More Like this

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. 

   more » « less
2. Ontogenetic Variation in Blade Toughness May Contribute to the Spread of Turbinaria ornata Across the South Pacific

   https://doi.org/10.1111/jpy.13292  

   Betancourt, Austin M.; Fenwick, Ileana F.; Howard, Hunter B.; Long, Alexys E.; Fong, Peggy; Barber, Paul H.; Fong, Caitlin R. (October 2022, Journal of Phycology)

   Coral reefs are shifting from coral to algal-dominated ecosystems worldwide. Recently, Turbinaria ornata, a marine alga native to coral reefs of the South Pacific, has spread in both range and habitat usage. Given dense stands of T. ornata can function as an alternative stable state on coral reefs, it is imperative to understand the factors that underlie its success. We tested the hypothesis that T. ornata demonstrates ontogenetic variation in allocation to anti-herbivore defense, specifically that blade toughness varied nonlinearly with thallus size. We quantified the relationship between T. ornata blade toughness and thallus size for individual thalli within algal stands (N=345) on 7 fringing reefs along the north shore of Moorea, French Polynesia. We found that blade toughness was greatest at intermediate sizes that typically form canopies, with overall reduced toughness in both smaller individuals that refuge within the understory and older reproductive individuals that ultimately detach and form floating rafts. We posit this variation in blade toughness reduces herbivory on the thalli that are most exposed to herbivores and may facilitate reproduction in dispersing stages, both of which may aid the proliferation of T. ornata. 

   more » « less
3. 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. 

   more » « less
4. A spatial numerical model for seagrass–herbivore interactions and the formation of reef halos

   https://doi.org/10.1007/s00338-025-02729-3  

   Llabrés, Eva; Innes-Gold, Anne A; DiFiore, Bartholomew; Sintes, Tomàs; Madin, Elizabeth (October 2025, Coral Reefs)

   Abstract Reef halos are circular patterns of bare sand surrounding patch reefs, formed by herbivorous fish grazing near their reef refuges. These formations serve as indicators of ecological processes, providing insights into interactions among herbivores, vegetation, and predators. Their size and prevalence are influenced by predator and herbivore densities, fishing pressure, and temperature, making them valuable proxies for assessing the impact of anthropogenic stressors on reef ecosystems. Halos can also be monitored using satellite imagery and artificial intelligence tools, offering a scalable method for evaluating ecosystem health. In this study, we present the first spatially explicit agent-based model to explore reef halo formation. By integrating seagrass growth models with herbivorous fish behavior, we capture the spatial complexity of halo dynamics. Our model reproduces observed field patterns, including halo size variability driven by temperature. In addition, the model uncovers new insights into the mechanisms behind the formation of sand corridors—vegetation-free pathways that link isolated halos—an aspect of halo dynamics that was previously unresolved. We propose that these corridors are shaped by limitations in rhizome growth rather than shifts in herbivore foraging behavior. These findings advance our understanding of the ecological processes driving halo formation and enhance the predictive value of halos as indicators of coral reef ecosystem health. The model offers a deeper insight into how reef systems respond to environmental pressures, providing a powerful tool for monitoring and managing reefs amid climate change and anthropogenic impacts. 

   more » « less
5. Social interactions among grazing reef fish drive material flux in a coral reef ecosystem

   https://doi.org/10.1073/pnas.1615652114  

   Gil, Michael A.; Hein, Andrew M. (January 2017, Proceedings of the National Academy of Sciences)

   In human financial and social systems, exchanges of information among individuals cause speculative bubbles, behavioral cascades, and other correlated actions that profoundly influence system-level function. Exchanges of information are also widespread in ecological systems, but their effects on ecosystem-level processes are largely unknown. Herbivory is a critical ecological process in coral reefs, where diverse assemblages of fish maintain reef health by controlling the abundance of algae. Here, we show that social interactions have a major effect on fish grazing rates in a reef ecosystem.We combined a system for observing and manipulating large foraging areas in a coral reef with a class of dynamical decision-making models to reveal that reef fish use information about the density and actions of nearby fish to decide when to feed on algae and when to flee foraging areas. This “behavioral coupling” causes bursts of feeding activity that account for up to 68% of the fish community’s consumption of algae. Moreover, correlations in fish behavior induce a feedback, whereby each fish spends less time feeding when fewer fish are present, suggesting that reducing fish stocks may not only reduce total algal consumption but could decrease the amount of algae each remaining fish consumes. Our results demonstrate that social interactions among consumers can have a dominant effect on the flux of energy and materials through ecosystems, and our methodology paves the way for rigorous in situ measurements of the behavioral rules that underlie ecological rates in other natural systems. 

   more » « less