Two closely-related species living in sympatry can only coexist if a shift towards a novel ecological niche occurs. For seabird species breeding in tropical regions, competitive pressures intensify due to high population densities in colonies and sporadic prey resources. Here, we used a stable isotope approach to infer inter-and intraspecific variation in foraging behaviour in two congeneric seabird species: Brown Booby (Sula leucogaster, N = 31) and Red-footed Booby (S. sula, N = 30) breeding on Raine Island in the Great Barrier Reef, Australia. Blood (red blood cells and plasma components) and feather samples were collected from breeding birds for δ 13 C and δ 15 N analysis. We found significant δ 13 C interspecific differences in feather and red blood cell tissue, but not in plasma, whereas δ 15 N values did not differ significantly between species in any of the tissues. Although interspecific differences were small, higher δ 13 C values found in Brown Boobies suggest differences in foraging strategy between the two species. Inter-sexual differences, either in size or ecology, did not translate into dietary niche segregation. This study suggests that nichepartitioning processes are flexible in the temporal scale, and can be constrained by reproductive behaviour and availability of prey resources.

Oceanic islands harbor unique yet fragile marine ecosystems that require evidence-based environmental management. Among these islands, the Galapagos archipelago is well known for its fish diversity, but the factors that structure communities within and between its islands remain poorly understood. In this study, water quality, physical habitats and geographical distance were assessed as potential predictors for the diversity and structure of fish assemblages. Differences in the structure of fish assemblages of the two studied islands (Santa Cruz and Floreana) were most likely driven by temperature and nutrient concentrations. In the relatively highly populated island Santa Cruz, the structure of fish assemblages was more affected by water conditions than physical habitats while the contrary was true for the more pristine area of Floreana. A wide variety of species with different geographical origins were distributed over the different islands, which indicates that most fish species are able to reach the islands of the archipelago. However, temperature gradients and elevated nutrient levels cause large differences in the structure of local fish assemblages. In addition, in Santa Cruz nutrient concentrations were negatively correlated with α diversity. Since pollution is a clear pressure on the fish assemblages of oceanic islands, environmental management of the coastal areas is of paramount importance.

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