A Rapidly Expanding Macroalga Acts as a Foundational Species Providing Trophic Support and Habitat in the South Pacific

Abstract

Foundation species facilitate associated communities and provide key ecosystem functions, making anthropogenically driven phase-shifts involving these species critically important. One well-documented such phase-shift has been from coral to algal domination on tropical reefs. On South Pacific coral reefs, the macroalga Turbinaria ornata has expanded its range and habitat but, unlike algae that often dominate after phase-shifts, T. ornata is structurally complex and generally unpalatable to herbivores. Therefore, it may serve a foundational role on coral reefs, such as providing habitat structure to more palatable primary producers and corresponding trophic support to fishes. We predicted increasing T. ornata density would facilitate growth of associated algae, resulting in a positive trophic cascade to herbivorous fish. An experiment manipulating T. ornata densities showed a unimodal relationship between T. ornata and growth of understory algae, with optimal growth occurring at the most frequent natural density. Epiphyte cover also increased with density until the same optimum, but remained high with greater T. ornata densities. Foraging by herbivorous fishes increased linearly with T. ornata density. An herbivore exclusion experiment confirmed T. ornata facilitated epiphytes, but resource use of epiphytes by herbivores, though significant, was not affected by T. ornata density. Therefore, T. ornata performs foundational roles because it provides novel habitat to understory and epiphytic macroalgae and trophic support to consumers, though likely this function is at the expense of the original foundational corals.