High resilience masks underlying sensitivity to algal phase shifts of Pacific coral reefs

Abstract

A single ecosystem can exhibit great biogeographic and environmental variability. While a given ecological driver might have a strong impact in one region, it does not necessarily hold that its importance will extend elsewhere. Coral reefs provide a striking example in that coral communities have low resilience in the Atlantic and remarkable resilience in parts of the species‐rich Pacific. Recent experimental evidence from the Atlantic finds that fishing of large herbivorous fish can be a strong driver of coral resilience. Here, we repeat the Atlantic experiment in the highly resilient forereef of Moorea (French Polynesia), which has repeatedly recovered from disturbances. A combination of cages, fish deterrents (FDs), and controls allowed us to simulate the consequences of fishing large herbivores on algal assemblages, coral recruitment, and the demographic rates of coral juveniles. We find that the impacts of removing large herbivorous reef fish vary with early coral ontogeny. Reduced herbivore access led to a modest macroalgal bloom and reduction in coral recruitment. However, larger juvenile corals (> 1 cm diameter) survived better and grew faster under these conditions because of a reduction in corallivory. To determine the net impact of losing larger herbivorous fish, we combined experimental results with estimated demographic parameters in an individual‐based model. Simulating coral recovery trajectories for five years, we find that protecting larger reef fish led to better recovery in 66–99% of simulations, depending on underlying assumptions (with the more credible assumptions being associated with greater likelihood of net positive impacts). While we find that fishing effects are detrimental to corals in both the Atlantic and Pacific systems studied, the nature of the interactions varied markedly. In the identical previously‐published study in the Atlantic, macroalgae exhibited a rapid bloom and caused a sufficiently large reduction in coral recruitment to force a predicted ecosystem shift to an alternative attractor. The commensurate macroalgal bloom in Moorea was weak yet the corals were two orders of magnitude more sensitive to its presence. We do not suggest that a reduction in recruitment in Moorea will lead to alternative attractors but the long‐term risks of a reduction in recovery rate are cause for concern as rates of coral mortality are projected to increase. The emerging picture is that Pacific reefs are less likely to experience macroalgal blooms but are surprisingly sensitive to such blooms if they occur.

Coral Reef Resilience, Tipping Points and the Strength of Herbivory

“…This contrasts with the notion that large-bodied herbivorous fishes are generally required to control seaweeds324854, but agrees with experimental findings of Cernohorsky et al 29. for an atoll in the Indian Ocean and Mumby et al 39. also for Moorea.…”

“…For this treatment, both turf algae and some foliose algae developed, but the predominant taxon after 1 year was Lobophora sp. with flattened thalli, a brown alga that is able to rapidly bloom following loss of coral2039. However, after three years, fleshy seaweeds such as T. ornata and S. pacificum had colonized some but not all of the replicates of this treatment (Fig.…”

“…also for Moorea. Of note, the experiment done in Moorea by Mumby et al 39. that showed no loss of control of seaweeds when large-bodied herbivores (mostly parrotfishes) were removed was a repeat of the same experiment done by this group on reefs in Belize48; for the study reefs in the Caribbean, excluding only large-bodied parrotfish led to substantial increases in macroalgae.…”

“…If herbivore populations are reduced by fishing, then macroalgae may be able to escape top down control on the reef, especially following a disturbance that kills coral on a landscape scale such that herbivory pressure functionally becomes diluted25. While numerous small scale experiments where herbivorous fishes were excluded resulted in the proliferation of macroalgae in the absence of herbivory29343536373839, we generally lack knowledge of the level of herbivory that would be sufficient to control fleshy seaweeds at the landscape scale (but see Graham et al 15…”

Fisheries productivity under progressive coral reef degradation

“…The predicted increase in the biomass and productivity of herbivores following initial reef degradation is encouraging and may help to buffer livelihoods against further reef decline. Through their grazing function, herbivores remove algae, promote coral recruitment and infer greater resilience on reef ecosystems (Doropoulos et al., ; Hughes et al., ; Mumby, Steneck, Adjeroud, & Arnold, ). Their increased prevalence in degraded or recovering reef habitats helps to maintain algae in a cropped state, prevent algal phase shifts and buy time for coral recruitment and recovery.…”