The coral reefs of Moorea, French Polynesia, are under increasing threat from degrading ecological states, particularly shifts from coral to macroalgae dominance. A recent study has highlighted the significance of herbivore functions, such as grazing and browsing, as potential buffers against these destructive transitions. Understanding the spatial covariation of herbivore activities could hold the key to assessing and enhancing coral resilience.
According to researchers Dana T. Cook, Sally J. Holbrook, and Russell J. Schmitt, shifts to macroalgae-dominated states can render coral ecosystems functionally depleted, compromising their ability to provide important services. "Persistent shifts to undesired ecological states, such as shifts from coral to macroalgae, are becoming more common," they note. Therefore, it is imperative to understand the dynamics of herbivorous fish and their impact on reef ecosystems.
The study, conducted across 20 lagoon sites, examined interactions between grazing and browsing rates of herbivorous fishes. Grazers typically consume early-stage macroalgae, preventing establishment, whereas browsers can remove mature macroalgae, potentially facilitating the recovery of coral habitats. Surprisingly, the researchers found no direct correlation between rates of grazing and browsing across the sites; instead, these functions frequently clumped together, creating distinct patterns of herbivore activity.
From their analysis, three primary combinations of grazing and browsing activity emerged: low grazing-low browsing, high grazing-low browsing, and high grazing-high browsing. No sites exhibited low grazing coupled with high browsing activity.
Spatial analysis identified environmental attributes, such as proximity to deep-water channels and coverage of different turf varieties, which influenced herbivore activity. Grazing hotspots were predominantly characterized by greater algal productivity and proximity to the reef crest. Conversely, browsing hotspots tended to occur farther from the reef crest, indicating how fishing practices may impact these fish species more than others.
The research concluded by underscoring the relevance of browsing intensity to the ability of coral reefs to revert to their original states following disturbances. They stated, "Our findings provide insights and simple diagnostic tools...that influence the vulnerability to and reversibility of shifts to macroalgae on coral reefs." Hence, recognizing zones of high browsing intensity can help assess which areas have the best potential for recovering from invasive macroalgae.
This study is part of the Moorea Coral Reef Long Term Ecological Research Site’s efforts to develop actionable insights for reef management. The team highlights the necessity for targeted management strategies specific to each location's dynamics, such as prescribing measures to reduce sedimentation or limiting fishing pressure to preserve herbivorous fish populations.
With rising human pressures and climate change accelerating the rate of ecological shifts, the research presents valuable guidelines for coral reef conservation efforts. Prioritizing the preservation and recovery of herbivorous fish could dramatically influence the capacity of coral reefs to withstand and bounce back from ecological disruptions.