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Abstract
The environmental variables and biological interactions that shape biological communities have long been a major focus of the science of ecology. When much of the physical structure of the community is biogenic, such as on coral reefs, these factors affect the living habitat itself. Coral reefs around most of Hawaiʻi are characterized by large seasonal waves, low coral cover, and low coral species richness. The antler coral, Pocillopora grandis, an uncommon member of the coral community, can grow to more than 1.5 meters in diameter. At this size, these highly branching colonies attract substantial assemblages of resident fishes and invertebrates in an otherwise low-lying reefscape that lacks other substantial sources of vertical relief. Antler coral may therefore provide disproportionately important habitat space relative to its total abundance. Surveys at three sites across the southern shore of Oʻahu revealed that reefscape-scale factors had little effect on resident assemblage structure. Most variation in fish and invertebrate assemblages was attributed to colony volume and amount of living tissue, yet there was evidence of agonistic interactions between species, especially at smaller colony sizes. Given these behavioral observations, I conducted a six-month experimental removal of two common residents, the Blue-eye Damselfish (an interference competitor) and the Arc-eye Hawkfish (a mesopredator). Resident fish and invertebrate abundance and species richness were significantly greater in the absence of these strong interactors. Finally, using photogrammetry, I measured host colony growth rates during this experiment on both manipulated and unmanipulated colonies. In the absence of Blue-eye Damselfish, predation on host colonies by coral-eating fishes greatly increased, resulting in reduced growth rates and sometimes mortality. This pattern provided evidence for a novel indirect mutualism on coral reefs in which a territorial corallivore indirectly benefits its host coral through defense against transient corallivores, reducing total corallivory and increasing host colony growth. Furthermore, on unmanipulated coral colonies, resident fish identity likely affected the magnitude of augmented growth acquired by the host, with species spending more time in direct contact with the colony providing a greater benefit. Despite limited colonization by other fishes when Blue-eye Damselfish and Arc-eye Hawkfish were already present, these two species provided multiple benefits that increased growth and survival of host P. grandis colonies.
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