The Invasion of the Pigs!

The Invasion of the Pigs!

Sus scrofa, also known as wild boars, feral pigs, and by many other names. (Photo source: mnn.com)

This seemingly innocent creature here is an example of a species of feral pig called Sus scrofa.  He may look kinda cute, but states like California, Texas, and Oregon want feral pigs gone!  So what's the big deal with feral pigs?  Keep reading and see why Sus scrofa is becoming a problem increasingly difficult to ignore.

General Facts:

·         The average wild sow weighs about 110 pounds (ranging from 77 to 330 pounds) and the average wild boar weighs 130 pounds.

·         Feral pigs are opportunistic omnivores that eat whatever plants or animals happen to be available and in the area.

·         They eat grasses, leaves, berries and other fruits, roots and tubers, corn and other agricultural crops, insects, crayfish, frogs, salamanders, snakes, mice, eggs of ground-nesting birds, young rabbits, fawns and young livestock (such as lambs, calves, and kids). They can also kill larger livestock, provided the livestock is weak from illness or injured.  Feral pigs will also readily scavenge carrion when fresh meat is difficult for them to procure.  More Sus scrofa facts

Dispersal and invasion:

Sus scrofa was introduced to North America in the 1500's when they escaped from the Spanish colonists.  Feral swine originated in Florida but have since spread to much of the Southern and Southeastern United States, as well as California and Oregon, due to Spanish settlers taking them west with them as they settled there.  Furthermore, these settlers often let the feral swine graze in nearby forests, where some likely escaped and eventually became the invasive population we see today.

Today, feral pigs and hybrids of the Russian wild boars are found in 23 states with the possibility of yet-undiscovered populations in several more states. The largest populations are in the coastal states surrounding the Gulf of Mexico and California

                                          Top: feral pig territory in 1982 vs. 2010 (photo source: media.smithsonianmag.com)                                             Bottom: feral pig dispersal in Australia (photo source: activemerchandise.net)

Effects on Ecosystems:

Sus scrofa degrade ecosystems through physical alteration of habitats via rooting, competition with native species of mammals, and overgrazing of native plant species.  An additional side effect of feral swine overgrazing is the way this overgrazing allows non-native weed species to invade.  Rooting by feral swine can cause damage so extensive that the soil can no longer support any native species of plants.  Numerous plant species have been negatively affected by feral swine, including the Vanilla lily (Arthropodium milleflorum) in Australia’s Namadgi National Park, (Metrosideros polymorpha), koa (Acacia koa) in Hawaii, and white oak (Quercus garryana) in Oregon.  

An example of feral pig rooting and its effects on the environment (photo source: motherjones.com)

Here are a few case studies regarding the effects of Sus scrofa  on ecosystems:

-In California, feral pigs prey primarily on small mammals such as California voles (Microtus californicus) which were the dominant prey species, totaling 109 individual voles and occurring in more than one-third of all stomachs of feral pigs. Botta’s pocket gophers (Thomomys bottae) also were common prey, with 26 individuals in 13% of stomachs.  Prey was represented by 20 species: 11 mammals, 5 birds, 3 snakes, and 1 frog.  This data was obtained by analyzing stomach contents of feral pigs in California.

-In Australia, Sus scrofa are also a problem for native species of turtles.  The Northern snake-necked turtle (Chelodina rugosa) is a favorite prey of the feral pigs there, and as a result, Aboriginal people that depend on these turtles as a source of protein are negatively affected.  Not to mention that decreased rates of turtle survival is a problem in itself, and only years with high levels of wet season rainfall see high rates of turtle survival.  Full study at http://www.springerlink.com.libproxy.uoregon.edu/content/4rk6lg2731571452/fulltext.pdf

-In Hawaii, the effects that humans have had on the environment, such as deforestation and urbanization, only serve to exacerbate the problems caused by feral pigs.  When pigs forage on native plants already threatened by the actions of humans, these species can be pushed even further toward the brink of extinction.  Hawaiian plants are especially susceptible to consumption by feral pigs due to the historic lack of larger herbivores like the feral pig on the islands.  According to some scientists, this means the plants have not had to develop defense mechanisms to predators like feral pigs.  The same holds true for the rooting and foraging of feral pigs in their search of food.  Native plant species are destroyed in these processes, and the soil damage done usually prevents native species from reclaiming their grounds.  Finally, feral pigs can alter the dynamics of entire ecosystems in Hawaiian rainforests through seed dispersal.  The nitrogen-fixing species of tree Myrica faya may increase the amount of earthworms in the soil near the tree.  This may then increase the number of feral pigs foraging and rooting near the tree, which would then cause further seed dispersal.  Full study at http://www.springerlink.com.libproxy.uoregon.edu/content/k76785727w31879j/fulltext.pdf.

Effects on Agriculture:

Sus scrofa rooting can have strong negative effects on agriculture as well as native species of plants.  Oregon’s agriculture losses due to invasive feral swine have the potential to be very high.  Although Oregon’s feral swine problem is not as bad as in other areas of the country, Oregon’s main crops like grain, grass, hay, and wheat have been shown to be favored by swine elsewhere in the United States.  Agriculture is particularly susceptible to feral swine infestation due to the ease of access and wide open areas typical of agricultural lands.  Here's a comprehensive list of feral pig impacts on Oregon agriculture.

Feral Pig Management:

In Wisconsin, the Department of National Resources views pigs as a significant threat to native plants and wildlife, and has encouraged the removal of the feral pigs by giving hunters no limit on the number of pigs they can kill, or what time of year they can hunt the pigs.  As an added incentive for hunters, feral pig meat is supposed to taste just as good if not better than domestic pork.  Wisconsin does not stand alone on this policy, however.  The consensus among states where feral pigs have become an epidemic seems to be that they want these pigs eradicated as soon as possible.   Oregon's action plan for how to deal with these feral pigs can be found at http://www.clr.pdx.edu/docs/feral%20swine%20action%20plan.pdf.

This is video from a helicopter feral pig hunt in Texas.  It's a good example of how states are dealing with this invasive species, and it's got a great song in it! (video source: http://www.youtube.com/watch?v=LGC1tjHCSVE)

 Sources:

Wilcox, J. T., & Van Vuren, D. H. (2009). Wild pigs as predators in oak woodlands of California. Journal               Of Mammalogy, 90(1), 114-118.

 Rouhe, A., & Sytsma, M. (2007). Feral swine action plan. Retrieved from http://www.clr.pdx.edu/docs/feral swine action plan.pdf

Fordham, D., Georges, A., Corey, B., & Brook, B. (2006). Feral pig predation threatens the indigenous harvest and local persistence of snake-necked turtles in northern Australia. Biological Conservation, 133(3), 379-88.

Salwey, M. K. (2009, March 10). Feral pigs. Retrieved from http://dnr.wi.gov/org/land/wildlife/publ/wlnotebook/pig.htm

Nogueira, S., Nogueira, S., & Fragoso, J. (2009). Ecological impacts of feral pigs in the Hawaiian islands. Biodiversity and Conservation, 18(14), 3677-83.

by Andrew Moore