Exact statistics are hard to come by, but there is plenty of anecdotal evidence to show that domestic pets and feral escapees are a major threat to our wildlife. Dogs have been responsible for the mauling and death of numerous cassowaries, tree kangaroos and possums. Sometimes these dogs are happily being walked by their owners and allowed to stray off the lead.
While there may be docile individuals, most cats instinctively kill anything they can catch or scratch including birds, skinks and geckos, snakes, native rodents, small possums and frogs. The hunting instinct of the cat is so strong, they hunt for pleasure as well as for feeding. Familiar to many cat owners is the dead animal left on the front doorstep as an unwanted gift!
Feral pigs are a major pest animal in the Wet Tropics. They are widely distributed, damage endangered ecosystems and compete with threatened species. Their habit of wallowing and rooting around the edges of watercourses and swamps disturbs natural vegetation, spoils water quality, causes erosion, allows weeds to grow and destroys the habitat of small native animals. Pigs compete for the food of other animals such as the cassowary and bettong. They spread weeds, diseases and parasites.
Pigs also cause damage to farmlands and crops. A study undertaken to measure pig damage on selected farms estimated costs to be $4,099 per annum for banana farms and $10,632 per annum for cane farms. Feral pigs may carry foot and mouth disease and there is the potential for this disease to spread into domestic animal populations at great expense to the industry.
Feral pigs are difficult to control because they are so adaptable. Their habitat is often inaccessible to humans. They reproduce so well that repeated control programs are required to reduce the population. Their omnivorous feeding habits give them a large variety of available food sources, and their home ranges are large. Control programs need to be conducted over a large area to be effective.
The cane toad is arguably one of Australia's worst environmental disasters, having now spread to New South Wales and the Northern Territory. Despite the warnings of some experts, 102 cane toads were shipped to Queensland from Hawaii in 1935 and they were released into cane fields in Gordonvale, just south of Cairns. As it turns out, the toads can't climb and their life cycle didn't coincide with that of the cane beetle which never came to the ground. So the cane toads didn't eat the cane beetles after all, but they did eat nearly everything else. Cane toads can reach well over 2kg in weight and achieve a snout to vent length of over 25cm. The toads also turned out to have a massive reproductive rate. Published accounts range from 20,000 eggs per female per year upwards to 60,000 eggs. Their tadpoles (often called toadpoles) develop faster than most Australian frog tadpoles. Toadpoles, like the toad itself, are toxic so an animal or fish that eats them dies.
Cane toads possess two glands behind the eyes that contain a milky poison. Snakes, birds and any other animal that eats a cane toad dies, often before it has even swallowed the offensive toad. Certain species of snake (such as the Keelback) can swallow cane toads without any side effects and some birds have learned to eat the toad by turning it over onto its back and devouring everything but the skin and those deadly glands. Cane toads compete with frogs for frogs food and breeding sites.
The Indian mynah is a very aggressive bird. It evicts other birds from their nests, dumps out their eggs and chases native birds from their roosting areas. The mynah prefers urban backyards, where it displaces many Australian species and is a strong competitor for food. This bird is brown with a black head and yellow feet, eye-patch and bill. It has a variable call and is often in pairs or small groups. Many other exotic animals are present in the region which are displacing native species (rather than preying on them). These include the Asian house gecko, sparrows, turtle doves, the big-headed ant and rats from Europe.