Analysis of the natural abundance of stable isotopes, mainly δ¹⁵N and δ¹³C, is an important tool in many fields of ecology. We used a relatively new approach to assess a species’ isotopic niche. This represents the area occupied by individuals in the δ-space – obtained by plotting δ¹⁵N against δ¹³C. As part of a broader NSF-FIBR project investigating feedbacks between evolutionary and ecosystem processes, we assessed the isotopic niche of two fish species, Hart’s killifish (Rivulus hartii) and the guppy (Poecilia reticulata), from three Trinidadian streams characterized by different predation and competition regimes. In each stream, fishes were sampled from three different communities: Rivulus+Poecilia+other fish predators, Rivulus+Poecilia, and Rivulus only. We hypothesize that the niche breadth of the two fish species is directly affected by different predation and competition regimes. 
Results/Conclusions

Preliminary results suggest that small-sized individuals of Rivulus and Poecilia display large niche overlap. In contrast, no niche overlap was observed in adults, probably due to resource partitioning. Ontogenetic niche shifts were inferred from isotopic signatures for individuals of different size classes. In both Rivulus and Poecilia, the δ¹⁵N signals were positively correlated with body size, possibly indicating a shift in trophic level. δ¹³C signatures increased with body size in Rivulus and decreased in Poecilia, potentially showing a change in carbon source from juveniles to adults. Our initial findings confirm the utility of the isotopic niche approach as a means of understanding community dynamics in tropical streams.