Pigs to the Rescue: An Invasive Species Helped Save Australia’s Crocodiles

It is a scene that has played out countless times across the swamps and wetlands of northern Australia: A family of feral pigs went down to the water’s edge to drink.

Just when the pigs are at their most vulnerable, the world’s largest crocodile species erupts from its camouflage in the water, sending piglets flying in a ferocious display of teeth and power. Even an adult pig, which can weigh up to 150 pounds, doesn’t stand a chance.

“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 you go and hunt for a shark, an animal that can swim five times faster than you?”

Frank Mazzotti, a crocodile and alligator expert from the University of Florida, agreed.

“A pig coming down to the water’s edge is like ringing the dinner bell,” he said.

Some scientists hope that the encounters between the crocs and the swine may be the first sign that the feral pig, an invasive species that has done great damage to Australia’s wild terrain, has finally met its match. The instances may also help to explain why crocodiles are doing so well, according to a recent study Dr. Campbell and other researchers published in the journal Biology Letters.

The saltwater, or estuarine, crocodile has lived for millions of years in Australia. The feral pig arrived in Australia with the first European settlers in the late 18th century. One is Australia’s largest apex predator that came close to extinction in the early 1970s. The other has spread across close to 40 percent of Australia’s land mass, and conservative estimates suggest that there may be 24 million in the country. Scientists blame feral pigs and other invasive species for widespread habitat loss and for Australia having the world’s highest rate of mammal extinctions.

As divergent as their evolutionary paths may appear, the interplay between the saltwater crocodile and the feral pig, between predator and pest, may be rewriting the complicated story of what happens when nonnative species take over an ecosystem. Whatever ecological destruction invasive species cause, the relationship between hungry crocodiles and voracious pigs in Australia highlights the unexpected cascades in nature brought about by invasive species. Similar surprises are being observed in Florida and elsewhere in the United States, where conservationists and wildlife officials must factor invasive species into their attempts to preserve local animals.

To understand whether unsuspecting pigs were helping to restore the Australian crocodile population, Dr. Campbell and her colleagues studied the carbon and nitrogen isotopes taken in recent years from bone samples of crocodiles dwelling in Darwin Harbor and Kakadu National Park. They then compared these to museum samples that had been taken from all 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 long term, you look at bones.”

The bone analysis revealed that over the past 50 years, feral pigs became the crocodile’s primary food source. This marked a fundamental shift in the diet of Australia’s oldest predator species to terrestrial species from mostly aquatic prey. “We expected to see some difference in the diet,” Dr. Campbell said. “But we were amazed by the difference between what they were eating back then and what they are eating now.”

The story of the saltwater crocodile and the dietary change that drove its recovery began in 1971, when the Northern Territory government banned crocodile hunting. Toward the end of World War II, there had been around 100,000 saltwater crocodiles. By 1971, there were barely 3,000, and the species was in danger of extinction.

In the decade after the hunting ban, a culling program significantly reduced the numbers of wild buffalo, another invasive species. This in turn expanded the ecological niche available to feral pigs. Smaller and more shy than buffaloes, the pigs were much more difficult to cull, and their population grew rapidly. In greater numbers and with a broader range, they became a ready food source for crocodiles.

There are now an estimated 100,000 wild saltwater crocodiles in the Northern Territory, and, Dr. Campbell said, “if it wasn’t for the availability of feral pigs in the environment, the population wouldn’t have recovered to the same level that they have.” The study noted that the recovery in saltwater crocodile numbers has been slower in areas where there are no feral pigs and where no dietary shift could occur.

Dr. Campbell acknowledges that more research is needed to understand whether predation by saltwater crocodiles is having any impact upon the overall feral pig population. But the early signs are promising.

“We believe that crocodiles are making a difference by creating barriers to movement by the feral pigs,” Dr. Campbell said. She added, “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.”

The study of Australian saltwater crocodiles is among the first to confirm that apex predators can benefit from large populations of invasive prey species. Scientists around the world had long suspected similar relationships.

Across the Gulf Coast of the United States, spanning from eastern Texas to northern Florida, for example, the American alligator was in dangerous decline in the mid-20th century. In 1938, the nutria, a large, semiaquatic rodent from Argentina, was introduced onto fur farms in Louisiana. The rodent escaped and took hold across the South, causing significant damage to coastal marsh habitat. Like the saltwater crocodile in Australia, alligator populations grew with the help of legal protections, culminating in the Endangered Species Act of 1973. But their recovery across much of the South was almost certainly aided by the presence of an abundant species of pest.

“Where the two species occur together, nutria are the main food item in the American alligator’s diet,” said Steven Platt, a herpetologist with the Wildlife Conservation Society.

And invasive animals have helped non-crocodilian animals, too.

Spanish explorers and settlers introduced pigs into the United States as a food source in the early 16th century. They are now present in at least 35 states, and, according to a study by the U.S. Department of Agriculture, feral hogs cause $1.5 billion in damage to crops, woodlands, levees and golf courses every year.

But the wild pigs may have helped one critically endangered animal — the Florida panther, of which there are only an estimated 150 adults and adolescents surviving in the wild. Studies of the diet of the cats found that feral hogs were the panther’s primary prey.

“Hogs may have saved Florida panthers from extinction,” said Mark Lotz, a panther biologist with the Florida Fish and Wildlife Conservation Commission. The cats had always preferred white-tailed deer, which were culled in the 1930s to control ticks that sickened cattle. With deer in decline, “the only place panthers remained was in southern Florida, where there was a sizable hog population,” he said.

Birds around the world have also shown an ability to profit from the rise of invasive species. And some of the consequences have been startling. In Florida, north of Lake Okeechobee, the spread of the fist-size island apple snail, which escaped from the aquarium trade, has prompted a remarkable change not just in the diet but in the body of the endangered snail kite.

“People were saying for a while that snail kites won’t be able to eat them because the snails are too big,” Dr. Mazzotti said. “Well, the snail kites grew bigger beaks, and the evolution occurred right in front of everybody’s eyes. The invasive apple snails are taking over, and the snail kites are going with them.”

Later studies confirmed the findings on beak size and found that those snail kites raised in wetlands with nonnative apple snails were in better condition and had a higher survival rate over a 10-year period. “We have also found that female snail kites prefer breeding with males with bigger beaks,” said Robert J. Fletcher Jr., a snail kite expert from the University of Florida.

Despite such adaptations and dietary shifts among apex predators, invasive species are still winning. In Australia in 2015, for example, the country’s then-Threatened Species Commissioner told the national broadcaster that “Australia has lost 29 mammals since European colonization, and feral predators are implicated in 28 of these extinctions.”

Florida faces similar invasion problems because it hosts an ideal combination of a subtropical climate, a thriving pet trade and multiple ports of entry. The result, said Ian Bartoszek, a wildlife biologist with the Conservancy of Southwest Florida, is that the state has “more established, nonnative animal species than any country in the world.” Or, as Dr. Mazzotti said about the Everglades, “I’m getting ready to call it the Everglades Invasive Reptile National Park.”

Even in the Everglades, there are pockets of good news. As Burmese pythons eat their way through the Everglades’ medium-sized mammals that consume reptile eggs it is possible, according to Mr. Bartoszek, that loggerhead sea turtles and the vulnerable American crocodile could benefit.

The impact on alligators is less clear. Although there is not yet data to confirm it, “It feels like the alligator is holding the line, and the alligator is most likely responsible for more python predation than we’ve given them credit for,” Mr. Bartoszek said. “The python has found its niche in the marshlands and those areas where there aren’t permanent water bodies where alligators don’t patrol. But in those deeper, more permanent water areas, the alligator has, I believe, locked onto the python, and is definitely doing us a service here.”

These are, so far, relatively small victories in the wider effort to combat invasive species. According to Mr. Bartoszek, 47 bird species, 24 mammal species and two reptile species have been found in the bellies of pythons.

And in the United States, like in Australia, it will take more than crocodiles and alligators to limit such pests. Where apex predators feed on invasive species, much remains uncertain. “Are there clear examples where a single species can and has benefited from an invasive species? You bet,” Dr. Mazzotti said. “What are the other repercussions? We’re a lot less certain about that.”