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Feral pigs in New Zealand as conservation pests and as potential hosts of bovine tuberculosis

The status, distribution, and ecology of wild pigs were assessed by Landcare Research whose analysis included assessments of the likely impacts of introduced wild pigs on indigenous conservation values, the potential role of wild pigs as a host for bovine tuberculosis (Tb) and an evaluation of the options for management of feral pigs.

Objectives:  To describe the legal status of feral pigs in New Zealand and review their distribution, density, biology and ecology. To collate and review existing information on the occurrence and prevalence of Tb in feral pig populations and assess the likely role of feral pigs as a maintenance host and vector of the disease. To assess the likely nature, scale and relative importance of feral pig impacts on indigenous conservation values. To describe the harvesting of feral pigs and their recreational, commercial and cultural value as a resource, including an assessment of their status in relation to the Treaty of Waitangi. To assess the likely annual number of, and reasons for, liberations of pigs into the wild and to suggest ways of preventing such liberations. To review options for feral pig management and control by the Department of Conservation and for the control of Tb within feral pig populations.

Main findings:  

Legal status:  The Wild Animal Control Act 1977 provides for ".. control of harmful species of introduced animals ..". The Conservation Act 1987 attributes "intrinsic value" to indigenous but not introduced species. Feral pigs may also be defined as pests and controlled upon production of a Pest Management Strategy under the Biosecurity Act 1993. Feral pigs are therefore pests where they adversely affect indigenous conservation values, where they have direct negative effects on production values, or where they pose an indirect risk to production values because they are a potential host for livestock diseases.

Distribution and density:  Feral pigs became widespread soon after their introduction to New Zealand in the late 18th century, but some early populations died out as land was developed for farming and for other unknown reasons. At present feral pigs occupy some 37% of New Zealand (approximately 92 000 sqkm). Their range is again increasing, primarily as a result of illegal liberations and the additional cover provided by the conversion of pasture to pine forests. Densities vary considerably, occasionally reaching over 100/sqkm in preferred habitats, but an estimated national population size of 110 000 suggests that average densities for the entire present range are likely to be about 1.2/sqkm. Densities appear to have declined in recent decades.

Movements:  Home range sizes in New Zealand of 30-200 ha have been recorded in rough pasture/beech forest habitats, compared with 550-16 000 ha in tussock grassland.

Diet:  Feral pigs are omnivorous, but in New Zealand appear to feed mainly on plant matter. However, animals, in particular earthworms, usually constitute a significant proportion of the diet, as pigs require protein-rich foods in order to breed successfully. 

Population dynamics:  By the standards of large mammals, feral pigs are particularly fecund and exponential rates of increase of up to 0.78 have been recorded; such populations could double in 11 months. Usually, however, most piglets die and populations can collapse when food becomes scarce. Pig populations appear to be more stable in areas such as grassland where the food supply is assumed to be relatively constant, but some forest-dwelling populations seem to fluctuate widely, perhaps as a consequence of the periodic abundance of fruit.

Bovine Tb:  Although infected feral pigs appear to occur in most or all areas where the disease is established in other wild animals and in livestock, few diseased pigs are found outside such areas. This suggests that feral pigs usually catch the disease from other animals. Although Tb transmission between pigs can and does occur, it is unclear whether the rate of transmission in the wild is sufficient to maintain the disease among pigs, or if they transmit it to other animals.

Impact on conservation values:  Pigs undoubtedly contributed to the disruption of indigenous ecosystems that followed the arrival of Europeans, but evidence is fragmentary. Their present impacts on the indigenous biota are also uncertain and it is unclear whether pigs are still changing ecosystems, or whether they have reached an equilibrium with the native biota. There appear to be relatively few situations where pigs are the pest primarily responsible for ongoing changes, although the high proportion of earthworms in their diet suggests a potential critical impact on litter and subterranean invertebrates. They appear to pose only a small direct threat to kiwi, but dogs (including those used to hunt pigs) can and do kill kiwi. Pigs also contribute, with possums, to ongoing decline in numbers of native snails, alter successional processes in native vegetation and restrict opportunities to restore indigenous ecosystems.

Feral pigs as a resource:  Most New Zealanders, and Maori in particular, view feral pigs more as a resource than as pests, contrary to their legal status. Such perceptions have important implications for the agencies that wish to control feral pigs as pests. In 1988, an estimated 23 000 hunters hunted pigs on approximately 300 000 days and killed about 100 000 pigs. About 12 500 of these pigs were sold, three-quarters of them illegally. Hunters continually (and illegally) liberate domestic and feral pigs to boost the number or quality of pigs within their established range, or to establish new hunting opportunities in areas previously free of pigs. At least 24 new populations have been established since 1989 and the number of pigs liberated annually is likely to exceed 100. 

Control:  Very little pig control is undertaken by state agencies, although a few hundred pigs are killed incidentally each year during DOC's goat control operations. However, DOC has eradicated some new populations (e.g. on Stewart Island in 1993) and regional councils occasionally kill pigs as a precaution against the spread of Tb. Strategic options for pig control are to do nothing, to implement sustained control, or to attempt eradication. The latter will usually be possible only on islands, or within fenced areas, or in small isolated areas on the mainland, and will often require use of several control techniques. Sustained control in forested areas is likely to rely mainly on hunting with trained dogs. The efficacy of this method can be enhanced by use of radio-telemetered dogs and 'Judas' pigs, especially to eradicate recently liberated or remnant populations.

Recommendations:  DOC should: (i) Attempt to stop the spread of feral pigs into areas without populations; (ii) where possible, eradicate populations from areas with high conservation values, as a precautionary measure; (iii) commence sustained control in areas with high conservation values confirmed by research as being at risk to pigs, and; (iv) elsewhere encourage (at minimal cost) recreational and commercial hunting.

AHB should: (i) Attempt to eradicate new populations of feral pigs known or thought to be infected with Tb when they occur outside areas in which Tb is already established in other wildlife, giving highest priority to areas with the most cattle; (ii) attempt to stop illegal liberations of pigs, with particular emphasis on translocation from within endemic areas, and; (iii) not specifically target pigs for control within endemic areas, but nevertheless attempt to reduce their numbers incidentally while undertaking control of other infected species.

DOC should commission research aimed at defining the importance of pig impacts on vegetation, kiwi, snails and other litter and soil invertebrates in both stable and unstable pig populations and in relation to the impacts of other pests.

AHB should attempt to determine whether Tb ever passes from feral pigs to other more likely vector species, particularly possums but also perhaps ferrets and/or deer.

Both agencies should develop improved methods for feral pig control, focused on improved use of dogs and better systems for the safe and efficient delivery of poisons.


Copyright [2012] by OSPRI
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