Consumers decrease variability across space and turnover through time during coral reef succession.

Consumers play an integral role in mediating ecological succession-the change in community composition over time. As consumer populations are facing rapid decline in ecosystems around the world, understanding of their ecological role is becoming increasingly urgent. Increased understanding of how changes in consumer populations may influence community variability across space and turnover through time during succession is particularly important for coral reefs, which are among the most threatened ecosystems globally, and where fishes play vital roles in structuring benthic succession. Here, we examine how consumers influence coral reef succession by deploying 180 paired settlement tiles, caged (to exclude fishes larger than approximately 15 cm) and uncaged, within Palmyra Atoll, a remote marine wildlife refuge with previously documented high fish abundance, and monitored benthic community development one and three years after deployment. We found that excluding large fishes lead to lower alpha diversity and divergent community states across space (i.e.,, high beta diversity among caged tiles), suggesting that benthic fish feeding maintains local diversity but tends to homogenize community composition with dominance by crustose coralline algae. In addition, when fish were experimentally excluded, the developing benthic community exhibited a greater change in species composition over time (i.e., high temporal beta diversity), indicating that fish feeding tends to canalize community successional trajectories. Finally, the caged and uncaged tiles became more similar over time, suggesting that fish feeding plays a more important role during early succession. Our results demonstrate that the loss of large fishes, for example from overfishing, may result in benthic communities that are more variable across space and time. Increased variability could have important implications for ecosystem function and coral reef resilience in the face of escalating global stressors.

Coupled beta diversity patterns among coral reef benthic taxa.

Unraveling the processes that drive diversity patterns remains a central challenge for ecology, and an increased understanding is especially urgent to address and mitigate escalating diversity loss. Studies have primarily focused on singular taxonomic groups, but recent research has begun evaluating spatial diversity patterns across multiple taxonomic groups and suggests taxa may have congruence in their diversity patterns. Here, we use surveys of the coral reef benthic groups: scleractinian corals, macroalgae, sponges and gorgonians conducted in the Bahamian Archipelago across 27 sites to determine if there is congruence between taxonomic groups in their site-level diversity patterns (i.e. alpha diversity: number of species, and beta diversity: differences in species composition) while accounting for environmental predictors (i.e. depth, wave exposure, market gravity (i.e. human population size and distance to market), primary productivity, and grazing). Overall, we found that the beta diversities of these benthic groups were significant predictors of each other. The most consistent relationships existed with algae and coral, as their beta diversity was a significant predictor of every other taxa's beta diversity, potentially due to their strong biotic interactions and dominance on the reef. Conversely, we found no congruence patterns in the alpha diversity of the taxa. Market gravity and exposure showed the most prevalent correlation with both alpha and beta diversity for the taxa. Overall, our results suggest that coral reef benthic taxa can have spatial congruence in species composition, but not number of species, and that future research on biodiversity trends should consider that taxa may have non-independent patterns.

Increased diversity and concordant shifts in community structure of coral-associated Symbiodiniaceae and bacteria subjected to chronic human disturbance.

Both coral-associated bacteria and endosymbiotic algae (Symbiodiniaceae spp.) are vitally important for the biological function of corals. Yet little is known about their co-occurrence within corals, how their diversity varies across coral species, or how they are impacted by anthropogenic disturbances. Here, we sampled coral colonies (n = 472) from seven species, encompassing a range of life history traits, across a gradient of chronic human disturbance (n = 11 sites on Kiritimati [Christmas] atoll) in the central equatorial Pacific, and quantified the sequence assemblages and community structure of their associated Symbiodiniaceae and bacterial communities. Although Symbiodiniaceae alpha diversity did not vary with chronic human disturbance, disturbance was consistently associated with higher bacterial Shannon diversity and richness, with bacterial richness by sample almost doubling from sites with low to very high disturbance. Chronic disturbance was also associated with altered microbial beta diversity for Symbiodiniaceae and bacteria, including changes in community structure for both and increased variation (dispersion) of the Symbiodiniaceae communities. We also found concordance between Symbiodiniaceae and bacterial community structure, when all corals were considered together, and individually for two massive species, Hydnophora microconos and Porites lobata, implying that symbionts and bacteria respond similarly to human disturbance in these species. Finally, we found that the dominant Symbiodiniaceae ancestral lineage in a coral colony was associated with differential abundances of several distinct bacterial taxa. These results suggest that increased beta diversity of Symbiodiniaceae and bacterial communities may be a reliable indicator of stress in the coral microbiome, and that there may be concordant responses to chronic disturbance between these communities at the whole-ecosystem scale.

Dynamic interplay: disentangling the temporal variability of fish effects on coral recruitment.

Ecosystems around the world are continuously undergoing recovery from anthropogenic disturbances like climate change, overexploitation, and habitat destruction. Coral reefs are a prime example of a threatened ecosystem and coral recruitment is a critical component of reef recovery from disturbances. Reef fishes structure this recruitment by directly consuming macroalgae and coral recruits or by indirectly altering the substrate to facilitate coral settlement (e.g., grazing scars). However, how these direct and indirect mechanisms vary through time remains largely unknown. Here, we quantified coral recruitment on settlement tiles with divots that mimic grazing scars and caging treatments to exclude or allow fish feeding over 3 years at Palmyra Atoll in the Pacific Ocean. We found that the positive and negative effects of fishes on coral recruitment varies through time. After 3 years, both grazing scars and fish grazing no longer predicted coral recruitment, suggesting that the role of fishes decreases over time. Our results emphasize that reef fish populations are important in promoting initial coral recovery after disturbances. However, over time, factors like the environment may become more important. Future work should continue to explore how the strength and direction of top-down control by consumers varies through time across multiple ecosystems.

Transformation of coral communities subjected to an unprecedented heatwave is modulated by local disturbance.

Corals are imminently threatened by climate change-amplified marine heatwaves. However, how to conserve coral reefs remains unclear, since those without local anthropogenic disturbances often seem equally or more susceptible to thermal stress as impacted ones. We disentangle this apparent paradox, revealing that the relationship between reef disturbance and heatwave impacts depends upon the scale of biological organization. We show that a tropical heatwave of globally unprecedented duration (~1 year) culminated in an 89% loss of hard coral cover. At the community level, losses depended on pre-heatwave community structure, with undisturbed sites, which were dominated by competitive corals, undergoing the greatest losses. In contrast, at the species level, survivorship of individual corals typically declined as local disturbance intensified. Our study reveals both that prolonged heatwaves projected under climate change will still have winners and losers and that local disturbance can impair survival of coral species even under such extreme conditions.

Global patterns and impacts of El Niño events on coral reefs: A meta-analysis.

Impacts of global climate change on coral reefs are being amplified by pulse heat stress events, including El Niño, the warm phase of the El Niño Southern Oscillation (ENSO). Despite reports of extensive coral bleaching and up to 97% coral mortality induced by El Niño events, a quantitative synthesis of the nature, intensity, and drivers of El Niño and La Niña impacts on corals is lacking. Herein, we first present a global meta-analysis of studies quantifying the effects of El Niño/La Niña-warming on corals, surveying studies from both the primary literature and International Coral Reef Symposium (ICRS) Proceedings. Overall, the strongest signal for El Niño/La Niña-associated coral bleaching was long-term mean temperature; bleaching decreased with decreasing long-term mean temperature (n = 20 studies). Additionally, coral cover losses during El Niño/La Niña were shaped by localized maximum heat stress and long-term mean temperature (n = 28 studies). Second, we present a method for quantifying coral heat stress which, for any coral reef location in the world, allows extraction of remotely-sensed degree heating weeks (DHW) for any date (since 1982), quantification of the maximum DHW, and the time lag since the maximum DHW. Using this method, we show that the 2015/16 El Niño event instigated unprecedented global coral heat stress across the world's oceans. With El Niño events expected to increase in frequency and severity this century, it is imperative that we gain a clear understanding of how these thermal stress anomalies impact different coral species and coral reef regions. We therefore finish with recommendations for future coral bleaching studies that will foster improved syntheses, as well as predictive and adaptive capacity to extreme warming events.