How a rapidly growing population of crocs has been impacting Australia’s Northern Territories ecosystem

A team of marine biologists, environmental researchers and land management specialists affiliated with several institutions in Australia, working with a colleague from Canada, has conducted a study of the ecological impact of a huge rise in the population of saltwater crocodiles in Australia’s Northern Territories.

In their paper published in the journal Proceedings of the Royal Society B: Biological Sciences, the group describes what they learned about changes in croc size, diet, and the sharp rise in nutrients they excrete into the water system.

Fifty-four years ago, the Australian government banned the hunting of saltwater crocodiles in its Northern Territories. Since that time, the population of crocs has grown from approximately 1,000 to approximately 100,000. The research team wondered about the ecological impact of such a rapid change, and more specifically, if it was possible to quantify the changes that had taken place.

The work by the team involved conducting two major studies. One involved analyzing data that has been amassed by various researchers over the past half-century and then using it to conduct bioenergetic modeling of croc size and population. They then used the models to make estimates about consumption rates of various foods the crocs have been consuming and what they were excreting, and how much.

The other study involved analyzing bones that have been recovered in the region over the years 1970 to 2022. From these, the team was able to learn more about what the crocs had been eating and how much by measuring carbon and nitrogen isotopes.

The researchers found that the size of the crocs has been growing slightly and that increases in population have led to a total biomass increase from an average of 10 kg to 400 kg per kilometer of river area. They also found that the amount of food they ate as a group increased approximately nine-fold. Additionally, the amount of phosphorous and nitrogen excreted rose 56 and 186-fold—most of which went into the water.

The researchers also found that the crocs had begun eating more terrestrial animals—at the beginning of the study period, 65% of prey came from the water. By 2019, approximately 70% of prey were land-based animals such as wild pigs, buffalo and occasionally wallabies.

The team concludes by suggesting the main takeaways from their study were that the croc population rise has led to massive amounts of excreted material into the water and that there have been huge reductions in feral pig and buffalo populations in the region.

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