Saltwater crocodile populations in the Northern Territory have been thriving lately, soaring back to the hundreds of thousands from near extinction over just a few decades. A recent study suggests an unusual reason for the positive trend: feral pigs.
Saltwater crocodiles have been around in Australia for about 65 million years. They’re formidable creatures, capable of growing to over 7 metres long and weighing over 900 kilograms. They can hold their breath for more than 7 hours at a time, and skilfully ambush large prey like buffalo, deer, and kangaroos.
Unfortunately, unregulated hunting caused croc populations to plummet in the 20th century. By the time protective legislation was passed in the 1970s, there were only a few thousand left in the wild.
In the Northern Territory, although a 1971 hunting ban started the crocodile’s recovery, a widespread buffalo culling program put a dent in a major food source for the species. That’s where the feral pig comes in.
An August study, headed by researchers at Charles Darwin University, looked at changes in the species’ diet as a potential factor in the recovery of NT croc populations. Scientists compared crocodile bones of museum specimens from the 1960s-80s to contemporary ones, analysing changes in ratios of carbon and nitrogen isotopes.
“Our results show that they have shifted from a marine-estuarine based diet, such as fish, marine turtles, to a more terrestrial-based diet of feral pigs and buffalo,” said Dr Mariana Campbell. In other areas of Australia where there aren’t so many feral pigs, croc populations haven’t seen the same success.
While crocodiles have been around since the Cretaceous period, pigs were only introduced to Australia in the late 18th century by European colonisers.
Feral pigs are the descendants of these domesticated animals, and are an extremely destructive species. Conservative estimates place the Australian feral pig population at 24 million, and the species has spread over 40% of our continent.
Feral pigs have had catastrophic ecological and agricultural impacts, and have contributed to Australia’s rate of mammal extinctions – the highest in the world.
But it looks like they’ve met their match, at least in the Northern Territory. In NT floodplains, crocs are now keeping feral pig populations in check. As Dr. Campbell believes, “crocodiles are making a difference by creating barriers to movement by the feral pigs.”
The research done by Dr. Campbell and her colleagues is one of the first studies to confirm that large populations of invasive prey species can benefit apex predators.
But it’s not all sunshine and rainbows: feral pigs are still a destructive force in most of the country.
The Australian Threatened Species Commissioner stated in 2015 that the country “has lost 29 mammals since European colonisation, and feral predators are implicated in 28 of these extinctions.”
“Are there clear examples where a single species can and has benefited from an invasive species? You bet,” says Dr Frank Mazzotti, a crocodile expert from the Uni of Florida. “What are the other repercussions? We’re a lot less certain about that.”
Cover photo by Michelle van der Hoek on Unsplash.
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