The scene was repeated over and over again in the swamps and swamps of northern Australia: a family of feral pigs descended to the water’s edge to drink. Just when the pigs are most vulnerable, the world’s largest species of crocodile explodes from its camouflage in the water, sending the piglets flying in a fierce display of teeth and power. Even an adult pig, which can weigh 150 kilograms, doesn’t stand a chance.
“Crocodiles eat whatever is easiest, and feral pigs [que voltaram a viver soltos na natureza] they are the perfect size,” said Mariana Campbell, a researcher at Charles Darwin University in Australia who studies saltwater crocodiles in the north of the country. “They are very lazy hunters. If you’re a crocodile, which is easier? Do you stay close to the shore and wait a few hours for a pig? Or are you going to hunt a shark, an animal that can swim five times faster than you?”
Frank Mazzotti, an expert on crocodiles and alligators at the University of Florida, agreed.
“A pig going down to the water’s edge is like ringing the dinner bell,” he said.
Some scientists hope the encounters between crocodiles and swine are the first sign that the feral pig, an invasive species that has wreaked havoc on Australia’s natural terrain, has finally found its mate. The cases may also help explain why crocodiles are doing so well, according to a recent study that Campbell and other researchers published in the journal Biology Letters.
The saltwater, or estuarine, crocodile has lived in Australia for millions of years. The wild pig arrived on the island with the first European settlers in the late 18th century. The first is Australia’s biggest predator, which came close to extinction in the early 1970s. The second spread across nearly 40% of Australia’s land mass. Australia, and conservative estimates suggest there may be 24 million of them in the country. Scientists blame feral pigs and other invasive species for widespread habitat loss and the fact that Australia has the highest rate of mammal extinction in the world.
As divergent as their evolutionary paths may seem, the interaction between the saltwater crocodile and the feral pig, between predator and pest, may be rewriting the complex story of what happens when non-native species dominate an ecosystem. Whatever the ecological destruction caused by invasive species, the relationship between hungry crocodiles and voracious pigs in Australia highlights the unexpected consequences wrought in nature by invasive species.
Similar surprises are being observed in Florida and elsewhere in the United States, where conservationists and wildlife officials must consider invasive species in their attempts to preserve local wildlife.
To understand whether pigs were helping to restore the Australian crocodile population, Campbell and his colleagues studied carbon and nitrogen isotopes taken in recent years from samples of crocodile bones that inhabit Darwin Harbor and Kakadu National Park. Then they compared them to museum samples that were taken from across Australia’s Northern Territory between the late 1960s and mid-1980s.
“Bone retains a signature that remains throughout the life of the animal. If you want to look at an animal’s diet in the short term, you study blood and plasma,” Campbell said. “If you want something a little older, look at collagen or skin. Longer term, you look at bones.”
Bone analysis revealed that in the last 50 years feral pigs have become the main food source for crocodiles. This marked a fundamental shift in the diet of Australia’s oldest predatory species, from primarily aquatic prey to land animals. “We were hoping to see some difference in the diet,” Campbell said. “But we were amazed at the difference between what they ate then and what they eat now.”
The story of the saltwater crocodile and the change in diet that led to its recovery began in 1971 when the Northern Territory government banned hunting the reptile. At the end of World War II, there were about 100,000 saltwater crocodiles. In 1971, there were only 3,000, and the species was at risk of extinction.
In the decade after the hunting ban, a culling program significantly reduced the number of wild buffalo, another invasive species. This, in turn, expanded the ecological niche available to pigs. Smaller and more timid than buffalo, pigs were much more difficult to slaughter and their population grew rapidly. In greater numbers and with a wider range, they became an easy food source for crocodiles.
There are now about 100,000 wild saltwater crocodiles in the Northern Territory and, according to Campbell, “if it weren’t for the availability of feral hogs in the environment, the population would not have recovered to the same level as it is.” The study noted that recovery in saltwater crocodiles has been slower in areas where there are no pigs and where there have been no changes in diet.
Campbell acknowledges that more research is needed to understand whether predation by saltwater crocodiles is having any impact on the general population of feral pigs. But the first signs are promising.
“We believe that crocodiles are making a difference by creating barriers to the movement of wild pigs,” Campbell said. “You can imagine: if I were a pig in the Northern Territory, I probably wouldn’t try to cross the River Mary because I wouldn’t make it to the other side.”
The study of Australian saltwater crocodiles is one of the first to confirm that predators at the top of the food chain can benefit from large populations of invasive species. Scientists around the world have long suspected similar relationships.
Across the US Gulf Coast, from East Texas to North 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 on fur farms in Louisiana. The rodent escaped and spread across the south of the country, causing significant damage to coastal marsh habitat.
Like Australia’s saltwater crocodile, US 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 abundant presence of a kind of plague.
“Where the two species occur together, nutria is the main staple of the American alligator’s diet,” said Steven Platt, a herpetologist at the Wildlife Conservation Society.
And invasive animals also helped non-crocodilian animals.
Spanish explorers and settlers introduced pigs to the United States as a food source in the early 16th century. Today they are present in at least 35 states, and according to a study by the Department of Agriculture, feral pigs cause damage of $1. 5 billion to plantations, forests, levees and golf courses every year.
But the pigs may have helped a critically endangered animal: the Florida panther, of which only about 150 adults and teenagers survive in the wild. Studies of the cat diet have found that feral pigs are the panther’s main prey.
“Pigs may have saved Florida panthers from extinction,” said Mark Lotz, a panther biologist at the Florida Fish and Wildlife Conservation Commission. The cats have always preferred white-tailed deer, which were culled in the 1930s to control ticks that sickened livestock. With deer in decline, “the only place panthers remained was South Florida, where there was a sizable pig population,” he said.
Birds from around the world have also shown the ability to profit from the emergence of invasive species. And some of the consequences were surprising. In Florida, north of Lake Okeechobee, the spread of the fist-sized island apple snail, which has escaped the aquarium trade, has brought about a remarkable change not only in the diet but also in the body of the snail hawk. , threatened with extinction.
“People have been saying for a while that snail hawks won’t be able to eat them because snails are too big,” Mazzotti said. “Well, the hawks got bigger beaks, and evolution happened before everyone. The invasive snails are taking over, and the hawks are enjoying it.”
Later studies confirmed the beak size findings and found that snail hawks reared in wetlands with invasive apple snails were in better condition and had a higher survival rate over a ten-year period. “We also found that female hawks prefer to mate with males with larger beaks,” said Robert Fletcher Jr., a snail hawk specialist at the University of Florida.
Despite these adaptations to the predators’ diet, invasive species are still winning. In Australia in 2015, for example, the country’s then Endangered Species Commissioner told the national broadcaster that “Australia has lost 29 mammals since European colonization, and feral predators are involved in 28 of these extinctions.”
Florida faces similar problems, as it is home to an ideal combination of subtropical climate, a thriving pet trade, and multiple ports of entry. The result, said Ian Bartoszek, a wildlife biologist at the Conservancy of Southwest Florida, is that the state has “more established non-native animal species than any country in the world.” Or, as Mazzotti said of the Everglades region, “I’m getting ready to call it the Everglades Invading Reptile National Park.”
Even in the Everglades, there are pockets of good news. Since Burmese pythons eat the medium-sized Everglades mammals that consume reptile eggs, it’s possible, according to Bartoszek, that sea turtles and the vulnerable American crocodile could benefit.
The impact on alligators is less clear. While there is still no data to confirm, “it looks like the alligator is holding the line, and the alligator is probably responsible for more python predation than we believe,” Bartoszek said. “The python has found its niche in the swamps and in areas where there are no permanent bodies of water, where the alligators don’t patrol. But in those areas of deeper, more permanent water the alligator, I believe, has become attached to the python and is definitely providing us with a service.”
These are, so far, relatively small victories in the broader effort to combat invasive species. According to Bartoszek, 47 species of birds, 24 species of mammals and two species of reptiles were found in the belly of pythons.
And in the United States, as in Australia, it will take more than crocodiles and alligators to limit these pests. Where cutting edge predators feed on invasive species, there is a lot of uncertainty. “Are there clear examples where a single species has benefited from an invasive species? Certainly,” Mazzotti said. “What are the other repercussions? We’re a lot less sure about that.”