Herbivorous Coral Reef Fishes Research Articles

Variation in the parasite communities of three co-occurring herbivorous coral reef fishes.

Parasites are important, diverse, and abundant components of natural ecosystems and can influence the behaviour and health of their hosts, inter- and intraspecific interactions, and ultimately community structure. Coral reefs are one of the world's most biodiverse ecosystems, yet our understanding of the abundance, diversity, and composition of parasite communities of coral reef fishes is limited. Here, the authors aimed to compare the abundance, richness and composition of parasite communities among three co-occurring herbivorous coral reef fishes (the barred rabbitfish Siganus doliatus, Ward's damsel Pomacentrus wardi and the obscure damsel Pomacentrus adelus) from an inshore reef of the Great Barrier Reef (GBR). In total, 3978 parasites (3869 endoparasites and 109 ectoparasites) from 17 families were recovered from 30 individuals of each of the three fish species (mean=44 ± 22 s.e. parasites per fish; range=0-1947 parasites per fish). The parasite communities of P. wardi and P. adelus were characterised by pennellid copepods, derogenid and lecithasterid digeneans and were distinct from those of S. doliatus that were characterised by a higher abundance of atractotrematid and gyliauchenid digeneans. The abundance and family richness of all parasites were greatest in S. doliatus (abundance: 22.1 ± 5.0 parasites per fish; richness: 3.2 ± 0.3 families per fish), intermediate in P. wardi (abundance: 4.8 ± 1.1 parasites per fish; richness: 2.3 ± 0.3 families per fish) and lowest in P. adelus (abundance: 1.4 ± 0.4 parasites per fish; richness: 0.9 ± 0.2 families per fish). Similarly, the abundance of endoparasites was greatest in S. doliatus (19.7 ± 5.1 endoparasites per fish), intermediate in P. wardi (2.6 ± 0.7 endoparasites per fish) and lowest in P. adelus (1.2 ± 0.4 endoparasites per fish). Ectoparasite abundances were also lowest for P. adelus (0.2 ± 0.1 ectoparasites per fish), and S. doliatus and P. wardi had comparable abundances of ectoparasites (1.3 ± 0.3 and 2.1 ± 0.5 parasites per fish, respectively). Similarities between the parasite assemblages of the two pomacentrids may be related to their similar behaviours and/or diets vs. those of the larger-bodied and more mobile rabbitfish. Investigating the causes and consequences of variation in parasite communities across a broader range of fish species will be critical to understand the potential role of parasites in coral reef ecosystems.

Co‐occurrence of herbivorous fish functional groups correlates with enhanced coral reef benthic state

AbstractAimBiodiversity loss is impacting essential ecosystem functions and services across the globe. Recently, our interest in the benefits of biodiversity for ecosystem function has shifted focus from measurements of species richness to functional diversity and composition. However, the additional importance of other community characteristics, such as species evenness and co‐occurrence, for diversity‐driven ecosystem function is less known. We used herbivorous coral reef fish as a model system to investigate how co‐occurrence of different functional groups, rather than purely functional diversity, within an assemblage might affect the coral reef benthic state.LocationWestern Atlantic.Time period2007–2017.Major taxa studiedHerbivorous reef fish.MethodsWe analysed benthic and fish assemblage data from 601 sites across 12 countries in the western Atlantic. Using diversity–interaction models, we investigated how the composition and relative abundances of reef fish functional groups were correlated with benthic cover and estimates of coral calcification rates. We used statistical interactions to explore the importance of co‐occurrence of herbivorous fish functional groups for the coral reef benthic state.ResultsWe found that co‐occurrence of herbivorous fish functional groups, in addition to functional diversity, was correlated with reduced algal cover and increased coral accretion. Moreover, pairwise statistical interactions between functional groups were significantly correlated with an improvement in the coral reef benthic state.Main conclusionsOur results support the idea that functional group co‐occurrence, in addition to functional diversity, within herbivorous fish offers additional benefits to the coral reef benthic state. We identify farming damselfish and excavating parrotfish as potential key determinants of the coral reef benthic state and highlight that co‐occurrence of cropping and scraping herbivores might promote coral accretion. Our findings support the argument that protecting herbivore abundance without regard to the species and functional groups present is not enough to preserve coral reef health and that fine‐scale community composition must be considered.

Differences of Macroalgal Consumption by Eight Herbivorous Coral Reef Fishes From the Xisha Islands, China

Herbivorous fishes play an important role in preventing the overabundance of macroalgae on coral reefs. Understanding the feeding selectivity and consumption of macroalgae by herbivorous fishes can be challenging in studies of their ecological role in the preservation and recovery of coral reefs. Coral reef decline, macroalgal overgrowth and overfishing are clearly visible in the Xisha Islands, China. However, there are seldom studies on the feeding behaviors of herbivorous fishes in this area. We used microscopy, 18S rRNA high-throughput sequencing, and stable isotope analyses to comprehensively examine the diet of eight herbivorous reef fish species common in the Xisha Islands, including one parrotfish, two chub, two unicorn fish, and three rabbitfish. Based on microscopic examination,Siganus argenteusfed on the highest number of macroalgae species (five species), followed byNaso unicornis(four species).Kyphosus cinerascens,K. vaigiensis,N. unicornisandS. punctatissimusfed on the entire macroalgal thallus, indicating their greater ecological importance compared with species that only consume algal fronds. According to the 18S rRNA high-throughput sequencing of fish intestinal contents, cluster analysis revealed that consumed macroalgae composition from the eight fishes always grouped together based on the fish species, but with low similarity.K. vaigiensisremoved the highest diversity of macroalgae species as well as the greatest quantity of macroalgae.Calotomus carolinuscan consume the red algaPneophyllum conicum, which is widely distributed on Indo-Pacific coral reefs and can overgrow and kill live corals.N. unicorniswas found to occupy the lowest trophic position based on stable isotope analysis. Multi-technique analyses revealed thatK. vaigiensis,N. unicornisandS. argenteusshowed a high consumption potential of macroalgae, suggesting that they are the key browsers that should receive priority protection in the Xisha Islands. A diverse herbivorous fish fauna is also very important in the Xisha coral reefs. These results not only demonstrated the various functions of different herbivorous fish species in macroalgal removal, but also provided insights into the management of herbivorous fishes on the coral reefs of the South China Sea.

Species differences drive spatial scaling of foraging patterns in herbivorous reef fishes

Herbivory is a core ecosystem function that is delivered heterogeneously across space. Disentangling the drivers of foraging patterns is key to understanding the functional impact of herbivores. Because intrinsic drivers of foraging like metabolism, nutritional requirements and movement costs scale allometrically, foraging movement patterns in terrestrial herbivores have been shown to also scale positively with body size. However, spatial patterns of herbivory can also be explained by orthogonal factors such as trophic position, competition and functional groupings. Here, we investigate body size and species traits as drivers of the spatial scaling of foraging patterns in herbivorous coral reef fishes. We quantified foraging patterns of 119 individuals from nine common herbivorous species using focal individual surveys. Body size, species identity, feeding substrata, social grouping and functional group were tested as predictors of three foraging metrics: foraging area, inter‐foray distance and tortuosity. Our resulting model revealed that species identity overshadowed body size as a predictor in models for all foraging metrics. While foraging area was explained best by species only, the resulting tortuosity and mean inter‐foray distance models included a small effect of body size that explained within‐species variation. We do not find strong support for size‐scaling of foraging patterns in our study species. These findings indicate that foraging allometry based on Optimal foraging theory cannot be generally applied to reef fish assemblages due to a diversity of foraging strategies, such as spatial partitioning and territoriality. Our work reveals the importance of behavioural ecology and taxonomic diversity in understanding herbivory, especially given the functional differences across species. With coral reefs under threat across the world, this is an important step to disentangling the spatial delivery of a core ecosystem function.

Quantifying the diet diversity of herbivorous coral reef fishes using systematic review and DNA metabarcoding

AbstractHerbivorous reef fishes are an important component of coral reef ecosystems and a focal point in reef management. Herbivore diets have been examined using a myriad of methods, making it difficult to compare between analyses and to examine consistency or variability in diet across large spatial scales or heterogeneous environments. Here, we present a method to quantify and validate the diet diversity of these important coral reef species. We compile a database of information on Pacific herbivorous reef fishes through a systematic review of published diet data, and for those studies in which appropriate information is reported, we calculate a single Simpson's Diversity Index value to represent the diet diversity of each. We then validate this approach by examining the diet diversity of five species using DNA metabarcoding to sequence the algae present in their stomach contents. The two approaches provided comparable estimates of diet diversity, and we find evidence for considerable diet variability both within functional groups and between closely related species. Although there is a wide range of diet specialization within each functional and taxonomic group, parrotfishes and detritivorous surgeonfishes appear to have more limited diets than other nominal herbivorous fishes in our survey. This may make them more vulnerable to habitat degradation, which is important considering that both groups face considerable fishing pressure in Hawaiʻi and elsewhere in the Pacific. Finally, we demonstrate how this approach to quantifying diet can be incorporated into trait matrices that offer valuable insight into the dissimilarity between species, regardless of their taxonomic and functional classification. This analysis offers a simplified quantitative index of diet diversity that can contribute to better management of herbivores and increased understanding of their functional effects on changing, degraded reefs.

Oceanographic chaos and its role in larval self-recruitment and connectivity among fish populations in Micronesia

A 30-year time series of the recruitment of rabbit fish, a herbivorous coral reef fish, on the island of Guam in the tropical western Pacific, showed variability that ENSO alone does not explain. To help explain this variability, a high-resolution biophysical model that includes directional swimming reveals how mesoscale turbulence and ENSO-driven changes in the ocean circulation control the self-recruitment of rabbit fish. ENSO drives island wakes that enhance the capacity to retain locally spawned larvae, and mesoscale turbulence generates much variability and promotes seaward dispersion at time scales larger than the Pelagic Larval Duration. The same processes are predicted to occur for the self-recruitment of grouper fish, a carnivorous coral reef fish, in Palau, Micronesia. The models suggests that 99% of these fish larvae are exported seaward from Guam and Palau. Those larvae are the ones that could provide connectivity between reefs and islands in Micronesia. This connectivity for the grouper fish was predicted using an altimetry-driven advection-diffusion oceanography model for 40 mass spawning events spread over 10 years. The mesoscale turbulence, and not the mean oceanographic currents, is the dominant process controlling the connectivity, which is thus chaotic. This finding applies also in the Galapagos archipelago and the Coral Sea fringing the Great Barrier Reef.

Fishing and habitat condition differentially affect size spectra slopes of coral reef fishes.

Marine food webs are structured through a combination of top-down and bottom-up processes. In coral reef ecosystems, fish size is related to life-history characteristics and size-based indicators can represent the distribution and flow of energy through the food web. Thus, size spectra can be a useful tool for investigating the impacts of both fishing and habitat condition on the health and productivity of coral reef fisheries. In addition, coral reef fisheries are often data-limited and size spectra analysis can be a relatively cost-effective and simple method for assessing fish populations. Abundance size spectra are widely used and quantify the relationship between organism size and relative abundance. Previous studies that have investigated the impacts of fishing and habitat condition together on the size distribution of coral reef fishes, however, have aggregated all fishes regardless of taxonomic identity. This leads to a poor understanding of how fishes with different feeding strategies, body size-abundance relationships, or catchability might be influenced by top-down and bottom-up drivers. To address this gap, we quantified size spectra slopes of carnivorous and herbivorous coral reef fishes across three regions of Indonesia representing a gradient in fishing pressure and habitat conditions. We show that fishing pressure was the dominant driver of size spectra slopes such that they became steeper as fishing pressure increased, which was due to the removal of large-bodied fishes. When considering fish functional groups separately, however, carnivore size spectra slopes were more heavily impacted by fishing than herbivores. Also, structural complexity, which can mediate predator-prey interactions and provisioning of resources, was a relatively important driver of herbivore size spectra slopes such that slopes were shallower in more complex habitats. Our results show that size spectra slopes can be used as indicators of fishing pressure on coral reef fishes, but aggregating fish regardless of trophic identity or functional role overlooks differential impacts of fishing pressure and habitat condition on carnivore and herbivore size distributions.

Fine-scale foraging behavior reveals differences in the functional roles of herbivorous reef fishes.

Efforts to understand and protect ecosystem functioning have put considerable emphasis on classifying species according to the functions they perform. However, coarse classifications based on diet or feeding mode often oversimplify species' contributions to ecological processes. Behavioral variation among superficially similar species is easily missed but could indicate important differences in competitive interactions and the spatial scale at which species deliver their functions. To test the extent to which behavior can vary within existing functional classifications, we investigate the diversity of foraging movements in three herbivorous coral reef fishes across two functional groups. We find significant variation in foraging movements and spatial scales of operation between species, both within and across existing functional groups. Specifically, we show that movements and space use range from low frequency foraging bouts separated by short distances and tight turns across a small area, to high frequency, far‐ranging forays separated by wide sweeping turns. Overall, we add to the burgeoning evidence that nuanced behavioral differences can underpin considerable complementarity within existing functional classifications, and that species assemblages may be considerably less redundant than previously thought.

Deep Insights into Gut Microbiota in Four Carnivorous Coral Reef Fishes from the South China Sea.

Investigations of gut microbial diversity among fish to provide baseline data for wild marine fish, especially the carnivorous coral reef fishes of the South China Sea, are lacking. The present study investigated the gut microbiota of four carnivorous coral reef fishes, including Oxycheilinus unifasciatus, Cephalopholis urodeta, Lutjanus kasmira, and Gnathodentex aurolineatus, from the South China Sea for the first time using high-throughput Illumina sequencing. Proteobacteria, Firmicutes, and Bacteroidetes constituted 98% of the gut microbiota of the four fishes, and 20 of the gut microbial genera recovered in this study represent new reports from marine fishes. Comparative analysis indicated that the four fishes shared a similar microbial community, suggesting that diet type (carnivorous) might play a more important role in shaping the gut microbiota of coral reef fishes than the species of fish. Furthermore, the genera Psychrobacter, Escherichia-Shigella, and Vibrio constituted the core microbial community of the four fishes, accounting for 61–91% of the total sequences in each fish. The lack of the genus Epulopiscium in the four fishes was in sharp contrast to what has been found in coral reef fishes from the Red Sea, in which Epulopiscium was shown to be the most dominant gut microbial genus in seven herbivorous coral reef fishes. In addition, while unique gut microbial genera accounted for a small proportion (8–13%) of the total sequences, many such genera were distributed in each coral reef fish species, including several genera (Endozoicomonas, Clostridium, and Staphylococcus) that are frequently found in marine fishes and 11 new reports of gut microbes in marine fishes. The present study expands our knowledge of the diversity and specificity of gut microbes associated with coral reef fishes.

Historical biogeography of herbivorous coral reef fishes: The formation of an Atlantic fauna

AbstractAimTo describe the global biogeography of key herbivorous coral reef fish groups since their presumed origins, using data from both fossil and extant species.LocationGlobal Cenozoic reefs.TaxonAcanthuridae (surgeonfishes), Siganidae (rabbitfishes) and Scarini (parrotfishes).MethodsWe applied the fossilized birth–death model to build chronograms including a comprehensive sampling of extant species and all the fossil occurrences described for each group. With the resulting chronograms, we built biogeographical models considering the geological changes in reef habitat availability since the ancient Tethys Sea. Finally, we used biogeographical stochastic mappings to trace the routes of colonization of the Atlantic Ocean by lineages in our focal taxa.ResultsWe found that the Palaeocene–Eocene was a period of significant lineage origination for surgeonfishes and rabbitfishes in the central Tethys Sea with the appearance of ancient genera. Most of these genera were probably extinct by the Eocene–Oligocene boundary as they do not correspond with modern taxa. Parrotfishes, however, originated in the early Oligocene, an epoch that corresponds with the geographical transition of the marine biodiversity hotspot. In all groups, extant genera had similar origin times and all expanded in the Miocene, mainly in the Indo‐Pacific. In the Atlantic, only one parrotfish lineage with Tethyan ancestry appears to have survived. It subsequently gave rise to extant endemic genera (Sparisoma and Cryptotomus). The other extant lineages in the Atlantic all have Indo‐Pacific origins and colonized more recently using different dispersal pathways.Main conclusionsThe Indo‐Pacific herbivorous fish fauna is the result of ongoing lineage expansion that started in the central Tethys. The Atlantic is a composite fauna with just one endemic lineage and at least four colonization events from the Indo‐Pacific.

The evolution of traits and functions in herbivorous coral reef fishes through space and time.

Herbivory by fishes has been identified as a key ecological process shaping coral reefs through time. Although taxonomically limited, herbivorous reef fishes display a wide range of traits, which results in varied ecosystem functions on reefs around the world. Yet, we understand little about how these trait combinations and functions in ecosystems changed through time and across biogeographic realms. Here, we used fossils and phylogenies in a functional ecological framework to reveal temporal changes in nominally herbivorous fish assemblages among oceanic basins in both trait space and lineage richness among functions. We show that the trait space occupied by extant herbivorous fishes in the Indo-Pacific resulted from an expansion of traits from the ancestral Tethyan assemblages. By contrast, trait space in the Atlantic is the result of lineage turnover, with relatively recent colonization by lineages that arose in the east Tethys/Indo-Pacific. From an ecosystem function perspective, the Atlantic supports a depauperate fauna, with few extant herbivorous reef fish lineages performing each function. Indo-Pacific fishes support both more functions and more lineages within each function, with a marked Miocene to Pleistocene expansion. These disparities highlight the importance of history in explaining global variation in fish functional composition on coral reefs.

Sediment addition drives declines in algal turf yield to herbivorous coral reef fishes: implications for reefs and reef fisheries

Coral reefs around the world are changing rapidly, with overfishing of herbivorous fishes and increased sediment inputs being two of the major local-scale stressors. We therefore assessed the effects of sediment loads and overfishing on the nutritional quality and yield to grazing fishes of algal turfs, within the epilithic algal matrix, on a coral reef at Lizard Island, Australia. Low, ambient and high sediment loads were maintained on turf-covered coral tiles, with and without grazer exclusion cages, for 1 month. Subsequently, algal turfs were removed and analysed for organic carbon and nitrogen content. Under grazer exclusion, sediment additions decreased algal turf biomass by approximately 63%, while algal turf biomass was the highest on tiles with sediments removed. In the presence of grazing fishes, algal turfs in all treatments were cropped by grazers to similar low biomass levels. Nitrogen content of algal turfs followed a similar trend. Effectively, added sediments decreased the potential yield of algal turf biomass and nitrogen to grazing fishes by an average of 2000 and 3300%, respectively. Sediments profoundly affect algal turf yield to grazing herbivorous fishes and, therefore, the productivity of algal turf-based food chains, potentially diminishing reef-based fisheries.

Experimental evidence of dietary ciguatoxin accumulation in an herbivorous coral reef fish

Ciguatoxins (CTXs) are potent algal toxins that cause widespread ciguatera poisoning and are found ubiquitously in coral reef food webs. Here we developed an environmentally-relevant, experimental model of CTX trophic transfer involving dietary exposure of herbivorous fish to the CTX-producing microalgae Gambierdiscus polynesiensis. Juvenile Naso brevirostris were fed a gel-food embedded with microalgae for 16 weeks (89 cells g−1 fish daily, 0.4 μg CTX3C equiv kg−1 fish). CTXs in muscle tissue were detectable after 2 weeks at levels above the threshold for human intoxication (1.2 ± 0.2 μg CTX3C equiv kg−1). Although tissue CTX concentrations stabilized after 8 weeks (∼3 ± 0.5 μg CTX3C equiv kg−1), muscle toxin burden (total μg CTX in muscle tissue) continued to increase linearly through the end of the experiment (16 weeks). Toxin accumulation was therefore continuous, yet masked by somatic growth dilution. The observed CTX concentrations, accumulation rates, and general absence of behavioural signs of intoxication are consistent with field observations and indicate that this method of dietary exposure may be used to develop predictive models of tissue-specific CTX uptake, metabolism and depuration. Results also imply that slow-growing fish may accumulate higher CTX flesh concentrations than fast-growing fish, which has important implications for global seafood safety.

Contrasting Responses in the Niches of Two Coral Reef Herbivores Along a Gradient of Habitat Disturbance in the Spermonde Archipelago, Indonesia

Habitat modification of coral reefs is becoming increasingly common due to increases in coastal urban populations. Coral reef fish are highly dependent on benthic habitat; however, information on species-specific responses to habitat change, in particular with regard to trophic strategies, remains scarce. This study identifies variation in the trophic niches of two herbivorous coral reef fishes with contrasting trophic strategies, using Stable Isotopes Bayesian Ellipses in R, along a spatial gradient of changing coral reef habitats. In the parrotfish, Chlorurus bleekeri, a roving consumer, the range of 15N (positive) and 13C (negative) and their niche area (positive) displayed significant relationships with the amount of rubble in the habitat. In contrast, the farming damselfish, Dischistodus prosopotaenia, showed a narrow range of both 15N and 13C, displaying little change in niche parameters among sites. This may indicate that parrotfish vary their feeding according to habitat, while the damselfish continue to maintain their turf and invertebrate resources. Assessing isotopic niches may help to better understand the specific trophic responses to change in the environment. Furthermore, the use of isotopic niches underline the utility of stable isotopes in studying the potential impacts of environmental change on feeding ecology.

Genetic connectivity in a herbivorous coral reef fish (Acanthurus leucosternon Bennet, 1833) in the Eastern African region

Knowledge of larval dispersal and connectivity in coral reef species is crucial for understanding population dynamics, resilience, and evolution of species. Here, we use ten microsatellites and one mitochondrial marker (cytochrome b) to investigate the genetic population structure, genetic diversity, and historical demography of the powder-blue tang Acanthurus leucosternon across more than 1000 km of the scarcely studied Eastern African region. The global AMOVA results based on microsatellites reveal a low but significant F ST value (F ST = 0.00252 P < 0.001; D EST = 0.025 P = 0.0018) for the 336 specimens sampled at ten sample sites, while no significant differentiation could be found in the mitochondrial cytochrome b dataset. On the other hand, pairwise F ST, PCOA, and hierarchical analysis failed to identify any genetic breaks among the Eastern African populations, supporting the hypothesis of genetic homogeneity. The observed genetic homogeneity among Eastern African sample sites can be explained by the lengthy post-larval stage of A. leucosternon, which can potentiate long-distance dispersal. Tests of neutrality and mismatch distribution signal a population expansion during the mid-Pleistocene period.

The importance of individual and species-level traits for trophic niches among herbivorous coral reef fishes.

Resolving how species compete and coexist within ecological communities represents a long-standing challenge in ecology. Research efforts have focused on two predominant mechanisms of species coexistence: complementarity and redundancy. But findings also support an alternative hypothesis that within-species variation may be critical for coexistence. Our study focuses on nine closely related and ecologically similar coral reef fish species to test the importance of individual- versus species-level traits in determining the size of dietary, foraging substrate, and behavioural interaction niches. Specifically, we asked: (i) what level of biological organization best describes individual-level niches? and (ii) how are herbivore community niches partitioned among species, and are niche widths driven by species- or individual-level traits? Dietary and foraging substrate niche widths were best described by species identity, but no level of taxonomy explained behavioural interactions. All three niches were dominated by only a few species, contrasting expectations of niche complementarity. Species- and individual-level traits strongly drove foraging substrate and behavioural niches, respectively, whereas the dietary niche was described by both. Our findings underscored the importance of species-level traits for community-level niches, but highlight that individual-level trait variation within a select few species may be a key driver of the overall size of niches.

Application of diet theory reveals context-dependent foraging preferences in an herbivorous coral reef fish.

Dietary preferences of grazers can drive spatial variability in top-down control of autotroph communities, because diet composition may depend on the relative availability of autotroph species. On Caribbean coral reefs, parrotfish grazing is important in limiting macroalgae, but parrotfish dietary preferences are poorly understood. We applied diet-switching analysis to quantify the foraging preferences of the redband parrotfish (Sparisoma aurofrenatum). At 12 Caribbean reefs, we observed 293 redband parrotfish in 5-min feeding bouts and quantified relative benthic algal cover using quadrats. The primary diet items were macroalgal turfs, Halimeda spp., and foliose macroalgae (primarily Dictyota spp. and Lobophora spp.). When each resource was evaluated independently, there were only weak relationships between resource cover and foraging effort (number of bites taken). Electivity for each resource also showed no pattern, varying from positive (preference for the resource) to negative (avoidance) across sites. However, a diet-switching analysis consisting of pairwise comparisons of relative cover and relative foraging effort revealed clearer patterns: parrotfish (a) preferred Halimeda and macroalgal turfs equally, and those two resources were highly substitutable; (b) preferred Halimeda to foliose macroalgae, but those two resources were complementary; and (c) also preferred turf to foliose macroalgae, and those resources were also complementary. Thus parrotfish grazing rates depend on relative, not absolute, abundance of macroalgal types, due to differences in substitutability among resources. Application of similar analyses may help predict potential changes in foraging effort of benthic grazers over spatial gradients that could inform expectations for reef recovery following the protection of herbivore populations.

Habitat effects on home range and schooling behaviour in a herbivorous fish (Kyphosus bigibbus) revealed by acoustic tracking

Large mobile herbivorous fish that specialise in browsing large brown algae are particularly important on coral reefs because their activities mediate algal–coral competition. Despite this important ecological role, we have a poor understanding of the movement patterns of such large herbivorous fish, including Kyphosus bigibbus. Nineteen K. bigibbus captured near adjacent but distinct patch reefs were tagged with internal acoustic tags and their movements monitored for up to 20 months by an array of 60 acoustic receivers. Home-range estimates showed that movements of individuals from each patch reef encompassed different spatial extents and resulted in differences in habitat used by the two groups of fish. The average 50 and 95% kernel utilisation distribution for long-term resident fish was 0.27±0.03 and 1.61±0.30km2 respectively, ranges that represent the largest values for a herbivorous coral reef fish recorded to date. There was a significantly higher degree of fidelity among fish from the same school, and to particular patch reefs, despite the proximity of the reefs and substantial overlap between schools of conspecifics. A coefficient of sociality was used on pairs of fish and showed that there was no evidence that individuals were consistently detected together when they were detected by receivers away from their home reef. The variability of movement patterns among individuals of K. bigibbus results in an increased niche footprint for this important browser, potentially increasing reef resilience.

Predator identity and time of day interact to shape the risk-reward trade-off for herbivorous coral reef fishes.

Non-consumptive effects (NCEs) of predators occur as prey alters their habitat use and foraging decisions to avoid predation. Although NCEs are recognized as being important across disparate ecosystems, the factors influencing their strength and importance remain poorly understood. Ecological context, such as time of day, predator identity, and prey condition, may modify how prey species perceive and respond to risk, thereby altering NCEs. To investigate how predator identity affects foraging of herbivorous coral reef fishes, we simulated predation risk using fiberglass models of two predator species (grouper Mycteroperca bonaci and barracuda Sphyraena barracuda) with different hunting modes. We quantified how predation risk alters herbivory rates across space (distance from predator) and time (dawn, mid-day, and dusk) to examine how prey reconciles the conflicting demands of avoiding predation vs. foraging. When we averaged the effect of both predators across space and time, they suppressed herbivory similarly. Yet, they altered feeding differently depending on time of day and distance from the model. Although feeding increased strongly with increasing distance from the predators particularly during dawn, we found that the barracuda model suppressed herbivory more strongly than the grouper model during mid-day. We suggest that prey hunger level and differences in predator hunting modes could influence these patterns. Understanding how context mediates NCEs provides insight into the emergent effects of predator-prey interactions on food webs. These insights have broad implications for understanding how anthropogenic alterations to predator abundances can affect the spatial and temporal dynamics of important ecosystem processes.

Natural bounds on herbivorous coral reef fishes.

Humans are an increasingly dominant driver of Earth's biological communities, but differentiating human impacts from natural drivers of ecosystem state is crucial. Herbivorous fish play a key role in maintaining coral dominance on coral reefs, and are widely affected by human activities, principally fishing. We assess the relative importance of human and biophysical (habitat and oceanographic) drivers on the biomass of five herbivorous functional groups among 33 islands in the central and western Pacific Ocean. Human impacts were clear for some, but not all, herbivore groups. Biomass of browsers, large excavators, and of all herbivores combined declined rapidly with increasing human population density, whereas grazers, scrapers, and detritivores displayed no relationship. Sea-surface temperature had significant but opposing effects on the biomass of detritivores (positive) and browsers (negative). Similarly, the biomass of scrapers, grazers, and detritivores correlated with habitat structural complexity; however, relationships were group specific. Finally, the biomass of browsers and large excavators was related to island geomorphology, both peaking on low-lying islands and atolls. The substantial variability in herbivore populations explained by natural biophysical drivers highlights the need for locally appropriate management targets on coral reefs.