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8 hours ago
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The growing saltwater crocodile population in the Northern Territory has led to the creatures gorging on nine times more prey than they did 50 years ago, with the apex predators contributing important nutrients to Top End waterways, new research suggests.
Saltwater crocodile populations have increased exponentially in recent decades, from less than 3,000 in 1971, when a ban on hunting was introduced, to more than 100,000 animals today.
According to new modelling, the NT crocodile population consumed less than 20kg of prey per square kilometre of wetland in 1979, increasing to about 180kg per square kilometre in 2019. The analysis was based on 50 years of NT government surveys which record crocodile size and density.
That increase coincided with a shift from predominantly aquatic prey, which comprised 65% of croc diets in 1979, to mostly land animals in 2019, with animals such as feral pigs, cattle and Asian water buffalo making up 70% of the diet.
As ectothermic (commonly known as “cold-blooded”) animals, crocodiles eat far less prey than other apex predators, according to research lead Prof Hamish Campbell of Charles Darwin University. “Crocodiles eat about 10% of the food of an equal-sized lion,” he said.
But because they have become concentrated in far higher densities in the Northern Territory, they have significant impacts, he added. “In terms of the amount they’re eating and the amount they’re excreting, it’s incredibly high purely because of their biomass … it’s equal or even greater than a lot of terrestrial endothermic [warm-blooded] populations, such as the lions on Serengeti or the wolves in Yellowstone,” Campbell said.
The Top End crocodile population consumes about six feral pigs per square kilometre of wetland floodplain each year, the researchers estimate.
In 50 years, the amount of nitrogen and phosphorus that crocodiles excreted into NT waterways increased 186-fold and 56-fold respectively, the study also found.
“They’re pulling that in from the terrestrial food web which is what makes it really impactful,” Campbell said. “They’re digesting it, and they’re excreting all those nitrates and phosphates into the water.
“That’s going to be having huge impacts on phytoplankton and zooplankton productivity, which are the building blocks of the food chain.”
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The ecological role of crocodiles has been hotly debated among researchers, with some previously arguing there had been little evidence to date for their importance as ecosystem-defining keystone species.
The research modelled prey rates and nutrient excretion on the energy inputs required for the growth in crocodile numbers and biomass in the NT over the half-century period.
To reveal the animals’ dietary habits over time, the researchers used stable isotope analysis of historical and contemporary crocodile bones.
The research was published in the journal Proceedings of the Royal Society B.
