Species
Feral goat (Capra hircus)
Goats arrived in Australia and New Zealand with the first European settlers. Current feral goat populations are descended from these and subsequent animals that were introduced for a variety of reasons. During the 19th Century, goats were set free on islands and on the mainland by mariners to ensure emergency supplies of food. However, most feral goat populations have a domestic origin. For example, cashmere goats were imported into South Australia in 1837, and in the 1860s there was an attempt to start a goat fibre industry using angora and cashmere goats imported from Asia. This industry collapsed in the 1920s and some herds were set free. Goats, particularly the milking breeds, were also taken around Australia and New Zealand with settlers, railway construction gangs, and miners. More recently, feral goat populations have established from goats used to control weeds in plantation forests and woody weeds in inland New South Wales and Queensland. Current feral goat populations reflect these mixed origins.
Domestic goats occur on all continents (except Antarctica), but their feral descendants are much less prevalent, occurring widely only in Australia, New Zealand, and on many small islands. In 1993 there were about 2.6 million feral goats in Australia but this number has fluctuated widely under the influence of extended dry periods and the effectiveness of management programs. In Australia, most feral goats live in the semi-arid pastoral areas used for sheep farming where food is usually abundant, regular water is provided by people, and natural predators such as dingoes and feral dogs are controlled to protect the sheep.
Under domestication, goats were selected for characteristics of value to people. Many of these characters ensure that feral goats are effective pests. Harvested populations of feral goats can increase by over 50% per annum if harvesting stops, because goats become sexually mature within their first year, have extended breeding seasons enabling them to have up to 2 litters a year, can conceive while still lactating, and can produce more than one kid per pregnancy. Their impact on the environment is enhanced by their herding behaviour which can increase the intensity of their browsing, and by their ability to use a wide variety of food plants in a wide variety of habitats.
In wetter, more productive areas, the home range of a feral goat may be as small as one square kilometre while in the Australian arid rangelands, it may be up to 600 square kilometres (average 380 square kilometres) for sub-adult males. The home ranges in the arid zone are centred on permanent water and the range decreases in droughts when goats have to drink more frequently.
Feral goats can carry many internal and external parasites, some of which can also infest sheep. The importance of cross-infection is unknown, although probably low. The bacterial disease melioidosis may limit the spread of goats, and the rickettsial disease Q-fever is carried by goats and may be transmitted to people. Goats also are potential hosts of exotic diseases such as Foot-And-Mouth Disease.
In Australia, feral goats have been estimated to cause losses to pastoralism of $25 million per year. The estimate does not include the costs associated with the impact of goats on the environment, of soil erosion, or pasture degradation which has not been calculated. Feral goats also adversely affect conservation values and biological diversity by damaging the vegetation and competing with native fauna. Their damage is most obvious and most severe on small islands.
It is difficult to differentiate the particular impact of feral goats on single species of indigenous plants because usually they are only one of several herbivores, both native and exotic, that browse the plants.
On the positive side, commercial exploitation of feral goats is an industry worth about $29 million annually. Many pastoralists in Australia now consider the capture and sale of feral goats to be an essential part of their business. Recently, feral goats have been crossed with the South African Boer goat to produce a heavier animal for export.
Local eradication is usually only possible on small islands and in some mainland pockets. In most areas, sustained management is required. Ideally managers need to understand the relationship between the density of feral goats and the damage that they cause so that they can determine how to maximise the benefits compared to the costs of management.
High density populations of goats in accessible, semi-arid areas are best reduced initially by mustering or trapping at water so that the costs of control can be offset by the sale of the captured goats. To further reduce the population or where the density of goats is too low to be economically harvested, aerial or ground shooting is an efficient method. Aerial shooting from helicopters is best in difficult terrain, and/or at low goat densities. Management of very low density colonising or remnant populations of goats is assisted by the use of radio-tagged Judas goats. Fences have been used to exclude goats from some areas although they need to be maintained regularly.
It is important that managers consider the advantages and disadvantages of each control method before use, and the relevant codes of practice followed. Usually, no single control method will be suitable or most efficient for long-term sustained management operations, and a combination of techniques need be used.
