plant#and#herbivore#traits#promoting#propagule#dispersal#by#aquatic#herbivores.# # Trait# Effect# References# Herbivores) # # Ability#to#chew#or#grind#food# The#presence#of#a#gizzard#or#grinding#teeth#reduces#propagule#survival.#Among#waterfowl,#heavier# gizzards#reduce#seed#survival#but#higher#grit#content#may#enhance#germination#of#undigested#seeds.# Figuerola#et#al.#(2002)# Furry#or#sticky#appearance#of# Animals#with#a#surface#on#which#propagules#can#attach#disperse#more#propagules# # animal#body# Diet#selection# Targeted#feeding#on#seeds#may#result#in#more#transport,#but#also#in#more#seed#predation# #thus# reducing#the#transport#through#untargeted#feeding#which,#particularly#when#mixed#with#large#bulks#of# food#(plant#parts,#animal#food,#debris),#may#result#in#high#propagule#survival.## Figuerola#et#al.#(2003)# Travelling#distance# Larger#travelling#distances#results#in#further#potential#dispersal,#particularly#for#migratory#species#that# cover#long#distances#in#single#leaps# Viana#et#al.#(2013b)# Habitat#use# Animals#with#specialized#use#of#aquatic#habitats#are#more#likely#to#deposit#the#propagules#in#suitable# habitat.#in#particular,#targeted#arrival#to#aquatic#habitats#at#stopovers#may#increased#the#deposition#of# Figuerola#and#Green#(2005)# # 62#

propagules##ingested#at#departure#sites,#especially#after#the#first#drinking#and#feeding#bout.# Plants) # # Propagule#dimensions# Small,#round#seeds#survive#digestive#tract#better#than#large,#elongated#seeds.#Small#size,#in#particular,# may#enhance#ingestion#mixed#with#vegetative#plant#material,#increasing#propagule#ingestion#( foliage#is# the#fruit,#sensu#janzen#(1984))#and#survival#to#gut#passage.# Mueller#and#van#der#Valk# (2002);#Soons#et#al.#(2008);# Figuerola#et#al.#(2010)# Hardness#and#permeability#of# seed#coat# Adaptations#for# epizoochorous#dispersal# Thicker,#harder#and#less#permeable#seed#coats#increase#survival#to#disperser s#gut#passage,#but#may# reduce#germination#in#the#absence#of#uningested#seeds.# Hooks,#rough#or#sticky#surface#have#been#proposed#to#enhance#dispersal#potential# Mueller#and#van#der#Valk# (2002);#Santamaria#et#al.# (2002);#Figuerola#et#al.# (2010)# Van#der#Pijl#(1982)# # Resistance#to#dessication# Organism s#and/or#propagule s#resistance#to#dessication#may#enhance#epizoochorous#dispersal#of# aquatic#organisms# Panov#and#Caceres#(2007);# Havel#et#al.#(2014)# 1496# 1497# # # 1498# # # # 63#

64# # Table#2.#An#overview#of#reported#conservation#conflicts#that#have#arisen#from#the#impacts#of#overgrazing#by#herbivores#on#macrophytes.# 1499# # 1500# Herbivore# Issue# Parties#in#conflict# Location(s)# Duration# Current#status# References# Green#turtle# (Chelonia)mydas)# Overgrazing#of#seagrasses#can# undermine#conservation#efforts# in#protected#areas#and#reduce# fish#catches#for#local#people# Turtle#and#seagrass# conservationists,# fishermen# IndoaPacific# oceans# 1980s#a# present# Ongoing# Arthur#et#al.#(2013);# Christianen#et#al.# (2014)# West#Indian# manatee# (Trichechus) manatus)# Overgrazing#has#hindered# efforts#to#restore#submerged# macrophyte#beds# Manatee#and# macrophyte# conservationists# Freshwater#and# brackish# ecosystems#in# southaeast#usa# 1990s#a# present# Ongoing# Hauxwell#et#al.#(2004)# Beaver#(Castor) spp.)# Impacts#on#aquatic#habitats,#via# effects#on#vegetation#and#wider# ecosystem#(e.g.#fish)# Conservationists,# animal#welfare# groups,#fishermen,# and#statutory#wildlife# management# Freshwater# habitats#across# North#America,# Russia# Unknown#a# present# Ongoing# Nolet#and#Rosell# (1998);#Collen#and# Gibson#(2001);#Halley# and#rosell#(2002)#

65# # agencies# Coypu#(Myocastor) coypus)#and# muskrat#(ondatra) zibethicus)# Overgrazing#on#emergent# macrophytes#degrades#aquatic# habitats# Conservationists,# animal#welfare#groups,# and#statutory#wildlife# management#agencies# Freshwater# lakes#and# wetlands#in# Europe# 1930s# # present# Resolved#by# 1970s#via# extirpation#of# coypu#in#britain;# ongoing# elsewhere#in# Europe# Gosling#and#Baker# (1989);#Barends# (2002)# Mute#swan# (Cygnus)olor)# Overgrazing#of#macrophytes# degrades#aquatic#habitats# Conservationists,# animal#welfare# groups,#and#statutory# wildlife#management# agencies# Freshwater# habitats#in# Europe#and#USA# 1950s#a# present# Ongoing# Perry#and#Perry# (2008);#Wood#et#al.# (2014b,#2015)# Geese#(Anser)spp.,# Branta#spp.,#and# Chen#spp.)# Overgrazing#of#emergent# macrophytes#degrades#wetland# habitats# Conservationists,# animal#welfare# groups,#and#statutory# wildlife#management# Canadian#Arctic# and#subaarctic# wetlands,# freshwater#lakes# 1970s#a# present# Ongoing# Kerbes#et#al.#(1990);# Nichols#(2014)#

agencies# and#wetlands#in# Europe#and# North#America# 1501# # 1502# # 1503# # 66#

1504# 1505# Table#3.#A#summary#of#key#herbivore#taxa#used#as#biocontrol#agents#in#the#management#of#macrophytes.# # Herbivore#biocontrol#agent# Target#macrophyte(s)# Herbivore#generalist#or# References# specific?# West#Indian#manatee# Wide#range#of##macrophyte#species,#including#Cabomba)aquatica,# Generalist# Allsopp#(1960)## (Trichechus)manatus)# Anacharis#spp.,#Leersia#spp.,#Utricularia#spp.# Geese#(Anser)spp)# Wide#range#of#macrophyte#species# Generalist# Ross#(1971);#Wilson#et# al.#(1977)## Grass#carp# Wide#range#of#macrophyte#species,#including#invasive#species#such#as# Generalist# Clayton#(1996);#Hanlon# (Ctenopharyngodon)idella)# Hydrilla)verticillata# et#al.#(2000)## Cichlid#fishes,#e.g.#blue#tilapia# Wide#range#of#macrophyte#species# Generalist# Schwartz#et#al.#(1986)## (Oreochromis)aureus)# Crayfish,#e.g.#papershell# crayfish#(orconectes)immunis)# Wide#range#of#submerged#macrophyte#species# Generalist# Letson#and# Makarewicz#(1994)## Weevils,#e.g.#milfoil#weevil# Speciesaspecific#biocontrol#agents#identified#for#many#macrophyte# Specialist# Creed#and#Sheldon# # 67#

(Euhrychiopsis)lecontei),# Hydrilla#tuber#weevil#(Bagous) affinis)# species,#e.g.#hydrilla#(hydrilla)verticillata),#eurasian#water#milfoil# (Myriophyllum)spicatum)# (1993);#Newman# (2004)# Apple#snails#(Pomacea#spp.)# Wide#range#of#macrophyte#species# Generalist# Rushing#(1973)## Dipteran#larvae,#e.g.#Asian# Speciesaspecific#biocontrol#agents#identified#for#many#macrophyte# Specialist# Wheeler#and#Center# hydrilla#leafamining#fly# species,#e.g.#hydrilla)verticillata# (2001);#Bownes#(2014)# (Hydrellia)pakistanae)# Lepidopteran#larvae,#e.g.# Speciesaspecific#biocontrol#agents#identified#for#many#macrophyte# Both#generalist#and# Wheeler#et#al.#(1998);# waterlettuce#moth# species# specialist#species#reported# Gross#et#al.#(2001);# (Spodoptera)pectinicornis)# Newman#(2004)# Hemiptera,#e.g.#Eccritotarsus) Speciesaspecific#biocontrol#agents#identified#for#many#macrophyte# Specialist# Coetzee#et#al.#(2007);# catarinensis# species,#e.g.#water#hyacinth#(eichhornia)crassipes)# Hernandez#et#al.#(2011)## Orthoptera,#e.g.#water# Speciesaspecific#biocontrol#agents#identified#for#many#macrophyte# Specialist# Bownes#et#al.#(2010)# hyacinth#grasshopper# species,#e.g.#water#hyacinth#(eichhornia)crassipes)# (Cornops)aquaticum)# 1506# 1507# # # 68#

1508# 1509# Table#4.#Different#methodologies#used#to#detect#and#quantify#herbivory#on#macrophytes.# # Methodology# Explanation# References# Exclosures# # Installation#of#cages#(fully#closed#or#open#at#the#bottom)#to#protect#macrophytes#from# different#herbivores#(fish,#muskrats,#waterfowl,#crayfish,#turtles)## Søndergaard#et#al.#(1996);# Körner#et#al.#(2002);#Hilt#(2006);# Christianen#et#al.#(2012);#Poore# et#al.#(2012);#veen#et#al.#(2013);# Van#der#Wal#et#al.#(2013);# Sarneel#et#al.#(2014)# Tethering# Shoot#herbivory#rates#(cm#shoota1#daya1)#are#estimated#for#marked#shoots#by# measuring#leaf#elongation#over#time.# Kirsch#et#al.#(2002);#Tomas#et#al.# (2005);#Prado#et#al.#(2007);# Pages#et#al.#(2014)## Underwater#videos# Video#recording#and#quantification#of#fish#activities#including#plucking#of#leaves## Körner#&#Dugdale#(2003);# Bennett#&#Bellwood#(2011);# Verges#et#al.#(2014b)# Visual#estimation#of#leaf#damage# # Francescini#et#al.#(2010)# # 69#

Natural#gradients# Measure#macrophyte#performance#(e.g.#growth#rate,#biomass,#etc)#across#naturallya occuring#gradients#in#herbivore#assemblage#properties#(e.g.#density)## Wood#et#al.#(2012b)# Drones# Identificaton#of#muskrat#damage#in#constructed#wetlands#by#digitizing#lowaaltitude# aerial#photographs## Brandt#et#al.#(2015)# Molecular#markers# # Assessments#of#genetic#variation#in#plants#across#environmental#or#geographical# (latitudinal)#gradients#using#different#molecular#markers# Mader#et#al.#(1998);# Hangelbroek#et#al.#(2002)#;#King# et#al.#(2002);#hidding#et#al.# (2014)# Stable#isotope#analyses# # Measurement#of#carbon,#nitrogen#and#hydrogen#stable#isotopes#in#resources#and# consumers#and#application#of#mixing#models# France#et#al.#(1996);#Solomon#et# al.#(2011);#dorenbosch#and# Bakker#(2012);#Mendonca#et#al.# (2013);#Vander#Zanden#et#al.# (2013);#Scharnweber#et#al.# (2014);#De#Kluijver#et#al.#(2015)# Gut#analyses#and#stable#isotope# analyses# Gut#analyses#in#fish#combined#with#stable#isotope#analyses#of#basal#food#resources#and# fish# Mao#et#al.#(2014)# # 70#

Telemetry# Wholealake#fish#telemetry# Hanson#et#al.#(2007)# Bird#counting#and#determination# # Dos#Santos#et#al.#(2012)# of#lignin#content#in#faeces# Laboratory#feeding#rate# determination#in#fish## Determination#of#fish#feeding#rates#in#aquaria## Körner#&#Dugdale#(2003)# Mechanistic#models# Simulations#of#foraging#herbivores#or#effects#on#plant#growth#can#predict#the#location,# timing,#and#magnitude#of#herbivore#effects#on#macrophytes# Hootsmans#(1999);#Van#Nes#et# al.#(2003);#nolet#et#al.#(2006);# Wood#et#al.#(2014a);#Nolet#et#al.# (2016);#Hidding#et#al.#(2016)# 1510# # 1511# # # 71#

1512# # 1513# # 72#

1514# # 1515# # 73#