Background/Question/Methods   Understanding interspecific interactions and the consequences of these interactions within a system is of profound importance to ecologists; of particular interest are predator-prey relationships. In the rivers of northern Trinidad, waterfalls restrict the movement of guppy (Poecilia reticulata) predators resulting in low predation pressure at upstream sites and high predation pressure at downstream sites. Therefore this system provides a natural experiment to test the influence of predation on trophic position and niche width of fish. Guppies, potential prey items, and predators were collected for stable isotope analysis (d15N and d13C) from up- and down-stream sites in five river systems (Aripo, Turure, Quare, Tunapuna, and Tacarigua) in Northern Trinidad. We expect guppies under high predation to be less selective about food choice resulting in a larger niche width. Additionally, we expect guppies under a high predation threat to have less access to high quality food items such as invertebrates causing them to feed at a lower trophic level.

Results/Conclusions   Preliminary results indicate stable isotope values were not statistically different among high (-26.4± 0.3‰) and low (-27.0±1.9‰) predation sites for d13C (Mann-Whitney Rank Sum Test, P=0.762). However, d13C values were significantly different (P<0.001) across different predation regimes at Airipo (-27.3±0.3‰ under lower predation threat and -23.8±0.3‰ under high predation threat) and at the high predation site Tacarigua (-33.5±0.4‰) compared to Tunapuna (-24.7±0.4‰) which serves as its low predation analog (P<0.001). Values of d15N were not significantly different among high (7.7±0.3‰) and low (7.5±0.3‰) predation sites for d15N (P=0.697) or within individual river systems suggesting that trophic position of the guppy does not vary under different predation regimes. Although niche width, measured as standard error around the mean, did not differ between high (0.3‰) and low (0.3‰) predation sites for d15N, niche width was slightly higher at the low (0.9‰) predation sites compared to high predation sites (0.3‰) for d13C.