Ecology and Trophic Relationships Among Fishes and Invertebrates in the Hawaiian Archipelago: Insights from Fatty Acid Signatures Analysis

Abstract

Healthy coral reefs have become increasingly rare, and their continuous degradation has serious implications for loss of marine biodiversity. There is an urgent need to assess the strength of top-down versus bottom-up effects on reef communities, to better understand how food web alterations can change the structure and function of these vulnerable marine systems. In this study, I used fatty acid (FA) analysis to investigate the trophic and ecological relationships among potential key forage species of the critically endangered monk seal in the Hawaiian archipelago. A series of multivariate tests performed on groups of closely related and ecologically equivalent species of fishes and invertebrates using a restricted number of FAs revealed that FA differences among groups primarily reflected diet, but could also be related to habitat and ecology. The same groups were subsequently analysed using an alternate method in quantitative FA signature analysis (QFASA) simulations, which allowed for the effects of using various subsets of FAs to be evaluated. Overall, species groups were relatively well characterized using both methods. When present, overlap in FA composition principally occurred among groups with similar diet/ecology, and were more prominent at higher trophic levels. A last set of analyses which combined the multivariate and QFASA simulation methods revealed that despite taxonomical relatedness and similarities in trophic ecology, individual species of carnivorous fish could be reliably distinguished using FAs. Therefore, while increasing the number of FAs used in the analyses might be useful to refine the resolution of distinctions, using a restricted number of FAs can also result in reliable differentiation among species. My results suggested that despite tremendous diversity, finer scale variations in FA composition could be detected among groups, and among species which shared the same diet and trophic ecology. These findings have important implications for the study of food web interactions in the Hawaiian archipelago, as they provide the foundation for using the same species groups in diets estimations of monk seal, as well as other top predators in this ecosystem. Moreover, they provide a framework for using multiple approaches to link FA patterns to the foraging ecology of individual species.