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Potentially Misidentified Species:
The only wild pig native to North America is the collared peccary
(Tayassu tajacu). In the US, this species is restricted to desert and thorn
scrub habitats of Arizona, New Mexico, and south Texas. Elsewhere in the
United States, the feral pig should be unmistakable.
II. HABITAT AND DISTRIBUTION
Regional Occurrence:
Feral Pigs occur within all US Gulf states, including Florida (Whitaker 1988).
IRL Distribution:
Feral Pigs occur in terrestrial habitats of all six counties within the IRL watershed. In fact, they occur in all 67 Florida counties (Belden 1993).
III. LIFE HISTORY AND POPULATION BIOLOGY
Age, Size, Lifespan:
Nowak (1991) reports that domestic hogs can reach 450 kg, and their feral counterparts are on the same order.
Abundance:
Belden (1993) states that in the United States, Florida's feral
pig population is second only to Texas. They occur in every
county in Florida and occupy a variety of habitat types, though
urbanized areas and areas with major agricultural operations lack
established populations. Trapping, hunting and agricultural
depredation control measures have been undertaken in much of the
state to control wild pig populations (Belden 1993).
Reproduction:
In temperate regions breeding in S. scrofa is confined to the spring, whereas
in subtropical climates the breeding season is protracted. In the
tropics, breeding can occur throughout the year. Regardless of
location, peek breeding coincides with the rainy season (Nowak
1991).
Both sexes usually reach sexual maturity in the first year of
life—between 8-12 months in males and as early as 5-8 months in
females (Johnson et al. 1982). Despite early onset of maturity,
female feral pigs usually do not breed prior to 18 months' age,
while males tend not to achieve reproductive success until they
are fully grown at approximately age five. The estrous cycle of
female pigs is approximately 21 days (Ingles 1965).
Adult males are solitary outside of the breeding season while
females and juveniles are gregarious (Gingerich 1994). Females
leave the group to nest and give birth. This nesting behavior is
atypical of ungulate (hoofed) mammals. Litters usually consist of
between 3 and 12 young and females generally produce one or two
litters each season throughout their reproductive lives (Ingles
1965, Gingerich 1994).
Embryology:
Gestation varies between 100 and 140 days. Young are weaned in 3-4
months and often leave their mother before the next litter if the
mother produces multiple litters in a breeding season (Nowack
1991). Early mortality rates can be high. Baber and Coblentz
(1986) reported that 58% of piglets died before weaning.
IV. PHYSICAL TOLERANCES
Temperature:
Feral pigs occur from temperate climates to the tropics and their
activity patterns are tied to the temperature regimes of the
various climates in which they are found. These mammals lack
sweat glands and rely greatly on behavioral means of body
temperature regulation (Gingerich 1994). In hot, tropical
climates, peak activity occurs in the early morning and late
afternoon (Diong 1982), or during the night (Sekhar 1998). In
temperate regions with pronounced seasonal variation, peak
activity times and foraging areas change to take advantage of
biologically accommodating temperatures (Belden and Pelton 1975).
For example, feral pigs on California's Santa Cruz Island are
most active at mid-day during wet winter months, and at night
during the warm, dry summer (Van Vuren 1984).
Other Physical Tolerances:
Feral pigs will only become established in hot climates if water
supplies are adequate to allow survival (Gingerich 1994).
V. COMMUNITY ECOLOGY
Trophic Mode:
Feral pigs are omnivorous. They use their tusks to root through
the ground in search of roots, tubers, bulbs, worms, insects, slugs
and snails, and other dietary items. Additionally they will
consume fallen acorns and other nuts, frogs, lizards and snakes,
rodents and other vulnerable mammals, and bird eggs (Lowery 1974,
Bratton et al. 1982, Laycock 1984, Baber and Coblentz 1987,
Gingerich 1994). Feral pig feeding activity can impact population
desities of preferred prey types (Meads et al. 1984).
Feral pigs are highly adaptable and opportunistic in terms of diet,
and seasonal dietary shifts occur as food items become either
scarce or more abundant. For example, Wood and Roark (1980) note
that in South Carolina feral pig populations, acorns and other nuts
and fruits make up the bulk of the diet in the fall and winter when
they are abundant. In the spring, pigs shift to foliage and
herbaceous vegetation, and to tubers and roots in the summer. As a
result of these dietary shifts, the degree of destructiveness
caused by rooting can also vary by season.
Associated Species:
Although adult feral pigs are safe from most predators other than
man, young animals are reportedly vulnerable to eagles and hawks,
owls, foxes, and bobcats (Laycock 1984, Gingerich 1994). In south
Florida, panthers are capable of taking adult pigs as prey
(Gingerich 1994).
VI. INVASION INFORMATION
Invasion History:
The historic native range of S. scrofa encompassed Europe and
extended through continental Asia south and east into Malaysia, and
into the islands of Sumatra and Java (Ickes et al. 2005). Sus scrofa is
now extinct across much of this historic range (Tisdell 1982).
Pigs were among the first mammals to be domesticated by man, beginning in China
some 7,000 years ago and possibly dating further back to 10,000 B.C. in the
region that is now Thailand (Nowack 1991). Several millennia of selective
breeding have yielded a domesticated animal that is morphologically quite
distinct from the wild type from which they derived.
The first introduction to the present-day United States may have been
intentional introduction of domesticated hogs to the Hawaiian islands by
Polynesians perhaps 1,000 years ago (Nowack 1991).
The first introduction of domestic hogs to the continental US is historically
documented. A vessel captained by the Spanish explorer Hernando De Soto and
carrying domestic hogs destined for the New World landed on the Gulf Coast in
1539 (Lowery 1974, Gingerich 1994). Intentional or accidental release of
animals derived from these stocks likely represent the source of the first
feral pig populations in the continental US and in the Gulf and southeast
regions.
Feral pigs currently found within the United States represent a combination of
descendant lines of European wild boars originally released for sport hunting
purposes and feral animals derived from escaped domestic pigs. These readily
interbreed where they co-occur (Whitaker 1988). The greater the percentage of
wild boar a feral pig contains, the more it will resemble the wild type in
appearance, typically bearing a bristly coat and mane, a straight tail, and
impressive tusks (Whitaker 1988).
Gingerich (1994) suggests that Florida's wild hogs may represent an amalgam of
lines derived from Spanish and Russian wild boars, European hunting stock, and
escaped domestic hogs.
Potential to Compete With Natives:
The feeding activities of feral pigs may preempt dietary resources
from co-occurring animal populations. More importantly, the
omnivorous nature and, particularly, the destructive rooting habits
of feral pigs make them particularly troublesome invaders.
Rooting digs up and overturns sizable patches of earth, destroys
vegetation and seed banks, and exposes tree roots. Soil nutrient
leaching is accelerated (Kotanen 1994, Singer et al.
1984, Arrington et al. 1999).
Ground nesting birds and other species may be negatively impacted.
Elsewhere in the United States where feral pigs occur, species such
as northern short-tailed shrews (Blarina brevicauda), southern
red-backed voles (Cleithronomys gapperi), and red-cheeked salamander (Plethodon jordani)
are deemed at-risk (Laylock 1984, Singer et al. 1984). Endemic
herbaceous vegetation such as Clingman's hedgenettle (Stachys
clingmanii) and Virginia chain fern (Woodwardia virginiana) may
also be impacted (Bratton et al. 1982).
In Hawaii, feral pigs kill several native tree species (by felling
or barking them) in pursuit of native tree ferns that are a
dietary staple (Diong 1982). On Santiago Island in Ecuador, egg
predation by feral pigs has reduced giant tortoise and sea turtle
population numbers (MacFarland et al. 1974, Green and Ortiz 1982, Coblentz and Baber
1987).
Where feral pigs occur in association with wetlands and coastal marshes,
pig foraging may add to the loss of these already imperiled
habitats. There is at least some indication, however, that plant
diversity in some instances may actually increase on localized
scales in response to disturbance by pigs, e.g., if pioneering
species move into areas that have been upturned by rooting pigs
(Arrinton et al. 1999, Ford and Grace 1998).
Baber and Coblentz (1987) indicate that feral pigs represent the
most successful non-native large mammal in the United States.
Possible Economic Consequences of Invasion:
Landowners and farmers regularly report damage and loss due to
feral pig activity. Delicate food crops like corn, oats, wheat,
and, soybeans are vulnerable, as are young trees planted in
silviculture operations. Home gardens often suffer damage from
these animals. A 1998 study by Frederick that surveyed 40
California counties estimated economic loss resulting from pig
rooting at $1.73 million (Frederick 1998).
Natural habitats are also susceptible to damage from feral pig
populations. A study by Singer et al. (1984) monitoring feral pigs
within Great Smokey Mountains National Park reports that the
destructive foraging of these invaders exposed several thousand
tree roots per hectare, reduced plant cover by as much as 80%, and
increased bare ground by nearly 90%. Forest litter and soil bulk
density were also greatly reduced while erosion and nutrient loss
from the forest floor to receiving river waters was doubled (see
also Peine and Farmer 1990). Habitat
alteration may include loss of native vegetation and spread of
opportunistic weeds into newly disturbed areas.
Feral pigs represent a potential source of disease. They carry
pseudorabies, (Aujeszky's disease, porcine herpesvirus 1), a viral
swine disease of considerable economic importance to the hog
industry. Cattle are susceptible as secondary hosts, and infection
results in the cattle disease known as mad itch. Rats, dogs, and
horses are also known secondary hosts, as are populations of wild
animals such as panthers (Fenner et al. 1993, Gingerich 1994).
Feral pigs are also a source of trichinosis and of swine
brucellosis which is potentially fatal in humans (Gingerich 1994).
Leptospirosis, foot-and-mouth disease, Japanese encephalitis and
the parasite Toxoplasma gondii are other disease agents harbored by
feral pigs (Tolleson et al. 1995, Hampton
et al. 2004, Gauss et al. 2005).
Newly emerging evidence has also implicated feral pigs in a
recent (2006) outbreak of E. coli spinach contamination in
California that killed at least three people and caused illness in
at least 200. The proposed infection pathway suggests that feral
pigs transmitted the pathogenic E. coli strain to spinach fields
from adjacent cattle pastures. Samples from cow manure in the
pastures tested positive for the same bacterial strain responsible
for the disease outbreak.
The World Conservation Union's Invasive Species Specialist Group
(ISSG) lists feral pigs as among "100 of the world's worst invasive alien
species" and recognizes them as potentially major drivers of extinction and ecosystem change.
VII.
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Report by:
J. Masterson, Smithsonian Marine Station
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