Final Comprehensive Conservation Plan - U.S. Fish and Wildlife ...

Pigs (Sus scrofa)
Hakalau Forest National Wildlife Refuge
Comprehensive Conservation Plan
Pigs that occur in Hawai„i are likely to be a blend descended from two ancestral types introduced on
separate occasions. Polynesians first brought pigs to the islands as a food source around 1,500 years
ago. Captain Cook subsequently brought European pigs to the islands in 1778 (Tomich 1986). Pigs
descended from European strains were generally larger, more fecund, and more nomadic than their
Polynesian counterparts (Van Driesche and Van Driesche 2000). Although pigs have been eradicated
from numerous islands worldwide, these animals remain highly abundant in Hawaiian island
ecosystems (Courchamp et al. 2003, Crux et al. 2005). They occupy every main island in the
Hawaiian archipelago. The pig population is largest on Hawai„i Island (Tomich 1986, USFWS
2007a).
Pigs are long and narrow in shape and predominately black in color and are generally hairy. They
measure 3.5 - 4.5 ft in length and average 2 ft in height. Pigs are elusive animals. They have been
reported to be highly active in the early morning and late afternoon in tropical climates (Diong 1982).
In the HFU, the maximum age of male pigs (boars) is 60 months, while females (sows) live a
maximum of 48 months (Hess et al. 2006).
The reproductive potential of pigs contributes to their invasive potential. These animals are
polyestrous, meaning that adult females have more than one estrus cycle (21 days) in a breeding
season (McGaw and Mitchell 1998). Pregnancy can occur year-round with peaks January - March
(Hess et al. 2006). The average sow in Hawai„i has 1.1 litters per year (Caley 1997). Reproductive
rates peak between 2- 4 years, but breeding has occurred by 10 month-old sows (Hess et al. 2006).
Although all ungulates have a negative impact in Hawaiian forests, it is generally agreed that pigs
pose the greatest threat to the survival of Hawai„i‟s forest birds and their habitats (Scott et al. 1986,
Van Driesche and Van Driesche 2000, USFWS 2008). Pigs are an omnivorous species that consume
fruits, seeds, plant material, as well as some invertebrates. In Hawai„i, pigs consume and damage
plant material in both wet and dry habitats and in agricultural and natural area settings. They root and
trample native vegetation, digging up the soil for earthworms, as well as underground plant parts
such as rhizomes and tubers (Stone et al. 1992). At Hakalau Forest NWR, pigs also eat native
Hawaiian plants such as bracken fern roots and hāpu„u (Jeffrey, pers. comm.). Pigs degrade habitat
for native invertebrate species such as the endangered picture-wing fly (Mitchell et al. 2005).
These animals facilitate the spread of seeds of nonnative species. Pigs act as vectors for invasive
plant species dispersing nonnative plants such as strawberry guava and banana poka (LaRosa 1992,
Stone et al. 1992, Barnett and Simonson 2008). On the Island of Hawai„i, areas without grazing
ungulates show a more diverse plant community with greater coverage of native overstory and
understory species (Cabin et al. 2000). However, the removal of pigs from Hawaiian forests does not
ensure reductions of nonnative plants (Anderson et al. 1992).
Pigs contribute to the prevalence of avian diseases by increasing breeding sites for mosquitoes. Both
on and adjacent to the HFU and KFU, pigs create abundant habitats for mosquito larvae by knocking
down and hollowing tree ferns to eat the starchy cores, leaving behind troughs that catch water and
provide mosquito breeding sites. By increasing the availability of standing water, pigs increase
mosquito populations and potentially increase infection rates of avian malaria and pox in native
forest birds. Most native forest birds have little resistance to these diseases (Van Driesche and Van
Driesche 2000, Atkinson et al. 2005). Some scientists believe that pig management should be
Chapter 4. Refuge Biology and Habitats 4-87