Interspecific variation in distributions and diets of coral reef butterflyfishes (Teleostei: Chaetodontidae)

Transcription

1 Journal of Fish Biology (2008) 73, doi: /j x, available online at Interspecific variation in distributions and diets of coral reef butterflyfishes (Teleostei: Chaetodontidae) M. S. PRATCHETT* AND M. L. BERUMEN *ARC Centre of Excellence for Coral Reef Studies, James Cook University, Townsville Q4811, Australia and Woods Hole Oceanographic Institution, Biology Department, MS#50, Woods Hole 02543, MA, U.S.A. (Received 22 January 2007, Accepted 6 August 2008) This study examined within-reef distributions for 19 species of butterflyfishes (Chaetodontidae) at Lizard Island, northern Great Barrier Reef, Australia, and compared spatial patterns of abundance among species with contrasting diets. Spatial variation in abundance of butterflyfishes was most prominent among physiognomic reef zones mainly due to significant zonation of eight species, including four obligate hard-coral feeders (Chaetodon trifascialis, Chaetodon baronessa, Chaetodon plebeius and Chaetodon lunulatus) and four generalist species (Chaetodon auriga, Chaetodon citrinellus, Chaetodon kleinii and Chaetodon rafflesi). Distributions of obligate hard-coral feeders were closely associated with spatial variation in percentage cover of scleractinian corals, but no more restricted compared with facultative hard-coral feeders or non-coral feeders. Species with highest dietary specialization (C. trifascialis and C. baronessa), however, exhibited the most pronounced zonation patterns and were restricted to habitats with greatest abundance of their preferred prey. While there are conspicuous links between dietary specialization v. spatial patterns in abundance of butterflyfishes, it remains unclear whether dietary specialization is the cause or consequence of more restricted distributions. Journal compilation # 2008 The Fisheries Society of the British Isles Key words: chaetodontidae; dietary specialization; habitat preferences; zonation. INTRODUCTION Spatial variation in the distribution and abundance of coral-reef fishes is prominent at many different scales (Williams, 1991; Roberts et al., 1992; Harmelin- Vivien, 2002). At small spatial scales, fishes are distributed according to the availability of specific resources or microhabitats and frequently exhibit striking variability in abundance among physiognomic reef zones (Bouchon-Navaro, 1980; Lecchini et al., 2003; Depczynski & Bellwood, 2005) or among habitats (Gratwicke & Speight, 2005). At broader spatial scales, the total density and relative abundance of reef fish species vary among reefs, along cross-shelf gradients (Anderson et al., 1981; Williams, 1982; Russ, 1984), with latitude Author to whom correspondence should be addressed. Tel.: þ ; fax: þ ; morgan.pratchett@jcu.edu.au 1730 Journal compilation # 2008 The Fisheries Society of the British Isles

2 DIETS AND DISTRIBUTIONS OF BUTTERFLYFISHES 1731 (Williams, 1983) and among oceans (Hourigan & Reese, 1988; Thresher, 1991; Harmelin-Vivien, 2002). Patterns and causal processes of spatial variation in abundance of coral-reef fishes usually vary with scale (Roberts et al., 1992; Syms, 1995). Distribution patterns of reef fishes, however, have rarely been studied concurrently at more than one spatial scale, such that the relative importance of smaller- v. broader-scale processes remains unclear. Small-scale variation in the abundance of coral-reef fishes is typically ascribed to habitat preferences of individual species combined with patchiness in the availability of food and shelter (Williams, 1991). At local scales (tens of metres), larval fishes often settle preferentially onto particular substrates in certain zones and thus exhibit highly predictable distribution patterns (Williams, 1980; Williams & Sale, 1981; Victor, 1986; Connell & Jones, 1991; O hman et al., 1998; Leis & Carson-Ewart, 2002). The distribution and abundance of fishes are much less predictable at larger scales (tens of kilometres), such as among locations within a single reef or between adjacent reefs, probably due to marked variation in larval supply and settlement rates (Sale et al., 1984; Choat et al., 1988; Williams, 1991). Variability in the magnitude and timing of settlement is very common, resulting in considerable spatial and temporal variation in the abundance of fishes (Sale et al., 1980; Doherty, 2002). Further, variability in the distribution and abundance of fishes may also result from post-settlement movement (Lecchini & Galzin, 2005; Pratchett et al., 2008a) and spatial variation in post-settlement survivorship (Roberts et al., 1992; Booth, 2002). In some cases, however, the distribution and abundance of fishes are highly predictable at small and large spatial scales (Findley & Findley, 2001) where populations and assemblages of coral-reef fishes are strongly influenced by habitat structure. The degree to which coral-reef fishes are influenced by the biological and physical structures of benthic reef habitats varies from family to family with differences in diet and shelter requirements (Lecchini et al., 2003; Wilson et al., 2006). Fishes from the family Chaetodontidae (butterflyfishes) are foremost in their association with reef benthos (Halford et al., 2004) because most butterflyfishes feed on live corals (Pratchett, 2005). Butterflyfishes that feed exclusively on live corals (obligate coral feeders) often exhibit variation in abundance in accordance with the percentage of reef substratum occupied by scleractinian ( hard ) corals (Harmelin-Vivien & Bouchon-Navaro, 1983; Bouchon-Navaro et al., 1985; Pratchett et al., 2006). Most notably, declines in coral cover caused by acute disturbance events (e.g. cyclones, coral bleaching or outbreaks of coral predators) often lead to significant declines in the abundance of these butterflyfishes (Bouchon-Navaro et al., 1985; Williams, 1986; Pratchett et al., 2006; Graham, 2007). In contrast, butterflyfishes that only occasionally feed on live corals (facultative corallivores) or species that never eat live corals (non-coral feeders) are much more resilient to coral loss (Pratchett et al., 2006; Graham, 2007). Spatial variation in the abundance of coral-feeding butterflyfishes is often correlated with hard coral cover (Harmelin-Vivien & Bouchon-Navaro, 1983; Bouchon-Navaro et al., 1985), from which it can be inferred that butterflyfishes preferentially settle or survive best in habitats with greatest availability of suitable prey (Pratchett et al., 2008a). Distribution patterns of butterflyfishes may,

3 1732 M. S. PRATCHETT AND M. L. BERUMEN however, be further constrained by interspecific competition, whereby dominant competitors monopolize habitats with greatest availability of prey corals and restrict subordinate species to other less-preferred habitats (Berumen & Pratchett, 2006a). There are several previous studies that demonstrate complementary habitat associations among coral-feeding butterflyfishes, viewed as evidence for spatial partitioning (Bouchon-Navaro, 1986; Pitts, 1991). Differential patterns of abundance among sympatric butterflyfishes, however, may also reflect interspecific variation in preferred prey and individualistic responses to food availability (Bouchon-Navaro, 1981). On the Great Barrier Reef (GBR), Pratchett (2005) revealed considerable convergence in feeding preferences of coral-feeding butterflyfishes, showing that 11 of 14 coral-feeding butterflyfishes fed predominantly on Acropora hyacinthus or Pocillopora damicornis. Given high levels of dietary overlap among sympatric butterflyfishes, this study brought into question the role of resource partitioning in promoting coexistence among coral-feeding butterflyfishes (Pratchett, 2005). It is apparent, however, that species with broadly similar feeding habits may co-occur within certain habitats (Pratchett, 2005) but have maximal abundance in contrasting habitats to minimize direct competition (Bouchon-Navaro, 1986; Pitts, 1991). The purpose of this study was to explore within-reef spatial variation in the abundance of sympatric butterflyfishes with contrasting dietary habits. Given that specialist species are expected to be more limited by the availability of critical resources (Brown, 1984), highly specialist butterflyfishes are expected to have more restricted distributions among zones and habitats compared to species that utilize a greater range of different resources. This study compares butterflyfishes with broadly different dietary composition, specifically testing whether butterflyfishes with strong reliance on scleractinian corals (e.g. obligate coral-feeding species) exhibit more restricted distributions across reef zones compared to butterflyfishes with limited reliance on corals (e.g. non-coral feeders). Among coral-feeding butterflyfishes, this study also tested whether species that consume a wide range of different corals are found across a greater range of habitats compared to highly specialist coral feeders. Moreover, this study tested whether species with broadly similar prey preferences are nonetheless localized in different reef habitats, which might signify competitive exclusion by dominant species. Differential arrangement of butterflyfishes among reef zones has been studied in the central Pacific (Gosline, 1965; Bouchon-Navaro, 1981), western Indian Ocean (Bouchon-Navaro & Bouchon, 1989; Zekeria et al., 2002) and Atlantic Ocean (Pitts, 1991), but this is the first such study on the GBR, within the centre of diversity for coral-reef butterflyfishes (Findley & Findley, 1985). The extent to which butterflyfishes are distributed among habitats in accordance with their preferred prey will provide important insights into the relative importance of competitive exclusion v. individual prey preferences in determining the distribution and abundance of fishes on coral reefs. MATERIALS AND METHODS This study was conducted at Lizard Island ( S; E), a continental island on the northern GBR, 35 km off the eastern coast of Australia. Sampling was conducted at four locations along extensive fringing reef on the eastern and southern sides

4 DIETS AND DISTRIBUTIONS OF BUTTERFLYFISHES 1733 of Lizard Island: North Reef, Washing Machine, Lizard Head and South Island. These four locations are separated by distances of km but are all characterized by extensive contiguous fringing reef with distinct physiognomic zonation (Fig. 1). At each location, sampling of both butterflyfishes and scleractinian corals was undertaken within four reef zones: (1) the reef flat (1 3 m deep), characterized by a shallow flat area of low topographic complexity extending from the shoreline to within 20 m of the reef edge; (2) the reef crest (1 3 m), a m wide band of high topographic complexity along the seaward edge of the reef flat; (3) the reef slope (3 11 m), represented by the sharply inclined reef front and (4) the reef base (7 16 m), a gently sloping area extending seaward from the bottom of the reef slope, mostly comprising soft sediment and rubble habitats (Nelson, 1993). The abundance and composition of butterflyfishes were quantified using underwater visual census (UVC) along replicate belt transects in each of the four zones (flat, crest, slope and base) at every location (North Reef, Washing Machine, Lizard Head and South Island). Transects were delineated using a 50 m fibreglass tape laid parallel to the reef crest within each zone. Divers then swam along the transect recording all N North Reef Washing Machine North Reef Depth (m) Flat 0 6 Crest Slope Base 12 Washing Machine 0 Flat Crest Lizard Island Depth (m) 6 Slope Base 12 Lizard Head 0 Flat Crest Depth (m) 6 Slope Base Lizard Head 12 South Island South Island Flat Crest 0 Depth (m) Slope Base Distance from crest (m) FIG. 1. Lizard Island, northern Great Barrier Reef showing study locations ( at each location. ). Inset shows depth profiles

5 1734 M. S. PRATCHETT AND M. L. BERUMEN butterflyfishes >50 mm total length (L T ) located within 2 m of either side of the transect path. A total of five replicate transects were surveyed in each zone at every location, giving a total of 80 transects. A total of 19 Chaetodon species [Chaetodon auriga Forsskål, Chaetodon aureofasciatus Macleay, Chaetodon baronessa Cuvier, Chaetodon citrinellus Cuvier, Chaetodon ephippium Cuvier, Chaetodon kleinii Bloch, Chaetodon lineolatus Cuvier, Chaetodon lunula (Lacepe` de), Chaetodon lunulatus Quoy & Gaimard, Chaetodon melannotus Bloch & Schneider, Chaetodon rafflesi (Bennett), Chaetodon trifascialis Quoy & Gaimard, Chaetodon plebeius Cuvier, Chaetodon rainfordi McCulloch, Chaetodon semeion Bleeker, Chaetodon speculum Cuvier, Chaetodon ulietensis Cuvier, Chaetodon unimaculatus Bloch and Chaetodon vagabundus L.] were recorded during this study, which were assigned to specific trophic groupings based on extensive feeding observations reported in Pratchett (2005). Moreover, the relative level of dietary specialization for each species of butterflyfish was reported in Pratchett (2007), where dietary specialization was calculated using selectivity functions ðxl2 2 Þ following Manly et al. (1993). Spatial variation in cover and taxonomic composition of scleractinian corals was documented using m video transects, following Carleton & Done (1995). Video transects were run independently of fish surveys (making it impossible to directly relate fish communities to benthic assemblages at the level of transects) and run parallel to the reef crest in each zone at every site. Video recordings were processed using random point sampling, where random pauses in the tape transport were combined with random placement of sampling points in the paused frame (Foster et al., 1991). Only one point was sampled per frame to minimize autocorrelation of sample points following Carleton & Done (1995). A total of 200 points were sampled on each transect in five runs (40 random points sampled on each run). Items underlying the sampling point on paused frames were identified to highest possible resolution, which included both taxonomic (e.g. genera or family) and morphological categorization. Variation in the abundance of butterflyfishes was analysed using two-way ANOVA to test for variation among locations (four levels, random factor) and zones (four levels, fixed factor). Physiognomic reef zones (flat, crest, slope and base) were readily distinguishable within each location and highly comparable among locations, justifying the use of an orthogonal, rather than a nested ANOVA model. Raw data were log 10 (x þ 1) transformed to compare proportional changes among treatments and residual plots were examined prior to the analysis to ensure normality and homoscedasticity of variance. Spatial variation in butterflyfish and coral assemblages was analysed using multivariate analyses of variance (MANOVA), comparing the relative abundance of different taxa among zones and among locations. Raw data were log 10 transformed prior to the analyses. Univariate homogeneity was tested using Cochran s test, and residual plots were examined to confirm MANOVA assumptions of multivariate homogeneity and normality. Pillai s trace statistic was used to determine the significance of the MANOVAs, following Olsen (1976). Canonical discriminate analysis (CDA) was used to display variation in the community structure for both butterflyfishes and corals. To assist with interpretation of the CDA, structural coefficients of each of the response variables (Chaetodon species and coral taxa, respectively) were plotted as vectors in twodimensional space. These vectors identify which variables are responsible for directional differences between the group centroids. The length of the vector is proportional to the F-ratio of the variable, and thus indicates the significance of each variable in separating group centroids in the direction of the vector. Simple linear correlation was used to relate spatial variation in overall abundance of butterflyfishes to variation in total live coral cover among zones and among locations (n ¼ 16 sites). Overall densities of butterflyfishes are expected to show only weak correlations with total live coral cover, with correlations driven mainly by species with a specific reliance on corals for food (i.e. corallivorous species). To test this, correlations were conducted separately for each feeding guild: (1) obligate coral feeders, (2) facultative coral feeders and (3) non-coral feeders. Species were assigned to feeding guilds following Pratchett (2005). Associations of individual butterflyfish species with specific coral taxa were not tested but are very apparent from direct comparisons of CDA results of butterflyfishes v. corals.

6 DIETS AND DISTRIBUTIONS OF BUTTERFLYFISHES 1735 RESULTS CORAL COVER AND COMPOSITION Mean S.E. coral cover recorded around Lizard Island was % and was fairly consistent among locations, but varied significantly among zones within locations (ANOVA; Table I). Within locations, coral cover was usually highest on the reef crest and declined with increasing depth. There was also marked variation in composition of corals among zones within locations (MANOVA; Table II). Most of the variation in coral composition was attributable to differences among zones, whereby reef flats were characterized by high abundance of Isopora spp. and digitate Acropora spp., whereas on the reef crest, coral communities were dominated by tabular Acropora spp. and Pocilloporidae [Fig. 2(a)]. At North Reef, however, the reef flat was also dominated by tabular Acropora spp. such that there was little difference in the composition of coral assemblages on the reef flat and reef crest at North Reef. Coral communities on the reef slope and base at all locations conspicuously lacked Acropora spp. Coral assemblages in these zones were much more diverse compared to the reef flat and crest and had relatively higher abundance of massive coral species, including Faviidae and Poritidae [Fig. 2(a)]. BUTTERFLYFISHES A total of 1030 individuals of Chaetodon butterflyfishes were recorded across all 80 transects sampled, corresponding to a mean S.E. of fishes per 200 m 2 (per transect). Overall densities of butterflyfishes varied slightly (but not significantly) among locations ranging from at North Reef (data pooled across all zones) to fishes per 200 m 2 at South Island. Within locations, densities of butterflyfishes were generally higher on the reef flat and reef crest compared to the reef slope and reef base [Fig. 3(b)]. TABLE I. ANOVA results for total coral cover (arcsine transformed) and overall abundance of butterflyfishes (log 10 transformed) among locations (North Reef, Washing Machine, Lizard Head and South Island) and among reef zones (flat, crest, slope and base) Source SS d.f. MS F P Total coral cover Location Zone Location zone <0001 Error All butterflyfishes Location Zone Location zone Error

7 1736 M. S. PRATCHETT AND M. L. BERUMEN TABLE II. MANOVA results for proportional cover of 12 major coral taxa (log 10 transformed) and relative abundance of 19 species of butterflyfishes (log 10 transformed) among locations (North Reef, Washing Maching, Lizard Head and South Island) and among reef zones (flat, crest, slope and base) Source Pillia s trace d.f. 1 d.f. 2 P Corals Location <0001 Zone <0001 Location zone <0001 Butterflyfishes Location <0001 Zone <0001 Location zone <0001 Variation in abundance of butterflyfishes among zones, however, varied significantly among locations (ANOVA; Table I). There were significant differences in the relative abundance of butterflyfishes among locations, among zones and among zones within locations (MANOVA; Table II). The relative abundance of different butterflyfishes, however, varied most among zones, apparent in the CDA by the grouping of centroids according to zone, rather than location [Fig. 2(b)]. Assemblages of butterflyfishes on the reef flat were characterized by high abundance of C. citrinellus and C. plebeius, whereas the reef crest was dominated by C. baronessa and C. trifascialis. The assemblages of butterflyfishes found on the reef slope were the least distinct of all zones, and mostly represented a convergence of species assemblages from the reef crest and reef base, where the reef base assemblage was dominated by C. kleinii and C. auriga [Fig. 2(b)]. Within trophic groups (obligate hard-coral feeders, facultative hard-coral feeders and non-coral feeders), species with the highest dietary specialization (based on XL2 2 ) exhibited the greatest variation in abundance among zones (Fig. 4). The most specialized obligate hard-coral feeders, C. trifascialis and C. baronessa, both exhibited significant and striking patterns of zonation (Fig. 4) with greatest densities recorded on the reef crest (Fig. 5). In contrast, C. aureofasciatus and C. rainfordi, which are the least specialized of the obligate coral-feeding species, were prevalent across all zones and did not exhibit significant variation in abundance among zones (Fig. 4). Densities (mean S.E.; number of fishes per 200 m 2 )ofc. rainfordi were very similar among the flat ( ), slope ( ) and base ( ), but slightly lower on the reef crest ( ), where C. trifascialis and C. baronessa predominated. Among trophic groups, there was a greater proportion of obligate hard-coral feeders (four of six species) that exhibited significant variation in abundance among reef zones compared to facultative hard-coral feeders (three of eight species) and non-coral feeders (one of five species). There was no evidence, however, that distributions of obligate hard-coral feeders were more restricted (greater variation in abundance among zones) compared to facultative hard-coral feeders (Fig. 4).

8 DIETS AND DISTRIBUTIONS OF BUTTERFLYFISHES 1737 (a) 6 00 Slope WM NR LH SI WM Faviidae Pocilloporidae NR Other SI Tabular Acropora Crest LH WM NR NR SI CV1 (48 2 ) 6 00 LH Base Poritidae LH SI Flat 6 00 Isopora Digitate Acropora WM 6.00 CV2 (19 2 ) (b) 8 00 SI 8 00 LH LH Slope SI WM C. auriga NR SI Crest LH WM C. plebeius C. trifascialis C. baronessa NR CV1 (39 0 ) 8 00 SI Base WM NR C. kleinii WM LH Flat NR C. citrinellus 8.00 CV2 (23 1 ) FIG. 2. Canonical discriminant analyses of (a) coral assemblages and (b) butterflyfish (Chaetodon spp.) communities among reef zones and locations [reef flat ( ), reef crest ( ), reef slope ( ) and reef base ( ); North Reef (NR), Washing Machine (WM), Lizard Head (LH) and South Island (SI); see Fig. 1].

9 1738 M. S. PRATCHETT AND M. L. BERUMEN (a) Coral cover ( ) North Reef Washing Machine Lizard Head South Island (b) Number of fishes per 200 m 2 (per transect) North Reef Washing Lizard Machine Head Location South Island FIG. 3. (a) Mean S.E. percentage cover of scleractinian corals and (b) mean S.E. densities of butterflyfishes among the reef flat ( ), reef crest ( ), reef slope ( ) and reef base ( ), at each sampling location (see Fig. 1). CORRELATIONS WITH TOTAL CORAL COVER Overall densities of butterflyfishes were strongly and positively correlated with live coral cover among zones and locations (Pearson s correlation: r ¼ 067, n ¼ 16, P < 001), but the correlation was even stronger when considering only obligate hard-coral feeders (Table III). Considered individually, none of the facultative hard-coral feeders or non-coral feeders exhibited significant correlations with scleractinian corals (Table III). In contrast, densities of most obligate hard-coral feeders (all except C. aureofasciatus and C. lunulatus) were strongly and positively correlated with spatial variation in percentage cover

10 DIETS AND DISTRIBUTIONS OF BUTTERFLYFISHES (a) (b) (c) * Univariate F-ratio * * * Dietary specialisation (X 2 L2) C. trifascialis C. baronessa C. plebeius C. lunulatus C. aureofasciatus C. rainfordi C. citrinellus C. kleinii C. unimaculatus C. rafflesi C. melannotus C. speculum C. ulientensis C. lunula C. ephippium C. auriga C. vagabundus C. lineolatus C. semion * * * * FIG. 4. Variation in abundance of butterflyfishes among zones (univariate F-ratios;,, ) and dietary specialization ( ) (after Pratchett, 2007) for (a) obligate hard-coral feeders, (b) facultative hardcoral feeders and (c) non-coral feeders. *P < 005 (ANOVA, Bonferroni-corrected significance levels). of scleractinian corals (Table III). The strongest correlation with total hard coral cover was found for C. baronessa (Table III). DISCUSSION Variation in the abundance and community structure of coral reef organisms (including corals and fishes) is often prominent when comparing among physiognomic reef zones (Done, 1982, 1983; Nelson, 1993; Lecchini et al., 2003; Depczynski & Bellwood, 2005; Penin et al., 2007). Accordingly, this study revealed marked differences in abundance and community structure of butterflyfishes among the reef flat, crest slope and base at Lizard Island in the northern GBR. Predictable assemblages occurred within each zone across all locations, with few species found across all four zones. Conspicuous zonation of butterflyfishes has also been reported from Aqaba, Red Sea (Bouchon-Navaro & Bouchon, 1989) and Moorea, French Polynesia (Bouchon-Navaro, 1981; Berumen & Pratchett, 2006b). Distinct zonation of butterflyfishes may be

11 1740 M. S. PRATCHETT AND M. L. BERUMEN Number of fishes per 200 m 2 (per transect) C. citrinellus C. plebeius Flat Crest Slope Base Flat Crest Slope Base C. baronessa C. trifascialis Flat Crest Slope Base Flat Crest Slope Base C. unimaculatus C. kleinii Flat Crest Slope Base Flat Crest Slope Base C. auriga C. lunulatus Flat Crest Slope Base Flat Crest Slope Base Reef zones FIG. 5. Mean S.E. densities of butterflyfishes (Chaetodon spp.) among reef zones for each of the eight species that exhibited significant variation in abundance among zones (see Fig. 4). established at settlement due to differences in larval supply and habitat selection by settling fishes (Sale, 1980; Williams, 1980; Doherty, 1983), though small-scale variations (e.g. among reef zones) in the distribution and abundance of coral reef fishes are likely to be modified by post-recruitment processes, such as movement and mortality (Connell & Jones, 1991; Jones, 1991; Booth, 2002). Striking patterns in the zonation of butterflyfishes are maintained despite considerable potential for butterflyfishes to move among different reef zones. Most notably, the maximum dimensions of individual territories for coral-feeding butterflyfishes (Berumen & Pratchett, 2006a) often exceed linear distances separating the four physiognomic reef zones. Strong zonation patterns are mostly

12 DIETS AND DISTRIBUTIONS OF BUTTERFLYFISHES 1741 TABLE III. Correlations between abundance of butterflyfishes and live coral cover for butterflyfish species in different feeding guilds Trophic group and species Mean density r All butterflyfishes ** Obligate hard-coral feeders *** Chaetodon aureofasciatus Chaetodon baronessa *** Chaetodon lunulatus Chaetodon plebeius * Chaetodon rainfordi * Chaetodon trifascialis * Facultative hard-coral feeders Chaetodon citrinellus Chaetodon kleinii Chaetodon lunula Chaetodon melannotus Chaetodon rafflesii Chaetodon speculum Chaetodon ulietensis Chaetodon unimaculatus Non-coral feeders Chaetodon auriga Chaetodon ephippium Chaetodon lineolatus Chaetodon semion Chaetodon vagabundus *P < 005. **P < 001. ***P < linked to specific patterns of resource use, at least for obligate hard-coral feeders (Berumen & Pratchett, 2006b), though interspecific competition has an over-arching influence of on spatial patterns in the abundance for some species (Bouchon-Navaro, 1986; Berumen & Pratchett, 2006a). Pratchett (2005) showed that virtually all coral-feeding butterflyfishes at Lizard Island (C. aureofasciatus, C. baronessa, C. citrinellus, C. kleinii, C. lunula, C. lunulatus, C. plebeius, C. rainfordi and C. trifascialis) prefer to feed on essentially similar corals, specifically A. hyacinthus and Pocillopora damicornis. Both these corals tended to be most abundant on the reef crest [Fig. 2(a)], but only C. baronessa and C. trifascialis were found in higher abundance on the reef crest compared to other reef zones (Fig. 4). Chaetodon baronessa and C. trifascialis are also the dominant competitors within this system (Berumen & Pratchett, 2006a) and might, therefore, exclude subordinate coral-feeding butterflyfishes from accessing preferred corals growing on the reef crest. In this study, C. rainfordi was conspicuously less abundant on the reef crest, compared to all other reef zones, and may actively avoid this area to minimize direct interactions with competitively dominant butterflyfishes. The contrasting zonation of corallivorous v. noncoral-feeding butterflyfishes may also reflect competitive exclusion, though

13 1742 M. S. PRATCHETT AND M. L. BERUMEN coral-feeding butterflyfishes are much less aggressive towards non-coral feeders (e.g. C. auriga and C. vagabundus) compared to other corallivores (Berumen & Pratchett, 2006a). While there was a greater proportion of obligate hard-coral feeders that exhibited significant variation in abundance among reef zones, patterns of zonation for obligate hard-coral feeders were not consistently stronger compared to facultative hard-coral feeders or non-coral feeders. Zonation of obligate hardcoral feeders is expected given their reliance on scleractinian corals, which vary greatly in abundance and composition among depth zones [Goreau, 1959; Stoddart, 1969; Done, 1982, 1983; Figs 2(a) and 3(a)]. It is less clear why facultative hard-coral feeders (e.g. C. citrinellus) and non-coral feeders (e.g. C. auriga) also exhibit striking patterns of zonation. Spatial variation in the quality and quantity of non-coral prey was not studied but could have a major influence on zonation of facultative and non-coral-feeding butterflyfishes (Bozec et al., 2005). Chaetodon citrinellus, for example, is reported to ingest large quantities of filamentous algae (Harmelin-Vivien & Bouchon-Navaro, 1983) and is most abundant on shallow reef flats, which is likely to be the zone with maximal algal productivity (Fox & Bellwood, 2007). In general, species with high dietary specialization are expected to be restricted to zones with highest abundance of essential prey resources and this is apparent among corallivorous and non-corallivorous butterflyfishes. It is also possible, however, that the dietary specialization is a consequence, rather than the cause, of restricted distributions. In some cases, dietary specialization may change in response to altered availability of prey resources (Pratchett et al., 2004; Berumen et al., 2005), while other species are obligate specialists (Berumen & Pratchett, 2008) and very sensitive to changes in the availability of their preferred prey. Variation in the abundance and structure of butterflyfish assemblages was much less apparent among locations compared to reef zones. At this larger spatial scale, it is likely that stochastic variation in larval supply and patterns of recruitment contributes to observed patterns of distribution and abundance (Bell et al., 1985). Ascertaining the importance of larval supply in driving patterns of abundance for butterflyfishes, however, has proved very difficult (Leis, 1989) due to the rarity of butterflyfishes in field collections of larval fishes (Leis & Miller, 1976; Leis, 1982; Leis & Goldman, 1984; Young et al., 1986). Despite potential differences in larval supply, this study shows that overall abundance of butterflyfishes, as well as the individual abundance of most obligate hardcoral feeders, is strongly correlated with percentage cover of scleractinian corals. Similarly, many previous studies (Reese, 1977; Findley & Findley, 1985; Roberts et al., 1988) have found strong correlations over relatively large spatial scales, suggesting that availability of corals greatly influences settlement patterns and post-settlement survival of butterflyfishes. In contrast, several studies (Luckhurst & Luckhurst, 1978; Bell et al., 1985; Fowler, 1990) have failed to detect any significant relationship between abundance of butterflyfishes and live coral cover due to large variation in the butterflyfish abundance at sites with high coral cover; sites with low coral cover, however, consistently supported low numbers of butterflyfishes (Bell et al., 1985). Apparently contradictory findings of these studies might be explained by lower proportions of obligate hard-coral feeders in local assemblages, and also synergistic effects of stochastic

14 DIETS AND DISTRIBUTIONS OF BUTTERFLYFISHES 1743 recruitment and resource limitation: locations with low coral cover will be unlikely to support large numbers of butterflyfishes, regardless of local recruitment rates, whereas at locations with very high coral cover, the abundance of butterflyfishes is more likely to be regulated by rates of recruitment. Scleractinian corals are an important resource for butterflyfishes, not only because they provide food, but topographic complexity provided by live coral moderates key biological processes, such as competition and predation (Pratchett et al., 2008b). Further, Bozec et al. (2005) argued that motile invertebrates targeted by non-coral-feeding butterflyfishes mostly shelter in areas with high coral cover. Consistent with these hypotheses, several studies have documented significant declines in the abundance of non-coral-feeding butterflyfishes (e.g. C. auriga) associated with severe coral depletion (Wilson et al., 2006). While generalist and non-coral-feeding butterflyfishes are much less reliant on live coral compared to their obligate coral-feeding counterparts, these species are nonetheless still dependent on the topographic complexity and shelter provided by intact scleractinian corals. Sano et al. (1987), for example, showed that noncoral-feeding butterflyfishes tended to be equally abundant on reefs with extensive live cover v. recently killed corals. When the dead coral eventually broke down leaving flat plains of unstructured coral rubble, however, all butterflyfishes (including non-coral-feeding species) disappeared (Sano et al., 1987). In conclusion, this study reveals marked interspecific variation in spatial patterns of abundance for congeneric butterflyfishes. Species with highest dietary specialization (e.g. C. baronessa and C. trifascialis) exhibited the most pronounced zonation patterns, but they were also the dominant competitors (Berumen & Pratchett, 2006a). It is unclear, therefore, whether dietary specialization is the cause or consequence of their restricted zonation. Tests of feeding selectivity suggest that many butterflyfishes (e.g. C. lunulatus) prefer to feed on A. hyacinthus (Pratchett, 2007) but have limited access to this coral within their specific habitat. It appears likely, therefore, that competitive exclusion is preventing some butterflyfishes from living on the reef crest where there is the greatest availability of A. hyacinthus, but this remains to be tested through experimental removal of C. trifascialis and C. baronessa. Research is also continuing to understand factors that influence dietary specialization among butterflyfishes and explicitly test whether dietary composition is flexible with respect to contrasting availability of different prey items (Berumen et al., 2005, Berumen & Pratchett, 2008). This research was conducted at James Cook University, with funding from a Merit Research Grant and Project AWARE. M.S.P. is also supported by the ARC Centre of Excellence for Coral Reef Studies. Field assistance was provided by R. Thomas, S. L. S. Watson and S. K. Wilson. The authors are grateful to the staff at Lizard Island Research Station for ongoing assistance and logistical support. References Anderson, G. R. V., Ehrlich, A. H., Ehrlich, P. R., Roughgarden, J. D., Russell, B. C. & Talbot, F. H. (1981). The community structure of coral reef fishes. American Naturalist 117,

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