Abstract
Tropical reefs are in global decline with seaweeds commonly replacing corals. Negative associations between macroalgae and corals are well documented, but the mechanisms involved, the dynamics of the interactions, and variance in effects of different macroalgal-coral pairings are poorly investigated. We assessed the frequency, magnitude, and dynamics of macroalgal-coral competition involving allelopathic and non-allelopathic macroalgae on three, spatially grouped pairs of no-take Marine Protected Areas (MPAs) and non-MPAs in Fiji. In non-MPAs, biomass of herbivorous fishes was 70–80% lower, macroalgal cover 4–9 fold higher, macroalgal-coral contacts 5–15 fold more frequent and 23–67 fold more extensive (measured as % of colony margin contacted by macroalgae), and coral cover 51–68% lower than in MPAs. Coral contacts with allelopathic macroalgae occurred less frequently than expected by chance across all sites, while contact with non-allelopathic macroalgae tended to occur more frequently than expected. Transplants of allelopathic macroalgae (Chlorodesmis fastigiata and Galaxaura filamentosa) against coral edges inflicted damage to Acropora aspera and Pocillopora damicornis more rapidly and extensively than to Porites cylindrica and Porites lobata, which appeared more resistant to these macroalgae. Montipora digitata experienced intermediate damage. Extent of damage from macroalgal contact was independent of coral colony size for each of the 10 macroalgal-coral pairings we established. When natural contacts with Galaxaura filamentosa were removed in the field, recovery was rapid for Porites lobata, but Pocillopora damicornis did not recover and damage continued to expand. As macroalgae increase on overfished tropical reefs, allelopathy could produce feedbacks that suppress coral resilience, prevent coral recovery, and promote the stability of algal beds in habitats previously available to corals.
Highlights
Competition has long been recognized as a critical process shaping the ecology and evolution of species and the structure and function of communities and ecosystems [1,2,3]
Coral-macroalgal competition can be a major determinant of benthic community structure, impacting food web dynamics, topographic complexity of the habitat, biodiversity, and ecosystem function [5,8,12,13,14,15]
We asked: (1) What is the frequency of coralmacroalgal contacts in the field? (2) Does frequency or extent of coral-seaweed contact differ between replicate Marine Protected Areas (MPAs) and nonMPAs? (3) Does the probability of contact with common coral species vary as a function of an alga’s allelopathic potency? (4) Do common corals differ in the speed and extent of damage caused by allelopathic macroalgae? (5) Does the impact of macroalgal allelopathy vary as a function of coral colony size? (6) Do corals recover from damage caused by allelopathic macroalgae? We address these questions to provide a better understanding of the frequency, dynamics, and outcome of coral-macroalgal contacts on natural reefs that are dominated by corals versus macroalgae
Summary
Competition has long been recognized as a critical process shaping the ecology and evolution of species and the structure and function of communities and ecosystems [1,2,3]. This is as true for marine as for terrestrial systems, but in marine systems competitive interactions commonly occur among dramatically different species (plants versus animals) rather than primarily among taxonomically similar species. Coral-macroalgal competition can be a major determinant of benthic community structure, impacting food web dynamics, topographic complexity of the habitat, biodiversity, and ecosystem function [5,8,12,13,14,15]. Macroalgae compete with corals via multiple direct and indirect mechanisms [8,13,16,17,18,19], but some macroalgae are allelopathic to corals on contact [11,20,21]
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