Niche-related processes in island intertidal communities inferred from stable isotopes data

It is well recognized that niche-based processes are intimately related to most important ecological theories such as natural selection and island biogeography. Current niche approaches involving stable isotope analysis (SIA) have been used to clarify niche population and community dynamics, mainly through carbon and nitrogen signatures in organisms modeled in an isotopic delta-space. We modeled over a thousand delta C-13 and delta N-15 samples from six intertidal rocky reefs, three in oceanic islands and three in mainland coastal habitats, to test whether island reef communities follow the prediction of ecological release and vacant niche theories and exhibit higher niche width, resource exploitation and trophic diversity if compared to mainland communities. Additionally, we used an endemic island species to verify if, as hypothesized, niche was conserved along the evolutionary history of the species in relation its mainland sister species. Our results indicate that niche width (standard ellipse area), resource exploitation range (delta C-13 range) and trophic heterogeneity were higher in island communities in comparison to mainland communities. We found that the niche of island endemic species was more similar to the niche of mainland sister species than to the niche of confamiliar sympatric species suggesting that island endemic species have retained their ancestral niche. Niche expansion processes in island populations commonly occurred in response to ecological release effects, which are associated with lower biotic resistance (competition and predation pressure) in depauperate island communities. Although niche expansion by competition release is well-known for island terrestrial fauna, to this day evidence for marine organisms, mainly at a community-scale, had remained weak. Based on our results island communities may be prone to biological invasions into vacant niches promoted by anthropogenic activities. More research is needed to understand how potential invasions can affect island intertidal reef communities.