Evaluating the precariousness of coral recovery when coral and macroalgae are alternative basins of attraction

Abstract

When coral and macroalgae are alternative attractors, the trajectory of the benthic community following a major disturbance is shaped in part by whether herbivores keep macroalgae suppressed, leaving reef surfaces in a state suitable for coral colonization. Because macroalgae tend to colonize much faster than coral, an important issue is how close ambient herbivory is to the level where control of macroalgae is lost, that is, the precariousness of the coral-invasible state relative to the switch-point to macroalgae. In Moorea, French Polynesia, reefs dominated by the macroalga Turbinaria ornata remained so for multiple population turnovers, whereas reefs where Turbinaria was removed by a simulated pulse disturbance remained free of macroalgae throughout the 5-yr study. A field experiment revealed that macroalgae was unable to proliferate on disturbed reefs until ambient herbivory was reduced by more than 50%. Low precariousness of the coral-invasible state facilitated a gradual transition to a coral community because it allowed sufficient time for coral to repopulate. Accumulation of coral on disturbed reefs was 3.5 times greater than on undisturbed, Turbinaria-dominated reefs. Long-term persistence of the coral-invasible state was facilitated by high abundances of grazing herbivores that prevent the establishment of macroalgae, whereas persistence of the Turbinaria-dominated state reflected the scarcity of browsing herbivores that consume mature macroalgae. By revealing processes that foster bistability of coral and macroalgae states, our work helps explain the rich dynamical behavior exhibited by coral reef communities in Moorea and elsewhere.

Long‐term ecological research and the COVID‐19 anthropause: A window to understanding social–ecological disturbance

“…With decreased commercial fishing due to COVID‐19, we expect that there will be greater top‐down control this year compared to years when fishing pressures are higher. On the coral reefs of Moorea, seaweeds are controlled by herbivorous fishes (Adam et al, 2011 ) that are targeted by the local small‐scale fishery (Leenhardt et al, 2016 ; Rassweiler et al, 2020 ), and research has shown that intensification of fishing (Holbrook et al, 2016 ) and alteration of fisher behavior (Rassweiler et al, 2021 ) can reduce resilience of the coral state and promote a switch to seaweed dominated reefs that can represent a difficult‐to‐reverse, alternative basin of attraction (Schmitt et al, 2019 , 2021 ). Relaxation of fishing would strengthen resilience of coral communities to other disturbances such as storms and heat waves (Holbrook et al, 2016 ).…”

Nonlinear dynamics, resilience, and regime shifts in aquatic communities and ecosystems: an overview

“…These researchers document the threshold density of sea urchins at which shifts occur and the long-term permanence of the algal state after urchin die-off. Benthic communities in Moorea (French Polynesia) are dominated either by coral or macroalgae, with the latter establishing more readily when herbivory drops below thresholds (Schmitt et al 2022). Intriguingly, the fate of fish herbivores on the reefs of Moorea is tied to fisher behavior that selects for low macroalgal sites (Rassweiler et al 2022).…”

How do fisher responses to macroalgal overgrowth influence the resilience of coral reefs?

“…However, six genera of herbivorous fishes (Naso, Scarus, Siganus, Chlorurus, Acanthurus, Calotomus) together comprise nearly 60% of the abundance and biomass of the catch (Rassweiler et al 2020). These herbivorous fishes are critical to the resilience of the reefs due to their potential to prevent proliferation of macroalgae (Han et al 2016;Schmitt et al 2019Schmitt et al , 2021.…”

“…Recent experimental work in Moorea has demonstrated this hysteresis and shown that at least some areas in the lagoons lie within the parameter space where either macroalgae-or coral-dominated states can persist (Schmitt et al 2019(Schmitt et al , 2021. Thus, we focused our remaining model analysis on scenarios in which parameters for fishing and fish dynamics yield alternative outcomes depending on the starting conditions (i.e., coral vs. macroalgal dominance).…”

“…There is experimental support that the macroalgae-and coral-dominated communities represent alternative stable states in the lagoons of Moorea (Schmitt et al 2019(Schmitt et al , 2021. Hysteresis arises because the macroalgae pass through a juvenile stage in which they are vulnerable to herbivory before reaching adult size, at which point they are extremely resistant to herbivory due to a combination of physical and chemical defenses (Davis 2018).…”