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Our EcoCheck series takes the pulse of some of Australia’s most important ecosystems to find out if they’re in good health or on the wane.

Australia’s Top End, Kimberley and Cape York Peninsula evoke images of vast, awe-inspiring and ancient landscapes. Whether on the hunt for a prized barramundi, admiring some of the oldest rock art in the world, or pursuing a spectacular palm cockatoo along a pristine river, hundreds of thousands of people flock to this region each year. But how are our vast northern landscapes faring environmentally, and what challenges are on the horizon?

Above 17° south, bounded by a rough line from Cairns, Queensland, to Derby, Western Australia, are the high-rainfall (more than 1,000mm a year) tropical savannas. These are the largest and most intact ecosystem of their kind on Earth. With the exception of some “smaller” pockets of rainforest (such as Queensland’s Kutini-Payamu (Iron Range) National Park), the vegetation of the region is dominated by mixed Eucalyptus forest and woodland with a grassy understorey.

Within the fire-prone Great Northern Savannas exist fire-sensitive communities such as these Allosyncapria ternata rainforests along the edge of the Arnhem Plateau in Kakadu National Park. Brett Murphy

There is a distinct monsoonal pattern of rainfall. Almost all of it falls during the wet season (December-March), followed by an extended dry (April-November). Wet-season rains drive abundant grass growth, which subsequently dries and fuels regular bushfires – making these landscapes among the most fire-prone on Earth. The dominant land tenures of the region are Indigenous, cattle grazing and conservation.

Cattle grazing is widespread in the Great Northern Savannas. Mark Ziembicki

These savannas are home to a vast array of plant and animal species. The Kimberley supports at least 2,000 native plant species, while the Cape York Peninsula has some 3,000. More than 400 bird and 100 mammal species call the region home, along with invertebrates such as moths, butterflies, ants and termites, and spiders. Many of the latter are still undescribed and poorly studied.

Many species, such as the scaly-tailed possum, are endemic to the region, meaning they are found nowhere else.

A large male antilopine wallaroo, endemic to tropical Australia. Euan Ritchie

The general lack of extensive habitat loss and modification, as compared to the broad-scale land clearing in southern Australia since European arrival, can give a false impression that the tropical savannas and their species are in good health. But research suggests otherwise, and considerable threats exist.

Fire-promoting weeds such as gamba grass, widely sown until very recently as fodder for cattle, are transforming habitats from diverse woodlands to burnt-out, low-diversity grasslands. Indeed, the fires themselves, which are considered too frequent and too late in the dry season at some locations, are now thought to be a primary driver of species loss.

Notable examples of wildlife in trouble include declines of many seed-eating birds, such as the spectacular Gouldian finch, and the catastrophic decline of native mammal species, most prominently in Australia’s largest national park, Kakadu.

Bauxite mining threatens the habitat of vulnerable Cape York palm cockatoos. Mark Ziembicki

Added pressures include bauxite mining, forestry and cattle grazing. The latter activity exerts strong pressures on the characteristically leached, nutrient-poor, tropical soils. Most recently, changes to Queensland’s land-clearing laws have led to virgin savanna woodland being cleared.

It is likely some threats may also combine to make matters worse for certain species. For instance, frequent fires, intensive cattle grazing and the overabundance of introduced species such as feral donkeys and horses all combine to remove vegetation cover. This, together with the presence of feral cats, makes some native animals more vulnerable to predation.

New threats

This globally significant ecosystem, already under threat, is facing new challenges too. Proposals to use the region as a food bowl for Asia are associated with calls for the damming of waterways and land clearing for agriculture.

This is against a backdrop of climate change, which among other effects may bring less predictable wet seasons, more frequent and intense storms (cyclones) and fires, and hotter, longer dry seasons. Such changes are not only likely to harm some species, but could also make those much-touted agricultural goals far more difficult to achieve.

Great opportunities exist in northern Australia, but we need to avoid the mistakes of the past. Mark Ziembicki

Great opportunities do exist in northern Australia, including carbon farming and expanded tourism enterprises. In some cases this might require difficult transitions, as already seen in parts of Cape York Peninsula, where often economically unviable cattle stations have become joint Indigenous and conservation-managed lands.

A key priority for the Great Northern Savannas should be to maintain people on country. It’s often thought that the solution to reducing environmental impacts is removing people from landscapes, but as people disappear so too does their stewardship and ability to manage and care for the land.

Importantly, and finally, we must also learn the historical lessons from southern Australia if we are to avoid making similar mistakes all over again, jeopardising the unique and precious values of the north.

Are you a researcher who studies an iconic Australian ecosystem and would like to give it an EcoCheck? Get in touch.

Euan Ritchie receives funding from Pozible, the Australia and Pacific Science Foundation, and the Australian Research Council. Euan Ritchie is affiliated with the Ecological Society of Australia and the Australian Mammal Society.

Brett Murphy receives funding from the Australian Research Council, the National Environmental Science Programme and the Hermon Slade Foundation.

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Australia's island wildlife is particularly vulnerable to invasive species. Roderick Eime/Flickr, CC BY

Australia has some 8,300 islands, many of them home to threatened species. But humans have introduced rodents and predators such as feral cats and foxes to many of these islands, devastating native wildlife and changing entire island ecosystems. Removing invasive mammals has proven to be a very effective tool for protecting island species.

As a result, the federal government has made it a priority to remove invasive vertebrates from islands where they pose the most severe threats to native plants and animals.

But choosing where to remove those invasives is difficult. We don’t have complete information about the distribution of native species and threats across the nation’s 8,300 islands, and we haven’t been able to predict where eradication will have the most benefit.

However, in a recent study published in Nature Communications, our global team of scientists looked at islands around the world to consider where we can get the biggest bang for our buck.

Eradicating cats, rats and pigs from Flinders Island in Tasmania would help save forty-spotted pardalotes. Francesco Veronesi, CC BY-SA It costs money to save species

The total cost of the recently completed rat and rabbit eradication on Macquarie Island was A$27 million. The proposed removal of rats from Lord Howe Island off New South Wales is expected to cost A$9 million.

Federal Environment Minister Josh Frydenberg has just announced funding to remove feral cats from five islands: Christmas Island, Dirk Hartog Island and the French Islands in Western Australia; and Bruny and King Islands in Tasmania.

Conservation dollars are limited, so it is important that these pricey interventions be focused on the islands where they will go the furthest toward conserving native island biodiversity.

Conversely, it is essential that we identify places where they won’t provide much benefit, either because a threatened species is likely to go extinct regardless of such interventions, or because the invasive species actually poses little threat.

It cost A$24 million to eradicate rats and rabbits from Macquarie Island. Macquarie Island image from www.shutterstock.com Island life

We analysed the effects of invasive mammals on 1,200 globally threatened species across more than 1,000 islands to develop a model for where eradicating invasive wildlife will provide the greatest benefits to island species.

We estimate nearly half of threatened species populations on islands could disappear without conservation efforts. But targeted eradication could prevent 40-75% of these losses.

We found that just a few types of invasive mammals – rats, cats, pigs, mongooses and weasels – are most strongly associated with the disappearance of native species from islands.

Importantly, our study shows that the impacts of invasive mammals vary widely across the type of native species (native amphibians, birds, reptiles or mammals) and the conditions of the islands on which they live.

For example, we found that removing invasive mammals from small, dry islands could halve the extirpation risk for threatened native birds and mammals, but doing so on large, wet islands would have less benefit.

Australia’s most important islands

Our study included thirty-three Australian islands, home to 17 species of globally threatened birds, mammals and amphibians including the woylie (or brush-tailed bettong), Tasmania devils, black-browed albatross and Cooloola sedgefrog.

Eighteen of these islands are also home to introduced rats, cats or pigs, which potentially threaten native species with extinction.

Traditionally, we might assume that eradicating cats and rats would always reduce bird extinctions. However, our study suggests otherwise.

Eradicating cats and rats could help northern quolls on some islands. Quoll image from www.shutterstock.com

Rat or cat eradication may have little benefit on some islands. This is either because these invasive species have relatively minor impacts in some island environments, or because the native population is likely to go extinct regardless of conservation interventions.

So our study shows that of these 18 islands, eradicating invasive species on only two would likely prevent extinction of three native species populations. These are the eradication of cats and rats on Groote Eylandt in the Northern Territory, which would avert the extirpation (that is, the island-level extinction) of the northern quoll and northern hopping mouse; and the eradication of cats, rats and pigs on Flinders Island in Tasmania, which would avert the extirpation of the forty-spotted pardalote.

While this sounds like a tiny number, remember we haven’t looked at all of Australia’s islands and the species that live on them. Indeed, we only included species considered threatened at a global level. For the other islands not included in our study, species threatened with extinction at regional or national scales may - or may not - benefit from eradicating invasive species. As more information comes in on these islands, our analysis can suggest which of these we should focus on.

The authors do not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and have disclosed no relevant affiliations beyond the academic appointment above.

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David Cragg-James
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