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Global warming is markedly changing diverse coral reef ecosystems through an increasing frequency and magnitude of mass bleaching events 1 – 3 . How local impacts scale up across affected regions depends on numerous factors, including patchiness in coral mortality, metabolic effects of extreme temperatures on populations of reef-dwelling species 4 and interactions between taxa. Here we use data from before and after the 2016 mass bleaching event to evaluate ecological changes in corals, algae, fishes and mobile invertebrates at 186 sites along the full latitudinal span of the Great Barrier Reef and western Coral Sea. One year after the bleaching event, reductions in live coral cover of up to 51% were observed on surveyed reefs that experienced extreme temperatures; however, regional patterns of coral mortality were patchy. Consistent declines in coral-feeding fishes were evident at the most heavily affected reefs, whereas few other short-term responses of reef fishes and invertebrates could be attributed directly to changes in coral cover. Nevertheless, substantial region-wide ecological changes occurred that were mostly independent of coral loss, and instead appeared to be linked directly to sea temperatures. Community-wide trophic restructuring was evident, with weakening of strong pre-existing latitudinal gradients in the diversity of fishes, invertebrates and their functional groups. In particular, fishes that scrape algae from reef surfaces, which are considered to be important for recovery after bleaching 2 , declined on northern reefs, whereas other herbivorous groups increased on southern reefs. The full impact of the 2016 bleaching event may not be realized until dead corals erode during the next decade 5 , 6 . However, our short-term observations suggest that the recovery processes, and the ultimate scale of impact, are affected by functional changes in communities, which in turn depend on the thermal affinities of local reef-associated fauna. Such changes will vary geographically, and may be particularly acute at locations where many fishes and invertebrates are close to their thermal distribution limits 7 . Fish and invertebrate communities transformed across the span of the Great Barrier Reef following the 2016 bleaching event due to a decline in coral-feeding fishes resulting from coral loss, and because of different regional responses of key trophic groups to the direct effect of temperature.

Fishes are the most diverse group of vertebrates, play key functional roles in aquatic ecosystems, and provide protein for a billion people, especially in the developing world. Those functions are compromised by mounting pressures on marine biodiversity and ecosystems. Because of its economic and food value, fish biomass production provides an unusually direct link from biodiversity to critical ecosystem services. We used the Reef Life Survey’s global database of 4,556 standardized fish surveys to test the importance of biodiversity to fish production relative to 25 environmental drivers. Temperature, biodiversity, and human influence together explained 47% of the global variation in reef fish biomass among sites. Fish species richness and functional diversity were among the strongest predictors of fish biomass, particularly for the large-bodied species and carnivores preferred by fishers, and these biodiversity effects were robust to potentially confounding influences of sample abundance, scale, and environmental correlations. Warmer temperatures increased biomass directly, presumably by raising metabolism, and indirectly by increasing diversity, whereas temperature variability reduced biomass. Importantly, diversity and climate interact, with biomass of diverse communities less affected by rising and variable temperatures than species-poor communities. Biodiversity thus buffers global fish biomass from climate change, and conservation of marine biodiversity can stabilize fish production in a changing ocean.

Understanding species’ roles in food webs requires an accurate assessment of their trophic niche. However, it is challenging to delineate potential trophic interactions across an ecosystem, and a paucity of empirical information often leads to inconsistent definitions of trophic guilds based on expert opinion, especially when applied to hyperdiverse ecosystems. Using coral reef fishes as a model group, we show that experts disagree on the assignment of broad trophic guilds for more than 20% of species, which hampers comparability across studies. Here, we propose a quantitative, unbiased, and reproducible approach to define trophic guilds and apply recent advances in machine learning to predict probabilities of pairwise trophic interactions with high accuracy. We synthesize data from community-wide gut content analyses of tropical coral reef fishes worldwide, resulting in diet information from 13,961 individuals belonging to 615 reef fish. We then use network analysis to identify 8 trophic guilds and Bayesian phylogenetic modeling to show that trophic guilds can be predicted based on phylogeny and maximum body size. Finally, we use machine learning to test whether pairwise trophic interactions can be predicted with accuracy. Our models achieved a misclassification error of less than 5%, indicating that our approach results in a quantitative and reproducible trophic categorization scheme, as well as high-resolution probabilities of trophic interactions. By applying our framework to the most diverse vertebrate consumer group, we show that it can be applied to other organismal groups to advance reproducibility in trait-based ecology. Our work thus provides a viable approach to account for the complexity of predator–prey interactions in highly diverse ecosystems.

The multifaceted effects of climate change on physical and biogeochemical processes are rapidly altering marine ecosystems but often are considered in isolation, leaving our understanding of interactions between these drivers of ecosystem change relatively poor. This is particularly true for shallow coastal ecosystems, which are fuelled by a combination of distinct pelagic and benthic energy pathways that may respond to climate change in fundamentally distinct ways. The fish production supported by these systems is likely to be impacted by climate change differently to those of offshore and shelf ecosystems, which have relatively simpler food webs and mostly lack benthic primary production sources. We developed a novel, multispecies size spectrum model for shallow coastal reefs, specifically designed to simulate potential interactive outcomes of changing benthic and pelagic energy inputs and temperatures and calculate the relative importance of these variables for the fish community. Our model, calibrated using field data from an extensive temperate reef monitoring program, predicts that changes in resource levels will have much stronger impacts on fish biomass and yields than changes driven by physiological responses to temperature. Under increased plankton abundance, species in all fish trophic groups were predicted to increase in biomass, average size, and yields. By contrast, changes in benthic resources produced variable responses across fish trophic groups. Increased benthic resources led to increasing benthivorous and piscivorous fish biomasses, yields, and mean body sizes, but biomass decreases among herbivore and planktivore species. When resource changes were combined with warming seas, physiological responses generally decreased species’ biomass and yields. Our results suggest that understanding changes in benthic production and its implications for coastal fisheries should be a priority research area. Our modified size spectrum model provides a framework for further study of benthic and pelagic energy pathways that can be easily adapted to other ecosystems.

As ocean temperatures rise, species distributions are tracking towards historically cooler regions in line with their thermal affinity1,2. However, different responses of species to warming and changed species interactions make predicting biodiversity redistribution and relative abundance a challenge3,4. Here, we use three decades of fish and plankton survey data to assess how warming changes the relative dominance of warm-affinity and cold-affinity species5,6. Regions with stable temperatures (for example, the Northeast Pacific and Gulf of Mexico) show little change in dominance structure, while areas with warming (for example, the North Atlantic) see strong shifts towards warm-water species dominance. Importantly, communities whose species pools had diverse thermal affinities and a narrower range of thermal tolerance showed greater sensitivity, as anticipated from simulations. The composition of fish communities changed less than expected in regions with strong temperature depth gradients. There, species track temperatures by moving deeper2,7, rather than horizontally, analogous to elevation shifts in land plants8. Temperature thus emerges as a fundamental driver for change in marine systems, with predictable restructuring of communities in the most rapidly warming areas using metrics based on species thermal affinities. The ready and predictable dominance shifts suggest a strong prognosis of resilience to climate change for these communities.

Global reef fish diversity is studied with metrics incorporating species abundances and functional traits; these identify diversity hotspots corresponding to the diversity of functional traits amongst individuals in the community, and greater evenness in the abundance of reef fishes at higher latitudes, findings that contrast with patterns reported previously using traditional richness-based methods. Cooler biodiversity hotspots revealed Traditional measures of biodiversity record species richness across different areas — in other words, they just count the number of species. This approach takes no account of the fact that different species will have different abundances, or that the range of functional traits present in a community is not dependent solely on the number of species. This paper presents a new measure of functional diversity, incorporating species abundances and functional traits into a global census of a vertebrate group —2,473 marine reef fish species — at 1,844 sites. The results reveal previously unknown diversity hotspots in temperate regions and in the Tropical Eastern Pacific, which are outside the species-rich tropical regions traditionally associated with high biodiversity. Species richness has dominated our view of global biodiversity patterns for centuries 1 , 2 . The dominance of this paradigm is reflected in the focus by ecologists and conservation managers on richness and associated occurrence-based measures for understanding drivers of broad-scale diversity patterns and as a biological basis for management 3 , 4 . However, this is changing rapidly, as it is now recognized that not only the number of species but the species present, their phenotypes and the number of individuals of each species are critical in determining the nature and strength of the relationships between species diversity and a range of ecological functions (such as biomass production and nutrient cycling) 5 . Integrating these measures should provide a more relevant representation of global biodiversity patterns in terms of ecological functions than that provided by simple species counts. Here we provide comparisons of a traditional global biodiversity distribution measure based on richness with metrics that incorporate species abundances and functional traits. We use data from standardized quantitative surveys of 2,473 marine reef fish species at 1,844 sites, spanning 133 degrees of latitude from all ocean basins, to identify new diversity hotspots in some temperate regions and the tropical eastern Pacific Ocean. These relate to high diversity of functional traits amongst individuals in the community (calculated using Rao’s Q 6 ), and differ from previously reported patterns in functional diversity and richness for terrestrial animals, which emphasize species-rich tropical regions only 7 , 8 . There is a global trend for greater evenness in the number of individuals of each species, across the reef fish species observed at sites (‘community evenness’), at higher latitudes. This contributes to the distribution of functional diversity hotspots and contrasts with well-known latitudinal gradients in richness 2 , 4 . Our findings suggest that the contribution of species diversity to a range of ecosystem functions varies over large scales, and imply that in tropical regions, which have higher numbers of species, each species contributes proportionally less to community-level ecological processes on average than species in temperate regions. Metrics of ecological function usefully complement metrics of species diversity in conservation management, including when identifying planning priorities and when tracking changes to biodiversity values.

Fish biodiversity can be measured by capturing and then sequencing free DNA present in water. Such environmental DNA (eDNA) methods offer an effective, noninvasive tool for species diversity measurement, although standardized protocols are not yet developed. We investigate how metrics of fish biodiversity revealed through eDNA analysis of water are influenced by sampling volume. Water samples were collected from the intertidal reef of Browse Island, a tropical, remote island in the Timor Sea. Aliquots from a single 20,700‐ml sample and multiple 2,000‐ml samples were filtered in various volumes (25–2,000 ml) across two membrane sizes (0.20 and 0.45 µm). A fish metabarcoding assay was used to characterize the fish diversity within aliquots. All samples, except one, yielded fish DNA sequences. Two hundred and nine operational taxonomic units (cf. species) representing 48 fish families were identified from the complete collection of DNA contained in all samples, comparable to the 200 fish species detected using conventional surveys at this location. Notable additions from eDNA methods were cryptic and nocturnal fish species. Nevertheless, large differences in taxonomic composition (<60% species overlap) between aliquots of identical volumes demonstrate that eDNA in seawater is patchy and that estimates of biodiversity are strongly influenced by the volume of water filtered. We suggest that eDNA studies maximize water volumes as much as logistically possible if the aim is to detect the greatest number of taxa and that species accumulation curves be provided as an indication of sampling adequacy. We investigate how metrics of fish biodiversity revealed through eDNA analysis of water are influenced by sampling volume. We demonstrate that eDNA in seawater is patchy and that estimates of biodiversity are strongly influenced by the volume of water filtered. We suggest that eDNA studies maximize water volumes as much as logistically possible if the aim is to detect the greatest number of taxa, and that species accumulation curves be provided as an indication of sampling adequacy.

The assessment of patterns in macroecology, including those most relevant to global biodiversity conservation, has been hampered by a lack of quantitative data collected in a consistent manner over the global scale. Global analyses of species’ abundance data typically rely on records aggregated from multiple studies where different sampling methods and varying levels of taxonomic and spatial resolution have been applied. Here we describe the Reef Life Survey (RLS) reef fish dataset, which contains 134,759 abundance records, of 2,367 fish taxa, from 1,879 sites in coral and rocky reefs distributed worldwide. Data were systematically collected using standardized methods, offering new opportunities to assess broad-scale spatial patterns in community structure. The development of such a large dataset was made possible through contributions of investigators associated with science and conservation agencies worldwide, and the assistance of a team of over 100 recreational SCUBA divers, who undertook training in scientific techniques for underwater surveys and voluntarily contributed skills, expertise and their time to data collection. Design Type(s) observation design Measurement Type(s) fish species abundance Technology Type(s) observational method Factor Type(s) Sample Characteristic(s) ocean • marine reef Machine-accessible metadata file describing the reported data (ISA-Tab format)

Human activities are altering the structure of ecological communities, often favouring generalists over specialists. For reef fishes, increasingly degraded habitats and climate-driven range shifts may independently augment generalization, particularly if fishes with least-specific habitat requirements are more likely to shift geographic ranges to track their thermal niche. Using a unique global dataset on temperate and tropical reef fishes and habitat composition, we calculated a species generalization index that empirically estimates the habitat niche breadth of each fish species. We then applied the species generalization index to evaluate potential impacts of habitat loss and range shifts across large scales, on coral and rocky reefs. Our analyses revealed consistent habitat-induced shifts in community structure that favoured generalist fishes following regional coral mortality events and between adjacent sea urchin barrens and kelp habitats. Analysis of the distribution of tropical fishes also identified the species generalization index as the most important trait in predicting their poleward range extent, more so than body or range size. Generalist tropical reef fishes penetrate further into subtropical and temperate zones than specialists. Dynamic responses of reef fishes to habitat degradation imply loss of specialists at local scales, while generalists will be broadly favoured under intensifying anthropogenic pressures. An increased focus on individual requirements of specialists could provide useful guidance for species threat assessments and conservation actions, while ecosystem and multi-species fisheries models should recognize increasing prevalence of generalists. Using a species generalization index calculated from a global dataset of reef fishes and their habitats, the authors show that generalist species respond more successfully to habitat disruption and are better able to move polewards in response to climate change.

Changing biodiversity alters ecosystem functioning in nature, but the degree to which this relationship depends on the taxonomic identities rather than the number of species remains untested at broad scales. Here, we partition the effects of declining species richness and changing community composition on fish community biomass across >3000 coral and rocky reef sites globally. We find that high biodiversity is 5.7x more important in maximizing biomass than the remaining influence of other ecological and environmental factors. Differences in fish community biomass across space are equally driven by both reductions in the total number of species and the disproportionate loss of larger-than-average species, which is exacerbated at sites impacted by humans. Our results confirm that sustaining biomass and associated ecosystem functions requires protecting diversity, most importantly of multiple large-bodied species in areas subject to strong human influences. Species identity and richness both contribute biodiversity-ecosystem functioning relationships. Here the authors apply a decomposition approach inspired by the Price equation to a global dataset of reef fish community biomass, finding that increased richness and community compositions favouring large-bodied species enhance biomass.