It took just 60 years for the red fox, one of Australia’s most devastating predators, to colonise the continent | Sean Tomlinson and Damien Fordham for the Conversation

To a newly arrived red fox, the abundant rolling grasslands and swamps of the Wadawurrung country, around what is now Victoria’s Port Phillip Bay, must have seemed like a predator’s paradise.

This landscape was filled with small marsupials and native birds, and free of European wolves or bears which generally kept fox numbers in check.

The first red foxes (Vulpes vulpes) to arrive in Australia were deliberately released by European colonialists in 1870 in three Victorian localities: Werribee, Corio (near Geelong) and Ballarat. They were introduced for the “noble” sport of fox hunting.

Small native animals became easy prey for foxes because they did not evolve with these predators and did not know how to avoid them.

The number of red foxes exploded and spread rapidly. How fast? Our new research shows it took just 60 years for one of Australia’s most devastating invasive predators to colonize the continent. Nowadays, foxes are found everywhere except the tropical north and Tasmania.

Their rapid spread offers clues as to how we might prevent future extinctions of native animals due to foxes and map the infiltration of Australia by other invasive species.

Mapping the spread

To model the arrival and spread of foxes across Australia, we relied on hundreds of historical “first sighting” documents collected from libraries, local authorities and state archives.

The first sightings of foxes were particularly publicized in the late 19th and early 20th centuries in Australia. This is due to the threats foxes pose to sheep and poultry.

We ran thousands of model simulations reconstructing the arrival and spread of foxes across Australia. We have imagined likely scenarios for fox survival, reproduction and dispersal based on what we know about their current behavior.

We then compared these simulated patterns of population growth and expansion to inferences of demographic change drawn from these historical records. Our best models were able to closely reconstruct the timing of foxes’ arrival in locations and regions as well as their current population size.

Our modeling showed that foxes were populating Australia at an incredible rate. Between 1870 and 1895 they had spread throughout south-eastern Australia. Then they spread more slowly north and west into arid regions. By 1940, however, they had reached the remote northwest.

Flourishing Foxes

Foxes mate in winter and females give birth to four to five young. In the fall, young foxes are alone. They can travel up to 300 km in search of new territories.

As omnivores, they eat everything from small mammals like rodents and rabbits to birds, insects and plants. In their native range, from Europe to the Middle East, foxes have been suppressed by predators such as bears and wolves, but in Australia their numbers have soared.

Unfortunately, the suppression of dingoes in Australia following European colonization is at least partly responsible for the explosion in fox numbers, as there are not sufficient densities of dingoes to control foxes.

Foxes thrive in areas modified by humans. We show that their populations are denser around urban centers and that they thrive after land is cleared for agriculture. Fox population growth rates in agricultural regions increased significantly in the 1950s, due to large-scale agricultural expansion following World War II.

This research also showed that in arid areas, fox population cycles follow a boom and bust cycle, whereas their numbers appear more stable in agricultural landscapes.

Driving extinction

European red foxes and domestic cats brought to Australia kill around 300 million of Australia’s native animals each year and remain the main culprit in past and current extinctions.

Australia’s fox population numbers around 1.7 million, and the Invasive Species Council estimates that up to 16 mammal species have become extinct, mainly or partly due to foxes. This represents approximately 40% of the total extinctions since the arrival of Europeans.

Our new research provides important information about which native species have been under threat for the longest period, identifying areas that were potentially important refuges from foxes.

The adaptable simulation models we used to track fox expansion can be used for other invasive species that have not yet infiltrated all of Australia, such as giant toads. We hope these models will help us map the spread of other invasive species such as cats and potentially halt the decline of native wildlife in Australia.

Sean Tomlinson is a research associate in ecology and evolution at the University of Adelaide. Damien Fordham is Associate Professor of Global Change Ecology at the University of Adelaide. This article was originally published in Conversation.