The Herpetological Journal

pp.57-64

Authors: L. Luiselli, G. C. Akani, E. Politano, E. Odegbune And O. Bello

Abstract: The diet of sympatric freshwater turtles was studied at two study areas in the Niger Delta (southern Nigeria), to test whether oil pollution affects the ecological relationships between free-ranging turtles. Two study areas, one unpolluted and one polluted by an oil spill, were used for our comparisons. Both areas had similar environmental conditions, i.e. a main river tract with banks covered by dense gallery forest, seasonal swamps in riverine forest, and almost permanent marshes with rich aquatic vegetation. Four species of turtle ( Trionyx triunguis, Pelusios castaneus, Pelusios niger, and Pelomedusa subrufa), were captured in the unpolluted area, whereas only two species (Pelusios castaneus and Pelusios niger) were captured in the polluted area. At the unpolluted area, the taxonomic composition of the diets of Pelusios castaneus and Pelusios niger was similar, whereas the diets of Pelomedusa subrufa and Trionyx triunguis were very different from the other two species and one another. In the polluted area, the taxonomic composition of the diet of Pelusios castaneus was significantly different from that of conspecifics in the unpolluted area, and consisted mainly of plant matter, annelids (earthworms and leeches), and gastropods. The taxonomic composition of the diet of Pelusios niger was also significantly different from that of conspecifics at the unpolluted area, and consisted mainly of annelids and gastropods, and secondarily of plant matter. Amphibian prey (eggs, tadpoles, and adults), which were one of the main food types for all turtles in the unpolluted area, practically disappeared from the diet of turtles at the polluted area. It was evident from this study that the two species that survived the oil spill event shifted considerably in their dietary preferences. In particular, in both species there was an obvious trend towards a reduction in the breadth of the trophic niche, with many fewer food categories eaten at the polluted area compared to the unpolluted area. It is suggested that such reduction in trophic niche breadth may depend directly on the reduced availability of most food sources (particularly amphibians, fish, and environmentally-sensitive invertebrates) in the polluted area, despite over 10 years having elapsed since the spill, and restoration operations at the site. It is likely that the above-mentioned reduction in trophic niche breadth also depended on the shifts in habitat use by the surviving turtles, which tended to concentrate into single habitat types in the polluted area, compared to the unpolluted area where they were more habitat generalists.

Keywords: Chelonia, community ecology, feeding ecology, pollution, resource partitioning