Invasive species do much more damage in Australia and NZ than elsewhere. But why?

If you had to pick the countries where invasive species pose the biggest problems, you probably wouldn’t think of Australia.

The continent has some of the world’s strictest biosecurity, a small human population and a comparatively short period of colonial occupation. It is also girt by sea. All of this should make introduced species less of an issue.

But Australia is one of the top three nations affected by feral species, after the United States and New Zealand.

More than 4,000 species have been introduced at last count. Of these, 3,000 are sufficiently out of control to be considered invasive. The damage they do to crops, infrastructure and nature now costs around 1.25% of GDP – almost as much as Australia spends on aged care.

So why are determined efforts and natural advantages often unable to stem Australia’s feral tide?

Important answers come from island biogeography, a field of study pioneered by Alfred Russell Wallace. He is the scientist who discovered evolution through natural selection, independently from Charles Darwin but at the same time.

Islands are rather isolated environments, which means unique species are more likely to evolve. But isolation also means their inhabitants are particularly vulnerable to invasive species.

Known as the island continent, Australia demonstrates this on a huge scale. Australia has a high number of species found nowhere else, and is one of only 17 megadiverse nations. But the isolation that protected these species over millions of years has left many vulnerable to new threats brought by humans.

Every Australian knows common mynah birds are bad, rabbits are worse and feral cats kill native animals unchecked.

But how do we decide what makes an introduced species “good” or “bad” – and will we ever concede that some species are here to stay?

Our new series on invasive species answers these questions.

Lopsided ecosystems, unfilled niches and unwary species

Australia’s biodiversity is largely an accident of its insular history.

Australia was near the South Pole when it broke away from the supercontinent of Gondwana, and the cold climate limited the diversity of animals present. The animals and plants that did make it here afterward found a sparsely inhabited continent.

These colonising lineages often diversified into a wide variety of species. The continent is thus short on lineages, but rich in species.

Most famously, Australia is a marsupial paradise, with a dizzying array of pouched mammals. The continent had almost no ground-dwelling placental mammals until rodents arrived from Asia about eight million years ago.

Australia also has the highest diversity of unique plants of any country on earth.

Across most of the world, largely harmless colubroid snakes account for most snake species. But to the consternation of ophidiophobes (those scared of snakes), most Australian snakes are venomous elapids who descended from relatives of deadly front-fanged cobras.

This lopsided biodiversity makes Australia unusually vulnerable to invasion for three reasons.

First, invasive species can fill ecological niches not well used by native species.

Second, they can be unfamiliar predators or competitors against which Australian species have little defence.

Third, super-invader species are more likely to evolve on larger continents and invade Australia rather than the reverse.

Cats and rabbits, super invaders

A super invader is a fast-breeding species able to eat a broad range of foods, tolerate many different environments and spread quickly. These traits help them quickly colonise new places and outcompete native species.

Species evolving on large, well-connected landmasses are more likely to be good at invading new areas, as their evolutionary adaptations have been honed by encounters with a broader range of adversaries.

Many of Australia’s worst invasive species are super invaders, from cane toads to fire ants. But feral cats and rabbits stand out from the pack. These are Australia’s two costliest invaders.

Cats and rabbits both evolved on Eurasia, the largest landmass. Domestic cats emerged in the Middle East, from a local wildcat. European rabbits are native to Europe’s southwest, where they are actually endangered.

Eurasia’s range of species is much broader than Australia’s. Many wildcats and related species would have co-evolved with marsupials, who lived in Eurasia until around 14 million years ago.

But bilbies evolved in isolation and would never have met any placental predators, let alone cats.

Feral cats are a global issue. But Australia is the epicentre for extinctions driven by cats.

Rabbits have become a problem in many countries, but their voracious appetites and ability to outcompete native species such as bilbies means they have done the most damage in Australia.

Both cats and rabbits have exploited ecological niches not well used by native Australian animals.

No marsupial predators have diets as broad as feral cats, nor do they roam everywhere from the baking deserts of Central Australia to the freezing forests of Tasmania.

Australia has no mid-sized marsupials able to breed like, well, rabbits and devour all greenery in sight. Bettongs prefer roots and fungi, while bilbies are omnivores. Both reproduce much more slowly.

Every other continent has native cat (felids) and rabbit relatives (lagomorphs). But Australia did not, and so our species were uniquely unprepared.

Before human colonisation, the main mammals preying on bilbies were quolls, which are smaller and slower than feral cats.

Many native plants – especially in drier regions – also produce seedlings vulnerable to the ravenous grazing of rabbits.

Worse, rabbits often avoid invasive weeds such as lantana (native to the Americas), as these co-evolved with rabbits or their relatives and acquired defences such as an unpleasant taste.

The blessing and curse of islands

What about New Zealand, which is even more isolated than Australia and has an even more lopsided biota? It’s here that introduced animals from Australia have done the most damage.

While Australia only has a few native land-dwelling placental mammals, New Zealand had none at all.

When wallabies and possums were introduced from Australia, it was easy for them to run rampant and become pests.

But the same species aren’t such a problem elsewhere, as they face much stiffer competition when trying to get a foothold on landmasses packed with placental mammals.

Life on an island can be splendid isolation – until new challengers arrive. This is why Australia (and New Zealand) need such tough biosecurity controls – and why they still suffer the most damage from invasive species.