Habitat and body size effects on the isotopic niche space of invasive lionfish and endangered Nassau grouper

Species invasions are a significant threat to global biodiversity and ecosystem function, and yet our knowledge of consequences for native species remains poor. The problem is exacerbated in highly speciose ecosystems like coral reefs. The invasion of the wider Caribbean by predatory lionfish (Pterois spp.) is one of the most successful marine colonizations ever documented, and its impact is anticipated to be substantial on native species. However, despite the ecological and commercial importance of iconic Nassau grouper (Epinephelus striatus), the impacts of the invasion on this IUCN Red-Listed species remain unexamined. Using data gathered from two critical habitats in the Bahamas, we investigate isotopic niche space overlap between lionfish, Nassau grouper and putative prey species. Despite their relatively small body size, we find that lionfish occupy the highest isotopic niche position on patch reefs, occupying much of the same space as the native apex predator. Contrary to expectation, lionfish trophic level (delta N-15) does not increase with body size, contrasting with confamilials in their native range. However, we find that tissue carbon (delta C-13) changes systematically with body size on deep forereef habitats, representing a length-specific shift in food resources, with smaller individuals partitioning resources from larger individuals in this habitat but not on shallow patch reefs. We conclude that, despite the difference in body size, lionfish are capable of directly competing for food resources with Nassau grouper, and that impacts on guilds such as planktivores and invertivores may vary systematically by habitat. Our study contributes to the growing body of research aimed at understanding how a species that is relatively rare in its native range achieved the most successful fish invasion ever documented.