The inherent complexity of high‐diversity systems can make them particularly difficult to understand. The relatively recent introduction of functional approaches, which seek to infer ecosystem functioning based on species’ ecological traits, has revolutionized our understanding of these high‐diversity systems. Today, the functional structure of an assemblage is widely regarded as a key indicator of the status or resilience of an ecosystem. Indeed, functional evaluations have become a mainstay of monitoring and management approaches. But is the heavy focus on broad metrics of functional structure grounded in empirical research? On tropical coral reefs, the ocean’s most diverse ecosystems, remarkably few studies directly quantify ecosystem functions and the term ‘function’ is widely used but rarely defined, especially when applied to reef fishes. Our review suggests that most ‘functional’ studies do not study function as it relates to ecological processes. Rather, they look at easy‐to‐measure traits or proxies that are thought to have functional significance. However, these links are rarely tested empirically, severely limiting our capacity to extend results from community structure to the dynamic processes operating within high‐diversity ecosystems such as coral reefs. With rapid changes in global ecosystems, and in their capacity to deliver ecosystem services, there is an urgent need to understand and empirically measure the role of organisms in various ecosystem functions. We suggest that if we are to understand and manage transitioning coral reefs in the Anthropocene, a broad definition of the word ‘function’ is needed along with a focus on ecological processes and the empirical quantification of functional roles. In this review, we propose a universal operational definition of the term ‘function’ that works from a cellular to a global level. Specifically, it is the movement or storage of energy or material. Within this broad definitional framework, all functions are part of a continuum that is tied together by the process‐based unifier of material fluxes. With this universal definition at hand, we then present a path forward that will allow us to fully harness the power of functional approaches in understanding and managing high‐diversity systems such as coral reefs. A plain language summary is available for this article.

The reef flat is one of the largest and most distinctive habitats on coral reefs, yet its role in reef trophodynamics is poorly understood. Evolutionary evidence suggests that reef flat colonization by grazing fishes was a major innovation that permitted the exploitation of new space and trophic resources. However, the reef flat is hydrodynamically challenging, subject to high predation risks and covered with sediments that inhibit feeding by grazers. To explore these opposing influences, we examine the Great Barrier Reef (GBR) as a model system. We focus on grazing herbivores that directly access algal primary productivity in the epilithic algal matrix (EAM). By assessing abundance, biomass, and potential fish productivity, we explore the potential of the reef flat to support key ecosystem processes and its ability to maintain fisheries yields. On the GBR, the reef flat is, by far, the most important habitat for turf‐grazing fishes, supporting an estimated 79% of individuals and 58% of the total biomass of grazing surgeonfishes, parrotfishes, and rabbitfishes. Approximately 59% of all (reef‐wide) turf algal productivity is removed by reef flat grazers. The flat also supports approximately 75% of all grazer biomass growth. Our results highlight the evolutionary and ecological benefits of occupying shallow‐water habitats (permitting a ninefold population increase). The acquisition of key locomotor and feeding traits has enabled fishes to access the trophic benefits of the reef flat, outweighing the costs imposed by water movement, predation, and sediments. Benthic assemblages on reefs in the future may increasingly resemble those seen on reef flats today, with low coral cover, limited topographic complexity, and extensive EAM. Reef flat grazing fishes may therefore play an increasingly important role in key ecosystem processes and in sustaining future fisheries yields.

Record-breaking temperatures between and led to unprecedented pan-tropical bleaching of scleractinian corals. On the Great Barrier Reef (GBR), the effects were most pronounced in the remote, northern region, where over 90 % of reefs exhibited bleaching.Mass bleaching that results in widespread coral mortality represents a major disturbance event for reef organisms, including reef fishes. Using 133 replicate 1 m 2 quadrats, we quantified short-term changes in coral communities and spatially associated reef fish assemblages, at Lizard Island, Australia, in response to the 2016 mass bleaching event.Quadrats were spatially matched, permitting repeated sampling of fish and corals in the same areas: before, during and 6 months after mass bleaching. As expected, we documented a significant decrease in live coral cover. Subsequent decreases in fish abundance were primarily driven by coral-associated damselfishes. However, these losses, were relatively minor (37% decrease) and compared to the magnitude of Acropora loss (> 95% relative decrease). Furthermore, at a local, 1 m 2 scale, we documented a strong spatial mismatch between fish and coral loss. Post-bleaching fish losses were not highest in quadrats that experienced the greatest loss of live coral. Nor were fish losses associated with a proliferation of cyanobacteria. Several sites did, however, exhibit increases in fish abundance suggesting substantial spatial movements. These results challenge common assumptions and emphasize the need for caution when ascribing causality to observed patterns of fish loss at larger spatial scales. Our results highlight the potential for short-term resilience to climate change, in fishes, through local migration and habitat plasticity.

The removal of macroalgal biomass by fishes is a key process on coral reefs. Numerous studies have identified the fish species responsible for removing mature macroalgae, and have identified how this varies spatially, temporally, and among different algal types. None, however, have considered the behavioural and morphological traits of the browsing fishes and how this may influence the removal of macroalgal material. Using video observations of fish feeding on the brown macroalga Sargassum polycystum, we quantified the feeding behaviour and morphology of the four dominant browsing species on the Great Barrier Reef (Kyphosus vaigiensis, Naso unicornis, Siganus canaliculatus, and Siganus doliatus). The greatest distinction between species was the algal material they targeted. K. vaigiensis and N. unicornis bit on the entire macroalgal thallus in approximately 90 % of bites. In contrast, Si. canaliculatus and Si. doliatus avoided biting the stalks, with 80-98 % of bites being on the macroalgal leaves only. This distinctive grouping into 'entire thallusbiters' versus 'leaf-biters' was not supported by size-standardized measures of biting morphology. Rather, speciesspecific adult body sizes, tooth shape, and feeding behaviour appear to underpin this functional distinction, with adults of the two larger fish species (N. unicornis and K. vaigiensis) eating the entire macroalgal thallus, while the two smaller species (Si. canaliculatus and Si. doliatus) bite only leaves. These findings caution against assumed homogeneity within this, and potentially other, functional groups on coral reefs. As functional redundancy within the macroalgal browsers is limited, the smaller 'leaf-biting' species are unlikely to be able to compensate functionally for the loss of larger 'entire thallus-biting' species.