Experiment mimics fishing on parrotfish: insights on coral reef recovery and alternative attractors

Dominance shifts in ecosystems can occur rapidly, resulting in alternative stable states. While some coral reef ecosystems shift and recover relatively quickly, others recover slowly or not at all over periods of centuries. We explore the role of large (fishing-susceptible) parrotfish in triggering algal phase shifts as alternative attractors that may lock reefs into coral-depleted alternative stable states. We designed an experiment to modestly reduce herbivory only from large parrotfish in the immediate vicinity of experimental coral settlement nursery habitats. We used vertical pegs ('parrotfish deterrents' or PDs) around coral settlement plates on 2 Belizean fore reefs. Time-lapse videos and a year's accumulation of bite-marks on plates confirmed that only herbivory from large parrotfish declined significantly due to PDs. Patches of macroalgae developed around PDs reducing coral recruitment in this treatment only. Two dominant reefdwelling coral genera (Porites and Agaricia) recruited to our settlement plates. The fast-growing, high-light requiring, reef-building coral Porites was more negatively affected by phase shifts; this coral failed to recruit at and above mid-levels of algal abundance. We illustrate the direct roles eco logical processes such as herbivory from large parrotfish play in regulating algal abundance, which in turn reduces the recruitment potential of reefs and thus the ecosystem's capacity to recover. Combining our empirical results with an individually-based ecological simulation model, we determined that these processes cascade to drive alternative states and create a 'hysteresis' effect delaying or preventing recovery of the coral reef ecosystem.