Northern Australia’s massive crocodile population is munching its way through huge numbers of feral pigs, with the apex predator’s changing diet having a significant impact on the environment.

Since they were protected in 1971, croc numbers in the Top End have exploded 25-fold from one every five kilometres of river to more than five per kilometre.

In terms of biomass, or the collective bulk, of the largest and some would argue most misunderstood reptile on the planet, that represents a whopping increase from less than 10kg per kilometre to 400kg.

Researchers have studied data from eight Northern Territory river systems and found the prehistoric predators are eating nine times as much as they did in 1979, with a major shift in diet from fish to wild pigs and water buffalo.

“They’re excreting all those nutrients in the water and this has also led to significant nitrogen and phosphorus input into the ecosystem,” Charles Darwin University research lead Hamish Campbell said.

“We don’t really know what the impact is yet but it’s significant. It’s really large.

“If you look at how much they’re eating, what they’re eating and where they’re excreting their waste products, the effects have to be pretty significant.”

“It might be that it’s great for barramundi but we just don’t know yet.”

The research, published in the Proceedings of the Royal Society B journal, is part of a larger study assessing the impact of crocodiles on the north Australian freshwater ecosystem.

The Top End’s estimated 100,000 crocodiles are devouring as many as six feral pigs per square kilometre of wetland annually.

This “landscape of fear” has had a significant impact on the pigs’ behaviour, Prof Campbell said.

“If you’re a pig in the 1960s or a buffalo, you could just wallow in those water holes to your heart’s content and feed but now you can’t do that because you’re going to get eaten.”

“Whilst we don’t have any evidence the crocs are managing feral pigs, because the pigs reproduce up to 50 piglets a year, I do imagine that without crocs, the level of damage would be much more severe from pigs and feral cattle and buffalo.”

While the ongoing crocodile population boom often leads to talk of culling, Prof Campbell was quick to dismiss it.

“Culling is an emotive argument, it’s a straw man,” he said.

“You cannot make culling decisions based around ecological data.

“It doesn’t matter if you’ve got 10 crocodiles in a water hole or one crocodile in the water hole, you still wouldn’t swim there, would you?

“You’re still going to die whether it’s one crocodile or 10 crocodiles that eat you. The only way to stop crocodile attacks is to kill all crocodiles.”

And a lot of people actually like crocodiles, he said.

“They’re a source of employment for a lot of Indigenous people; they’re an Indigenous totem and a lot of tourism comes to the Territory because of crocodiles, so we’re not going to go back to that state of all-out war on crocodiles like we did in the 70s.”

The next research step is to study the impact of crocodiles on vegetation.

“So we’re looking at sediment cores because we’re trying to go back in time to look at what the ecosystem was composed of and during different crocodile (population) densities,” Prof Campbell said.

“And then we’re using satellite (data) to try to look at how vegetation has changed over the last 20-30 years.”

Abe Maddison
(Australian Associated Press)