Estuarine crocodiles in a tropical coastal floodplain obtain nutrition from terrestrial prey

Abstract

The estuarine crocodile (Crocodylus porosus) is one of the largest and most widespread crocodilians in the world. Although considered an apex species, the role of the estuarine crocodile in aquatic foodwebs is poorly understood; we know what crocodiles ingest, but not what nourishes them. In this study, we used a combination of stable isotope measurements (δ13C, δ15N, and δ34S) and direct feeding observations to identify the source of nutrition of estuarine crocodiles in Kakadu National Park, Northern Australia. Our results show that most crocodiles sampled (size 850 – 4200mm, with 76% of them being > 2.5 m) consume a large variety of prey, however a large proportion of their nutrition is derived from terrestrial prey. Introduced species such as water buffaloes (Bubalus bubalis) and pigs (Sus scrofa) could contribute between 53 and 84% to the nutrition of the sampled crocodiles. The isotopic composition of large crocodiles (total length > 3 m) suggested possible increase in marine prey consumption with size (R2 = 0.30; p = 0.005). Additionally, we found crocodiles sampled in the dry season had on average higher terrestrial contributions compared to crocodiles sampled during the wet season (84.1 ± 2.4% versus 55.4 ± 7.0%). Overall, we found that terrestrial prey are important source of nutrition for many crocodiles in this region where introduced herbivorous mammals are abundant.

Potential Pollution Sources from Agricultural Activities on Tropical Forested Floodplain Wetlands Revealed by Soil eDNA

“…We used a "mixing polygon" approach to identify possible contributions of the plant end-members to the total soil isotopic mix [34], which gives a similar result to mixing models [35]. We did not use a mixing model, as these would suggest that we included all possible sources to the soil mixture, which in this case, were mostly unknown at time of sampling [34].…”

Managing threats and restoring wetlands within catchments of the Great Barrier Reef, Australia

“…Pigs can also affect the trophic chain as they feed directly on many native species such as freshwater turtles (Waltham & Schaffer, ). In turn, pigs have become a new source of nutrition for crocodiles in the region (Adame et al, ).…”

Dietary shifts may underpin the recovery of a large carnivore population

“…A small sub-sample from each was weighed (0.8-1.0 mg) into tin capsules and analysed for stable isotopes (δ 13 C and δ 15 N) at the Stable Isotope Core Laboratory at Washington State University (WA, USA). The δ 13 C and δ 15 N values from those bone samples were corrected using diet-tissue discrimination factors (1.4‰ and 3.0‰, respectively) [24,26,27]. The carbon isotope values were adjusted for the Suess effect by applying a correction factor to the 'historic' cohort based on the atmospheric CO 2 [28] for each crocodile's year of death.…”

“…Our 'contemporary' cohort had a reduced range of body sizes compared to the 'historic' cohort. To provide a more comparative range of body sizes, we sourced additional stable isotope values from Adame et al [26]. These estuarine crocodiles (n = 41, size range = 83.5 to 420 cm, details in the electronic supplementary material, table S1) were captured from the same region between 2012 and 2014 (details on scute tissue isotopic discrimination in [26]).…”

“…To provide a more comparative range of body sizes, we sourced additional stable isotope values from Adame et al [26]. These estuarine crocodiles (n = 41, size range = 83.5 to 420 cm, details in the electronic supplementary material, table S1) were captured from the same region between 2012 and 2014 (details on scute tissue isotopic discrimination in [26]). We first tested if it was a valid assumption to combine these two groups of crocodiles, whose collagen had been sampled from different tissues (bone and scute).…”