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1. Patterns and implications of spatial covariation in herbivore functions on resilience of coral reefs

   https://doi.org/10.1038/s41598-024-83672-1  

   Cook, Dana T; Holbrook, Sally J; Schmitt, Russell J (December 2025, Scientific Reports)

   Abstract Persistent shifts to undesired ecological states, such as shifts from coral to macroalgae, are becoming more common. This highlights the need to understand processes that can help restore affected ecosystems. Herbivory on coral reefs is widely recognized as a key interaction that can keep macroalgae from outcompeting coral. Most attention has been on the role ‘grazing’ herbivores play in preventing the establishment of macroalgae, while less research has focused on the role of ‘browsers’ in extirpating macroalgae. Here we explored patterns, environmental correlates and state shift consequences of spatial co-variation in grazing and browsing functions of herbivorous fishes. Grazing and browsing rates were not highly correlated across 20 lagoon sites in Moorea, French Polynesia, but did cluster into 3 (of 4) combinations of high and low consumption rates (no site had low grazing but high browsing). Consumption rates were not correlated with grazer or browser fish biomass, but both were predicted by specific environmental variables. Experiments revealed that reversibility of a macroalgal state shift was strongly related to spatial variation in browsing intensity. Our findings provide insights and simple diagnostic tools regarding heterogeneity in top-down forcing that influences the vulnerability to and reversibility of shifts to macroalgae on coral reefs. 

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2. Modeling the effects of selectively fishing key functional groups of herbivores on coral resilience

   https://doi.org/10.1002/ecs2.4749  

   Cook, Dana T; Schmitt, Russell J; Holbrook, Sally J; Moeller, Holly V (January 2024, Ecosphere)

   Abstract Mounting evidence suggests that fishing can be a major driver of coral‐to‐macroalgae regime shifts on tropical reefs. In many small‐scale coral reef fisheries, fishers target herbivorous fishes, which can weaken coral resilience via reduced herbivory on macroalgae that then outcompete corals. Previous models that explored the effects of harvesting herbivores revealed hysteresis in the herbivory–benthic state relationship that results in bistability of coral‐ and macroalgae‐dominated states over some levels of fishing pressure, which has been supported by empirical evidence. However, past models have not accounted for the functional differences among herbivores or how fisher selectivity for different herbivore functional groups may alter the benthic dynamics and resilience. Here, we use a dynamic model that links differential fishing on two key herbivore functional groups to the outcome of competitive dynamics between coral and macroalgae. We show that reef state depends not only on the level of fishing but also on the types of herbivores targeted by fishers. Selectively fishing browsing herbivores that are capable of consuming mature macroalgae (e.g., unicornfish) increases precariousness of the coral state by moving the system close to the coral‐to‐macroalgae tipping point. By contrast, selectively harvesting grazing herbivores that are only capable of preventing macroalgae from becoming established (e.g., parrotfishes) can increase catch yields substantially more before the tipping point is reached. However, this lower precariousness with increasing fishing effort comes at the cost of increasing the range of fishing effort over which coral and macroalgae are bistable; increasing hysteresis makes a regime shift triggered by a disturbance more difficult or impractical to reverse. Our results suggest that management strategies for small‐scale coral reef fisheries should consider how functional differences among harvested herbivores coupled with fisher selectivity influence benthic dynamics in light of the trade‐off between tipping point precariousness and coral recovery dynamics following large disturbances. 

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3. Herbivorous sea urchins (Echinometra mathaei) support resilience on overfished and sedimented tropical reefs

   https://doi.org/10.1038/s41598-024-52222-0  

   Fong, Caitlin R; Smith, Nefertiti; Catalan, Elijah; Caraveo, Blanca Alvarez; Barber, Paul H; Fong, Peggy (December 2024, Scientific Reports)

   Abstract Human impacts are dramatically changing ecological communities, motivating research on resilience. Tropical reefs are increasingly undergoing transitions to short algal turf, a successional community that mediates either recovery to coral by allowing recruitment or transitions to longer turf/macroalgae. Intense herbivory limits turf height; subsequently, overfishing erodes resilience of the desirable coral-dominated reef state. Increased sedimentation also erodes resilience through smothering and herbivory suppression. In spite of this critical role, most herbivory studies on tropical reefs focus on fishes, and the contribution of urchins remains under-studied. To test how different herbivory and sedimentation scenarios impact turf resilience, we experimentally simulated, in situ, four future overfishing scenarios derived from patterns of fish and urchin loss in other reef systems and two future sedimentation regimes. We found urchins were critical to short turf resilience, maintaining this state even with reduced fish herbivory and increased sediment. Further, urchins cleared sediment, facilitating fish herbivory. This study articulates the likelihood of increased reliance on urchins on impacted reefs in the Anthropocene. 

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4. Elevated nutrients and herbivory negatively affect Dictyota growth dynamics

   https://doi.org/10.3354/meps13788  

   Ramseyer, TN; Tronholm, A; Turner, T; Brandt, ME; Smith, TB (August 2021, Marine Ecology Progress Series)

   Caribbean coral reefs are experiencing a shift to algal dominance at the expense of stony corals. Determining the factors leading to algal phase shifts is crucial for assuring the survival of Caribbean coral reefs. In this study, factors controlling the growth of the abundant brown macroalgae Dictyota spp. were investigated by varying herbivory pressure (caging) and nutrients (fertilizer addition) on coral reefs near St. Thomas (US Virgin Islands). Experiment 1 measured Dictyota heights and percent cover at 3 sites (11-20 m depth) and showed no growth response to nutrient addition and a weak negative response to herbivory. To confirm results of Experiment 1, a caging and nutrient manipulation (Experiment 2) was conducted at one site (14 m depth) using the dependent variable Dictyota biomass. A strong negative response of growth to nutrient addition was shown, presumably because of nutrient inhibition, and an equally negative response to herbivory (loss of ~50% biomass over 21 d). The inhibitory effect of fertilization on growth was confirmed in a third experiment that showed increasing biomass loss over 4 treatment levels of increasing fertilizer addition (0 [ambient], 5, 10, 20 g). Overall, Dictyota was not nutrient limited at any sites, and was weakly controlled by herbivore populations. Factors responsible for Dictyota abundance on Caribbean reefs may reflect decreased herbivory caused by overfishing and reductions in coral cover and do not appear to be affected by recent changes in nitrogen or phosphorus load. This study reinforces the need for conservation and management of herbivores in coral reef ecosystems, to mitigate the effects from anthropogenic stressors. 

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5. Positive feedbacks in coastal reef social-ecological systems can maintain coral dominance

   https://doi.org/10.1093/icesjms/fsae182  

   Pellowe, Kara E; Durfort, Anaëlle; Burkepile, Deron E; Mouillot, David; Lade, Steven J (May 2025, ICES Journal of Marine Science)

   Rodil, Ivan (Ed.)

   Abstract Understanding the mechanisms underlying nutrient (nitrogen and phosphorus) and carbon cycling in reefs is critical for effective management. Research on reef nutrient and carbon cycling needs to account for (i) the contributions of multiple organisms, (ii) abiotic and biotic drivers, and (iii) a social-ecological perspective. In this paper, we review the mechanisms underlying nutrient and carbon cycling in reef social-ecological systems and analyse them using causal loop analysis. We identify direct and indirect pathways and feedback loops through nutrient and carbon cycles that shape the dominant benthic state of reefs: coral, algal, and sponge-dominated states. We find that two of three anthropogenic impact scenarios (size-selective fishing and land use change) have primarily negative consequences for coral and macroalgae via the nutrient and carbon cycles. A third scenario (runoff) has fewer negative impacts on sponges compared to other benthos. In all scenarios, frequent positive feedback loops (size-selective fishing: 7 of 12 loops; runoff: 6 of 9 loops; land use change: 8 of 11 loops) lead to system destabilization; however, the presence of multiple loops introduces avenues whereby reefs may retain coral dominance despite anthropogenic pressures. Context-specific information on the relative strength of loops will be necessary to predict future reef state. 

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