New research suggests that a growing feral pig population supported the conservation success story of one of Australia’s largest carnivores, the estuarine crocodile.

Griffith University researchers contributed to the study published in Biology Letters which compared the diet of present-day crocodile populations in Kakadu and around Darwin with that of museum specimens collected in the same area about 50 years ago.

Professor Stuart Bunn, Director of the Australian Rivers Institute

Professor Stuart Bunn, Director of Griffith’s Australian Rivers Institute, said “the museum specimens were collected at a time when the crocodiles had been heavily hunted and their population was extremely low.”

The research suggests the hunting pressure that removed crocodiles from the river and floodplain systems, meant those remaining fed predominantly on marine prey. But results show over the last 50 years that there has been a significant, clear-cut shift in preference within the estuarine crocodile population in the Northern Territory, away from marine food webs.

“As the crocodile population has recovered, they’ve moved back into the extensive river/floodplain systems in the Northern Territory and now seem to be much more dependent on terrestrial prey,” Professor Bunn said.

“Competition for food resources within the recovering crocodile population, together with an increase in hoofed animals like feral pigs on the floodplains, are likely key drivers of this dietary shift.

“Without the local surge in feral pig abundance over the last 50 years and the crocodiles’ shift in diet, the substantial growth in their numbers would not have been possible.”

The crocodiles’ move back into river/floodplain systems like this in Kakadu, Northern Territory has driven their shift to terrestrial prey. Credit: Stuart Bunn

The work is based on a comparison of the carbon and nitrogen signatures in the bones of museum specimens with those of present-day animals.

“We measured the naturally occurring carbon and nitrogen isotopes extracted from the crocodile’s bones and other tissue which are derived directly from the animal’s diet,” said Professor Bunn. “This gives truth to the old adage ‘you are what you eat’.”

“Crocodiles in the Northern Territory today had significantly lower values of both carbon-13 and nitrogen-15 isotopes, reflecting a shift from marine food sources to terrestrial prey.

“Stable isotope analysis is a very powerful tool to understanding not only animal diets, but also the overall flow of energy in food webs within and between aquatic and terrestrial ecosystems.”

Given the growth in crocodile numbers and biomass in the Northern Territory, the authors suggest they are helping to control the feral pig population in that state.

“At the same time, these apex predators are increasing the flow of land-based nutrients into river, floodplain and estuarine aquatic ecosystems,” Professor Bunn said.

This study highlights the significance of prey availability in contributing to large carnivore population recovery.

15: Life on Land
UN Sustainable Development Goals 15: Life on Land