A Rapidly Expanding Macroalga Acts as a Foundational Species Providing Trophic Support and Habitat in the South Pacific
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.
Keywordscoral reefs epiphytes foraging behavior foundation species herbivory macroalgae phase-shift
Thank you to the undergraduate students from UCLA’s field courses 2012–2016, and ImageJ processing by Von Phan and Meera Solanki. A special thanks for 2014 field assistance from Briana Fodor and funding from Aquarium of the Pacific. Funding in the field for SJB and PF was provided by UCLA’s OID and the EEB Department; RJC was funded by these sources and the NSF GRFP; and CRF was funded by a Sigma Xi Grant-in-Aid of Research (GIAR). Funding while writing was provided to SJB by the Eugene Cota-Robles Fellowship Program and NSF GRFP. Thank you to the Gump South Pacific Research Station and the French Polynesian Department of Research.
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