Introduced and feral pigs
Wild pigs or their domestic and feral descendants have been widely distributed by man as a source of food, and naturalized populations have become established on all continents except Antartica, and on a great many oceanic islands. The overwhelming majority of these feral populations derive from Eurasian wild boar, although the Sulawesi warty pig has also been domesticated and introduced in some areas.
Feral pigs are a conservation problem where the co-occur with threatened native taxa, as in parts of the Philippines, because they can potentially hybridize with these species, adding to their extinction threat. Also, given their evident adaptibility, their omnivorous diet, and their high reproductive rate, feral pigs generally have a profound and often negative impact on the ecosystems to which they have been reintroduced. In many areas, they also cause serious damage to agriculture, pasture and forestry, and they act as a reservoir for various pathogenic organisms transmissible to humans and livestock.
With a few notable exceptions - see the IUCN SSC Pigs, Peccaries and Hippos Action Plan - feral pigs should be treated as exotic pests and strictly controlled or eradicated as appropriate. This is not easy as is shown below for the northern Americas and Australia where the range or feral pigs is still expanding despite decades of control measures. Also, in South America, feral Sus scrofa appears to be spreading and significant numbers now co-occur with native peccary populations. Total eradication of feral pig populations has been successful though on a number of islands, including Santiago Island in the Galápagos Archipelago, Ecuador and Santa Cruz Island in California.
In North America, feral pigs have been present since the 16th century. Until recently, their range was restricted to the southern states (the red areas on the county map to the left).In the mid-1990s, wildlife management researchers began to document a substantial expansion of feral pig range (the green areas). From 2004 to 2007, additional range has been occupied (the orange area), in the northern states and in Canadian provinces. Much of the range expansion has been caused by accidental and intentional releases of pigs that are largely Eurasian wild boar in origin. In these northern areas, densities are very low.
In Australia, feral pigs inhabit an estimated 45% (3.43 million square kilometers). They occur in all states and territories, and on some large coastal island (see here for details). Feral pigs cost the agricultural industry over $100 million each year8, largely via predation of lambs, competition with livestock and damage to property and crops. As a benefit, Australia’s supply of feral pig or ‘wild boar’ meat to international markets — mainly Europe and Asia — generates around $20 million each year.
Feral pigs in Australia cause extensive damage to natural habitats by turning over vast areas of soil when rooting for food. They also wallow and foul up water sources, trample and consume native vegetation and facilitate the spread of weeds and Phytophthora. Grounddwelling fauna like frogs and turtles are easy prey for digging pigs. Some in Australia see feral pigs as a food and recreational resource, and hunting plays a role in managing this highly destructive species. But the scope of the damage caused by feral pigs to propertyand landscape, as well as their potential for disease transmission, far outweigh any benefits of hunting and harvesting. For example, the social consequences of a foot and mouth disease outbreak in Australia would be catastrophic for individual landholders and farming communities. Harmful effects would resonate nationally, not just in regional Australia (see http://www.feral.org.au/).
Relevant Literature
Cruza, F., C. J. Donlan, K. Campbella, and V. Carriona. 2004. Conservation action in the Galàpagos: feral pig (Sus scrofa) eradication from Santiago Island. Biological Conservation 121:473-478.
Parkes, J. P., D. S. L. Ramsey, N. Macdonald, K. Walker, S. McKnight, B. S. Cohen, and S. A. Morrison. 2010. Rapid eradication of feral pigs (Sus scrofa) from Santa Cruz Island, California. Biological Conservation 143:634-641.
West, P. 2008. Assessing Invasive Animals in Australia. National Land & Water Resources Audit and Invasive Animals CRC, Canberra.