Using stomach contents, stable isotopes and morphometrics to quantify resource partitioning among fruit-eating fishes in Amazon floodplain habitats

Background/Question/Methods

Resource partitioning has been demonstrated in a wide array of communities including invertebrates, vertebrates and plants.  Floodplains of the Amazon River are characterized by diverse assemblages of frugivorous fishes including many species that are closely related and morphologically similar.  Here we test the null hypothesis that frugivorous fishes of the Amazon do not partition food resources during the annual flood pulse, the period when fishes invade the forest.  We conducted intensive fishing and monitored fruit availability during eight months (high and falling-water periods) in an oligotrophic river and an adjacent oxbow lake in the Colombian Amazon.  Stomach contents and stable isotope data were analyzed to evaluate trophic niche width and overlap among species.  We examined the influence of seasonality and food availability on patterns of food consumption.  We also evaluated relationships between morphological variation, phylogenetic relationships and trophic niche similarity. 

Results/Conclusions

Species richness and assemblage composition of frugivorous fishes were different between the river and the oxbow lake.  Consumption of fruits and seeds was significantly higher during the peak of the flood pulse in both sites, and this was correlated with greater availability of these resources.  Despite finding a large diversity of fruits and seeds in fish stomachs, only a few taxa dominated volumetrically.  Trophic niche overlap was relatively high among all seven species but it varied with both seasonality and phylogenetic distance.  Overlap among pairs of non-congeneric species, for instance, was lower than the overlap among pairs of congeneric species and it also decreased as the water receded from the forest.  Δ13C isotopic signatures were significantly different between species at both sites.  Such differences could be influenced by several mechanisms, including 1) large variation in isotopic signatures within and among fruit and seed taxa, 2) differential digestion and assimilation of material by fish species, and 3) variation in feeding rates and assimilation among fish species reflected in stable isotope data but not in stomach contents.  Findings reveal variable degrees of food resource partitioning among sympatric frugivores fishes.  We discuss the implications of these results in the context of fisheries conservation and of forest recruitment dynamics, since frugivorous fishes are known to be seed dispersers in floodplain forests.