Quantifying the ecological role of crocodiles: a 50-year review of metabolic requirements and nutrient contributions in northern Australia.

Abstract

The ecological roles of large predators are well recognized, but quantifying their functional impacts remains an active area of research. In this study, we examined the metabolic requirements and nutrient outputs of the estuarine crocodile population (Crocodylus porosus) in northern Australia over a 50-year period, during which the population increased from a few thousand to over 100 000 individuals. Bioenergetic modelling showed that during this period, the crocodile population's annual prey consumption increased from <20 kg km^-2 in 1979 to approximately 180 kg km^-2 in 2019. Further, the prey consumption increase was accompanied by a significant dietary shift from predominantly aquatic prey (approx. 65% in 1979) to a terrestrial-based diet (approx. 70% in 2019). A substantial portion of these terrestrial-derived nutrients was excreted into the water, significantly increasing the input rates of nitrogen (186-fold) and phosphorus (56-fold). The study shows that, despite being ectothermic, the high biomass of crocodiles within the environment generated nutrient inputs comparable to terrestrial endothermic predator populations. While crocodiles are apex predators, they are not considered to influence ecosystems in the same manner that large-bodied endothermic predators do. However, in the oligotrophic freshwater systems of northern Australia, the large volume of crocodile biomass is likely to impact the ecosystem through top-down and bottom-up processes.