In the land down under, one ecological nightmare is helping crocodiles fill their bellies.
The saltwater crocodile has been prowling Australia’s waterways for millions of years, eating whichever animal was unfortunate enough to fall in its jaws. Still, despite their fearsome nature, this species came to the brink of extinction in the 1970s. But now, they are recovering while also performing a hugely important ecological service: the species is growing fat by eating the invasive, habitat-destroying feral pig.
Researchers say this interplay between the native saltwater croc and feral pigs, the descendants of animals brought to Australia by European colonists, could be the first sign that the invasive species has finally met with a predator that can help keep its population in check. At the same time, it gives real grounds for hope that the crocs themselves may recover, now that they have access to a steady and convenient source of food.
“Crocodiles eat whatever is easiest, and feral pigs are the perfect size,” said Mariana Campbell, a researcher at Charles Darwin University in Australia who studies saltwater crocodiles in the country’s north. “They’re pretty lazy hunters. If you’re a crocodile, what is easiest? You stay near the bank and wait a few hours for a pig? Or do you go and hunt for a shark, an animal that can swim five times faster than you?”
Since first arriving in Australia in the late 18th century, feral pigs — the wild, descendants of escaped domestic pigs — have spread over almost 40% of the continent. They are one of the most successful invasive species plaguing Australia today, and are held responsible for widespread habitat loss and as important contributors to Australia’s rate of mammal extinctions, which is the highest in the world.
Their stunning success is owed to the fact that pigs had no natural predators in Australia when they were first introduced, and they could outcompete local species for food, water, and territory. And although Australia’s ecosystems were irrevocably changed by the presence of feral pigs, a new paper highlights some of the unexpected changes that invasive species can produce in an ecosystem.
The research, led by Dr. Campbell, aimed to understand whether the presence of feral pigs was actually helping restore Australia’s crocodile population. For the study, the team collected carbon and nitrogen isotopes from bone samples collected over the last few years from crocodiles in Darwin Harbor and Kakadu National Park. Those were then compared to similar readings obtained from crocodile bones in museum collections taken from across Australia’s Northern Territory between the late 1960s and the mid-1980s.
“The bones retain a signature that remains across the life of the animal. If you want to look at an animal’s diet in the short term, you look at blood and plasma,” Dr. Campbell said. “If you want something a little further back, you would look at collagen or skin. For the long term, you look at bones.”
Based on their analysis, the team explains that over the last 50 years, feral pigs made up a growing proportion of the crocodiles’ diets, eventually becoming their primary food source. This was a very significant shift in diet for the saltwater crocodiles, who primarily ate aquatic prey before. The team was actually “amazed by the difference” in diet they saw, despite starting their research with the expectation of finding some difference in this regard.
The team explains that the recovery of the saltwater crocs began in 1971, when the Northern Territory government banned crocodile hunting. This move was prompted by the immense decline the species was experiencing — its numbers dwindled from around 100,000 in the 30’s and 40’s, to under 3,000 in 1970.
In the decade following this ban, a culling program also led to a dramatic reduction in the numbers of wild buffalo (another invasive species). This set the stage for the feral pig population to rapidly expand, as they could fill the same ecological niche left empty by the buffalo. Smaller and more averse to human contact, the pigs proved more difficult to cull or control, and their population expanded rapidly. They also proved themselves more adaptable than the buffalo, covering a larger range. This, in turn, put them in contact with the crocodiles.
The team is confident that without the ample supply of feral hogs roaming around Australia’s waterways “the [saltwater crocodile] population wouldn’t have recovered to the same level that they have,” explains Dr. Campbell. Data in the field supports this view: the recovery of crocodile populations in areas that have not been invaded by feral pigs has been much slower than the areas where they are present.
It seems that nature does have a sense of humour, however. The pigs, which have caused so much ecological damage in Australia, have helped prop up one of its most endangered species of predator, nursing it back to relative health. The crocs, meanwhile, are helping safeguard intact ecosystems by creating barriers to the movement of feral pigs.
“You can imagine: If you are a pig in the Northern Territory, you probably wouldn’t try and swim across the Mary River because you won’t get to the other side,” Dr. Campbell says.
Researchers around the world have hypothesized that invasive species, especially those that are wildly successful and can grow in numbers rapidly, can benefit local predator species, especially apex predators that can take their pick of kills. And, despite ongoing work on this subject in areas such as the Gulf Coast of the United States, this hypothesis has not been validated so far — but the current findings seem to suggest that it does hold some water.
The findings showcase the intricate web of interdependencies that form in the wild. Although feral hogs have caused untold damage to Australia’s wildlands as well as crops and other inhabited landscapes, their presence aided in the recovery of crocodiles. At the same time, they did cause a lot of other species of mammals to go extinct, and have helped to push Australia’s rate of mammal extinction to be the world’s highest.
All in all, the study helps underscore just how beautifully complex a mechanism natural life can be, with numerous moving parts all influencing those around them. And, apart from that, that crocodiles really like the taste of pork.
The paper “Dietary shifts may underpin the recovery of a large carnivore population” has been published in the journal Biology Letters.