Terrestrial Diet Dependence in an Unprotected Nile Crocodile (Crocodylus niloticus) Population
Nile Crocodile (Crocodylus niloticus) populations that exist outside of protected areas are under threat in South Africa. They are believed to predominantly feed on fish, but they also take prey from the terrestrial system, which brings them into conflict with humans and hampers their management. Here, we use stable light isotope analysis to explore the diet of an unprotected Nile Crocodile population in the Olifants River, Mpumalanga and Limpopo Provinces. Nitrogen stable isotope ratios were obtained from fish and crocodile populations along the length (±430 km) of the river. The catchment is severely polluted with elevated δ15N values at nutrient hotspots, which provides a basis for tracking the trophic response of crocodiles to the longitudinal profile of fish δ15N values. Crocodiles did not respond to changes in the δ15N values of fish populations and dietary predictions based on size-specific diet to tissue discrimination factors suggests a nonaquatic food base. These results suggest terrestrial diet dependence in one of the few viable Nile Crocodile populations from outside protected areas, posing unique challenges to their conservation.ABSTRACT
Map of the Middle Olifants River catchment showing the location of sampling sites (green and red circles) between Lake Loskop and Lake Flag Boshielo, Mpumalanga and Limpopo Provinces.
Relationship of δ15N for all trophic levels across seasons between Lake Loskop and Lake Flag Boshielo, Mpumalanga and Limpopo Provinces, South Africa. The dotted line represents a 1 : 1 relationship. Crosses represent vegetation, triangles represent invertebrates, and circles represent fish average δ15N values with bars indicating standard deviation. The relationship across trophic levels conforms to the equation: Winter δ15N = 0.82 (Summer δ15N) + 2.22 with R2= 0.97. Vegetation across seasons conforms to the equation: Winter δ15N = 1.01 (Summer δ15N) − 0.98 with R2 = 0.99. Samples are from sites 1–6 in the summer and winter of 2018.
Variation in δ15N values as a function of Nile Crocodile size at and between Lake Loskop and Lake Flag Boshielo in the Middle Olifants River catchment (2018, sites 1–6), Mpumalanga and Limpopo Provinces, South Africa. Open circles represent crocodiles of unknown sex, closed circles represent male crocodiles, and diagonally crossed circles represent female crocodiles.
Samples from this study and others (Dabrowski et al., 2013; Lübcker et al. 2016; Huchzermeyer et al., 2017; Woodborne et al., 2021, all from below Lake Loskop) representing variation in δ15N values as a function of site for Nile Crocodile, fish, and predicted δ15N values for Nile Crocodile diets in the Olifants River. Dotted lines represent the interquartile range for the inferred dietary niche of Nile Crocodiles in the Olifants River. Dashes represent the δ15N values for fish communities, and crosses represent the predicted diet of Nile Crocodiles based on the corrections of Woodborne et al. (2021).
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