Invasive animals: The unexpected savior of the Australian crocodile

The scene has played out countless times across the wetlands of Northern Australia: A saltwater crocodile prostrate in the swamp, waiting for a wild pig to reach the water's edge before attacking. Even an adult wild pig weighing nearly 70 kg cannot escape the crocodile's teeth.

Such interactions, the scientists say, are evidence that feral pigs - an invasive animal that causes major damage to Australia's wildlife - may have helped saltwater crocodiles escape the brink. extinction.

The unexpected relationship between crocodiles and wild pigs, or more broadly, between predators and pests, may be rewriting the story of how to preserve native ecosystems. Experts and officials in Australia and the United States will also need to take into account the role invasive species play in the conservation of native animals.

Saltwater crocodile attacks wild pigs in Australia.

Wild pig saves crocodile before extinction

To see if wild boar could help restore Australian crocodile populations, Dr. Mariana Campbell and colleagues from Charles Darwin University (Australia) studied carbon and nitrogen isotopes obtained in recent years from samples. Crocodile bones live in Darwin Harbor and Kakadu National Park.

These samples will be compared with specimens collected from crocodiles living throughout the Northern Territory between the late 1960s and mid-1980s.

'Animal bones will record the characteristics of the animal's life. If you want to understand the animal's diet in the short term, you need to look at blood and plasma," Campbell said. 'If you want to go further, you need to study the skin. But if we want to understand the long-term, we have to look at the bones."

The results of bone analysis show that over the past 50 years, wild pigs have become the main food source of saltwater crocodiles. This finding shows that the diet of saltwater crocodiles has a fundamental change, from mainly aquatic prey to terrestrial prey.

The story of the change in the diet of saltwater crocodiles dates back to 1971, when the Northern Territory government banned hunting of this animal.

The reason for the ban was a sharp decline in the population of saltwater crocodiles, from about 100,000 at the end of World War II to less than 3,000 in 1971. After millions of years living in Australia, the crocodile species Saltwater has reached the brink of extinction.

Wild boars were brought to Australia with European settlers in the late 18th century.

In the decade following the ban on crocodile hunting, authorities simultaneously launched a program to eradicate wild buffalo, another invasive species. As a result, the wild buffalo population dropped sharply, giving way to wild pigs.

Because of their small body and shyness compared to wild buffalo, wild pigs are difficult to hunt and eradicate. The population of wild pigs increased rapidly and spread to many places, thereby becoming an available food source for crocodiles.

Currently, the Northern Territory of Australia has an estimated 100,000 saltwater crocodiles living in the wild. 'If it weren't for the availability of wild pigs, crocodile populations would not have recovered to their current levels,' Dr Campbell said.

Research by Ms Campbell and colleagues, recently published in the scientific journal Biology Letters, also shows that the recovery rate of saltwater crocodile populations is often slower where there are no wild pigs.

Dr Campbell acknowledges that more research is needed to better understand whether saltwater crocodile predation has an impact on the overall feral pig population. But the early signals are very promising.

'We believe alligators are creating barriers to prevent feral pigs from moving,' Ms Campbell said.

The consequences are not clear

Ms. Campbell's team's study of Australian saltwater crocodiles is among the first to confirm that top predators can benefit as invasive populations grow. Scientists around the world have long assumed that this relationship exists.

For example, the American alligator population across the US Gulf Coast had dwindled to dangerous levels in the mid-20th century. By 1938, the beaver rat - a large semi-aquatic rodent from Argentina - was discovered. taken to farms in Louisiana to be raised for fur.

Kites eat slugs in Florida.

After fleeing the farm, the beaver rat took root throughout the American South, causing extensive damage to the coastal marsh environment. The rapid reproduction of beaver rats, along with legal protections, has also helped restore populations of alligators throughout the South.

'When these two species appear side by side, the beaver rat is the main food source in the American alligator's diet,' said Steven Platt, a reptile researcher with the Wildlife Conservation Society (USA), said. .

Birds also sometimes benefit from the resurgence of pests. In Florida, the spread of the round-mouthed yellow snail has caused the slug-eating kite here to develop a larger beak to eat larger prey.

Subsequent studies confirmed that slug-eating kites growing in marshes with round-mouthed golden apple snails had healthier bodies and higher survival rates over a 10-year period.

However, despite dietary shifts and adaptations by predators, invasive species are still prevailing. In the US as well as in Australia, relying on crocodiles alone will not be enough to combat invasive species.

'Are there clear examples of an organism benefiting from an invasive species? Definitely yes,' said Dr. Frank Mazzotti, an alligator expert at the University of Florida (USA). 'But what about the other consequences? We can't be sure about that yet."