Sus scrofa

Outcompeting of native peccaries in Texas (Galetti et al. 2015) and Brazil (Keuroghlian et al. 2009); and outcompeting of fox populations in California (Melstrom 2014).

One indirect effect widely reported in Hawai‘i is the spread of avian malaria with feral pig activity. Specifically, wallowing and rooting behaviors create pools of standing water that provide habitat for nonnative mosquitoes carrying avian malaria, and a lack of resistance to avian malaria in native Hawaiian forest birds has resulted in remarkable population reductions and even extinctions of numerous endemic avifauna (Atkinson and LaPointe 2009).

Feral pigs damage plants via rooting, in addition to direct herbivory. Rooting behaviors cause extensive damage to both seedlings and adult plants (Diong 1982, Campbell and Long 2009, Cole et al. 2012, Cole and Litton 2014, Murphy et al. 2014).

Feral pigs variably impact soil nutrient cycling and availability as well, with several studies documenting increased soil nitrogen with feral pig activity (Siemann et al. 2009, Wirthner et al. 2012, Bueno et al. 2013), but others have observed increased cycling and availability of soil nitrogen with feral pig removal (Long et al. 2017).

Feral pigs have been shown to alter soils differently in various habitats. For example, in Tennessee’s deciduous forests, soil bulk density was decreased by feral pigs (Singer et al. 1984), but soil bulk density increased with feral pig presence in Spanish alpine habitats (Bueno et al. 2013). Removal of feral pigs from Hawaiian wet forests resulted in decreased soil bulk density within 6.5 yr (Long et al. 2017). Physical alteration of soil organic matter via fragmentation during feral pig foraging was also recorded in some (Hobbs 1996, Siemann et al. 2009, Wirthner et al. 2012), but not all cases (Bruinderink and Hazebroek 1996).

Feral pigs eat strawberry guava fruits and then release the seeds via their feces, providing excellent conditions for further recruitment and dispersal (Diong 1982, Huenneke and Vitousek 1990). Other invasive plants known to be spread by feral pigs in the Pacific region include banana poka (Passiflora tarminiana) (Beavon and Kelly 2015), mesquite (Prosopis pallida) (Lynes and Campbell 2000), and the freshwater pond apple (Annona glabra) (Setter et al. 2002), all of which show high levels of germination or recruitment following feral pig–mediated dispersal. The replacement of native plants with invasive plants due to feral pig activity results in a positive feedback loop generating progressively more opportunities for invasive plants to establish.

One noteworthy direct effect is egg predation and the destruction of important nesting sites used by Pacific island ground-nesting avifauna (Challies 1975).

Feral pigs are known to consume the roots, buds, flowers, and fruits of plants (Diong 1982, Ralph and Maxwell 1984, Tomich 1986, Katahira et al. 1993, Pratt et al. 1999, Cole and Litton 2014), and up to 95% of feral pigs’ diets seasonally can come directly from vegetation (Diong 1982, Cuevas et al. 2010). Feral pigs are also well-established seed predators, limiting the recruitment of some plant species, particularly in continental mast-seeding environments (Lott et al. 1995, Sweitzer and Van Vuren 2008, Sanguinetti and Kitzberger 2010). In turn, feral pigs readily disburse many nonnative seeds, particularly those found in fleshy fruits (Diong 1982).

Compaction of soils has been shown to negatively impact arthropod communities (Vtorov 1993). Additional negative effects include the destruction of turtle habitats in Australia stemming from trampling in mudflats surrounding marshes (Doupé et al. 2009);

Feral pigs have also been shown to aid in dispersing aquatic invertebrates ( Vanschoenwinkel et al. 2008).