Feral pig or wild boar (Sus scrofa)
History and distribution
Captain Cook presented some pigs to the Maori and released others during his second and third voyages to New Zealand during 1773–1777. The first official introduction to Australia was with the first fleet in 1788. Feral pigs are habitat generalists and have colonised subalpine grasslands and forests, dry woodlands, tropical rainforests, semi-arid and monsoonal floodplains, swamps and other wetlands in many parts of the Northern Territory, Queensland, New South Wales, and other states and territories. Their distribution in inland or seasonally dry areas of Australia is restricted to the vicinity of watercourses and their associated floodplains. In the more forest-covered parts of eastern Australia and south-west Western Australia, populations are still spreading, often through deliberate or accidental releases. In New Zealand, feral pigs are also distributed throughout the North and South Islands where there is suitable habitat.
Once established, colonies of feral pigs build up rapidly in many areas. Estimates of population size vary between 3.5 million and 23.5 million, inhabiting 38% of Australia, but their distribution and abundance can vary markedly from year to year according to environmental conditions.
The biology and ecology of feral pigs are two of the major reasons why they are such an important and successful vertebrate pest in Australia. Their large robust bodies, snouts specially developed for rooting up the ground, omnivorous diet and adaptive activity patterns allow them to live in a wide range of habitats. Adult males usually weigh between 75 and 115 kilograms and sows 75 kilograms, although the weight varies with the quality of the habitat. In cooler and wetter areas feral pigs can reach 175 kilograms while boars over 200 kilograms are regularly caught in New Zealand. Feral pigs will breed continuously if there is sufficient, high quality food but in most areas they are seasonal breeders. For example, in the NSW highlands, young are mainly born in summer and autumn whereas under suitable conditions they breed all year on the flood plains of western NSW.
Sows breed at about 25 to 30 kilograms, which they reach at about seven to 12 months, with an average litter of six. Two litters a year are possible in good conditions although one litter a year is more common. Feral pigs have a reproductive rate that is closer to rabbits than other large herbivores and, potentially, populations can increase by 86% a year. Hence feral pigs can quickly recover from the impact of management or drought.
Feral pigs are opportunistic omnivores that prefer green vegetation, a wide variety of animal material, fruits and grain. Other foods include underground, starch-rich plant material such as roots and bulbs. They will also take fungi, earthworms, snails, eggs of ground nesting birds, turtle eggs, lambs and carrion. Like domestic pigs, feral pigs need a diet high in protein (more than 15%) in order to breed and raise their young.
Feral pigs cause agricultural damage through predation of newborn lambs, reduction in crop yields, damage to fences and water sources, and competition with stock for feed by consuming or damaging pasture. They also are considered a major threat to stock as a potential carrier of exotic diseases, with the major concern being their role as a reservoir for Foot-And-Mouth Disease should it ever become established in Australia or New Zealand. However, they are also an economic resource for game meat, an industry that is worth approximately $20 million a year.
While feral pigs are also considered an environmental pest, there have been no studies that clearly demonstrate and quantify the damage. Their major damage is likely to be habitat degradation through selective feeding, trampling and rooting for underground parts of plants and invertebrates, as well as predation on, competition with, or disturbance of a range of animals. The most obvious perceived environmental damage by feral pigs is based on their rooting up of soils, grasslands or forest litter, particularly along drainage lines, moist gullies and around swamps and lagoons, or after rain, when the ground is softer.
Poisoning, primarily using 1080 poison in grain or meat baits, is used in rural communities to manage the damage due to pigs. It requires appropriate free-feeding with non-toxic bait to attract pigs before the poison bait is used. Free-feeding also helps to reduce the risk of loss to non-target animals. Shooting from helicopters is efficient and provides a quick knockdown to protect susceptible enterprises from short-term damage. Pig populations can recover rapidly between shooting and poisoning episodes. Shooting from the ground, with or without dogs, is generally considered to play an insignificant role in damage control except where it is intensively conducted on small accessible populations.
Trapping can be effective, but results are variable, being affected by season, trap type and site, pre-baiting techniques and trapping frequency. An advantage of trapping is that it can be fitted into routine property activities and the trapped animals sold to the game meat market. There are currently no biological or fertility control agents suitable for use against feral pigs. Best results have been obtained from integrated management using a range of control techniques.