They go by many names — pigs, hogs, swine, razorbacks — but whatever you call them, feral pigs (Sus scrofa) are one of the most damaging invasive species in North America. They cause millions of dollars in crop damage yearly and harbor dozens of pathogens that threaten humans and pets, as well as meat production systems.
Omnivores on the Hoof<p>Much concern over the spread of feral pigs has focused on economic damage, which was estimated in the early 2000s at <a href="https://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1037&context=nwrcinvasive" target="_blank">about $1.5 billion annually</a> in the U.S. Since then, feral pig populations in the U.S. have grown by about 30 percent and the area they occupy has expanded by about 40 percent, so their economic impact likely has increased.</p><p>Feral pigs have a unique collection of traits that make them problematic to humans. When we told one private landowner about the results from our study, he responded: "That makes sense. Pigs eat all the stuff the other wildlife do — they just eat it first, and then they go ahead and eat the wildlife too. They pretty much eat anything with a calorie in it."</p><p>More scientifically, feral pigs are extreme generalist foragers, which means they can survive on many different foods. A <a href="https://doi.org/10.1111/mam.12015" target="_blank">global review</a> of their dietary habits found that plants represent 90 percent of their diet — primarily agricultural crops, plus fruits, seeds, leaves, stems and roots of wild plants.</p><p>Feral pigs also eat most small animals, along with fungi and invertebrates such as insect larvae, clams and mussels, particularly in places where pigs are not native. For example, one recent study reported that feral pigs were digging up eggs laid by endangered loggerhead sea turtles on an island off the coast of South Carolina, <a href="https://doi.org/10.1016/j.actao.2019.103442" target="_blank">reducing the turtles'</a> nesting success to zero in some years.</p><p>And these pigs do "just eat it first." They compete for resources that other wildlife need, which can have <a href="http://dx.doi.org/10.1007/s10530-012-0229-6" target="_blank">negative effects on other species</a>.</p><p>However, they likely do their most severe damage through predation. Feral pigs kill and eat rodents, deer, birds, snakes, frogs, lizards and salamanders. This probably best explains why the decrease in diversity we observed was similar to other studies of <a href="https://www.sciencedirect.com/science/article/pii/S0065250416300514" target="_blank">invasive predators</a>. And our findings are consistent with a global analysis showing that invasive mammalian predators — especially generalist foragers like feral pigs — that have no natural predators themselves <a href="https://doi.org/10.1073/pnas.1602480113" target="_blank">cause by far the most extinctions</a>.</p>
Altering Ecosystems<p>Many questions about wild pigs' ecological impacts have yet to be answered. For example, they may harm other wild species through indirect competition, rather than eating them or depleting their food supply.</p><p>In one well-known case, feral pigs indirectly caused the near extinction of indigenous foxes on California's Channel Islands. The pigs were released on the islands over 150 years ago, but did not apparently affect fox populations for over a century. That changed when golden eagles began breeding on the islands in the <a href="https://www.nps.gov/chis/learn/nature/golden-eagle.htm" target="_blank">mid-1990s</a>, after hunting and exposure to the insecticide DDT eliminated bald eagles, which were golden eagles' natural enemy.</p><p>Unlike bald eagles, which eat mainly fish, golden eagles hunt on land. Wild pigs served as easy prey for them allowing the eagles to become established. The eagles also ate island foxes, and <a href="https://doi.org/10.1073/pnas.012422499" target="_blank">nearly eradicated them</a>as a result.</p><p>Ultimately the foxes were restored through an <a href="https://www.nps.gov/chis/learn/nature/fox-saving.htm" target="_blank">elaborate initiative</a>. First, wildlife managers removed pigs from the islands. Then they restored bald eagles to ward off the golden eagles. Finally, managers reintroduced foxes that had been raised through captive breeding.</p>
Feral swine damage native habitats by rooting up grasses and rubbing on trees. Their activities can create opportunities for invasive plants to colonize new areas.