Effects of Oceanographic Variability on the Trophic Ecology of Eight Species of Coral Reef Fishes From the Remote Central Pacific

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Miller, Scott Donald
coral reef fishes; trophic ecology; oceanographic variability; Central Pacific
thesis / dissertation description
Ecologists have long been interested in the factors that influence species’ ecological niches. Factors such as inter- and intraspecific competition, predation risk, and resource availability have been demonstrated to cause either expansions or constrictions in populations’ niches. Coral reefs represent complex systems rich in ecological interactions to study these effects on species’ niches. However, much of our knowledge of the ecological processes responsible for regulating community structure and function in tropical coral reefs originated in areas that were already strongly influenced by anthropogenic factors, such as fishing and eutrophication. In this study, I examined the role of natural oceanographic variation on the trophic ecology of eight common coral reef species spanning multiple trophic guilds. These fishes were collected from the Southern Line Islands, a chain of five remote, uninhabited islands spanning a strong primary productivity gradient in the central Pacific. A combination of stomach content and stable isotope analyses (δ15N, δ13C) were used to elucidate the spatial variability of diet composition, trophic niche width, and degree of individual dietary specialization in these species. I found evidence of spatial variation in diet composition for most of the species, with planktivorous species incorporating more larvaceans and foraminifera at productive islands and herbivores incorporating more invertebrates or filamentous algae at productive islands, depending on the species. Additionally, across species, populations tended to either have larger dietary niche widths and degree of individual specialization at the more productive islands or show no sign of change. The planktivores exhibited the strongest effect of 2 increasing niche width and specialization using metrics calculated from their stomach contents, while the carnivores and herbivores exhibited stronger effects from metrics calculated with the isotopic data. Across all 8 species, mean isotopic variability in δ13C was greatest at the most productive islands, indicating greater utilization of basal sources at more productive islands. At the island level, the community of fishes became more dispersed in isotopic space with increasing productivity, indicating increasing trophic diversity at the community level. The two most productive islands exhibited striking similarities in all community metrics relative to the communities from the three least productive islands. These findings suggest that increases in trophic diversity as a function of increasing oceanographic productivity are a result of increased resource availability at the island level, which subsequently drives increases in the potential community niche space. With higher prey productivity, consumers are therefore relaxed from interspecific competition and can occupy different areas of the community niche space, increasing their population niche width to reduce intraspecific competition. Taken together, these results highlight the importance of considering natural oceanographic variability when evaluating the structure and health of coral reef ecosystems and provide a strong foundation for future research to examine the food web and trophodynamics in systems that lack human impacts.