Part III: Conservation and Management328
the early attempts at control in Australia, the New Zealand pig
population has been left to establish and spread unregulated.
The establishment of feral pigs throughout New Zealand has
benefitted from both their adaptable diet and the lack of defen-
sive mechanisms in New Zealand native vegetation. The spread of
pigs throughout the country was aided by various pig releases by
European expeditions and by Māori settlers (Challies 1975). More
recently, illegal liberations of feral pigs for hunting purposes have
been responsible for pig population range expansion (McIlroy
2001). The most recent estimates suggest that pigs now occupy
approximately 93,000 km^2 (35 per cent) of the New Zealand
land area (Fraser et al. 2000) extending their range into Otago,
Southland and the West Coast of the South Island (Figure 30.4).
The New Zealand feral pig is descended from domestic pig breeds
such as Captain Cookers, Tamworths, Berkshires, Canterbury
grey/blues and large whites (McIlroy, 2001). However, the natu-
ralized feral pig can be distinguished from domestic pigs as they
are smaller and more muscular, with massive forequarters and
smaller, shorter hindquarters (King 2005).Impacts of Feral Pigs in Australia and New Zealand
Feral pigs have an extremely wide range of impacts on environ-
mental, agricultural, and social values in Australia and New
Zealand. Much of this breadth can be attributed to the diver-
sity of habitat types occupied by feral pigs, combined with
their generalist feeding behaviour and their ability to change
the physical structure of their environment through foragingactivity such as rooting through the soil in search of subterranean
food such as geophytes, plant roots, fungi, and arthropods.
Despite the great variety and perceived importance of pig dam-
age, there have been few comprehensive studies of feral pig
impacts, particularly in New Zealand. Most studies of physi-
cal damage to the environment and agricultural resources have
been observational or have examined only part of the system
in isolation. The conclusions of these studies have been largely
speculative. However, their accumulated weight, combined with
a handful of manipulative studies, strongly supports the com-
mon view that feral pigs are one of Australia’s most destructive
vertebrate pests. In New Zealand, feral pig research is even more
limited, although recent research has highlighted the many
environmental impacts pigs may have in New Zealand forests
(Krull et al. 2013a, 2013b; Krull & Egeter 2016).Environmental Impacts
In both Australia and New Zealand, the main sources of damage
to environmental and biodiversity values are generally thought
to be predation, habitat degradation, and competition for food.
Observational studies indicate that selective and indiscriminate
herbivory or ground rooting can alter plant community struc-
ture and composition (e.g. Alexiou 1983; Thompson & Challies
1988; Webber et al. 2010; Krull et al. 2013a). Further investi-
gations suggest that these impacts could be compounded by
dispersal of invasive plant propagules (Lynes & Campbell 2000;
Setter et al. 2002) or pathogens (Krull et al. 2013b; Li et al. 2014;
Bassett et al. 2017), but his has not yet been conclusively demon-
strated in the field.
The strongest inference for pig impacts on plant communities
has come from studies in which pig density has been repeatedly
manipulated or where pigs have been excluded from small-scale
plots for extended periods. These have shown that pig foraging
activity can inhibit recruitment and plant community struc-
ture or composition in landscapes ranging from sub-alpine
grasslands and woodlands (Hone 2002, 2012) to tropical and
subtropical lowland forests (Mitchell et al. 2007; Melzer et al.
2009; Taylor et al. 2011), ephemeral tropical lagoons (Doupé
et al. 2010), and temperate forest in New Zealand (Krull et al.
2013a) (Figure 30.5). Sites exposed to pig damage generally
have less surface-level biomass than do sites that are protected
(Mitchell et al. 2007; Taylor et al. 2011), and the magnitude of
this impact can be positively related to pig abundance or activ-
ity (Hone 2012; Krull et al. 2016). However, exclusion plot
studies have not always provided clear or consistent evidence
of major impacts (Elledge et al. 2010). Contrary to Australia,
New Zealand’s ecosystem has evolved in the absence of ground-
dwelling mammals and mammalian biotic disturbance agents
and may be more vulnerable to the impacts of feral pig ground
disturbance (Krull et al. 2013a).
Feral pigs are also thought to adversely affect many native
animal populations, either directly through predation or
indirectly through exploitative competition or habitat modi-
fication. Pigs are known to prey on many species, particu-
larly slow- moving or immobile animals such as aestivating
freshwater turtles (Fordham et al. 2006), marine turtle eggs
(Whytlaw et al. 2013), ground-nesting birds (Challies 1975;Figure 30.4 Estimated distribution of feral pigs on mainland New Zealand
in 2007. Pigs are also present on the Chatham Islands (not shown). Adapted
from Biodiversity Data Inventory (2008) (Crown Copyright: Department of
Conservation Te Papa Atawhai, 2016)..03212:55:46