How do wild pigs affect riparian systems?

If you live in the southeastern United States, there’s a good chance you’ve encountered wild pigs or their damage. They can live along stream beds and dig in fields or along roadsides. In the U.S., wild pigs are an invasive species and don’t have many predators. They can affect water quality, compete with native species for resources, and spread disease. This can have significant impacts on other wildlife, the environment, and to humans and domestic animals.

So where did these wild pigs come from? Domestic swine originate from Eurasian boar, which are native to Europe and parts of Asia. The first domestic swine in North America were brought over by European settlers in the 16th century. Eventually some of these animals escaped, became feral, and established breeding populations.

map of united states showing wild pig populations by county
The estimated distribution of wild pig populations in the United States. Their presence along stream banks has been shown to increase E. coli contamination. Credit: USDA  

The first wild boar were imported from Germany in the late 1800’s and released onto a game preserve in New Hampshire. Many introductions and releases of wild boar and domestic pigs have occurred since then. Wild pigs have spread across North America into an estimated thirty-one states. Today, most populations in the U.S. are hybrids of domestic swine and wild boar and are referred to by many names – including feral hog, feral pig, wild boar, and feral swine. Essentially all these animals are considered the same species (Sus scrofa).

While wild pigs can survive in a surprisingly wide range of habitats, they are frequently found in wetlands and along streams, called riparian areas. These habitats have an abundance of resources typically available, such as food sources, water, and shelter.

Having wild pigs living along streams is concerning. Riparian habitat is rapidly declining and provides important ecological and public health benefits. Riparian areas provide habitat for wildlife and plant species and water filtration and storage. Even wild salmon spend part of their lives along stream banks.

several wild pigs rooting and disturbing soil in a riparian area
Wild pigs rooting and disturbing the soil in a riparian area. This behavior can cause a lot of damage to the habitat. Credit: Stephen Ditchkoff

Wild pigs can cause changes at multiple “zones” of a watershed. Besides digging (“rooting”), they also roll around in the mud (“wallowing”). Both disturb the soil which can negatively impact plants, soil invertebrates, and ground-dwelling animals. This soil disturbance can also affect nutrient cycling by disrupting natural soil processes.

Pig traffic across or up and down the stream bank can impact bank stability, leading to soil erosion. It can damage vegetation along the stream that create an important buffer between water and land. Wild pigs may also use the stream channel itself, which can increase erosion and sediment in the water and affect nutrient cycling. They also use the stream as their personal bathroom, introducing feces and urine. All these actions can have negative effects on aquatic plant and animal species.

stream with evidence of rooting and soil erosion caused by wild pigs
This photo taken at one of our study sites shows evidence of rooting, which can cause damage to stream banks. Credit: Sara Bolds

For my master’s research at Auburn University, I studied the impacts of wild pig presence in small, forested streams to determine if they were impacting water quality and increasing fecal bacteria levels. As wild pigs are a huge problem in Alabama, it’s important to know if wild pig activity in riparian areas is a public health risk. This is similar to how gulls along beaches have been shown to be related to beach closings.

We collected water samples from small streams at two properties: one with a high density of wild pigs and one without an established population. We analyzed water samples for total suspended solids (such as sediment), dissolved organic carbon, organic and inorganic nitrogen, and fecal bacteria, specifically Escherichia coli (E. coli) and other fecal coliform bacteria. We also sent water samples to a lab to test for swine fecal bacteria DNA to confirm that pig feces were entering the stream water.

Our analyses found that concentrations of organic carbon and nitrogen were greater in the streams with wild pigs than those without pigs. We attribute that to pig feces and urine in the water. However, there wasn’t a difference in concentrations of total suspended solids or inorganic nitrogen.

two people holding bottles near stream testing water quality
Water was collected from streams to test water quality and fecal bacteria levels. Credit: Sara Bolds

There was a considerable difference in E. coli concentrations between the streams with pigs and those without pigs. E. coli concentrations in the streams with wild pigs were forty times the concentrations in the streams without pigs. The levels in streams with pig populations were higher than the recommended limits set by the U.S. Environmental Protection Agency. Lab analyses found swine fecal bacteria DNA in most of the samples from the property with wild pigs, so we were able to confirm that wild pig feces were entering our streams since there weren’t any domestic pig facilities near our study sites. We did not find a difference in fecal coliform concentrations.

Our research showed that wild pigs can be a threat to water quality in riparian areas by introducing fecal material and disease-causing organisms. This indicates that it may be important to control wild pig populations upstream of major drinking water sources and recreational areas to protect public health.

Answered by Sara Bolds, Auburn University

This blog is based on research published in the Journal of Environmental Quality.

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