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

Abstract

Abstract. 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.

Experimental support for alternative attractors on coral reefs

“…Turbinaria plants have both structural and chemical defenses against herbivory that develop ontogenetically and become most effective in mature thalli ( 53 , 71 ). As a result, palatability to fish herbivores declines sharply once an individual grows out of the highly vulnerable early recruit stage ( 62 ). Germlings of T. ornata settle very close to the parent thalli, with the bulk dispersing <0.2 m from the parent ( 72 ).…”

“…Germlings of T. ornata settle very close to the parent thalli, with the bulk dispersing <0.2 m from the parent ( 72 ). Once a patch of mature Turbinaria forms, the early palatable life stages that settle under the canopy of much less palatable adults gain an associational defense against herbivores that greatly enhances their probability of surviving to maturity ( 62 ).…”

“…Disturbed lagoon reefs can either return to the coral state or transition to a distinctly different, persistent assemblage characterized by low cover of coral and dense stands of macroalgae ( Fig. 1 B and C ), primarily the brown macroalga Turbinaria ornata ( 53 , 60 – 62 ). This suggests a potential for macroalgae to be an alternative attractor to the coral state under prevailing conditions in the lagoons of Moorea.…”

Macroalgae size refuge from herbivory promotes alternative stable states on coral reefs

“…Thus, macroalgae may achieve a size refuge due to the acquisition of structural and/or chemical defenses that make them less palatable to herbivores, and/or because of an increased ability of herbivores to detect and avoid feeding on large individuals. For example, the tropical brown alga, Turbinaria ornata , a dominant macroalga on Pacific coral reefs, becomes resistant to herbivory once it reaches approximately 2 cm in height [ 24 ].…”

Landscape‐scale patterns of nutrient enrichment in a coral reef ecosystem: implications for coral to algae phase shifts

“…We focused on a 90‐d period assuming that N tissue content in the thalli of Turbinaria ornata (hereafter, Turbinaria ), the species of macroalgae we used to map N availability, reflects the nutrient environment during a period of up to three months prior to its collection. This estimate is based on the growth rates and longevity of Turbinaria thalli (Davis 2018, Schmitt et al 2019). However, because little is known about N storage and turnover in Turbinaria we also describe rainfall and wave conditions during the 45‐d period prior to Turbinaria collection (Appendix : Figs.…”