Pacific Science (1987), vol. 41, nos. 1-4© 1988 by the University of Hawaii Press. All rights reserved

The Fish Communities of a Coral Reef Transect!

RENE GALZIN2 AND PIERRE LEGENDRE3

ABSTRACT: As a contribution to the discussion on the causes of the high fishspecies diversity found on coral reefs, a coast-to-sea transect has been studiedin the lagoon of Moorea Island (French Polynesia) in order to uncover the spatialscales at which recurrent assemblages (i.e., communities) can be identified . Thetransect was divided into 22 stations where fishes were sampled. According tothe null hypothesis (chaos), the fish species should be distributed at randomalong the transect. Th is was tested first by a method of constrained clusteringthat performs a statistical test of cluster fusion , based on a null hypothesis thatcorresponds to chaos. Groups of stations were found with, at most, a 5% chanceof resulting from a random distribution of species among the groups. Thedistribution of species among the stations pro vided a second test of the nullhypothesis; the observed number of ubiquitous species was found to be signifi­cantly smaller than expected under the hypothesis of chaos and, in the same way,the species limited to a single group of stations were found to be significantlymore numerous than expected under chaos. Both patterns are consistent withreports from other reefs of the Indo-Pacific.

number of papers (Sale 1974, 1977, 1978 .. . );it has been referred to by various authors asthe "chaos" hypothesis. The main argument isthat, although reef fishes are habitat special­ists with regard to broadly defined habitatswithin the reef, the allocation of space withinhabitat occurs through a chance process thatSale calls a " lottery" for living space (Sale1977, 1978).

This paper reports on a program of obser­vations designed to unco ver the spatial scaleat which community structure can be identi ­fied. This program is actually a part of a largerresearch program aimed at measuring the sta­bility and variability of fish communities oncoral reefs in the Pacific Ocean (French Poly­nesia). Using statistical methods that takechaos as their null hypothesis, the fish com­munity will be shown to be significantly struc-tured in space following the major habitatsdiscernable on the reef.

Thi s paper contributes further to the dis­cussion on the causes of the high fish speciesdiversity on coral reefs, in directly testing pre­dictions from chaos theory (Ho) as to thenumber of ubiquitous species and the numberof area-typical species on the reef. Unfortu­natel y, our program of observations is not

158

1 Manuscript accepted April 1987.2 Lab oratoire de Biologie marine et de Malacologie,

Ecole pratique des Haut es Etudes, 55 rue de Buffon, F­75005 Paris, France, and Antenne du Museum nationald'Histoire naturelIe et de I'Ecole pratique des HautesEtudes, Centre de I'Envir onnement, B.P. 1013, Moorea,Polynesie fran caise,

3 Dep art ement de Sciences biologiques, Universite deMontreal, c.P. 6128, Succursale A, Montreal, QuebecH3C 317, Canada .

TRADITIONAL EXPLANATIONS OF the high di­versity encountered within coral reef fishcommunities state that, like other tropicalsystems , coral reefs are equilibrial systems inwhich species compete for , and partition, anumber of limiting resources. For example ,Smith (1977, 1978) suggests that living spaceis a limiting factor which imposes a regularand predictable structure on reef-fish assem­blages . This view can be termed the theory of"order."

Sale (1984) argues that the predictions fromthis theory must be tested against a null hy­pothesis (Ho) stating that there is no patternother than that of a random redistribution ofindividuals and species within each genera­tion , among resources which have patchy dis­tributions. Thi s theme has been developed in a

The Fish Communities of a Coral ReefTransect-GALZIN AND L EGENDRE 159

*:.

22

3034

..0 250 350 840 1020 m

-~/........ .................................

~~- - I

~..........

1 2 3 4 5 6 7 8 9 10 11 12 14 15 16

FRINGING RE EF CHANNEL BARRIE R REEF OUTER SLOPE

FIGURE I. Geomorphological description of the Tiahura reef transect. The section numbers correspond to:I-Sandy accumulation 9-Scattered detr itic zone2- Reef flat with microatolls IO-Coral heads area3-Dying filling-up reef flat II -Outer biogenic ridge4-Coral head 12-Furrowed platform5-Channel coral slope 13-Precipitous slope6-Channel sandy slope 14-Buttresses and valleys7-Coral sand area 15-0uter slope spur s and grooves8- Reef flat with scattered coral s 16-Sand yarea

appropriate to test the theory of lottery; asshown by Sale (1977), predictions from lotterytheory could primarily be tested from a sam­pling through time.

MATERIAL AND METHODS

Study Area

A coast-to-sea transect, 1020 m long, wasestablished at "Tiahura" in the lagoon lo­cated at the northwest end of the high vol­canic Moorea Island (149°50'W , 17°30'S) inFrench Polynesia. The geomorphology of thetransect is as follows (Figure 1):

- The fringing reef, 250 m wide from thebeach, is shallow (less than 1.5 m). The coralsand beach (1) is followed by a zone ofdetriticsediments (2), including several microatolls,considerably degraded and colonized byalgae. This is followed by a highly crumbled,dying reef flat (3) which is being filled in. Fol­lowing is a zone of coral patches (4), someisolated from one another, others aggregatedin large coral heads.

- The channel, 9 m deep and 100 m wide ,is colonized by coral constructions on thebeachward slope (5), while the seaward slopeis gently sloping and sandy (6).

- The barrier reef, 490 m wide, begins witha zone of fine coral sand (7) covered by 2.5 mofwater or less, at high tide. Then comes a reefflat (8) with sparsely distributed coral patches,increasing in number towards the ocean; thesecoral patches grow on the old coral limestonebench which is covered, outside the patches,by a few centimeters of sand. This is followedby an area scattered with pebbles and detritus(9), preceding a zone of coral masses closinginto a coral table (10). The barrier reef endswith a slightly elevated zone oflimestone (11),supporting a mixed reef ridge.

- The outer slope begins with a furrowed,upper platform (12), with 0 to 3 m of water athigh tide , followed by a sharp slope (13) 3 to 8m in depth. The next zone is subhorizontal (14),sloping gently from 8 to 14 m, with coral but­tresses and valleys . From 14 to 30 m deep, theslope is sharp again, with spurs and grooves(15). Zone no . 16 is an area of strewn coralsand, which ends at a depth of 65 m, at a sharpslope towards the abyss.

Sampling

The transect was divided into 22 samplingstations. On the reef, each station was 50 mlong (Figure 2), except for station 17, the last

160 PACIFIC SCIENCE, Volume 41, 1987

22

30

1010100 200 300 400 500 60 0 700 800 B40 890 900 990 1020 m-~/ -..... .................................

~··::::::::: :: :::::I..............

...........

fA)Stations I 2 3 4 5 6 7 8 9 10 " 12 13 14 15 16 17 18 20 21 ----IB l o.05~ Alpho~O.25 1 2 3 <I

IClo.30 :!'Alpho 1 20 2b z, 2d 3 <I

Sandy Inner slope of Sandy bar rier Reef fla t withlolFish communiti es fring ing the fringing Channel reef scattered coral Reef front O uter slope

reef reeF potches

IEJGeomorpholog y I 2 I 3 4 6 7 81

9 101

11 12 14 II5 16

FIGURE 2. Profile of the Tia hura reef tra nsect, showing (A) The 22 sampling stations. (B)The four groups of sta tionsobtained by constrained clustering. (C) The seven groups ofstations obt ained also by constrained clustering. (D) Thecorresponding fish community names. (E) The geomorpho logy; the 16sections are described in the legend of Figure I .

one of the barrier reef, which was only 40 mlong . On the outer slope, stations were dividedat depths of 3, 8, 15, 22, and 30 m, follow­ing Harmelin-Vivien and Bouchon-Navaro(1983), who described fish distributions onthat slope.

Sampling was carried out from I October to18 November 1982 using two complementarytechniques (Smith and Tyler 1972; Galzin1979). The presence or absence of each of 280species was noted for all 22 stations. First,visual observations were made during ran­doml y directed 45 min swims within each sta­tion by a trained observer (R.G.) while snor­kel or scuba diving. Station 1 is too shallowfor diving and was sampled by net fishing,supplemented with visual observations undercalm surface conditions. Secondly, single cor­al patches of about 0.5 m' were selected withineach station and poisoned with rotenone by adiver (R.G.) who expelled the solution froma plastic bag at the base of the patch. Thefish, which were usually nocturnal or crack­inhabiting species, were then collected with afine-meshed hand net.

Num erical Analysis

The sampling described above was designedto test the null hypothesis that the fish com­munity is not structured in space on the reef(chaos theory). It will first be analysed using a

statistical method described as "chronologi­cal clustering" by Legendre et al. (1985). Thismethod partitions multi -species data seriesinto homogeneous segment s, following hier­archical agglomeration. The method is said tobe "constrained" in that only samples or seg­ment s of samples that are adjacent along theseries are allowed to cluster. The present pa­per extends this method to space series. Cutsin the data series are produced only when thenull hypothesis is rejected by a randomiza­tion statistical test , at the Alpha-level of sig­nificance. The null hypothesis states that thetwo groups tested do actually form a singlegroup. This statistical hypothesis seems ap­propriate for testing the ecological null hy­pothesis of chaos, since Legendre et al. (1985)have shown that the null hypothesis is rejectedby the randomization test with a frequencyAlpha, when applied to randomly generateddata (corresponding to chaos), in a MonteCarlo simulation. Connectedness of 50% wasused for the intermediate-link linkage ag­glomeration that precede s the test of clusterfusion. The similarity matrices subjected toconstrained clustering were computed amo ngstations from the species presence and absencedata, using the binary coefficients of Jaccardand of Kulczynski; on similarity coefficients,see for instance Legend re and Legendre(1983). The stati stica l tests of predictionsfrom the null hypothesis of chaos, for the

The Fish Communities ofa Coral Reef Transect-c-Gxt.zrx AND L EGENDRE 161

number of ubiquitous species and for thenumbers of species characterizing each groupof stations, are described with the Results.

RESULTS

Both the Jaccard and the Kulczynski co­efficients of similarity produced exactly thesame partitioning of the 22-sample spaceseries into groups, for all the Alpha-levelsconsidered. At the Alpha = 5% level of sig­nificance, four major groups of stations werefound . They are shown in Figure 28. As rec­ommended by Legendre et al. (1985), largerAlpha-levels were tried in order to find finerpartitions. The partition of the data seriesremains exactly the same for all Alpha-levelsof significance from 5% to 25%. When Alphareaches 30%, a finer partition is found (Figure2C). This seven-group solution is nested in thefour-group partition, although this is not aproperty of this clustering method; this showsthe robustness of the four-group solution, sincethe seven-group partition brings only a refine­ment to the large central group 2. The sevengroups were found at all Alpha-levels between30% and 50% (the largest Alpha-level used inthis study), which shows again the stabilityof the seven-group partition. The four fishcommunities can be characterized as follows(Figure 20):

I- Sandy fringing reef communityII -Lagoon community:

(a) Inner slope of the fringing reef(b) Channel(c) Sandy barrier reef(d) Reef flat with scattered coral

patchesIII- Reef front communityIV-Outer slope community.

A posteriori tests were performed amongthe seven groups of stations in order to detectaffinities between non-adjacent segments ofthe partition. Using the same randomizationtest as the clustering procedure does, signifi­cant relationships were found between seg­ments 2a and 2c on the one hand (the proba­bility of the null hypothesis of fusion was

80%) and between segments 2a and 30n theother (the probability of H, was 40%).

Fourty-six families are represented amongthe 280 fish species recorded along the tran­sect. The most important are the Labridae (40species), the Pomacentridae (25), the Chae­todontidae (19), the Acanthuridae (19), theApogonidae (14) and the Scaridae (13).

Contingency tables comparing the pre­sence/absence of each species to the 7-groupclassification of stations show that the Acan­thurid Ctenochaetus striatus is found at all 22sampling stations of the transect, while fiveother species show up in each of the sevengroups of stations (Pseudopeneus multifascia­tus, Chaetodon vagabundus, Stegastes nigri­cans, Stethojulis interrupta and Acanthurustriostegus), giving a total of six ubiquitousspecies (2%). On the other hand, 15 species(5%) were found at least once in each of thefour major groups of stations 1 to 4. A directprediction from the hypothesis of chaos wastested as follows: under the hypotheses of ran­dom distribution among stations, and of in­dependence of adjoining stations, the prob­ability of presence of each species at leastonce in each group of stations (ubiquity) wascomputed. These probabilities were calculatedfrom the hypergeometric distribution, takingas probability p the actual proportion of sta­tions occupied by each given species (the hy­pergeometric distribution was used instead ofthe binomial, because we are sampling with­out replacement from a finite population). As­suming that the species are not associated toone another, the probabilities of ubiquity foreach of the 280 species were combined bysumming. The expected number of ubiquitousspecies is 36 for the 7-group classification ofstations, and 67 for the 4-group classifica­tion . The differences between observed andexpected numbers of ubiquitous species arevery highly significant (p < 0.001), using aPearson's chi-square test or a G-test of good­ness of fit, or by computing the confidenceintervals of the observed proportions of ubiq­uitous species.

On the other hand, only 28 species arelimited to a single habitat, considering the4-group classification. Only the species pre­sent in at least half of the stations of any

162

given group are counted here . Two species arefound in a majority of stations of the fringingreef (group 1) and nowhere else: Pseudochei­linus tetra taenia (Labridae) and Amblygobiusdecussatus (Gobiidae). Three species charac­terize the lagoon (group 2): the AcanthuridAcanthurus nigricauda, the Ostraciontid Os­tracion cubicus, and the Tetraodontid Canthi­gaster valentini. Five species are typical of thereef front (group 3): Platybelone argala (Bel­lonidae), Crenimugil crenilabis (Mugilidae),Kyphosus vaigiensis (Kyphosidae), Cirripectesvariolosus (Blenniidae), and Acanthurus gut­tatus (Acanthuridae). Finally, 18 species arefound in a majority of the stations of the outerslope (group 4) and nowhere else: 1 Muraenid,1 Synodontid, 3 Serranids, 2 Lutjanids, 1 Po­macanthid, 1 Pomacentrid, 2 Labrids, 1 Sea­rid, 1 Eleotrid, 4 Acanthurids, and 1 Balistid.Using the same method as above, the numberof species expected to characterize a majorityof stations of each group, while being foundnowhere else, was computed under assump­tions corresponding to chaos theory. Forgroups of stations 1 to 4, these values arerespectively 0.090, 0.008, 0.090 and 0.006. APearson's chi-square test, or a G-test of good ­ness of fit, shows the observed numbers oftypical species (reported above) to be signifi­cantly different (p < 0.001), and larger thanthese expectations, thus allowing us to rejectthe null hypothesis corresponding to chaos.

DISCUSSION AND CONCLUSIONS

The major finding of this study is that thefish assemblages display more order than isconsistent with a null model based on randomdistributions, and that most of this "order"can be explained by predictable associationsbetween species and habitats.

Comparison of the fish communities, iden­tified abo ve by constrained clustering, withthe geomorphology of the transect shows in­teresting relationships (Figure 2). The fringingreef community occupies the shallow sandysection (1 and 2) of the fringing reef. Thecommunity found on the inner slope of thefringing reef penetrates far deeper into thefringing reef (sections 3 to 5) than would be

PACIFIC SCIENCE, Volume 41 ,1987

expected from the geomorphology; this isdue to the channel water influence, describedabove , and the consequent vitality of corals inthese sections . To the fish, the channel itself isthe sandy part, found in geomorphologic sec­tion 6 and a part of section 7. On the barrierreef, the fish communities of the sandy barrierreef and of the coral patches (2c and 2d) fitalmost exactly geomorphologic sections 7 to9; detritic zone 9 does not bear a characteristicfish community. This is followed for the fishby a well-identified reef front community,found in geomorphologic zones 10 to 12, andan outer slope community occupying zones 13to 15. No sampling took place in zone 16.

The similarity of the communities of theinner slope of the fringing reef (2a), of thesandy barrier reef (2c), and of the reef front(3), is of particular interest. The affinity be­tween these three reef sections may be due towater circulation on the reef. Each of thesethree sections seems to act as a " reef front,"and it would be interesting to test the hypoth­esis that these " fronts" receive the planktonicfish larvae first, directly from the ocean. Ac­cording to this hypothesis, other sections ofthe reef would then be colonized afterwards,by the fish that have settled on these "fronts."

Among the factors responsible for the parti­tioning of the Tiahura reef transect into fourmajor communities, the percentage of livingcoral has been found to be of prime impor­tance by Bell and Galzin (1984). This samevariable has also been found by Sale (1984) tobe a significant predictor of the number offishspecies found on a patch reef, together withthe reef surface area. Hydrodynamic activityis here another important factor, as shown bythe a posteriori clustering of station groups 2a,2c and 3, described above, because these reefsections seem to behave as reef fronts. Thesame phenomenon has recently been pointedout by other workers (Williams 1982,Williamsand Hatcher 1983, Russ 1984). This findingalso agrees with the theory of Legendre andDemers (1985) concerning the importance ofergoclines (transition zones from high to lowlevels of auxiliary energy) in the dynamics ofecosystems.

As far as we know, only two other paperspresent data comparable to ours : the work of

The Fish Communitiesofa Coral Reef Transect-e-Gxtzrx AND LEGENDRE 163

Goldman and Talbot (1976) on One Tree Is- COr--

land Reef, in the Great Barrier Reef System of 0\

'""-~

Australia, and that of Harmelin-Vivien (1979) « ~ O\

~<-

'"" 00\

on the Tulear reefs in Madagascar. The parti- co '" .0-< U -~

::c z ~ <:

tion of the reef into five major habitats, con- ~ f-o "'" ~ -g :~ ;.,

sidered by Goldman and Talbot, is given by..Jo

~ee>"C

i?l • ;:l

the geomorphology, while Harmelin-Vivien en a] ~s " ~

used non-constrained hierarchical clustering -c "CS§0 _ .... tIl

of the [species x zones] data matrix to recog- '"" o ~ "0 "'::C~

nize three major fish communities. Despite the ~differences in methods, these authors' results <2

'""are quite comparable to ours in terms ofubiq- ~ 0",~ '"" 0.uitous species , and of species limited to a sin- ~ - @ 9

gle habitat (Table 1). For the species limited to U ~ !-< r/}","'1"'1 \0

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one habitat, the above-mentioned authors en ~ ~5had counted all species present at least once in

c,~o0

0any given habitat and absent from all others. z

<The figures given in Table 1 for the present til

~

study's data follow this rule , instead of the o 0 ,""'"0. ~ '"" «"species found in a majority of stations of any en U @ t:til \0 0 0

one habitat" rule that was used above for the ::l '" '"" '" r---:~ lri0 0. U «

5 til <2 :I: "'""'""'"statistical test. ;g, '""~

With regard to chaos theory: three statisti-C! o ~ 0::;

cal tests of hypotheses, each with a null hy-::J

""pothesis corresponding to chaos, have shown

0

~ ~til

the disagreement of our observations with o...J~

::lal o ~

chaos theory. These are : (1) A partition of <: z o t:: 0 \oN ","

f-o '" :>' ::l ", \CioO V)the transect into four major groups has been o C! o.0: ::; til

'"evidenced by constrained clustering. This ll.. ::l0:

method of clustering, which respects the spa- ~

tial relationship of the stations with each other .(

~ ""within the ecological continuum, shows dif- <: ..J ~ ~ferences between the successive groups at the o ~ ~ U 'I"'INO

Ii: 0\ '1"'1 00

Alpha = 5% level of significance; this means u o :E '" M'I"'IN

« '"" ~ i};that the structure that has been evidenced has ll..

6at most a 5% chance to be the result of ran- 0zdom allocation of species among those four -

~ "" til til til

groups. (2) The number of ubiquitous species '"~ <: <: <:

is significantly smaller (p < 0.001) than the i?l!i!

.g.g.g~

o o o

" " "number of ubiquitous species that should ~ '""til til til

"" 'I"'IM","

have been observed if the species had been0

..J

distributed at random among the stations on«0:0

the transect, supposing that adjacent stations o'" :?are independent. (3) The species limited to ~:I: "@

only one of the four groups of stations are f-o b

""tIl ~

significantly more numerous (p < 0.001) than ;:l .... ~0

~ ~ .~zwhat would have been expected from the same 0 "" "C ~ "til

~c OIl <:

hypotheses. Data available in the literature <2 ~ ~ ;.,« tii""go

indicate that fish species show comparable0. ';;;EC:E0 ,, ~~

distribution patterns on other reefs (Table I). o ~aa

It could be argued, of course, that chaos,,"oJ:<: - ~

o~E=

164

does not necessarily imply the independenceof adjacent stations, as was used in the hyper­geometric simulations leading to tests (2) and(3) above. Even in that case, test (1) still holds,since it was constructed to take the spatialadjacency of samples into account.

One should finally notice that after reject­ing chaos as the explanation of ichthyologicaldiversity along the Tiahura reef transect, oneis not left with a single alternative hypothesis.Specific tests have to be designed to decideamong various alternative theories. For in­stance, we need to look at the patterns ofsettlement and juvenile survivorship of thecoral reef fishes . Several authors (Williams1980, Doherty 1982 and 1983, Victor 1983)have proposed that population abundanceswithin habitats are primarily determined byrecruitment fromthe pelagic larval phase. Inaddition, finer observations at the microhabi­tat level are needed to define the affinity orexclusion relationships among species result­ing possibly from their co-adaptation. Final­ly, longer-term studies are needed to mea­sure the stability of communities and of theenv ironment.

The original [species x stations] data table isavailable from the authors.

ACKNOWLEDGMENTS

Contribution No. 311 of the Group d'Eco­logie des Eaux douces, Universite de Mon­treal. We are grateful to J. D. Bell, P . J. Do­herty, M . L. Harmelin-Vivien, P. J. Sale, andtwo anonymous referees, for valuable discus­sions and comments. This study was sup­ported by the Centre National de la RechercheScientifique (CNRSjRCP 806).

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PACIFIC SCIENCE, Volume 41, 1987

DOHERTY, P. J. 1983. Tropical territorial dam­selfishes: is density limited by aggressionor recruitment? Ecology 64 : 176-190.

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WILLIAMS, D. McB., and A. I. HATCHER.1983. Structure of fish communities onouter slopes of inshore, mid-shelf and outershelf reefs of the Great Barrier Reef. Mar.Ecol. Prog. Ser. 10:239-250.