Habitat characteristics and priority effects shape fish and invertebrate assemblages inhabiting the coral <i>Pocillopora grandis</i> in Hawaiʻi

The structure of ecological communities is driven by an interplay of abiotic and biotic factors, yet the history of this interplay is less often considered. Priority effects, or how prior residents affect later‐arriving individuals, can strongly affect local assemblage trajectory, potentially leading to indirect effects on ecosystem state or function. On Hawaiian reefs, pocilloporid corals provide the primary source of branching habitat for fishes and invertebrates, with Pocillopora grandis , the largest species, often hosting substantial, multispecies assemblages. Smaller P. grandis colonies support assemblages dominated by an interference competitor, the blue‐eye damselfish Plectroglyphidodon johnstonianus , and/or a mesopredator, the arc‐eye hawkfish Paracirrhites arcatus , as well as mutualistic guard crabs of the genus Trapezia . Visual surveys showed that the assemblage size and abundance of other resident species, particularly the competitively inferior damselfish Dascyllus albisella and some guard crab species, increased greatly with colony size. During a six‐month press removal of P. johnstonianus and P. arcatus on small P. grandis colonies off the south shore of Oʻahu, Hawaiʻi, we tracked the colonization of fishes and invertebrates every two weeks in the presence and absence of these two strong interactors. By the end of the experiment, colonizing fish and invertebrate abundance were approximately 7‐ and 4‐fold greater in the absence of the competitor and the predator, respectively. Likewise, fish and invertebrate species richness was approximately 6‐ and 1.7‐fold greater, respectively. The damselfish D. albisella , of a genus known to benefit corals, was almost entirely prohibited by the strong interactors from colonizing these corals. However, trapezid guard crabs, while not directly manipulated during the experiment, were minimally affected. These results indicate that at smaller colony sizes, assemblages of fishes and invertebrates inhabiting P. grandis in Hawaiʻi are likely inhibited by priority effects, potentially reducing mutualist interactions and affecting the host colony and local ecosystem function.