By Roberta Attanasio, IEAM Blog Editor
Wild pigs—also known as wild hogs, wild boars, feral swine, or razorbacks— are wreaking havoc around the world, from trashing European cities to invading the mystical Malaysian island of Pulau Besar by crossing coastal waters. In the US, wild pigs, aptly designated an “infestation machine,” are considered to be the most damaging invasive species.
In the 1500s, Spanish explorer Hernando DeSoto introduced domestic stocks of wild pigs to what is now the southeastern US. Throughout the following centuries, wild pig populations became established and spread to several US regions. Then, they interbred with Eurasian or Russian wild boars brought to the US in the early 1900s for hunting purposes, which led to the generation of many hybrid populations. Today, US wild pigs (Sus scrofa) are a combination of escaped domestic pigs, Eurasian wild boars, and hybrids of the two. They have been reported in at least 35 states, with a rapidly expanding population currently estimated at over 6 million. Alabama, Arkansas, Florida, Georgia, Louisiana, Mississippi, North Carolina, Oklahoma, South Carolina, and Texas are the states with the largest wild pig populations and documented levels of damage caused by the species.
In addition, a survey published in May 2019 found a rapid expansion of the wild pigs’ range in Canada. Ruth Aschim, the study lead author, said: “Wild pigs are ecological train wrecks. They are prolific breeders making them an extremely successful invasive species.”
The Smithsonian Magazine provides a good picture of what invasion by wild pigs looks like: “About 50 miles east of Waco, Texas, a 70-acre field is cratered with holes up to five feet wide and three feet deep. The roots below a huge oak tree shading a creek have been dug out and exposed. Grass has been trampled into paths. Where the grass has been stripped, saplings crowd out the pecan trees that provide food for deer, opossums, and other wildlife. A farmer wanting to cut his hay could barely run a tractor through here. There’s no mistaking what has happened—this field has gone to the hogs.”
Wild pigs can act as reservoirs for many infectious diseases that affect domestic animals—for example classical swine fever, brucellosis and trichinellosis. They are also reservoirs for human pathogens—for example hepatitis E, tuberculosis, and leptospirosis. Because of the threat that wild pigs pose to human and animal health, as well as to agriculture and ecosystems, the US Department of Agriculture announced this year that it is offering $75 million in funding for the eradication and control of feral swine through the Feral Swine Eradication and Control Pilot Program, thus expanding the investments from the previous years. While the focus of this initiative is mostly on restoration and feral swine removal and control, new research indicates that we could also see benefits related to biodiversity.
Indeed, results from a recent study show that wild pigs do threaten biodiversity. For the study—carried out in the Mississippi River Alluvial Valley—the investigators monitored the richness of vertebrate species using data obtained by strategically placing camera traps near animal activity, as to maximize detection without using attractants. Camera traps are remotely activated cameras equipped with motion or infrared sensors, and are used to capture wild animals on film.
Notably, the study focused on forest fragments. Because most of the of historic bottomland hardwood forests found in the Mississippi River Alluvial Valley have been converted into agriculture, the remaining forested wetlands are highly fragmented (broken into small patches). The investigators monitored 36 forest patches between 10 and 10,000 acres in size. They found that in patches where wild pigs were present, mammal and bird communities were 26 percent less diverse when compared with similar forest patches in which wild pigs were absent. Thus, wild pigs seem to drive out a variety of wildlife species.
The investigators speculate that the decline in native vertebrate species could be caused by several types of species interactions and competition, such as predation, habitat degradation, interference, or exploitative competition; but, it is likely that wild pigs cause the most severe damage through predation. They kill and eat rodents, deer, birds, snakes, frogs, lizards, and salamanders. Importantly, there are no wild pig predators.
The investigators also point out that the observed decline of native species could be an artifact caused by many of these species simply avoiding areas where wild pigs were present. Indeed, in presence-absence survey methods, non-detection errors and false absences are a common sampling problem. In other words, a species may be declared absent from a landscape unit simply as a result of not detecting the species using the prescribed sampling methods.
However, other researchers have also raised concerns about wild pigs and their deleterious effects on biodiversity. A study published in 2018 concluded that this invasive species presents a severe threat to imperiled species in the contiguous US, pointing out that we’re in the midst of Earth’s sixth mass extinction event and underscoring the need for rapid and aggressive management action.