Trophic Ecology of African Dwarf Crocodiles (Osteolaemus spp.) in Perennial and Ephemeral Aquatic Habitats
Crocodilians occupy diverse aquatic and riparian habitats, and through their movements and ontogenetic niche shifts, link the flow of energy and nutrients between aquatic and terrestrial food webs. We analyzed the trophic ecology of African dwarf crocodiles Osteolaemus tetraspis and O. osborni at one site for each species in Cameroon by analyzing carbon and nitrogen stable isotope ratios. We hypothesized that Osteolaemus species inhabiting a perennial river would be primarily associated with aquatic food webs, whereas those inhabiting a small ephemeral stream would assimilate significant fractions of terrestrial-derived material into their biomass. We also hypothesized that Osteolaemus species would undergo ontogenetic trophic shifts, including changes in vertical trophic position (TP). We found that crocodiles in both systems assimilated material from both terrestrial and aquatic-based food chains, and therefore provide a trophic link between habitats. In the perennial river, aquatic gastropods, fish, and amphibians were estimated to have higher maximum feasible contributions to crocodile biomass than terrestrial invertebrates. Prey contributions to crocodile biomass in the ephemeral stream could not be estimated because model results suggested that at least one important prey had not been sampled. Even though isotopic overlap was high between juveniles and adults, and between sexes, ontogenetic shifts in TP were apparent in both Osteolaemus populations.Abstract
Map showing locations and photos of the aquatic habitats of the Campo Ma'an and Dja River sites in Cameroon.
Biplot of stable isotope ratios of basal production sources, dwarf crocodiles (Osteolaemus osborni, O. tetraspis) (triangles), and their potential prey (circles) from the Dja River (A) and creek at Campo Ma'an (B). Abbreviations of samples are listed in Table 1. Values were not adjusted for trophic fractionation. Points are means; error bars are standard deviations.
Polygons and histograms of proportional contributions of sources to Osteolaemus osborni (n = 3) isotope signatures of collagen (OstC) and keratin (OstK) at the Dja River site. The convex hull area connects sources delineating the mixing space for dwarf crocodile diet. Values of potential prey sources were adjusted for trophic fractionation. Histograms show the mean percentile of feasible proportional contributions of sources, with ranges of values in parentheses. Abbreviations for sources are listed in Table 1.
Polygons and histograms of proportional contributions of sources to Osteolaemus tetraspis (n = 24) isotope signatures of collagen (OstC) and keratin (OstK) at Campo Ma'an. The convex hull area connects sources delineating the mixing space for dwarf crocodile diet. Values of potential prey sources were adjusted for trophic fractionation. Histograms show the mean percentile of feasible proportional contributions of sources, with ranges of values in parentheses. Abbreviations for sources are listed in Table 1.
Relationship between body size and δ15N and δ13C from keratin and collagen tissues of Osteolaemus tetraspis at Campo Ma'an National Park. Broken lines are multivariate adaptive regression splines (MARS). Gray bars indicate break points in the data from MARS on the basis of both tissue types.
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