Feral animals of the tropical savannas

The feral cat: one newcomer that has the advantage over many native species in the survival stakes Photo: Greg Calvert Getting the cat into the bag: just what toll are feral cats exacting on wildlife? As an adjunct to a survey of fauna in the Desert Uplands, conducted by Dr Alex Kutt, the diets of feral cats were examined via stomach contents, which were systematically collected across the bioregion and the directly adjacent areas of the Mitchell Grass Downs and Northern Brigalow Belt. Samples were obtained by local professional kangaroo and pig shooters. A total of 194 cat guts were collected over two years comprising 1300 prey items. Initial sorting revealed some broad patterns: of all prey items identified, 16 per cent were birds, 33 per cent reptiles, 5 per cent amphibians, 25 per cent mammals, and 21per cent insects. And volume of prey was frightening: one cat had consumed two birds, one dunnart, one dragon and seven geckoes while another, one dunnart, three dragons, eight geckoes and one tree skink. Multiplied over a year, this translates to a significant amount. The influence of feral cats on native fauna, particularly in recently modified habitats, is not well understood, and regional information is sorely needed. Hopefully this data will contribute to the debate on the impact of feral cats, which currently does not receive nearly enough attention. from Savanna Links 19

Over the past 200 years, colonising peoples have brought to Australia a huge number of animals—mainly mammals—that did not evolve here. The results of these introductions have often been dramatic, changing the natural environment irrevocably because of the newcomers. Generally, animals (and plants) were brought here for a reason—hence, we speak of deliberate introductions—but often small numbers 'escaped' from human control and started self-sustaining feral populations.

Other introductions have been accidental; stowaway rats or cats jumping ship, for example, or small invertebrates arriving on imported produce or attached to ocean-going liners.

Even the dingo, often seen as quintessentially Aussie, is very closely related to the South-East Asian dog. Dingoes are in fact, fairly new arrivals introduced by humans probably about 3 to 5000 years ago.

Why ferals flourish

Whether accidental or deliberate, most animals introduced to Australia by humans thrived, usually because here they have escaped the predators and parasites that plague them in their areas of origin. As well, there are often fewer competitors. And the wide-open spaces with few large natural land barriers (except deserts) also helps some species spread.

Examples of ferals

Many of the best-known examples of introduced animals and their effects come from southern and central Australia. But the tropical savannas of the north have also suffered. Tropical Australia is a huge area of great conservation, agricultural and tourism potential, but it is now home to the following 'alien' species:

• Asian water buffalo
• Donkey
• Pig
• Horse
• Cane toad
• Cat
• Feral (unbranded) cattle
• Dingo/feral dog

Small populations also exist of:

• Banteng cattle (originally from Indonesia) on Cobourg Peninsula Sambar deer ( Cervus unicolor ), an Asian deer, with a population on Cobourg peninsula (but in Monsoon rainforest)
• Chital deer ( Cervus axis ) in parts of tropical western Queensland
• Rusa deer in some Torres Strait islands and North-East Island (next to Groote Eylandt)
• In the more arid parts of the zone are camels, spreading from their desert range.
• Feral goats occur in parts of Queensland's and Western Australia's savannas.
  

  Wild pigs could be the biggest threat to natural ecology in Australia's north Photo: Jim Mitchell

Australia's biggest scourge, the rabbit, is rare in the semi-arid tropics, but feral pigs are plentiful. Many experts see wild pigs as the biggest threat to the natural ecology of the north of the country.
 

Click on the sub-menu at left to find out more about selected individual feral animals. 

Impact of feral animals

Little research has taken place on the long-term impacts of ferals grazing in the savannas. Scientists have ideas of the likely effects, but little hard data that applies to large areas over many years. For example, experience in semi-arid areas of Africa suggests that camels can have a large effect on the land but the situation in equivalent parts of Australia has simply not been studied in sufficient detail.

The same holds true for feral pigs and the widely distributed feral cat. What effects, if any, these animals are having on native ecosystems are largely unquantified.

What we do know is that in some places, at certain times, some feral species can cause undesirable changes to native ecosystems, threaten the survival of other species, and become a serious nuisance and cost to farmers. But changes caused by ferals may be occurring slowly and insidiously without us being fully aware of the situation until it is too late.

In many parts of Australia dingoes have cross-bred with other dog types that have escaped domestication. Dingoes/feral dogs are a nuisance to pastoralists because they prey on lambs and calves and, when hunting in packs, will even harass fully grown adults. It is considered quite possible that dingoes caused thylacines to become extinct on the Australian mainland over the last few thousand years. But dingoes are now part of the accepted Australian fauna. Indeed, they may even help in the battle against newer ferals, for example, by keeping down fox numbers in temperate areas.

For most feral species, the future may be something similar to the dingo story—adaptation both by the species and by the rest of Australia to it, along with targeted damage control, rather than removal.

Management options

Management of feral animal populations and any associated problems is very difficult in the vast and sparsely populated areas of northern Australia. For a start, there is little information on the extent and size of feral populations and on the effects they may be causing.

It is not known whether impacts are always proportional to the density of the feral population. This is a key question when it comes to control. It is surely better to respond to measurable or observable impacts rather than the absolute density of a species.

There is also the problem of conflicting interests. Agriculturalists, traditional owners, tourism operators and conservationists all have different agendas. Whose viewpoint will be given priority? Control options are likely to be expensive and so can often only be justified if the target animal is causing clear economic loss. This monetary emphasis can mean that damage to unique but non-economic ecosystems may go unchecked.