Herbivorous Fish Biomass Research Articles

Herbivory through the lens of ecological processes across Pacific coral reefs

AbstractCoral reefs are in global decline primarily due to climate change. Herbivory is often viewed as key to maintaining coral‐dominated reefs, and herbivore management is gaining traction as a possible strategy for promoting reef resilience. The functional impact of herbivorous fishes has typically been inferred from total biomass, but robust estimates of ecological processes are needed to better inform management targets. Here, we provide a framework to calculate rates of herbivory across Pacific reefs. We synthesized available observations of foraging metrics in relation to fish body size and found considerable variation, even among closely related species. We then applied these allometric functions to survey data and calculated rates of herbivory for acanthurids and scarines, which make up the vast majority of herbivorous fish biomass in the Pacific. Estimated rates of algal consumption, area scraped, and bioerosion varied across islands, with noticeable differences that may align with the relative influence of human population density among underlying herbivore functional groups. We found no evidence of compensatory relationships among herbivore processes whereby decreasing rates in one type of herbivory is offset by increasing rates in another. We observed nonlinear, positive relationships between fish biomass and rates of herbivory. Yet, for a given biomass, the corresponding rates of herbivory varied among regions, and we observed instances where islands with the greatest biomass did not also have the highest rates of herbivory. Islands with the largest size classes of herbivores did not consistently exhibit greater rates of herbivory, and we did not find a clear, consistent pattern between the number of fish species and corresponding rates of herbivore processes. Cropping Acanthurus spp. provided the greatest proportion of algal consumption at every island, yet no single species accounted for the majority of this process, whereas we identified parrotfish species that provided >75% of scraping or bioerosion at certain islands. Our results emphasize the importance of considering the species and size composition of herbivore assemblages when estimating processes, rather than relying on total biomass alone. Lastly, we highlight gaps in foraging observations and additional work needed to further broaden our ability to quantify the ecological processes of herbivores.

Biomass of key grazing fishes is an important determinant of coral growth and fouling control in coral nurseries

Restoration is an emerging tool for coral reef conservation, yet despite small‐scale successes there are concerns about high costs and ecological setbacks. Integration between reef ecology and restoration could help address such concerns. A prime example is the use of grazing by herbivores to reduce coral nursery cleaning costs. However, the relation between herbivore communities and cleaning benefits remains unquantified. This study aimed to measure links between herbivorous fish communities, grazing intensity and coral nursery performance. Six reefs were selected in southern Kenya, equally divided across three levels of fisheries management (fished < reserve < no‐take). Fish surveys determined herbivorous fish biomass and composition, and remote underwater videos recorded grazing intensity on coral nurseries. Accumulated fouling and coral growth were measured at the end of the 4‐month study. Grazing intensity was sixfold lower and fouling density fourfold higher in the fished areas compared to protected zones. Higher fouling strongly correlated with lower coral growth: exponential growth constants in fished areas were respectively twice and three times as low compared to marine reserves and no‐take zones. Across study sites grazing was dominated by bristletooth tangs (Ctenochaetus spp.), except where these were outcompeted by territorial damselfish. Thus, better coral nursery performance in protected areas can be partially linked to higher grazing intensity, which in turn is determined by both fish biomass and local species composition. We recommend protecting herbivorous fish species and placing coral nurseries in areas with high biomass of key grazers to improve coral nursery performance and reduce maintenance costs.

Analisis Kesehatan Terumbu Karang Berdasarkan Kelimpahan Ikan Herbivora Di Perairan Labuan Bajo

Coral reef ecosystem is one of the ecosystems located in coastal areas which has a very important role for the community, both economically and ecologically. Rock or scleractinian corals are the main building blocks of coral reef ecosystems. The connectivity of coral reef ecosystems with other ecosystems has a very large influence on the life cycle of reef fish such as spawning, rearing, foraging, and sheltering places. Herbivorous fish are that eat turf algae or macroalgae that inhibit the recruitment of new corals. Labuan Bajo is one of the tourism destinations in Indonesia, especially marine tourism. Coral reefs in Labuan Bajo cover 10.8% of the total area of coral reefs in Indonesia located in Nusa Tenggara. The purpose of this study was to determine the health of coral reefs in the waters of Labuan Bajo. This research will be carried out from September to October 2021. The coral reef data collection method used is the Underwater Photo Transect (UPT), and for reef fish is using the Underwater Visual Census (UVC) method. Based on the research results, the percentage of hard coral is 20.16% of the total stations and rubble is 32.24%. The abundance of fish in Labuan Bajo waters ranges from 1608.89 - 4600.00 ind/ha, where the average abundance is 2919.78 ind/ha. All stations got a health index value of 3, this is indicated by the low coral cover, high coral resilience, and low herbivorous fish biomass obtained.

The effect of artificial reef design on the attraction of herbivorous fish and on coral recruitment, survival and growth

Fish assemblages of different types of artificial reefs can differ greatly in abundance, biomass and composition, with some reef types harboring over five times more herbivores than others. It is assumed that higher herbivorous fish abundance results in a higher grazing intensity, affecting the benthic community by means of enhanced coral recruitment, survival and growth. Territorial fish species might affect this process by chasing away other fish, especially herbivores. In this study we compared the fish assemblage, territorial behavior and grazing intensity by fish on two artificial reef types: reef balls and layered cakes, differing greatly in their fish assemblage during early colonization. In addition, the effect of artificial reef type on benthic development and coral recruitment, survival and growth, was investigated. Although layered cakes initially harbored higher herbivorous fish biomass, this effect was lost during consecutive monitoring events. This seems to be the result of the higher territorial fish abundance around the layered cakes where almost four times more chasing behavior was recorded compared to the reef balls. This resulted in a more than five times lower fish grazing intensity compared to the reef-ball plots. Although macroalgae were effectively controlled at both reefs, the grazing intensity did not differ enough to cause large enough structural changes in benthic cover for higher coral recruitment, survival or growth. The high turf algae cover, combined with increasing crustose coralline algae and sponge cover likely explained reduced coral development. We recommend further research on how to achieve higher grazing rates for improved coral development on artificial reefs, for example by facilitating invertebrate herbivores.

Impact of Coral Bleaching on Coral Reef Fishes in Sekotong Bay, West Lombok Regency

Coral reef fish is one of the main components making up the ecosystem of coral reefs. There is a strong mutual dependence between reef-forming corals and fish that inhabit coral reefs. Various fish species use coral as a food source and habitat. This article describes the impact of coral bleaching on the diversity, density, and biomass of coral reef fish in the waters of Sekotong Bay, West Lombok. Underwater visual census (UVC) methods are used to obtain data on species, the number of individuals per species, and fish size at transect length 70 meters and width 5 meters. Simple linear regression analysis that is used to assess the impact of coral bleaching on the condition of coral reef fish shows that coral bleaching affects the diversity and density of reef fish. This analysis also showed that coral bleaching affected herbivorous fish biomass but not carnivorous fish. This study concludes that the degradation of coral fish in the Sekotong Bay of West Lombok occurs due to various factors, especially the declining health conditions of coral reefs

Post-bleaching alterations in coral reef communities

We explored the extent of post-bleaching impacts, caused by the 2014–2016 El Niño Southern Oscillation (ENSO) event, on benthic community structure (BCS) and herbivores (fish and sea urchins) on seven fringing reefs, with differing protection levels, in Zanzibar, Tanzania. Results showed post-bleaching alterations in BCS, with up to 68 % coral mortality and up to 48 % increase in turf algae cover in all reef sites. Herbivorous fish biomass increased after bleaching and was correlated with turf algae increase in some reefs, while the opposite was found for sea urchin densities, with significant declines and complete absence. The severity of the impact varied across individual reefs, with larger impact on the protected reefs, compared to the unprotected reefs. Our study provides a highly relevant reference point to guide future research and contributes to our understanding of post-bleaching impacts, trends, and evaluation of coral reef health and resilience in the region.

Oceanic productivity and high-frequency temperature variability—not human habitation—supports calcifier abundance on central Pacific coral reefs

Past research has demonstrated how local-scale human impacts—including reduced water quality, overfishing, and eutrophication—adversely affect coral reefs. More recently, global-scale shifts in ocean conditions arising from climate change have been shown to impact coral reefs. Here, we surveyed benthic reef communities at 34 U.S.-affiliated Pacific islands spanning a gradient of oceanic productivity, temperature, and human habitation. We re-evaluated patterns reported for these islands from the early 2000s in which uninhabited reefs were dominated by calcifiers (coral and crustose coralline algae) and thought to be more resilient to global change. Using contemporary data collected nearly two decades later, our analyses indicate this projection was not realized. Calcifiers are no longer the dominant benthic group at uninhabited islands. Calcifier coverage now averages 26.9% ± 3.9 SE on uninhabited islands (compared to 45.18% in the early 2000s). We then asked whether oceanic productivity, past sea surface temperatures (SST), or acute heat stress supersede the impacts of human habitation on benthic cover. Indeed, we found variation in benthic cover was best explained not by human population densities, but by remotely sensed metrics of chlorophyll-a, SST, and island-scale estimates of herbivorous fish biomass. Specifically, higher coral and CCA cover was observed in more productive waters with greater biomass of herbivores, while turf cover increased with daily SST variability and reduced herbivore biomass. Interestingly, coral cover was positively correlated with daily variation in SST but negatively correlated with monthly variation. Surprisingly, metrics of acute heat stress were not correlated with benthic cover. Our results reveal that human habitation is no longer a primary correlate of calcifier cover on central Pacific island reefs, and highlight the addition of oceanic productivity and high-frequency SST variability to the list of factors supporting reef builder abundance.

Fisheries restrictions and their cascading effects on herbivore abundance and macroalgae removal at Kenyan coral reefs

The increase of macroalgae at degraded reefs impedes several ecosystem services and calls for effective methods to facilitate a return to coral dominance. Removal of macroalgae (browsing) is typically realized by fish, but the role and identity of browsers at the heavily-fished East African coast is still largely unknown. This study investigated how browsing pressure at Kenyan reefs (−4.700, 39.396) related to fisheries management and herbivore community. From October 2018 to January 2019, consumption during 24-h buffet assays using the brown macroalgae Sargassum and Padina was determined and video recorded at six sites: two in fished zones, two in marine reserves (traditional fishing allowed) and two in no-take zones. Herbivorous fish composition, biomass and sea urchin density were also determined. Consumption of Sargassum and Padina was nearly three-fold lower in the fished zones (26% and 28% of macroalgal biomass consumed, respectively) compared to the no-take zones (62% and 82%), with intermediate consumption in the marine reserves (48% and 71%). Herbivore biomass was seven-fold higher in no-take zones and included substantially more browsers (mainly unicornfishes, Naso spp.) and scrapers (scarids), which were associated with the higher browsing pressure. Browsers and scrapers were predominantly responsible for the consumption of macroalgae as determined by video recordings, though key species differed across sites. In contrast, damselfish-dominated fished sites were associated with high sea urchin densities and low browsing pressure. These results indicate that fishing restrictions are likely to support reef resilience by increasing herbivorous fish biomass of key species and thereby promote macroalgae removal.

A contemporary baseline of Madagascar's coral assemblages: Reefs with high coral diversity, abundance, and function associated with marine protected areas.

Madagascar is a major hotspot of biodiversity in the Western Indian Ocean, but, as in many other regions, coral reefs surrounding the island confront large-scale disturbances and human-induced local stressors. Conservation actions have been implemented with encouraging results for fisheries, though their benefit on coral assemblages has never been rigorously addressed. In this context, we analyzed the multiscale spatial variation of the composition, generic richness, abundance, life history strategies, and cover of coral assemblages among 18 stations placed at three regions around the island. The potential influences of marine protected areas (MPAs), algal cover, substrate rugosity, herbivorous fish biomass, and geographic location were also analyzed. Our results highlight the marked spatial variability, with variation at either or both regional and local scales for all coral descriptors. The northeast coastal region of Masoala was characterized by the high abundance of coral colonies, most notably of the competitive Acropora and Pocillopora genera and stress-tolerant taxa at several stations. The southwest station of Salary Nord was distinguished by lower abundances, with depauperate populations of competitive taxa. On the northwest coast, Nosy-Be was characterized by higher diversity and abundance as well as by high coral cover (~42–70%) recorded at unfished stations. Results clearly underline the positive effects of MPAs on all but one of the coral descriptors, particularly at Nosy-Be where the highest contrast between fished and unfished stations was observed. Biomass of herbivorous fishes, crustose coralline algae cover, and substrate rugosity were also positively related to several coral descriptors. The occurrence of reefs with high diversity, abundance, and cover of corals, including the competitive Acropora, is a major finding of this study. Our results strongly support the implementation of locally managed marine areas with strong involvement by primary users, particularly to assist in management in countries with reduced logistic and human resources such as Madagascar.

Assessing of coral reef resilience on Lembeh Island, North Sulawesi

Resilience is the ability of a system to reach equilibrium after a temporary disturbance. When an ecosystem is disturbed, then the recovery of the system is very dependent on the diversity of species that are still left. The remaining communities determine the direction of new successional communities formed after the disturbance has passed, including coral, fish, and other biota communities. This study aimed to determine the level of resilience of coral reef ecosystems on Lembeh Island. The observation was carried out from January 16-19, 2019, at a 7-meter depth. Determination of the coral reef index will refer to coral reef resilience index table. It is found that the resilience of coral reefs on Lembeh Island currently has a high potential for recovery (resilience) from natural stressors based on the 16 parameters. Seven parameters have very high resilience values: the proportion of bleaching resistant species, community resistance to bleaching, taxonomic diversity, size/age distribution, the abundance of reef fish on live coral, and the abundance of coral benthos on live coral and coral disease levels. Lembeh Island is dominated by hard corals measuring between 5.1 to 25 cm (size M) by ~46%, then sizes more than 25 cm (size L) by ~37% and juvenile size (size S) by 17%. The coral health index on Lembeh Island has the capacity to recover if the condition of coral cover is minimally maintained or improved and lower fleshy seaweed cover by increasing the abundance and biomass of herbivorous fish.

Impacts of overfishing and sedimentation on the feeding behavior and ecological function of herbivorous fishes in coral reefs

Coral reef environments are currently experiencing a global crisis. For restoration and conservation of such environments, substratum suitable for coral recruitment is important. Herbivorous fishes are likely to facilitate the survival and growth of coral reefs by removing sediment and epilithic algae on corals in the course of their feeding behavior. However, this facilitative function can be greatly impaired by anthropogenic stressors such as overfishing, which decreases the biomass of fishes, and by excessive sediment deposition due to land development, which might decrease the frequency of feeding. To examine these hypotheses, we conducted observations and experiments at 4 reefs with different degrees of anthropogenic stress in subtropical Japan. Our findings were as follows: (1) Underwater visual censuses, sediment trap surveys, and an analysis of fishery statistics revealed that biomass, population density, and species richness of herbivorous fishes were generally lower in reefs with high densities of fishermen and high sedimentation rates. (2) Caging experiments showed that, compared to control plots, caged plots inaccessible to herbivorous fishes had greater accumulation of sediment and epilithic algae. (3) Sediment removal experiments found that, compared to control plots, experimental plots with the sediment removed were more heavily grazed by some functional groups of herbivorous fishes (grazers). These results suggest that excessive deposition of sediment and overfishing depress the facilitative function of herbivorous fishes, which can further exacerbate the deposition of sediment.

Manipulation of fish community structure effectively restores submerged aquatic vegetation in a shallow subtropical lake

Fish community manipulation and regulation has been largely overlooked as a mitigation strategy for restoring submerged aquatic vegetation (SAV) in shallow lakes of the middle and lower Yangtze River Basin (MLYRB). An in-situ fish exclusion experiment and a large-scale lake manipulation were conducted to test the hypothesis that the reasonable removal of benthivorous and herbivorous fish would facilitate the restoration and reconstruction of SAV in shallow lakes within the MLYRB. The in-situ exclusion experiment was conducted from April to October in 2017. Electrofishing was used to remove benthivorous and herbivorous fish from the exclosures. SAV were then artificially planted in the same pattern and density in both exclosures and adjacent open sites, and responses were measured for seven consecutive months. The mean percent coverage and biomass of SAV in the exclosures increased quickly and remained significantly higher than those in open sites over the duration of the experiment. Water quality also improved as turbidity, chlorophyll-a, total phosphorus and total nitrogen in the exclosures remained significantly lower than those in the open sites. After the in-situ experiment, a larger scale manipulation of fish in the entire submerged macrophyte zone (SMZ) was implemented from 2017 to 2020. After removing more than 2/3 of the benthivorous and herbivorous fish biomass by October 2020 in the SMZ, both the species richness and spatial coverage of SAV increased from 2 to 9 and from 1.7% to 32.2%, respectively. Our results provided clear evidence that fish are strong regulators of SAV productivity and that their reasonable removal facilitates ecological recovery. Therefore, we propose that fish community manipulation as implemented in this study be given more attention in addition to the reduction of external nutrient loading when designing projects to restore SAV in shallow lakes of the MLYRB.

Succession of submerged vegetation in a hydrologically reclaimed opencast mine during first 10 years

Succession of submerged vegetation was monitored from the early stages for a period of 10 years by Self‐Contained Underwater Breathing Apparatus (SCUBA) divers in Milada Lake. Milada Lake is the result of a flooded surface coal mine, the first large‐scale hydrological recultivation in the Czech Republic. The main focus was on apparent changes in the percentage cover of primary producers (macrophytes and macroalgae) during early successional stages. In total, data on 33 environmental factors (chemical, physical, and biological) were collected and the importance of their impact was statistically analyzed. Besides macrophytes, we also monitored macroalgae that mainly occupied deep areas (down to 12 m) and have been neglected in literature in contrast to macrophytes. Uniquely, the trophic state of the study site decreased in contrast to most water bodies worldwide. Changes in the plant species composition during succession, including replacement of the dominant group of Characeae with Vaucheria sp. (both macroalgae), correlated with a decrease in nitrate concentration, N–NO3. Furthermore, the herbivorous fish biomass and availability of uncovered lake bottom played important roles in the succession of macrophytes and macroalgae. Although some changes in submerged vegetation during succession of a newly created lake are to be expected, the main shifts observed apparently correlated with the studied factors. The conclusions may be useful for the prediction of succession in similarly created lakes in the future.

First insights into the impacts of benthic cyanobacterial mats on fish herbivory functions on a nearshore coral reef

Benthic cyanobacterial mats (BCMs) are becoming increasingly common on coral reefs. In Fiji, blooms generally occur in nearshore areas during warm months but some are starting to prevail through cold months. Many fundamental knowledge gaps about BCM proliferation remain, including their composition and how they influence reef processes. This study examined a seasonal BCM bloom occurring in a 17-year-old no-take inshore reef area in Fiji. Surveys quantified the coverage of various BCM-types and estimated the biomass of key herbivorous fish functional groups. Using remote video observations, we compared fish herbivory (bite rates) on substrate covered primarily by BCMs (> 50%) to substrate lacking BCMs (< 10%) and looked for indications of fish (opportunistically) consuming BCMs. Samples of different BCM-types were analysed by microscopy and next-generation amplicon sequencing (16S rRNA). In total, BCMs covered 51 ± 4% (mean ± s.e.m) of the benthos. Herbivorous fish biomass was relatively high (212 ± 36 kg/ha) with good representation across functional groups. Bite rates were significantly reduced on BCM-dominated substratum, and no fish were unambiguously observed consuming BCMs. Seven different BCM-types were identified, with most containing a complex consortium of cyanobacteria. These results provide insight into BCM composition and impacts on inshore Pacific reefs.

The Rabbitfish Siganus virgatus as Key Macroalgae Browser in Coral Reefs of the Gulf of Thailand

Coral reef resilience is greatly influenced by herbivory. There is a need to identify key fish species fulfilling this critical function in biogeographically distinct regions. This experimental in situ study investigated fish herbivory in coral reefs of the lower Gulf of Thailand characterized by a considerably low herbivorous fish biomass and diversity, but high live coral and low macroalgal cover. This provided an intriguing situation for macroalgal browsing research. Visual census techniques assessed the abundance of local herbivorous fish species, and filmed single-choice assays using the macroalga Turbinaria evaluated mass-standardized bites (ms-bites) and biomass removal. Multiple-choice assays offering four locally abundant macroalgae identified specific biomass removal and ms-bites to uncover selection and avoidance patterns of observed fish species. The rabbitfish Siganusvirgatus constituted only 39% of herbivore biomass but accounted for 90% of ms-bites. In multiple-choice assays, fishes took most (61%) bites on Sargassum, followed by Padina (28%) and Turbinaria (11%), while Lobophora was avoided. S. virgatus exhibited the most generalized browsing pattern of all species observed. Coinciding with recent studies, our findings suggest that S. virgatus plays a key functional role in reefs characterized by low diversity of herbivores and low functional redundancy.

O‘ahu’s marine protected areas have limited success in protecting coral reef herbivores

Herbivorous fishes and urchins can contribute to reef resilience by removing algae that are competitive with corals, yet herbivorous fishes are frequently targeted by fishers. The deleterious effects of fishing on coral reef herbivore populations are thought to be decreased through the establishment of no-take marine protected areas (MPAs). We conducted surveys to compare the biomass, diversity, and size structure of herbivorous reef fish within three functional guilds (i.e., grazers, scrapers, and browsers) and herbivorous urchin populations inside and outside of four of O‘ahu’s current MPAs to determine if they were effectively protecting herbivorous fishes and assess any resulting effects on sea urchin competitors. We also characterized the benthos, including the identification of common taxa in algal turfs, to evaluate interactions between herbivory and benthic communities. Urchins and grazing fishes accounted for the majority of herbivore biomass across regions of O‘ahu, and the dominance of urchins suggests that they significantly contribute to herbivory. Coral cover was a significant predictor of urchin biomass, but urchins did not have a significant relationship with herbivorous fish biomass or MPA protection. MPA protection and depth had positive effects on herbivorous fish biomass when examined in aggregate. However, these results were largely influenced by a large MPA protection effect in the Hanauma Bay MPA, compared to weaker MPA effects in Kāne‘ohe Bay, Waikīkī, and Pūpūkea. Hanauma Bay had stark differences in the condition of reef habitats between the MPA and fished areas which may also confound the interpretation of a strongly significant protection effect. Depth and habitat complexity were significant predictors of both grazer biomass and aggregate herbivore community composition, suggesting that these bottom-up factors may have a greater impact on herbivorous fish biomass than protection. Turf communities were speciose and displayed similar trends to herbivore communities across regions, but within-region trends in the Hanauma Bay region suggest that turf species composition is not only dictated by top-down factors. Within-guild fish diversity was low across site and region, suggesting herbivory was dominated by a single or few species. The small size structure and densities of herbivorous fishes in MPAs across all regions limits their population-level functional and reproductive contributions. While MPAs serve as a popular fisheries management tool for coral reef environments, the limited benefits of MPAs for herbivorous fish and urchin populations around O‘ahu suggest a general need for improved fisheries management and enforcement, especially prior to the consideration of an expansion of the existing MPA network.

Landscape-scale patterns of nutrient enrichment in a coral reef ecosystem: implications for coral to algae phase shifts.

Nutrient pollution is altering coastal ecosystems worldwide. On coral reefs, excess nutrients can favor the production of algae at the expense of reef-building corals, yet the role of nutrients in driving community changes such as shifts from coral to macroalgae is not well understood. Here we investigate the potential role of anthropogenic nutrient loading in driving recent coral-to-macroalgae phase shifts on reefs in the lagoons surrounding the Pacific island of Moorea, French Polynesia. We use nitrogen (N) tissue content and stable isotopes (δ15 N) in an abundant macroalga (Turbinaria ornata) together with empirical models of nutrient discharge to describe spatial and temporal patterns of nutrient enrichment in the lagoons. We then employ time series data to test whether recent increases in macroalgae are associated with nutrients. Our results revealed that patterns of N enrichment were linked to several factors, including rainfall, wave-driven circulation, and distance from anthropogenic nutrient sources, especially human sewage. Reefs near large watersheds, where inputs of N from sewage and agriculture are high, have been consistently enriched in N for at least the last decade. In many of these areas, corals have decreased and macroalgae have increased, while reefs with lower levels of N input have maintained high cover of coral and low cover of macroalgae. Importantly, these patchy phase shifts to macroalgae have occurred despite substantial island-wide increases in the density and biomass of herbivorous fishes over the time period. Together, these results indicate that nutrient loading may be an important driver of coral-to-macroalgae phase shifts in the lagoons of Moorea even though the reefs harbor an abundant and diverse herbivore assemblage. These results emphasize the important role that bottom-up factors can play in driving coral-to-macroalgae phase shifts and underscore the critical importance of watershed management for reducing inputs of nutrients and other land-based pollutants to coral reef ecosystems.

Negative trophic relationship between parrotfish biomass and algal cover on Philippine coral reefs

The importance of herbivory in preventing phase shifts to algal dominance on coral reefs is widely acknowledged. However, there is a paucity of information on which and how much herbivores are needed to effectively prevent an overgrowth of different algal types in natural environments. Our study showed the overall negative relationship between herbivorous fish biomass and benthic algal cover on Philippine coral reefs. The strongest negative relationships were between parrotfish biomass and both total algal and algal turf covers, and between total herbivore biomass and macroalgal cover. We further identified 4.0 mt/km2 and 2.0 mt/km2 for parrotfish, and 1.0 mt/km2 for all herbivores, as the potentially critical biomass levels needed to keep total algal, algal turf, and macroalgal covers, respectively, at low levels. The mean (±SE) cover of all algae was about 37.0 ± 0.7% in areas with ≥4.0 mt/km2 parrotfish biomass compared with 46.2 ± 0.8% and 44.0 ± 1.1% in areas with 2.0–3.9​ mt/km2 and <2.0 mt/km2 parrotfish biomass, respectively. Algal turf cover of 10.1 ± 0.7% in areas with <2.0 mt/km2 parrotfish biomass was almost twice as high as those with ≥2.0 mt/km2 parrotfish biomass. Macroalgal covers were only 4.7 ± 0.5% and 10.6 ± 1.1% in areas with 1.0–2.0 and >2.0 mt/km2 total herbivore biomass, respectively, compared with 39.8 ± 1.6% in those with <1.0 mt/km2 total herbivore biomass. Our findings support the hypothesis that phase shifts to algal dominance on coral reefs would not happen as long as there are no major losses of important herbivore species and herbivore biomass levels do not fall below critical levels.

Caribbean reefs of the Anthropocene: Variance in ecosystem metrics indicates bright spots on coral depauperate reefs.

Dramatic coral loss has significantly altered many Caribbean reefs, with potentially important consequences for the ecological functions and ecosystem services provided by reef systems. Many studies examine coral loss and its causes—and often presume a universal decline of ecosystem services with coral loss—rather than evaluating the range of possible outcomes for a diversity of ecosystem functions and services at reefs varying in coral cover. We evaluate 10 key ecosystem metrics, relating to a variety of different reef ecosystem functions and services, on 328 Caribbean reefs varying in coral cover. We focus on the range and variability of these metrics rather than on mean responses. In contrast to a prevailing paradigm, we document high variability for a variety of metrics, and for many the range of outcomes is not related to coral cover. We find numerous “bright spots,” where herbivorous fish biomass, density of large fishes, fishery value, and/or fish species richness are high, despite low coral cover. Although it remains critical to protect and restore corals, understanding variability in ecosystem metrics among low‐coral reefs can facilitate the maintenance of reefs with sustained functions and services as we work to restore degraded systems. This framework can be applied to other ecosystems in the Anthropocene to better understand variance in ecosystem service outcomes and identify where and why bright spots exist.

Habitat zonation on coral reefs: Structural complexity, nutritional resources and herbivorous fish distributions.

Distinct zonation of community assemblages among habitats is a ubiquitous feature of coral reefs. The distribution of roving herbivorous fishes (parrotfishes, surgeonfishes and rabbitfishes) is a particularly clear example, with the abundance of these fishes generally peaking in shallow-water, high-energy habitats, regardless of the biogeographic realm. Yet, our understanding of the factors which structure this habitat partitioning, especially with regards to different facets of structural complexity and nutritional resource availability, is limited. To address this issue, we used three-dimensional photogrammetry and structure-from-motion technologies to describe five components of structural complexity (rugosity, coral cover, verticality, refuge density and field-of-view) and nutritional resource availability (grazing surface area) among habitats and considered how these factors are related to herbivorous fish distributions. All complexity metrics (including coral cover) were highest on the slope and crest. Nutritional resource availability differed from this general pattern and peaked on the outer-flat. Unexpectedly, when compared to the distribution of herbivorous fishes, none of the complexity metrics had a marked influence in the models. However, grazing surface area was a strong predictor of both the abundance and biomass of herbivorous fishes. The strong relationship between grazing surface area and herbivorous fish distributions indicates that nutritional resource availability may be one of the primary factors driving the distribution of roving herbivorous fishes. The lack of a relationship between complexity and herbivorous fishes, and a strong affinity of herbivorous fishes for low-complexity, algal turf-dominated outer-flat habitats, offers some cautious optimism that herbivory may be sustained on future, low-complexity, algal turf-dominated reef configurations.