Feral Hogs at Congaree National Park

Feral hogs are an invasive species that are negatively impacting the floodplain forest ecosystem at Congaree National Park.

NPS Photo

An Overview (1 of 2)

Feral Hogs are Invasive

The feral hog (Sus scrofa), also called the wild pig, is a highly-destructive, invasive mammal found at Congaree National Park and throughout much of the United States. An invasive species is any non-native organism (plant, animal, fungus, or bacterium) that threatens the ecosystem, economy, or public health when it moves into a new area. People may introduce non-native species intentionally (such as those sold for gardens, yards, and pets) or unintentionally (such as beetles that infest shipping materials, moths that infest firewood, plant seeds that stick to tires, or mussels that attach to boats). Invasive species quickly spread out of control. In the process, they disrupt natural food chains, damage natural habitats, destroy agricultural products, reduce tourism, and carry diseases. Invasive species management is a major economic, scientific, and policy concern for public and private landowners alike.

Feral hogs today are the descendents of escaped food animals, the Eurasian wild boar, or a cross between the two. History records that feral hogs were brought to North America by Europeans as early as 1539, and continue to be imported even today. Feral hogs have significant, negative impacts on plants, animals and habitats across the country - including the floodplain and upland forest ecosystems protected by Congaree National Park.

Feral Hogs in Congaree National Park

Staff and visitors at Congaree National Park have noticed an increase in feral hog behavior and impacts. Evidence of feral hog behavior, such as feeding sign (or "rooting") can be seen throughout the park. In these areas, leaves and soil are completely turned up. Many wildlife trails throughout the park are also related to feral hog behavior.

Feral hogs are not normally a danger to visitors. Most feral hog encounters reported to park staff involve visitors who are hiking along the boardwalk and briefly observe hogs running away into the forest. As with any wild animal, however, feral hogs should never be approached or harassed.

How do Feral Hogs Threaten the Forest?

Feral hogs threaten the floodplain forest of Congaree National Park because they are:

• Destructive - Feral hogs root up acres of forest floor, make mud wallows, and create trail networks.
• Able to reproduce quickly - A single sow, which reaches sexual maturity by age two and may live ten years (or more), can produce two litters of up to ten piglets each year.
• Omnivores that will eat almost anything - Their diet includes worms, roots, reptiles, acorns, amphibians, berries, eggs, fungi, freshwater mussels, leaves, invertebrates, fruit, and much more.
• Intelligent - Domestic pigs are well known for their intelligence and problem solving ability. Feral hogs are no different. For example, feral hogs have been observed to intentionally ram paw paw trees and then eat the fallen fruit.
• Highly adaptive - Feral hogs are habitat generalists that can survive in a wide range of ecosystems.
• Aggressive competitors with native animals - Animals such as wild turkey and white-tailed deer rely on many of the same resources used by feral hogs for food, water and shelter.
• Not naturally controlled - Feral hogs have few natural predators. Floods and harsh winters provide limited control of feral hog populations at Congaree National Park, but recent droughts and mild winters may have minimized this effect.
• Potential vectors for disease - Feral hogs can carry pseudorabies, brucellosis, and other diseases that pose a veterinary risk to domestic animals including pigs and cattle. Brucellosis is also "zoonotic," which means that it can affect humans in direct contact with infected feral hogs. Feral hog waste can also contaminate water with disease-causing bacteria.

Resources at Risk

Feral hogs threaten many resources at Congaree National Park. Their presence and behavior negatively impact forest ecosystem health as well as visitor experiences. Examples include the following:

• Native vegetation - Native plants are at risk of being eaten or uprooted. One example is Carolina Bogmint (Macbridea caroliniana), a plant species of state conservation concern. Feral hog behavior may also be impacting regeneration of tree seedlings and encouraging the spread of invasive plants.
• Native wildlife - In addition to habitat destruction by feral hogs, native wildlife including soil invertebrates, small mammals, salamanders, and ground-nesting birds are at direct risk of being eaten. Other wildlife, such as deer and turkey compete with feral hogs for food. Feral hogs may also cause disease outbreaks through direct contact or by contaminating water.
• Soil - Feral hog behaviors, such as rooting and wallowing, cause erosion and significantly change the structure, chemistry, and biology of the remaining soil. This can have dramatic, long-term impacts on the plant communities and the entire ecosystem.
• Streams and wetlands - Shallow stream beds and seasonal wetlands can be damaged by rooting and wallowing. Water can be polluted by bacteria in feral hog waste and top soil eroded from feral hog behaviors.
• Archaeological sites - Historic homesteads, earthworks, and prehistoric sites are at risk of disturbance - and even destruction - from feral hog behavior.

What is Congaree National Park Doing?

Congaree National Park takes the threats posed by feral hogs seriously. To help address the problem, Congaree National Park has partnered with scientists from several government agencies and universities to answer basic questions about feral hog populations, movements, diseases, and impacts on park lands. These results are highlighted in a separate research summary. Based on these results and similar studies, resource management staff are developing a plan to reduce feral hog impacts across Congaree National Park.

What Can You Do to Help?

• Use common sense if you encounter feral hogs. Do not approach or harass them, keep your distance, and avoid separating a sow (mother hog) from her piglets.
• Increase awareness of invasive species. These include exotic animals and plants (even some pretty ones!) found around the home, yard, garden, and community. Stay informed about related public policy initiatives.
• Prevent the introduction of invasive species. Choose native animals and plants instead of non-native ones. Never intentionally introduce or "dump" domestic animals or plants at Congaree or anywhere.
• Prevent the spread of invasive species. Keep non-native animals and plants contained. Thoroughly wash cars, trucks, ATVs, boats, trailers, and other equipment when traveling to and from outdoor destinations. Do not transport firewood or soil.
• Check out the "Additional Information" links at the bottom of this page.

This feral hog was trapped and tranquilized during a research project. It was measured, weighed, and fitted with both an identification tag and radio collar. It was then released to track its movements across Congaree National Park.

NPS Photo

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The feral hog (Sus scrofa), also called the wild pig, is an invasive mammal found at Congaree National Park and throughout much of the United States. Feral hogs impact the floodplain forest ecosystem at Congaree National Park because they are omnivores that compete aggressively with native animals for food, water, and shelter. In addition, their feeding ("rooting") and wallowing behaviors disturb the natural soil, destroy native habitats, reduce water quality, and may encourage the spread of invasive plants. Furthermore, feral hogs reproduce quickly, have few natural predators, and can spread disease.

Staff and visitors at Congaree National Park have noticed a sharp increase in feral hog behavior and impacts. To help address the problem, Congaree National Park has partnered with scientists from several government agencies and universities to answer basic questions about feral hog populations, movements, diseases, and impacts on park lands. This summary highlights research results from three studies. Based on these results and similar studies, resource management staff are developing a plan to reduce feral hog impacts across Congaree National Park.

Monitoring Feral Hog Distribution at Congaree

In 2000, the park partnered with the U.S. Geological Survey's South Carolina Cooperative Fish and Wildlife Research Unit at Clemson University to study feral hog disturbance within the park. This study focused on repeated mapping of visible feral hog disturbance (primarily evidence of rooting and wallowing) in several plots randomly located throughout the park.

A total of 12,000 one-meter plots were established in the park. The condition of every square was systematically categorized and recorded every other month from 2000-2003. The resulting maps showed patterns in feral hog behavior and were compared with vegetation maps in order to determine local habitat preferences. Changes in the maps over time provided information about feral hog movement patterns, as well as how the forest recovered after feral hog disturbance.

The results indicated that approximately ten percent of the floodplain forest floor showed evidence of rooting and other feral hog disturbance at any given time. In some areas, however, feral hog disturbance affected 100 percent of the forest floor. Feral hog disturbance was generally concentrated in the floodplain as opposed to the pine flatwoods above the bluffs.

This was especially true during droughts and summer months. Disturbance was also greater in the remote, southeastern portions of the park.

There were differences in feral hog disturbance between three habitat types found in the park:

• Seepage forest habitats, such as "Muck Swamp" (the low, wet area on the boardwalk by the Harry Hampton Visitor Center), were affected by intense and persistent feral hog disturbance. This disturbance remained visible for several years. This disturbance may be affecting populations of Carolina Bogmint (Macbridea caroliniana), a plant species of conservation concern.
• Cypress-tupelo habitats, including the edges of Weston Lake, were significantly affected by soil and vegetation damage, especially when water levels were low.
• Bottomland hardwood forest habitats, such as areas found along the Kingsnake and Oakridge trails, were also affected. Disturbance was locally less intense and persistent, but because this habitat is very widespread it actually encompasses (in terms of absolute numbers) more total disturbance than the other habitat types. This disturbance may be impacting recruitment (sprouting and survival) of oak and other tree species.

Radio Telemetry and Feral Hog Tracking

In 2005, Congaree National Park partnered with the U.S. Geological Survey's South Carolina Cooperative Fish and Wildlife Research Unit at Clemson University to track feral hog movements. Feral hogs were trapped, tranquilized, given an ear tag for ID, fitted with a radio transmitter collar, and released. Collared feral hogs were regularly relocated over a two year period by researchers using a radio antenna. This approach, called "radio telemetry," is a common tool for monitoring wildlife.

During this study, scientists trapped and collared 23 feral hogs. Relocation data were analyzed to determine local "home range" size (an estimate of the total space individual feral hogs used to survive), habitat preferences, and factors affecting feral hog mortality.

Feral hogs were observed across the park, but seemed to prefer habitats dominated by oak trees (Quercus sp.) that presumably provide acorns for food. During droughts, feral hogs were concentrated around water features such as sloughs and guts. The edges of these features are exposed at low water and provide a rich food base.

The "home range" size for individuals varied from 470-540 acres, which was small when compared to other studies. Many home ranges overlapped. Together these data indicated a large, densely concentrated feral hog population within the park.

Only four of the 23 collared feral hogs were confirmed alive at the end of the study. Thirteen of the 23 collared feral hogs moved across the park boundaries during the study. These data suggest that hunting on adjacent lands has a potential impact on feral hog populations but apparently is not, by itself, enough to reduce the total population. No floods were observed during the study, but anecdotal evidence suggests that floods are important in controlling feral hog populations.

Disease Monitoring in Feral Hogs

In 2006, the park partnered with scientists at the U.S. Department of Agriculture (USDA) and the University of Georgia to study the presence of pseudorabies and brucellosis in the park's feral hog population. The goal of this ongoing project is to determine the potential veterinary health risks posed by these diseases, which can be transmitted to domestic livestock including pigs, cattle, and other animals found on local farms. Other wildlife such as deer may also be at increased risk of infection from feral hogs. Furthermore, brucellosis is zoonotic, which means that it can be transmitted to humans in direct contact with infected feral hogs or their bodily fluids. The human form is called undulant fever.

A total of 119 feral hogs were trapped in the park from 2006 to 2010. Thirty-six of these feral hogs (30 percent) tested positive for pseudorabies, while thirty-seven (31 percent) tested positive for brucellosis. USDA scientists continue to monitor the prevalence of feral hog diseases at Congaree National Park and across South Carolina.

Future Directions

These research results indicate that active management may be most effective along the bluffs when the park is flooded, or in the cypress-tupelo habitat during dry periods. Working with adjacent land owners is also critical. Based on these results and similar studies, resource management staff are developing a plan to reduce feral hog impacts across Congaree National Park.

Additional Information

• Find "Wild Boar" information at the USDA National Invasive Species Information Center.
• See the USDA brochure on "Feral/Wild Pigs: Potential Problems for Farmers and Hunters."