Coral species composition drives key ecosystem function on coral reefs

Rapid and unprecedented ecological change threatens the functioning and stability of ecosystems. On coral reefs, global climate change and local stressors are reducing and reorganizing habitat-forming corals and associated species, with largely unknown implications for critical ecosystem functions such as herbivory. Herbivory mediates coral–algal competition, thereby facilitating ecosystem recovery following disturbance such as coral bleaching events or large storms. However, relationships between coral species composition, the distribution of herbivorous fishes and the delivery of their functional impact are not well understood. Here, we investigate how herbivorous fish assemblages and delivery of two distinct herbivory processes, grazing and browsing, differ among three taxonomically distinct, replicated coral habitats. While grazing on algal turf assemblages was insensitive to different coral configurations, browsing on the macroalga Laurencia cf. obtusa varied considerably among habitats, suggesting that different mechanisms may shape these processes. Variation in browsing among habitats was best predicted by the composition and structural complexity of benthic assemblages (in particular the cover and composition of corals, but not macroalgal cover), and was poorly reflected by visual estimates of browser biomass. Surprisingly, the lowest browsing rates were recorded in the most structurally complex habitat, with the greatest cover of coral (branching Porites habitat). While the mechanism for the variation in browsing is not clear, it may be related to scale-dependent effects of habitat structure on visual occlusion inhibiting foraging activity by browsing fishes, or the relative availability of alternate dietary resources. Our results suggest that maintained functionality may vary among distinct and emerging coral reef configurations due to ecological interactions between reef fishes and their environment determining habitat selection

Nitrogen enrichment in macroalgae following mass coral mortality

Scleractinian corals are engineers on coral reefs that provide both structural complexity as habitat and sustenance for other reef-associated organisms via the release of organic and inorganic matter. However, coral reefs are facing multiple pressures from climate change and other stressors, which can result in mass coral bleaching and mortality events. Mass mortality of corals results in enhanced release of organic matter, which can cause significant alterations to reef biochemical and recycling processes. There is little known about how long these nutrients are retained within the system, for instance, within the tissues of other benthic organisms. We investigated changes in nitrogen isotopic signatures (δ15N) of macroalgal tissues (a) ~ 1 year after a bleaching event in the Seychelles and (b) ~ 3 months after the peak of a bleaching event in Mo’orea, French Polynesia. In the Seychelles, there was a strong association between absolute loss in both total coral cover and branching coral cover and absolute increase in macroalgal δ15N between 2014 and 2017 (adjusted r2 = 0.79, p = 0.004 and adjusted r2 = 0.86, p = 0.002, respectively). In Mo’orea, a short-term transplant experiment found a significant increase in δ15N in Sargassum mangarevense after specimens were deployed on a reef with high coral mortality for ~ 3 weeks (p < 0.05). We suggest that coral-derived nutrients can be retained within reef nutrient cycles, and that this can affect other reef-associated organisms over both short- and long-term periods, especially opportunistic species such as macroalgae. These species could therefore proliferate on reefs that have experienced mass mortality events, because they have been provided with both space and nutrient subsidies by the death and decay of corals. © 2021, The Author(s)

Coral reef ecosystem services in the Anthropocene

Coral reefs underpin a range of ecosystem goods and services that contribute to the well‐being of millions of people. However, tropical coral reefs in the Anthropocene are likely to be functionally different from reefs in the past. In this perspective piece, we ask, what does the Anthropocene mean for the provision of ecosystem services from coral reefs? First, we provide examples of the provisioning, regulating, cultural and supporting services underpinned by coral reef ecosystems. We conclude that coral reef ecosystem service research has lagged behind multidisciplinary advances in broader ecosystem services science, such as an explicit recognition that interactions between social and ecological systems underpin ecosystem services. Second, drawing on tools from functional ecology, we outline how these social–ecological relationships can be incorporated into a mechanistic understanding of service provision and how this might be used to anticipate future changes in coral reef ecosystem services. Finally, we explore the emergence of novel reef ecosystem services, for example from tropicalized coastlines, or through changing technological connections to coral reefs. Indeed, when services are conceived as coming from social–ecological system dynamics, novelty in services can emerge from elements of the interactions between people and the ecosystem. This synthesis of the coral reef ecosystem services literature suggests the field is poorly prepared to understand the changing service provision anticipated in the Anthropocene. A new research agenda is needed that better connects reef functional ecology to ecosystem service provision. This research agenda should embrace more holistic approaches to ecosystem service research, recognizing them as co‐produced by ecosystems and society. Importantly, the likelihood of novel ecosystem service configurations requires further conceptualization and empirical assessment. As with current ecosystem services, the loss or gain of services will not affect all people equally and must be understood in the context in which they occur. With the uncertainty surrounding the future of coral reefs in the Anthropocene, research exploring how the benefits to people change will be of great importance

Spatial scaling properties of coral reef benthic communities

The spatial structure of ecological communities on tropical coral reefs across seascapes and geographies have historically been poorly understood. Here we addressed this for the first time using spatially expansive and thematically resolved benthic community data collected around five uninhabited central Pacific oceanic islands, spanning 6° latitude and 17° longitude. Using towed-diver digital image surveys over ~140 linear km of shallow (8–20 m depth) tropical reef, we highlight the autocorrelated nature of coral reef seascapes. Benthic functional groups and hard coral morphologies displayed significant spatial clustering (positive autocorrelation) up to kilometre-scales around all islands, in some instances dominating entire sections of coastline. The scale and strength of these autocorrelation patterns showed differences across geographies, but patterns were more similar between islands in closer proximity and of a similar size. For example, crustose coralline algae (CCA) were clustered up to scales of 0.3 km at neighbouring Howland and Baker Islands and macroalgae were spatially clustered at scales up to ~3 km at both neighbouring Kingman Reef and Palmyra Atoll. Of all the functional groups, macroalgae had the highest levels of spatial clustering across geographies at the finest resolution of our data (100 m). There were several cases where the upper scale at which benthic community members showed evidence of spatial clustering correlated highly with the upper scales at which concurrent gradients in physical environmental drivers were spatially clustered. These correlations were stronger for surface wave energy than subsurface temperature (regardless of benthic group) and turf algae and CCA had the closest alignments in scale with wave energy across functional groups and geographies. Our findings suggest such physical drivers not only limit or promote the abundance of various benthic competitors on coral reefs, but also play a key role in governing their spatial scaling properties across seascapes

Guiding coral reef futures in the Anthropocene

Anthropogenic changes to the Earth now rival those caused by the forces of nature and have shepherded us into a new planetary epoch – the Anthropocene. Such changes include profound and often unexpected alterations to coral reef ecosystems and the services they provide to human societies. Ensuring that reefs and their services endure during the Anthropocene will require that key drivers of coral reef change – fishing, water quality, and anthropogenic climate change – stay within acceptable levels or “safe operating spaces”. The capacity to remain within these safe boundaries hinges on understanding the local, but also the increasingly global and cross-scale, socioeconomic causes of these human drivers of change. Consequently, local and regional management efforts that are successful in the short term may ultimately fail if current decision making and institution-building around coral reef systems remains fragmented, poorly coordinated, and unable to keep pace with the escalating speed of social, technological, and ecological change

Tropical coastal fish

Climate change will affect populations and communities of marine fishes in many ways, ranging from indirect effects associated with habitat degradation and altered resource availability to direct effects of rapidly changing environmental conditions. In the short-term (up to 2030), the impact of climate change on Australia’s tropical coastal and demersal fishes is largely tied to the fate of critical benthic habitats, especially for coral reef environments, which are highly vulnerable to elevated temperature and ocean acidification. There is good evidence and high consensus that climate-induced coral bleaching affects the community structure and abundance of reef-associated fishes, especially when it leads to the structural collapse of reef habitat. In the longer-term (after 2030), sea level rise and altered rainfall patterns will also significantly alter coastal wetlands that are important nursery areas for estuarine and nearshore species. In addition to the effects of habitat degradation, warmer ocean temperatures will cause distributional shifts in some tropical fishes, increasing the geographic ranges of some species and decreasing the ranges of others, including some commercially important species. Life history traits and population dynamics will be affected by warmer temperatures, with potential implications for fisheries yields. Altered oceanic circulation and ocean acidification could have very significant effects on populations and communities of coastal fishes. However, these impacts are still poorly understood and are likely to become most apparent in the longer term. There are a many critical knowledge gaps in our understanding of the effect of climate change on tropical marine fish, including the impact of warmer temperatures on adult reproduction, and the development, survival and behaviour of larvae; the effect of ocean acidification on the development, survival and behaviour; and the degree to which fish will acclimate or adapt to the expected rapid climate change. Non-reefal environments and commercially important species are especially understudied in relation to climate change impacts. Key strategies in mitigating effects of climate change on coastal marine fishes are to maintain and restore habitat quality, incorporate climate uncertainty into fisheries management plans, and limit impacts of other human activities in coastal regions

Reef fishes weaken dietary preferences after coral mortality, altering resource overlap

The direct and indirect effects of climate change can affect, and are mediated by, changes in animal behaviour. However, we often lack sufficient empirical data to assess how large-scale disturbances affect the behaviour of individuals, which scales up to influence communities. Here, we investigate these patterns by focusing on the foraging behaviour of butterflyfishes, prominent coral-feeding fishes on coral reefs, before and after a mass coral bleaching event in Iriomote, Japan. In response to 65% coral mortality, coral-feeding fishes broadened their diets, showing a significant weakening of dietary preferences across species. Multiple species reduced their consumption of bleaching-sensitive Acropora corals, while expanding their diets to consume a variety of other coral genera. This resulted in decreased dietary overlap among butterflyfishes. Behavioural changes in response to bleaching may increase resilience of coral reef fishes in the short term. However, coral mortality has reduced populations of coral-feeders world-wide, indicating the changes in feeding behaviour we document here may not be sufficient to ensure long-term resilience of butterflyfishes on coral reefs

Maximizing regional biodiversity requires a mosaic of protection levels

Protected areas are the flagship management tools to secure biodiversity from anthropogenic impacts. However, the extent to which adjacent areas with distinct protection levels host different species numbers and compositions remains uncertain. Here, using reef fishes, European alpine plants, and North American birds, we show that the composition of species in adjacent Strictly Protected, Restricted, and Non-Protected areas is highly dissimilar, whereas the number of species is similar, after controlling for environmental conditions, sample size, and rarity. We find that between 12% and 15% of species are only recorded in Non-Protected areas, suggesting that a non-negligible part of regional biodiversity occurs where human activities are less regulated. For imperiled species, the proportion only recorded in Strictly Protected areas reaches 58% for fishes, 11% for birds, and 7% for plants, highlighting the fundamental and unique role of protected areas and their environmental conditions in biodiversity conservation