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GlobalEcology and BiogeographyLetters (1998) 7, 61-71

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Mangrovesas alienspecies: the case of Hawaii

JAMES A. ALLEN U.S.D.A. ForestService, Institute of PacificIslands Forestry,1151 PunchbowlSt., Rm. 323, Honolulu,HI 96813 U.S. A. email:jimallen@gte. net

Abstract. Prior to the early 1900s, therewere no (Himantopus mexicanus knudseni),colonization of in the Hawaiian Archipelago. In 1902, habitats to the detrimentof native species (e.g. in Rhizophoramangle was introducedon the island of anchialine pools), overgrowing native Hawaiian Molokai, primarilyfor the purpose of stabilizing archaeologicalsites, and causingdrainage and aesthetic coastal mud flats.This species is now well established problems. Positive impacts appear to be fewer,but in Hawaii, and is found on nearly all of the major includeuses of local importance,such as harvestingB. islands. At least five other species of mangrovesor gymnorrhizaflowers for lei-making,as well as some associated species were introducedto Hawaii in the ecological servicesattributed to mangroveselsewhere, early 1900s,and whilenone has thrivedto the degree such as sedimentretention and organicmatter export. of R. mangle, at least two have established self- From a researchperspective, possible benefitsof the presenceof mangrovesin Hawaii includean unusual maintainingpopulations (Bruguieragymnorrhiza and to evaluatetheir functional role in coastal Conocarpuserectus). Mangroves are highlyregarded opportunity ecosystemsand the chance to examineunique or rare in most partsof the tropicsfor the ecosystemservices species interactions. theyprovide, but in Hawaii theyalso have important negative ecological and economic impacts. Known Key words. Species introductions,alien species, negativeimpacts include reductionin habitat quality Hawaii, Rhizophoramangle, gymnorrhiza, for endangeredwaterbirds such as the Hawaiian stilt Conocarpuserectus, mangroves.

INTRODUCTION nativespecies and communities,alteration of ecosystem processes,impacts on agricultureand infrastructure, Mangrovesare thepredominant form of vegetationin and aestheticimpacts (Vitousek & Walker,1989; Loope, the intertidalzone of tropicalestuaries, lagoons, and 1992). shelteredshorelines. Their vast natural distribution The Hawaiian Archipelago,located in the central also includesmany subtropical coastlines (Duke, 1992). Pacific Ocean between 18? and 30?N and 1550 and Where they occur naturally,mangroves perform 180?W, has no nativemangrove species, despite having ecological functionsof substantialdirect or indirect both suitable climateand geomorphicsettings. Since value to human society,such as the productionof their introductionin the early part of this century, wood, provisionof habitat for mangrovecrabs and however,mangroves have flourishedto such a degree fish,and improvementof water quality (Odum, Mclvor thatmany people have become concernedabout their & Smith,1982; Ewel, Eong & Twilley,1998). impacts,and expensiveprojects aimed at controlling As they provide valued goods and services,there them have been undertaken.Although they have have been some attemptsto introducemangroves into receivedless attentionthan many other alien speciesin areas where they do not occur naturally.Where Hawaii, withinthe range of suitable habitat, mangroves introductionshave been successful and mangroveshave have largelyvalidated Egler's (1947: p.407) prediction developed self-maintainingpopulations (e.g. Hawaii, that they would effect'a change as sweeping, as Tahiti), theyare by definitionalien species. As such, complete,and as strikingas any whichhas occurred the goods and servicesthey provide must be weighed in the Hawaiian Islands'. This paper addressessome against any damage caused. Among the causes for aspects of the change Egler that saw coming, and concernabout alien species in generalare effectson includes an overviewof the introductionand spread

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Table 1. Known introductionsof mangrovesin the Hawaiian Islands

Species Year Source Plantinglocation Comments

Rhizophoramangle 1902 Florida SouthwesternMolokai Highlysuccessful; the probable source formost R. manglein Hawaii

Rhizophoramangle ? ? Kalihi Fishpond,Oahu Possiblyonly one ,but was reportedlythriving'

Rhizophor-amangle 1922 ? Heeia Marsh, Oahu Rhizophor-amangle 1960 ? Kealia Pond, Maui 3,000 propaguleswere planted to keep down dust duringpond drawdowns

Bruguieragymnorrhiza2 1922 Philippines Up to 11 sitesaround Oahu3 Has shownlimited tendency to spread

B. parviflora 1922 Philippines Up to 11 sitesaround Oahu3 Last recordedin 19484 Ceriopstagal 1922 Philippines Up to 11 sitesaround Oahu3 Last recordedin 19224 Rhizophor-amucronata 1922 Philippines Up to 11 sitesaround Oahu3 Last recordedin 19284

Conocaipuserectus Before1910 Florida? Oahu C. erectusvar. sericeus 1946 Bahamas Oahu Still widelyplanted as an ornamental

'McCaughey (1917). 2Although treatedas B. gymnor-rhizain recent literature and in Wagneret al. (1990), earlierreports (e.g. Walsh 1967) referredto thisspecies as B. sexangula.Samples fromOahu sentrecently to Dr Norman Duke, of theAustralian Institute of Marine Science, were identifiedas B. sexangula.I have retainedthe currentuse of B. gymnorrhizain thispaper, but the readershould be aware that some or all of the treesreferred to as B. gymnorrhizamay in factbe B. sexangula. 3McEldowney(1922); Degener (1940, 1945); Wester(1981). 4Wester(1981).

of mangroves,an analysis of their ecological and B. parvifora(Roxb.) W. & A. ex Griff., socioeconomic impacts, and a brief descriptionof (Perr.) C.B. Robinson, and L. recentefforts to controlmangroves in the state. (Table 1). A total of approximately14,000 propagules of thefirst four of thesespecies were obtainedin 1922 fromthe Philippinesby the Hawaiian Sugar Planter's THE INTRODUCTION AND SPREAD Association and planted on eleven sites around the OF MANGROVES IN HAWAII island of Oahu (McEldowney, 1922; Degener, 1940, 1945). The plantings of C. tagal apparentlyfailed The earliestknown introduction of a mangrovespecies completely,and R. inucronata and B. parvifora occurred on the island of Molokai in 1902. An persistedfor 20-30 years,but of the fourspecies only unreported number of mangle L. B. gymnorrhizais known to exist still on Oahu or propaguleswere obtained fromFlorida by the U.S. anywhereelse in Hawaii (Wester,1981). C. erectushas ExperimentStation and plantedby the American Sugar been introducedat least twice(Wester, 1981) and the Company on mud flatson the southwesternpart of variety (C. erectus var. sericeus Griseb.) is still the island (Munro, 1904; Degener,1940, 1945). Cooke commonlyplanted as an ornamental.Both formsof (1917: p.366) stated that the purposes of this C. erectushave escaped cultivationand established introductionwere to hold back 'soil washed down by small wild populationson some islands. everyheavy rain into the sea, and also as a pasture Of the threemangrove species or close associates forbees'. (R. mangle,B. gymnorrhiza,and C. erectus)known Records exist for the introductionof five other to be present in Hawaii, R. mangle is by far mangroveor closely associated species, includingR. the most widespreadand common; most mucronataLamk., Bruguieragymnorrhiza (L.) Lamk., swamps are essentiallymonospecific stands of this

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t 9 I| S He'eia Mangrove

0 0 Kauai Oahu 0

t _ , > Molokai I I ~~Hawaii

Lanai Maui C I Kahoolawe

Fig. 1. Approximatelocations of known mangrovestands in Hawaii. Map is updated fromWester (1981) with information obtaineddirectly by the authorand fromconsultation with individuals familiar with specific islands.

species.Although documentation was found only for THE ECOLOGICAL ROLE OF plantingson Molokai, Oahu and Maui (MacCaughey, MANGROVES IN HAWAII 1917; Fosberg, 1948; Anonymous,1960), R. mangle presentlyis found on all of the main islands with Effecton existingwetland plant the possible exceptions of Kahoolawe and Niihau communities (Fig. 1). Evidently,R. manglebegan to spread veryrapidly Prior to the arrivalof humans,intertidal in followingits introduction.Fifteen years after the first Hawaii apparentlyhad fewspecies of vascularplants. planting,seedlings had become establishedat least The aquatic Ruppiamaritima L. could be foundin the 8km from the original Molokai planting site, and lowestportions of the intertidalzone, and herbaceous healthy-lookingpropagules were found on the other or smallshrub species such as Sesuviumportulacastrum side of the island, over 90 km away from the (L.) L., Heliotropiumcurassacvicum L., and Lycium original site (Cooke, 1917). R. mangle is still sandwicenseA. Gray werepresent at the upper end of continuingto spread, especially along the eastern the intertidalzone in some areas (Gagne & Cuddihy, shore of the island of Hawaii (the 'Big Island') and 1990), but the bulk of the intertidalzone may have the western shore of Maui (B. Hobdy, Hawaii been inhabitedprimarily by algae and fungi(Egler, Division of Forestryand Wildlife;K. Smith, U.S. 1947). Fish and WildlifeService; L. Wester,Univ. of Hawaii, Intertidalwetlands that existed prior to human pers. comm.). settlementhave sincebeen heavilymodified, beginning In marked contrastto R. mangle,B. gymnorrhiza with the arrival of Polynesians 1500 or more years and C. erectushave shown littletendency to spread ago. Most nativeHawaiians lived near the coast, and beyond their originalplanting sites (Fosberg, 1948; developed extensive agriculturaland aquacultural Wester,1981; personal observations).B. gymnorrhiza systemsin areas thatwere originally low-lying uplands, is currentlyknown to existonly on foursites on Oahu naturalwetlands or shallowopen water(Kikuchi, 1976; and C. erectusis reportedas 'sparinglynaturalized' on Kirch, 1982). Coastal wetlandswere further altered by theislands of Oahu, Lanai, and Maui (Wagner,Herbst European and Asian immigrants(Cuddihy & Stone, & Sohmer,1990; p.547). 1990) who, in addition to their many physical

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This content downloaded from 170.144.163.23 on Tue, 29 Oct 2013 16:21:31 PM All use subject to JSTOR Terms and Conditions 64 JamesA. Allen modificationsof wetlands, also introducedplant species Hawaii Sea GrantProgram, pers. comm.). The impacts that quicklybecame dominantin the intertidalzone, of mangroveson species native to anchialine pools such as Batis maritimaL. and Paspalumvaginatum Sw. have not been documented,but are likelyto be quite (Egler, 1947; Elliot, 1981). significant.The pools have a large algal component, Mangroves,therefore, have generallynot displaced whichis likelyto be reducedby a mangroveoverstorey, nativeplant communitiesdirectly. Most of the larger and theyalso have highlyspecialized and vulnerable stands of mangrovesare on areas of new sediment fauna (Chai et al., 1989). deposition (due in large part to human-induced On Molokai, R. manglecan be foundon seasonally- erosion), withinformer Hawaiian fishponds,or on floodedsites that in itsnatural range might be occupied disturbedsites, including those occupied by B. maritima by mangrovespecies not presentin Hawaii, such as and P vaginatum.Egler (1947), describedthe following Avicenniagerminans (L.) Stearn.On thesesites, which scenariofor the intertidalzone, whichseems to have are subjectedto periods of droughtand hypersalinity occurred in many locations: (1) Pre-Hawaiian and levels,whole cohortsof sapling-sizedtrees appear to Hawaiian communitiesof R. maritima,algae, fungi, have died back duringthese periodic stress events. and sessile animals; (2) introductionof B. maritima The possibility that mangroves have displaced sometimeprior to 1859 and subsequentdevelopment communitiesdominated by tree species such as Hibiscus of pure B. maritimameadows; (3) introductionand tiliaceusL. or Thespesiapopulnea (L.) Sol. ex Correa spread of mangrovesonto the meadows; and (4) the (themselvespossibly Polynesian introductions)has eventual replacementof B. maritimameadows by been mentionedby some,but thereis littleevidence to mangroves.Evidence for the widespreaddisplacement support this contention.Walsh (1967), for example, of B. maritimacommunities is providedby de Ausen stated that the mangrovesat Heeia Swamp, Oahu, (1966) and Richmond& Mueller-Dombois(1972). extendedover an area dominatedby H. tiliaceus,and To a lesser degree, mangroveshave also become that only a few dyingindividuals of this species were establishedin freshwaterenvironments, sites above the stillpresent. However, both historicalphotographs of intertidalzone and other atypical locations. Walsh the area (Devaney et al., 1982) and accounts that the (1967) measuredsalinity levels along a seriesof stations mangroveswere planted in a (Degener, in Heeia Swamp,Oahu, whichcontains R. mangleand 1945) indicatethat previouslythe area had no more B. gymnorrhiza.The uppermoststation was in a small than scatteredtrees, most of whichwere probablyon pool at the inlandedge of the mangroves,adjacent to highmicrosites. A few treespecies do occur near the a freshwatermarsh dominatedby Brachiariamutica upperlimit of the intertidalzone and at themouths of (Forssk.) Stapf. During the course of a year of rivers,where they may competewith mangroves, but measurements,the salinityof the pool neverexceeded most of the habitatstypically occupied by mangroves 0.19 ppt. elsewherein the tropicsmay have had no treespecies In some areas, most notably on the west coast of at all in Hawaii. Mangroves,therefore, are not only the island of Hawaii but also to a lesser degree on alien species in Hawaiian wetlands, but they also Maui, mangroves have become established in representan entirelynew life formin the ecosystems anchialine pools. These pools are exposed portions they invade, causing dramatic effects on plant of the groundwatertable found predominantlyon communitystructure (albeit mostlyof communitiesof geologicallyyoung, porous lavas in the coastal tropics otheralien ) and thereforealmost certainlyon and subtropics(Chai, Cuddihy& Stone, 1989). They ecosystemfunctioning (Ewel & Bigelow,1996). have no directsurface connection to the sea, but may exhibittidal fluctuations due to subsurfaceconnections. Anchialinepools close to the sea may be overwashed Mangrove stand structure duringstorms, which results in depositionof sediments and the occasional introduction of mangrove Only a few descriptionsof mangrovestand structure propagules.Once mangrovesreach these pools, they in Hawaii are available (Table 2). In additionto sites soon colonize all suitableshoreline habitat, and they suchas thosesummarized in Table 2, wheremangroves have completelyfilled in some shallowpools. The have formedclosed-canopy forests, there are some sites can shade the ponds heavilyand greatlyincrease the wheremangrove stands are moreopen. Siteswith either rate of organic matter accumulation, leading to widelyscattered trees or a thinbut nearlycontinuous prematurepool senescence(D. Brock, Universityof crowncover typicallyhave a B. maritimaunderstorey,

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Table 2. Structuralcharacteristics of Hawaiian mangroves

Site Overstorey Tree Mean Basal Stand Remarks species density dbh area height (cm) (m2/ha) (m)

Kalau'apuhi Fishpond, R. mangle 1,529/ha' 14.92 30.3 203 Averageof sevenplots along Molokai 500 m transectfrom sea to landwardedge. Source: Bigelow et al. (1989)

Kukuku Fishpond, R. mangle 1,150/ha' 9.12 7.7 5 Averageof threeplots along Molokai 100m transectfrom sea to landwardedge. Source: Bigelow et al. (1989)

Heeia Swamp, Oahu R. mangle,B. 815/ha4 11.3 approx. Based on point-quartersample gymnorrhiza, 593/ha 3.3 14 of 20 points.Source: Lee H. tiliaceus 341/ha 5.2 (1971) Heeia Swamp, Oahu R. mangle 1,958/ha5 13.4 37.5 21 Based on point-quartersample of 20 pointsin lowerpart of swamp. Source: 0. Steele, unpubl. data

NuupiaPonds, Oahu R. mangle 24,430/haf6 3.9 37.2 approx. Based on 11 25 m2 plots. 6 Source: E. Cox, unpubl.data

'Trees greaterthan 7.62 cm dbh, averagefor all treeswas 3,786 trees/hafor Kalau'apuhi Pond and 3,050 trees/hafor Kukuku Pond. 2Mean fortrees greater than 7.62 cm dbh. 3The tallesttree measured was 36.6 m. 4Minimumtree diameter not specified. 5Mean fortrees greater than 3.0cm dbh. 6Includes all freesgreater than 1.5m in height.

and may be in a transitionalphase to a closed canopy Pluchea indica(L.) Less., H. tiliaceus,and T populnea, mangroveforest (de Ausen, 1966). as well as B. maritima. One particularlynotable characteristic of mangrove Another pattern of zonation was described by standsin Hawaii is the highdensity of seedlings.Lee Bigelowet al. (1989) forR. manglestands on Molokai, (1971) reportedan average of forty-sevenR. mangle wherethe largesttrees were consistently found on the and twenty-twoB. gymnorrhizaseedlings per square landwardside (Fig. 2). They attributedthis to a rapid metrein Heeia Swamp, and many other sites have rate of sedimentdeposition on the seawardside of the comparableseedling densities. High seedlingdensities stands,followed by subsequentcolonization, resulting may be attributableto a low rate of predation on in a gradationof tree age fromthe older,landward- propagules (Steele, 1998; V. Yap, Univ. of Hawaii, to the younger,seaward-edge of the stands. unpubl. data). Even thoughmany stands are monospecificand most Primary productivity are relativelysmall (<20 ha), thereis some evidenceof zonation. In Heeia Swamp, Wester(1981) noted that Only one studyof net primaryproductivity has been tall, dense R. mangle dominate along Heeia Stream conducted on the tree component of Hawaiian and along the lower portionsof the swamp,whereas mangrovestands. Over a 2-yearperiod in a R. mangle away fromthe streamand furtherinland the canopy stand at Nuupia Ponds, Oahu, litterfallaveraged becomesmore open, and B. gymnorrhizabecomes more 2.52kg dry weightm-2 year-', of which 1.09kg dry common.In additionto thetwo mangrove tree species, weightm-2 year-' was leaf fall (Cox & Jokiel,1996). Wester also noted the presence of the occasional Total litterfallexceeds values reportedfor R. mangle

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ofnet canopy photosynthesis (based on lightextinction throughthe canopy) ranged from 5 to 16g C m-2day-' (Cox & Jokiel,1996), which is higherthan sites in Florida (Lugo & Snedaker,1974) and similarto estimatesfrom Australia and Papua New Guinea (Boto,Bunt & Wellington,1984; Clough, Ong & Gong, in press).

Sediment retention and water quality improvement Mangrovesappear to have a generallypositive influence on waterquality in Hawaii.Sediment retention, for example,can be quitehigh in Hawaiianmangroves, andmay contribute toimproving the quality of offshore waters.In onepossibly extreme case, 10 cm of sediment wasdeposited in 16months at a samplingstation (No. 1) on theupstream side of themangroves at Heeia Swamp,Oahu (Walsh,1967). On Molokai,turbidity was loweron coralreefs adjacent to mangrovesthan on reefswith no adjacentmangroves and a negative Fig. 2. An exceptionallylarge growing relationshipwas foundbetween mangrove basal area onthe landward edge of a stand,south-central Molokai. Photo andturbidity ofadjacent waters (Bigelow et al., 1989). byK. Ewel. The authorsattributed these patterns to effective sedimentretention by mangroves.In additionto an apparentrole in reducingsuspended sediments, Walsh standsin Florida(Odum, McIvor & Smith,1982) and (1967) reportedthat the high nitrate and phosphate formost mangroves in southeastAsia and Australia levelsin Heeia Streamwere reduced significantly in (Twilley,Lugo & Patterson-Zucca,1986; Saenger & theupper reaches of theswamp, indicating that the Snedaker,1993). mangrovesmay be servingas a sinkfor these nutrients. Part of the reasonfor the highlitterfall rate at In contrast,mangroves inareas with restricted water NuupiaPonds may be theunusually high density of flows,such as fishpondsand anchialinepools, may the stand evaluated (Table 2). Most of the difference, havesignificant negative effects on waterquality. Leaf however,appears to be due to the large production litterinput without subsequent export of muchof (and subsequent fall) of propagules, which is the thematerial, for example, is suspectedof decreasing primaryreason for pronounced peaks in litterfall dissolvedoxygen concentrations (Cox & Jokiel,1996). observedduring August-November (Cox & Jokiel, Mangrovesmay compound their impacts in areas with 1996).Propagule production is muchhigher than in limitedtidal flushing by blockingchannels and water reportsof other Rhizophora stands (e.g. Sasekumar & controlstructures, further restricting thealready limited Loi, 1983).The highrate of propaguleproduction exchangeof water. mightconceivably be due to a lack of agents that damage and developing fruits, but this Food chain support possibilityhas notbeen investigated. Annualaboveground biomass production at Nuupia The faunalcomponent of mangrovesin Hawaii is Pondswas estimatedto be 29.1t har' yr1-and total depauperatecompared with mangroves elsewhere in abovegroundbiomass was estimatedto be 266t ha-' the Pacific.This may be due in part to Hawaii's (E. Cox, Universityof Hawaii,pers. comm.), which isolation,which has limited colonization even of marine arehigher than many productive R. manglestands in and intertidal'organisms (Hourigan & Reese, 1987; theGulf of Mexico and comparable to some Rhizophora Kay & Palumbi,1987). The relativelyshort time that spp. - dominatedstands in Asia (Lugo, Brown& mangroveshave been in Hawaiiand theyoung age of Brinson,1988; Saenger & Snedaker,1993). Estimates moststands may also be importantfactors. Another

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This content downloaded from 170.144.163.23 on Tue, 29 Oct 2013 16:21:31 PM All use subject to JSTOR Terms and Conditions Mangrovesas alienspecies 67 factorimportant in some swampsis theharsh physical species have all sufferedlarge declinesin population and chemicalconditions. Walsh (1967) concludedthat size during this century,due to a combination of the paucityof animal species in the upper reaches of hunting, habitat loss, predation by introduced Heeia Swamp could be at least partiallyexplained mammals, invasion of wetlands by alien plants, by the flocculentnature of the sediments,low redox hybridization,disease, and possibly environmental potential,moderately high levels of hydrogensulfide, contaminants(U.S. Fish and WildlifeService, 1985). and wide fluctuationsin salinity,water temperature, Whereasmangroves were not primarilyresponsible for and pH. theirpopulation declines, they may be a major factor Onlyat themost seaward of his six samplingstations limitingthe abilityof waterbirdsto recover. did Walsh find a moderatelydiverse fauna, which The most directimpact mangroves have had on the includedportunid and grapsoidcrabs, littorine snails, endangeredwaterbirds is the invasionof foragingand polychaetes,and gobies. Even at thisstation he found nestinghabitat. None of the specieswill forageor nest no oysters,barnacles, holothurians, tunicates, sponges, in mangroves,so many areas where mangrovesare amphipods,or isopods, althoughsome of these,such establishedare thereforeexisting or potentialhabitat as oystersand barnacles,can be found in mangrove lostto thewaterbirds. Mangroves also frequentlyblock siteselsewhere (e.g. Pearl Harbor, Oahu). Bird lifein drainage outlets fromponds where water levels are Hawaiian mangrovesis also depauperate.The black- manipulatedto provide waterbirdhabitat, requiring crownednight heron (Nycticorax nycticorax hoactli) is expensive maintenanceand affectingwildlife refuge the only residentnative bird species known to use managers'ability to lowerwater levels at optimaltimes mangroves. Occasionally great blue herons (Ardea forthe birds(J. Beall, U.S. Fish and WildlifeService, herodias)stray into Hawaii and mayuse themangroves, pers. comm.). and at least fouralien bird species are knownto nest Mangroves are known to provide shelterfor some in mangroves(M. Rauzon, Eco-Horizons, Inc., pers. waterbirdpredators, most of which are also alien comm.). In contrast,Odum et al. (1982) compiled a species. The native black-crownednight herons and list of 181 species that use Florida mangrovesfor introducedcattle egrets (Bubulcus ibis), both of which feeding,nesting, roosting, or otheractivities. They also prey on other waterbirdchicks, nest in mangroves cited several studies from other regions that found and probablyhave benefitedfrom their establishment. betweenforty-five and 137 bird species in mangrove Introduced mammals, including Polynesian rats ecosystems. (Rattus exulans), black rats (R. rattus) and Indian The roleof Hawaiian mangrovesin exporting organic mongooses (Herpestes auropunctatus)are found in material to adjacent offshore waters is poorly some mangrovestands; all may prey on waterbirds documented.Because theymay produceexceptionally (Shallenberger,1977). high amounts of litterfall(Cox & Jokiel,1996), the All four endemicwaterbird species are affectedby potentialfor substantial exports appears high.Export mangroves,but the Hawaiian stiltis probablyaffected potentialis limitedat some sites,however, by restricted the most. Its optimalforaging habitat is on shallowly connectionsto offshorewaters. Mangroves established floodedmarshlands and exposed tidal flats(U.S. Fish in derelictfishponds and sites above the influenceof and WildlifeService, 1985), sites ideal for mangrove daily tides may exportrelatively low proportionsof colonization.Also, Hawaiian stiltshave been observed theirlitterfall, especially in coarse or particulate,as nesting on sites recently cleared of mangroves, opposed to dissolved, forms. Indeed, Walsh (1967) indicatingthat mangrovesmay be limitingnest site describedthe upper portion of Heeia Swamp as a zone availability (M. Rauzon, Eco-Horizons, Inc, pers. of decay and organicmatter accumulation, indicating comm.). thatmuch of the litterfallwas retainedin situ.

Effectson endangered waterbirds SOCIOECONOMIC IMPACT OF MANG ROVES IN HAWAII Four species of endemic waterbirdsare presenton the main Hawaiian islands-the Hawaiian duck (Anas Effectson archaeological sites wyvilliana),Hawaiian coot (Fulica alai), Hawaiian stilt (Himantopus mexicanus knudseni), and Hawaiian Literallythousands of archaeologicalsites have been moorhen (Gallinula chloropus sandvicensis).These mapped in Hawaii (Kirch, 1985). Althoughmost sites

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This content downloaded from 170.144.163.23 on Tue, 29 Oct 2013 16:21:31 PM All use subject to JSTOR Terms and Conditions 68 JamesA. Allen are above the intertidalzone, thereare nevertheless occurredrecently in thetown of Kailua, on Oahu. Local many sites impacted by mangroves, including businesspeoplecomplained about theodour emanating fishponds, temporary habitation sites, and lithic fromthe mangrovesin a nearbychannelized stream, manufacturingsites. At the lattertwo typesof sites, and personnelfrom the Hawaii Departmentof Land the primaryarchaeological impact has been to make and Natural Resourcescleared out a portionof them access to and excavationof the sites very difficult (Mike (less than 0.5 ha), at an estimatedcost of US $40,000 Degas, SCS/CRMS, Inc., pers. comm.). The impact (Aguiar, 1996). of mangroveson fishponds,however, has been more serious. Economic uses of mangroves Hawaiian fishpondstypically included a large,arc- shaped stone wall extendingout fromthe shore onto Mangroves are very rarely used for any economic a reefor tidal flat,along withassociated water control purpose in Hawaii. Occasional stumpsof R. mangle structures.At least 449 fishpondswere constructed are encountered;most treesare believedto be cut by (Kirch, 1985), and they are found on all the major recentimmigrants from other Pacific islands, who use islands. Fishponds are particularlysignificant sites the wood formaking small structuressuch as cooking because they were closely associated with chiefs of huts.The mostimportant economic use of mangroves particularareas, and theirconstruction and subsequent is the harvestingof B. gymnorrhizaflowers for leis managementwere believed to have been a majorfactor (traditionalHawaiian necklacesof strungflowers). The in the development of the highly-stratifiedsocial Hawaiian name forB. gymnorrhizais kukuna-o-ka-la, structureof pre-Europeancontact Hawaiian society meaning'rays of the sun'. Leis made fromthis species (Kikuchi, 1976). are highlyvalued because the flowerslast for weeks, In theyears following the arrival of Europeans,most or, when dried,for years (Henry,1993). In fact,one fishpondswere abandoned, and many silted in due reason B. gymnorrhizais not morecommon in Hawaii to erosion from agriculturalplantations and urban may be that many trees are cut down to facilitate developments.Mangroves have becomeestablished on harvestingof the flowers. the walls and in the interiorsof manyfishponds, and in some cases have caused walls to deteriorate.The resurgenceof interestin traditionalHawaiian culture MANGROVE CONTROL PROJECTS and lifestylesin recent years has included several IN HAWAII attemptsto restorefishponds to productiveuse (Wyban, 1992). Such attemptsare much more difficultand Several effortshave been made to controlmangroves expensive where mangroves are present. Apple & in Hawaii. Most have beenprojects undertaken to clear Kikuchi(1975) recommendthat, where mangrove mangrovesfrom small areas (<2 ha), such as adjacent are presentin the walls, theybe dismantled,the roots to watercontrol structures on nationalwildlife refuges, removed, and the walls rebuilt-a difficultand withinanchialine pools, and along canals and streams expensiveprocess. wherethey are impedingdrainage. A fewlarger control projectshave also been implemented,mainly to protect endangered waterbird habitat or to restore Effectson waterways archaeologicalsites. All controlefforts known to the Mangroves have colonized the banks of canals and author have employed mechanical, rather than streams,as well as the shorelinesof harbours,where chemicalor biological,techniques. theirpresence has oftenbeen regardedas a nuisance. A projectunderway to clearup to 5 ha of mangroves Althoughthey may not be responsiblefor the siltation fromNuupia Ponds is one of the largercontrol efforts of fishponds,canals, and the mouths of streams, undertakenin Hawaii to date. Heavy machinery mangroveshave occasionallybeen blamed because they (mainlya trackedbucket excavator)is being used to are quick to colonizeareas of recentdeposition. Where clearmangroves from thirteen dredge spoil islandsand waterwaysare sufficientlynarrow, mangroves may also to reducethe surfaceof the islandsbelow thelow low- trap large debris that would otherwise float water mark to reduce futuremangrove colonization. downstream,resulting in impededdrainage. Water may Mangrovesat more sensitivelocations, such as on or stagnatein suchareas, resulting in offensiveodours and near fishpondwalls and on highlyunstable soils, are increasesin mosquitopopulations. One such situation being removedby crewswith chainsaws, who cut the

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This content downloaded from 170.144.163.23 on Tue, 29 Oct 2013 16:21:31 PM All use subject to JSTOR Terms and Conditions Mangrovesas alienspecies 69 treesand prop roots as close to the substratesurface alien species(e.g. B. maritimaand P vaginatum)would as practicable.Because thereis no local marketfor the also need to be controlled. wood, it is beingchipped for disposal. The difficulties In Hawaii, most habitatprotection and restoration in workingon unstable soils and in areas with both efforts are currentlytargeted at high elevation endangeredspecies and archaeological sites greatly ecosystems,which are more intactand contain more increaseremoval costs. The estimatedcosts range from highlyendangered species than do lowlandand coastal $108,000/haon siteswhere machinery can operateto ecosystems.Given these other prioritiesand today's at least$377,000/ha on siteswhere mangroves are being limitedbudgets, the most reasonable strategyfor the removed by chainsaw crews workingfrom floating forseeable future may be to continue to control walkways(L. Bookless,Kaneohe Marine Corps Base, mangroveson themost sensitive sites. Such effortswill pers. comm.). involvethe initialremoval of large treesand periodic follow-up effortsto remove new mangroverecruits. Developing a small-scalelocal marketfor mangrove wood (e.g. forfuelwood, charcoal, or poles) mightbe CONCLUSIONS thebest way to achievethis level of control.Biocontrol approachesdesigned to reducethe production of viable Although not native to the Hawaiian Islands, propagules or seedling establishmentmay prove mangroves (esp. R. mangle), have become effectivein keeping down the costs of post-clearing well-established,and currentlyoccupy much of the maintenanceon sensitivesites. intertidal habitat potentially suitable for their The likely continued presence of mangroves in development.Because the Hawaiian intertidalzone Hawaii mayprovide useful opportunities for ecologists. was largelydevoid of trees(and indeed,of practically The presenceof mangroves on tropicalcoastlines where all vascular plants), the establishmentof mangroves theydid not previouslyoccur, for example,provides has introduceda dramaticallydifferent life form than an opportunityto take a new perspectiveon their thosethat existed previously, with concomitant changes functionalroles within tropical coastal ecosystems, in ecosystemstructure and function. such as by comparingnearshore fisheries in areas with Mangroves are playingsome of the same roles in and withoutmangroves. It also providesan opportunity Hawaii for which they are valued in their native to examinethe developmentof new ecosystemswith habitats, such as sedimentretention, water quality rare or unique species interactions.R. mangleand B. improvement,and theproduction and exportof organic gymnorrhiza,for example, may not occur together matter. They may also be providing shoreline naturally,and it would be interestingto investigate protectionand otherimportant goods and services.On why R. manglehas been so much more effectiveat theother hand, they have colonizedimportant foraging colonizing new sites than B. gymnorrhiza.Perhaps and nestinghabitat of fourendemic (and endangered) standsof thetwo speciesin Hawaii could be contrasted Hawaiian waterbird species, overgrown native with similar stands found on South Pacific islands Hawaiian archaeological sites, invaded anchialine consistingof B. gymnorrhizaand R. samoensis,which pools, and caused localized drainage and aesthetic some considerto be merelya synonymfor R. mangle problems. (Spalding,Blasco & Field, 1997). Takingadvantage of Theirpresence in the intertidalzone is also a major such opportunitiesmay lead to importantadvances in factor,though by no means the onlyone, limitingthe our understandingof mangrovesand theirlarger role potentialfor restoration of nativewetland ecosystems. in coastal ecosystems,and therebymake the best of Practicallyspeaking, however, restoration of anything what manyconsider to be an unfortunatesituation. morethan a smallproportion of the originalintertidal ecosystemsof Hawaii may not be feasiblein the near future.The total acreage of mangrovesin Hawaii is ACKNOWLEDGMENTS not large (probably <1000 ha), but it is widely distributedand occurson numerousfederal, state, and I would like to thankAlice Keesing forher assistance privateproperties, making a coordinated,state-wide in gatheringinformation reported in this paper, and eradicationeffort extremely difficult. Furthermore, the KatherineEwel, LyndonWester, and two anonymous originalgeomorphology of much of the coastal zone reviewersfor theirconstructive commehits on earlier has been highlyaltered, and severalother aggressive draftsof this manuscript.The willingnessof Evelyn

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