Comparing trophic levels estimated from a tropical marine food web using an ecosystem model and stable isotopes

Comparing the outputs of food web models with those from other independent approaches is necessary to build confidence in the use of these models to help manage fisheries. Mass-balance models such as Ecopath with Ecosim (EwE) and stable isotope analysis are widely used to describe food webs, but the results from these methodologies are rarely compared. In this study, an Ecopath model was developed to study the food web in the Bitung area, North Sulawesi, Indonesia and compare it with results from stable isotopes. Stable isotope data were available for 19 out of 50 functional groups defined in the model, including fishes, crustaceans, squids, sea cucumbers and other invertebrates. The trophic levels and niches of these functional groups estimated from the Ecopath model were compared with those calculated from nitrogen and carbon isotope data. The trophic levels of 19 functional groups were estimated to range from 2.00 (sea cucumber) to 3.84 (coral trout). Trophic levels estimated from Ecopath were correlated with those derived from stable isotopes (r2spearman = 0.71, n = 19, p < 0.001). On the average, Ecopath overestimated trophic levels of the functional groups in the model by about 2.4% compared to those calculated from stable isotopes, which is very encouraging. It is still suggested, however, that trophic level estimation should be cross-validated by using mass-balance models and SIA whenever possible.