3 years ago

Associational refuge facilitates phase shifts to macroalgae in a coral reef ecosystem

Coral reefs are among the most diverse and productive marine ecosystems and are currently threatened by natural disturbances such as predator outbreaks and tropical storms that can facilitate community shifts from coral to macroalgae. Such shifts have received considerable attention since macroalgae are known to inhibit coral recruitment, and thus hinder or prevent recovery to the coral‐dominated state. It is critical to understand mechanisms that permit the establishment and persistence of macroalgae. Though many macroalgae are chemically and physically defended against herbivores as adults, these defenses may be lower during early life stages. One potential mechanism that could facilitate the establishment and persistence of macroalgae is an associational refuge whereby early life stages are protected from herbivory in close proximity of older, less palatable conspecifics. Using Turbinaria ornata as a model macroalga, I tested whether an associational refuge between different life stages could facilitate the establishment and long‐term persistence of Turbinaria in the lagoons of Moorea, French Polynesia. Results suggest that Turbinaria may only be limited by herbivory as recruits, which can lead to the establishment of Turbinaria if the alga survives the brief period when it is especially vulnerable to herbivores. I also tested whether vulnerable recruits experience a refuge from herbivory when they are associated with relatively unpalatable adults, and found that survival of recruits was higher when they were associated with adults than when alone. I also found that the value of a refuge can be spatially variable because the association with adults has a growth and survivorship cost to recruits where herbivory was low. These mechanisms can promote persistent macroalgal assemblages and limit the capacity of herbivores to control macroalgae and reverse a phase shift.
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