Chapter 30: Feral pigs in Australia and New Zealand329
Miller & Mullette 1985; Flux 2002), frogs (Krull & Egeter 2016),
snakes in Australia (Gentle et al. 2015; Wishart et al. 2015),
and giant land snails (Powelliphanta spp.) in New Zealand
(Coleman et al. 2001). Predation by feral pigs poses a serious
threat to the persistence of snake-necked turtle Chelodina
rugosa populations in northern Australia (Fordham et al. 2008),
but little is known about the population-level impacts of feral
pig predation on other animal species or populations (Bengsen
et al. 2014b). Similarly, degradation of aquatic habitats resulting
from soil and plant disturbance by feral pigs is likely to have seri-
ous adverse impacts on aquatic or amphibian fauna (Richards
et al. 1993; Baber et al. 2006; Doupé et al. 2009), but the extent
and importance of any such impacts have not been described.
Finally, in Australia, dietary and foraging studies suggest that
pigs are likely to compete with some threatened species for food
(e.g. Laurance & Harrington 1997), but again, conclusive evi-
dence of adverse impacts on populations is lacking.
Agricultural and Economic Impacts
The broad distribution and adaptability of feral pigs also
allows them to exploit a wide range of agricultural systems. In
Australia, dietary studies indicate that crops can be an impor-
tant food source when they are available (Gentle et al. 2015;
Wishart et al. 2015), and pigs can cause considerable damage
to the production of crops such as cereals (e.g. sorghum, maize,
wheat, oats, and barley), tropical fruits (e.g. bananas, melons),
and sugarcane through consumption and trampling. In the
Northern Territory, pigs destroyed up to 50 per cent of sorghum
and maize crops surveyed, but usually less than 10 per cent
(Caley 1993). Across 19 farms in north Queensland, pig damage
to sugarcane production was estimated to have cost 3.5 per cent
of total production value, or AUD 237,000 in 2002 (over USD
230,000 in 2014 terms; Mitchell & Dorney 2002). During the
same period, total loss to banana production across 11 farms in
the same region was estimated at less than one per cent of total
production (over USD 20,000 damage in 2014 terms; Mitchell
& Dorney 2002). Despite the oft-made argument that reliable
economic estimates of damage are necessary for cost-effective
damage reduction (Caley 1993; Hone 1994, 2007), the economic
costs of crop destruction by pigs have not been estimated for
most agricultural systems affected by pigs in Australia. The eco-
nomic impact of pigs in New Zealand has also had little atten-
tion. However, in North Canterbury one farmer claimed a loss
of NZD 32,000 in stock units and another NZD 10,000 loss of
pasture (Parkes 2006).
Pigs can also cause major losses to animal production
enterprises, most notably by killing lambs. A manipulative
experiment in the semi-arid rangelands of western New South
Wales found that predation rates increased asymptotically with
increasing pig densities, peaking at about 30 per cent of lambs
in a flock being killed (Choquenot et al. 1997). Similar preda-
tion rates have been recorded in other regions (Plant et al. 1978;
Pavlov et al. 1981), and it has been noted that some individual
pigs may be more prone to killing lambs than others (Pavlov &
Hone 1982). Records of lamb predation in New Zealand are
sparse. One farmer reported a loss of over 50 per cent of his
lambs due to pig predation, although this may be a localized
problem in areas with high feral pig densities (Parkes 2006).Figure 30.5 Feral pig damage
around the edge of a lagoon in
Rinyirru (Lakefield) National Park, in
tropical North Queensland. The belt
of damage widens as the dry season
progresses and the surface area of
water recedes (photo by Jim Mitchell)..03212:55:46