LAGUNAS COSTERAS, UN SIMPOSIO. Mem. Simp. Intern. Lagunas Costeras. UNAM- UNESCO, Nov. 28-30, 1967. Mexico, D. F.: 515-536, 5 figs. (1969) THE MANGROVE COMMUNITY, ASPECTS OF ITS STRUCTURE, FAUNISTICS AND ECOLOGY K. RUTZLER ':. ABSTRACT In a brief survey of the literature dealing with mangroves in the sense of a littoral habitat, the points of focus and results of earlier works are summarized. The various biotopes within the community are classified in accordance with their dominant floristic and faunistic components. It is felt that not enough consideration has been given to the sedentary faun.a which is reaching dominant importance on mangrove roots in certain regions. The limited abundance and peculiarity of the substrate raises interesting problems concerning interspecific relations and dynamics of associations. A large number of habitats in mangrove swamps are influenced by the sea. These have been greatly neglected by systematists and ecologists in the past. With coordinated international and interdisciplinary cooperation it should be possible, with reasonable effort, to determine the community structure, i. e. the correlation and self-deter- mination of the bioconotic units. INTRODUCTION Back to the days of Alexander the Great we can follow the amazement of travellers first confronted with the ghostly evergreen forest raising from the sea. After Nearchus, commander of Alexander's fleet reported on the mangroves he had observed bordering the Arabian Sea, Theophrastus (305 B.C.), stu- dent of Aristotle, gave the first recognizable description of Rhizophora mangle and its habitat. With Linnaeus a period of morpho- logic and systematic bot'lnical studies began in the second half of the eighteenth century. Because of the peculiar embryology of Rhizophora and the adaptation of mangrove trees to growth in salt the interest of biologists soon focused on the ecology of mangrove plant communities and associated vegetation. Studies ant their successions and physiology were carried out by Walter and Steiner (1937) for East Africa, by Davis ( 1940) for Florida, by Chapman (1943) for Jamaica and by Galley et al (1962) for Puerto Rico. Besides the osmotic properties of Rhizo- jJhora, Avice1tnia, Laguncularia and Sonne- ratia the function of the stilt roots and pneumatophores has also engaged many physiologists until Scholander et al (1955) proved the hypothesis that they served for ventilation of the rest of the root system which is buried in the anaerobic mud. An important part of· the literature on mangroves has been devoted to practical human interests, from mosquito control (Gilroy and Chwa tt, 1945) to economic exploitation for building timber, fire wood, and tannic acid; not to mention the fact that the roots of the living trees serve as hiding places for fish and as substrate for much desired oysters (Holdridge 1939, Mattox 1949). Mangroves were even plant- ed artificially to avoid erosion of railway embankments (Bowman, 1917) . .. Smithsonian Institution, Washington, D. c., U.S.A. - 515 -
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LAGUNAS COSTERAS, UN SIMPOSIO. Mem. Simp. Intern. Lagunas Costeras. UNAMUNESCO, Nov. 28-30, 1967. Mexico, D. F.: 515-536, 5 figs. (1969)
THE MANGROVE COMMUNITY, ASPECTS OF ITSSTRUCTURE, FAUNISTICS AND ECOLOGY
K. RUTZLER ':.
ABSTRACT
In a brief survey of the literature dealing with mangroves in the sense of a littoralhabitat, the points of focus and results of earlier works are summarized. The variousbiotopes within the community are classified in accordance with their dominantfloristic and faunistic components. It is felt that not enough consideration has beengiven to the sedentary faun.a which is reaching dominant importance on mangroveroots in certain regions. The limited abundance and peculiarity of the substrate raisesinteresting problems concerning interspecific relations and dynamics of associations.A large number of habitats in mangrove swamps are influenced by the sea. Thesehave been greatly neglected by systematists and ecologists in the past. With coordinatedinternational and interdisciplinary cooperation it should be possible, with reasonableeffort, to determine the community structure, i. e. the correlation and self-determination of the bioconotic units.
INTRODUCTION
Back to the days of Alexander the Greatwe can follow the amazement of travellersfirst confronted with the ghostly evergreen
forest raising from the sea. After Nearchus,commander of Alexander's fleet reported on
the mangroves he had observed bordering theArabian Sea, Theophrastus (305 B.C.), student of Aristotle, gave the first recognizabledescription of Rhizophora mangle and itshabitat. With Linnaeus a period of morphologic and systematic bot'lnical studies beganin the second half of the eighteenth century.
Because of the peculiar embryology ofRhizophora and the adaptation of mangrovetrees to growth in salt w~ter the interestof biologists soon focused on the ecology ofmangrove plant communities and associatedvegetation. Studies ant their successions andphysiology were carried out by Walter andSteiner (1937) for East Africa, by Davis( 1940) for Florida, by Chapman (1943)
for Jamaica and by Galley et al (1962) forPuerto Rico.
Besides the osmotic properties of RhizojJhora, Avice1tnia, Laguncularia and Sonneratia the function of the stilt roots andpneumatophores has also engaged manyphysiologists until Scholander et al (1955)proved the hypothesis that they served forventilation of the rest of the root systemwhich is buried in the anaerobic mud.
An important part of· the literature onmangroves has been devoted to practicalhuman interests, from mosquito control(Gilroy and Chwatt, 1945) to economicexploitation for building timber, fire wood,and tannic acid; not to mention the factthat the roots of the living trees serve ashiding places for fish and as substrate formuch desired oysters (Holdridge 1939,Mattox 1949). Mangroves were even planted artificially to avoid erosion of railwayembankments (Bowman, 1917) .
.. Smithsonian Institution, Washington, D. c., U.S.A.
- 515 -
516 RUTZLER
This leads to the interesting aspects oftheir role in geology and sedimentology. Thefunction of the mangroves in this respecthas sometimes been denied, but more frequently overemphasized. However, it is afact that during the changes of tides debrisaccumulates among the roots. This creates,with the aid of massive growing epiphytes,stagnant bodies of water favorable for deposition. In addition, dead leaves and otherplant remains pile up and become imbedded.There is no way for the roots themselves tohold the sediments as has been frequentlystated; on the contrary, they might aid destruction by breaking through .limestone rock[as in the example of the pneumatophores ofAvicennia (Howard, 1950)]. Thus, in coordination with favorable geomorphologicaland hydrographical conditions, mangrovesare definitely able to help build up land fora protruding terrestrial forest.
Prior to the turn of this century nozoological research had been done in themangrove community if one neglects thereports of voyagers complaining about mosquito attacks and praising the palatableoysters and crayfish. Later, the mangroveforest was recognized as a littoral habitatwhere truly terrestrial and typical marineorganisms lived in close relationship and periodically overlaped with the change of tides.Also, attention was drawn to the fact thatthe mangrove is a physiographic unit, theprincipal components of which are organnisms; therefore, the problems, are predominantly of a biological nature.
TOPOGRAPHY AND CLASSIFICATION
There are different types of mangroves depending on the various geomorphic featuresof the coaSt. Plant ecologists have demonstrated that there are distinct topographicalsuccessions which are similar in old worldand new world mangroves. Pioneer red mangroves (Rhizophora) protrude farthest intothe sea or border channels and· lagoons. Ty-
pically, they are followed by black mangrove(Avicennia) - salt marsh associes whichare not regularly flooded. Then comes aseldomly flooded transition zone (e. g. Conocarpus, HibiSCUS) which connects to dryland, frequently rain forest. According tothe substrate the mangrove types are classified as reef, sand, mud, and peat mangroves. This classification also expresses different hydrographical conditions, which areof interest to the marine biologist. Reef andsand mangroves are the most exposed towards the open sea with sufficient waterexchange to provide a typical marine environment. One of these has been studiedclosely by the Great Barrier Reef Expedition at Low Isles (Stephenson et at 1931),and it was found that all the sedentaryfauna from mangrove roots, i. e. corals,sponges, hydroids, anemones and ascidiansand also the holothurians clearly belongedto the reef fauna proper.
Mud mangroves usually occur near rivermouths, thus showing a steep salinity gradient from marine to fresh water. Thiseffect was the subject of an exemplarystudy by Walsh (1967) in Hawaii.
Peat mangroves grow along shelteredshores and in lagoons without fresh waterinfluences other than subsoil water and rain.This latter type is of great interest for themarine biologist because the environment issuch that a typical marine flora and faunacan exist but the factors are so variable andsometimes extreme that the organisms mustbe strongly selected.
This type of mangrove is rather commonin the Western Atlantic and has stimulatedthe interest of various systematists and ecologists as can be learned from comprehensive studies by Mattox (1949) in PuertoRico and by Gerlach (1959) in Brazil.
VERTICAL ZONATIONS
As in all coastal marine habitats the vertical distribution of communities is deter-
Figure 1. Typical mangrove channels as represented by Bone Fish Creek, East Bimini. On the permanently submerged partsof Rhi=ophora roots limitation of substrate causes spacial competition between sedentary organisms. HT=High tide level. LT=
Low tide level.
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518 RUTZLER
mined by the exposure to air. The supralittoral is only a narrow zone because thereis no wave action causing spray. However,it is extensive since it is mainly formed bythe mangrove trees. Some crabs (Aratus,Sesarma) and gastropods (Littorina, N eritina) are dominant here closely followingthe changing sea level.
In the medio-littoral, the substrate andthe living habits and morphologicaland physiological properties of organisms influence the community structure. In orderto avoid coverage by mud sedentary organisms must be confined to vertical stablesubstrates like stems, air-roots, and seedlingsof the mangrove trees. Because they are exposed, however, the danger of desiccationduring low tide is greatest. A characteristicintertidal algal community the "Bostrychietum" has a high osmotic resistance andwithstands desiccation. As field and laboratory experiments have proven (Biebl,1962), it is not very resistant to light. Solarradiation, however, is comparatively lowbecause of the - protecting canopy of themangrove leaves. Animals with closeableshells like oysters (Crassostrea) and barnacles (Balanus, Chthamalus) or with tubeswithin which they can retract, such as serpulids, remain inactive but protected duringthe dry period.
On the extended mud flats (the most investigated habitat to date) representativesof the vagrant macrofauna alternate in theiractivity periods. Fish, such as gobiids andblenniids remain buried in mud or in clumpsof algae during low tide. Crabs such as Ucaremain in their humid, but air-containingburrows during high tide; others, like Aratus, avoid the rising water by climbingtrees. The famous mud-skipper (Periophthalmus) is probably one of the very fewtruly amphibious mangrove animals.
Small, or true micro-organisms are, fromthe viewpoint of their micro-environment,inhabitants of the infra-littoral. During low
tide they have to withstand high or, in (;a~c
of rainfall, low salinities but not desiccation.They either remain protected in crevices,bore holes in their substrate (mangroveroots, oyster shells), live endophyticly inclumps of algae, live endozoicly in oystersor spDnges, or live interstitially in mud orsand. In these cases the chemical and physical conditions provided by the substratedetermine the community structure.
It is difficult to obtain all references onsystematics and distribution of mangrovedwelling organisms since they are frequentlyhidden in some specialized taxonomic work.However, I have tried to summarize froma literature study all organisms characteristic for the different environments in bothEastern and Western mangroves. The resul tsare presented in table 1.
INFRA-LITTORAL TIDE CHANNELS
The proper infra-littoral, since the occurrence of mangrove trees is limited by waterdepth, is almost exclusively restricted to anumber of channels through which the waterof the changing tides flows and to lagoonsor ponds devoid of trees. Here the onlysolid substrate uncovered by mud are the roots of bordering Rhizophora. The stiltroots of these trees extend to deeper waterthan the stems. At the current-exposed outersides of the channel bends ("Prallhang") thesoil is washed out, thus exposing a wickerwork of mud-roots under a mud-cornice.This overhang is left in the upper mediolittoral because the water current is veryslow near the peak of the tide. Examples ofthis forma tion can be found In narrowparts of mangrove channels where thecurrent is accelerated such as in Bone Fish--or other creeks in the mangroves of EastBimini, Bahamas (figure 1). 1
1 Observations from Bimini mentioned herewere made during work under Office of Naval Research Contract NONR 552 (07). I amindebted to Mr. Robert Mathewson, director ofthe Lerner Marine Laboratory.
Figure 2. ]Jos/rycbieflllll grows on the tntertiebl parts of the redmangrove roots. At high tide numerous fishes find here hiding space.
Figure 3. Tedania ignis is one of the most common mangrovesponges. Because of its toxic properties, which even effect thehuman skin on touch and because of its flame-red colot it is com-
monly called the fire-sponge.
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TABLE 1
1\fARINE ORGANISMS CHARACTERIZING U1FFERENT TYPES OF MANGROVE SWAMP HABITATS
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BV,B LT E,W f. Gerlach, 1958BV,B LT W Gerlach, 1958BV,B C W Gerlach, 1958BV,B HT W Gerlach, 1958BV,B HT E Stephenson 1'/ (Ii, 1931BV,B HT E Stephenson 1'/ ai, 1931BV,B LT E Stephenson 1'1 ai, 1931BV,B HT E Stephenson e/ ai, 1931
BV LT E Stephenson 1'/ ai, 1931
BV,B HT E Stephenson et ai, 1931
T ABLE I ( CONTIN U A TION)
MARINE ORGANISMS CHARACTERIZING DIFFERENT TYPES OF MANGROVE SWAMP HABITATS
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MOLLUSCA(GASTROPODA) 15
Alderia 'Uda BV HT W Gerlach, 195 8Pyra::lIs palltstris BV HT F Stephenson et a.l, 1931Telesco/lilt m tclcsco/,ill m BV HT E Stephenson rf ai, 1931
MOLLUSCA(BIVALVIA) 5-t-
Area tu.berclIlosll BS 1'11' W f. Gerlach, 1958Pel/illil pllilltam BV,n I-lT E Stephenson et a.!, 1931
PISCES 5-1-Gobillcllu.s smaragdus BV,B lIT W Gerlach, 1958Pcr;ophtbalmlls koe!rrlltcri BV,13 HT E Stephenson et ai, 19.11
REPTILIACrocodyllls poroSIlS BV C E f. Gerlach, 1958
Figure 4. Mass development of Cbolldril/a .,,1/cllla on mangroveroor prohibiting any orher sedentary organisms to settle.
Figure 5. On the shaded backside of dominating Ircillia !asciclliatafour different species of sponges occur epizoicly.
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The bottom of these channels is coveredwith a thin layer of mud containing shellfragments and diatoms, which is inhabitedby a rich microfauna. The underlaying peatis rich in H 2S and organic carbon (Kornicker, 1958) and therefore poorly inhabited. In areas more exposed to tidal currents there are occasional stands of seagrass(Thalassia, CYlIlodocea) and green algae(PeniciIl1ls, Call1crjJa).
The intertidal parts of stilt roots andseedings are covered with a thick layerof bostrychictll In (figure 2). No oysters orbarnacles are present in the Bimini mangrove. Just below low tide level, heavy clusters of sponges cover the stilt roots, down tothe point where the bottom sediments set alimit. Dominant are the species Tedaniaignis, lrcinia fasc.'clliata and Chondrilla1111C1lla (figures 3, 4). The only space competitors are the tunicate Ectcinascidia tttrbinata, some hydroids, mainly E1Idendri1t11lsp, and Myrio1lellla sp, and the anemoneCVlldylactis sp.
Behind a curtain of stilt roots the abovementioned mud-caves are located; thereagain, on the exposed mud-roots of RhizofJhora, sponges dwell abundantly. The domina ting species are Anthosig II'tl'1la 'variam,Myriastra sp, and Haliclona sp.
SPACLAL COMPETITION
The' very restricted a'vailability of substrate suggests studies on the spacial competition of sponges and other sedetttaria. Earlier observations on isolated hard-bottomstructures have shown (Riitzler, 1965) thatthe lack of substrate can be overcome bycertain species which are able to settle andsurvive 'on other species through specializedmorphological and chemical adaptations. Acomparable situation can be found on themangrove roots where numerous small sponge crusts, hydroids, and some bryozoansgrow on the dominant primary settlers. Alsoa horizontal zonation according to light canbe observ.ed, the epizoa preferring the semi-
LAGUNAS COSTERAS, UN SIMPOSlO 533-
obscure surfaces which are faced toward the·mud bank (figure 5).
Nothing is known yet about the effect oftannic acid and other chemical substances.precipitated by the mangrove roots on thesettlement of larvae. Other limiting factorsare undoubtedly food supply and physicalaction of the tide current. The latter isnaturally strongest in the undercuts. Sponges'of a certain size originating there arefrequently torn loose, leaving space for newsuccessions. A study concentrating on these·environmental factors and their biocoenoticconsequences is currently under way with,the aid of an instrument combination recently asserribled (Forstner and Riitzler~
1969). Main emphasis is given to long-termin situ measurements of micro-climatic facters (mainly light quality and quantity~
water turbulences, temperature, salinity andoxygen), with the aim of identifying anaarranging micro-environments according to·their ecological significance.
ENVIRONMENTAL AFFINITIES
The question arises finally, to which otherhabitats the mangrove community can berelated. In fact, there are four environmentcomplexes superimposed.
The tree tops are strictly terrestrial~
usually inhabited by the flora and fauna ofa tropical rain forest. Lower branches andparts of the stem are comparable with thesupra-littoral and pneumatophores, parts ofthe stilt roots and bases of tree stemswith the medio-littoral of unexposed shadedrocky coasts. The mud flats are true marinemedio-Iittoral soft bottom environments,which have been compared with the shoalsof moder:lte latitudes, such as the North.German U\'V'attenmeer" (Gerlach, 1958).. The channels and lagoons, even in themiddle of the swamp, represent a marginalbiotope. The muddy bottom with decomposing plant remains underneath and the composition of the sedentary organisms growingon plant structures as sole substrate indicate-
534 RUTZLER
<t close rehtionship to shallow seagrass meadows in open bays. The main difference isthat the outflowing tide carries water masseswhich, for several hours, were subject to·considerable temperature and salinity changes and to all the metabolic processes ofthe intertidal swamp community.
CONCLUSIONS
Plant systematists and sociologists have·devoted much work to the mangrove community during the past decades. A great<tmount of data is also available on topo-
graphical, geological and economical aspects. Although the salinity ranges are widethe mangrove forest is restricted to the intertidal marine environment and sets thestage for the transition from marine to terrestrial life.Th~ large number of organic, inorganic,
and behavioristic factors involved wouldjustify an international monographic studyin order to get away from the practice ofpresenting numerous valuable but uncorreJated single studies and to arrive at an ecoiogical synthesis.
LITERATURE CITED
BIEHL, R. 1962. "Protoplasmatisch-oekologischeUntersuchungen an Mangrovealgen von Puerto Rico." Protoplasl//(/ 55: 572-606.
BOWMAN, H. H. M. 1917. "The ecology andphysiology of the red mangrove." Proc. American Phil. Soc. 56: 589-672.
-CHAPMAN, V. J. 1943. "1939 Cambridge University Expedition to Jamaica. I. A study ofthe botanical processes concerned in the development of the Jamaican shore-line. II. Astudy of the environment of Avicennia nitM"Jacq. in Jamaica." J. Linllean Soc. Bot. 52:407 -486.
DAVIS, C. C. and WILLIAMS, R. H. 1950. "Brackish water plankton of mangrove areas inSouthern Florida." Ecology 31: 519-531.
DAVIS, J. H. 1940. "The ecology and geqlogicrole of mangroves in Florida." Carnegie Inst.Washington Publ. 517: 303-412.
FORSTNER, H. and RUTZLER, K. 1969. "Problems and methods of microclimatic measurements in the littoral zone." (In press.)
'GERLACH, S. 1958. "Die Mangroveregion tropischer Kuesten als Lebensraum." Z. Morph.Oekol. Tiere 46: 636-730.
GILROY, A.B. and CHWATT, L.J. 1945. "Mosquito-control by swamp drainage in the coastal belt of Nigeria." Alln. Trol', Med. 39:19-40.
GLYNN, P. W. 1964. "Common marine invertebrate animals of the shallow waters of Puerto Rico." Hist. Nat. Puerto Rico: 1-53.
GOLLEY, F., ODUM, H.T. and WILSON, R. F.1962. "The structure and metabolism of aPuerto Rican red inangrove forest in May."Ecology 43:9-19.
HARTMANN-SCHRODER, G. 1959. "Zur Oekologie der Polychaete des Mangrove-Estero-Gebietes von EI Salvador." Bedr. z. neotrop.Fau.na 1: 69-183.
BECHTEL, G . .J. 1965. "A systemaIic study ofthe Demospongiae of Port Royal, Jamaica."8ull. Peabody Mus. Nat. Hist. 20: 1-94.
HOLDRIDGE, L. R. 1939. "Some notes on theMangrove swamps of Puerto Rico." Caribb.Forester 1: 19-29.
HOWARD, R. A. 1950. "Vegetation of the Bimini Island group, B. W. I." Ecol. Monogr. 20:317-349.
KORNICKER, L. 1958. "Ecology and taxonomyof recenr marine ostracodes in the Biminiarea, Great Bahama Bank." IllSt. Mar. Sci. 5:194-300.
LAUBENFELS, M. W. de 1950. "An ecologicaldiscussion of the sponges of Bermuda." TrailS.Zool. Soc. 27: 155-201.
MACNAE, W. 1963. "Ma~grove swamps in SouthAfrica." J. Ecol. 51: 1-25.
---. 1967. "Zonation within mangroves associated with estuaries in North Queensland."111: Lauff, G. H. (Ed). Estuaries. AmericanAssoc. Adv. Sci., Pub!. Washington, D. C. 83:-132-441.
MARGALEF, R. 1962. "Comunidades naturales."Inst. Bioi. Mar. Univ. Puerto Rico, Spec.Publ., 469 p.
MATTOX, N. T. 1949. "Studies on the biology ofthe edible oyster Ostrea rhizophorae Guilding,in Puerto Rico." Ecol. M01lOgr. 19: 339-356.
NEWELL, N., IMBRIE, J., PURDY, E. G. andTHURBER, D. 1959. "Organism communities
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OSBURN, R. C. 1940. "Bryozoa of Puerto Ricowith a summary of the West Indian bryozoanfauna." Scieut. Surl'. Puerto Rico and VirgiuIslands 16: 321-486.
RODRiGUEZ, G. 1963. "The intertidal estuHinecommunities of Lake MHacaibo, Venezueb."Bull. Mar. Sci. Gulf Caribb. 13: 197-218.
RUTZLER, K., 19'65. "Substntstabiliuet als oekologischer Faktor, dargestellt am Beispiel adriatischer Porifera." lilt. Revue ges. H)'drobiologie 50:281-292.
SCHOLANDER, P. F., van DAM, L. and SCHOLAN-
LAGUNAS COSTERAS, UN SIMPOSIO 535
DER, S.L., 195 5. "Ga~ exchange in the rootsof mangroves." AIII.erican ]. Bot. 42 :92-98.
STEPHENSO:-l, T. A., STEPHE:-iSON, A., TANDY,G. and SPENDER, M. 1931. "The structureand ecology of Low Isles and other reefs."Great BHrier Reef Expedition 1928-29. Sci.Re/,. 3:17-112.
\VALSH, G. E. 1967. "An ecological study of aHawaiian mangrove swamp." In: Lauff, G.H. (Ed), Estuaries. American Assoc. Adv.Sci., Publ., Washington, D. C. 83 :420-431.
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