Hawaiian pigs aggressive and spreading everywhere.

Everybody’s got a pig story.

Unless you live in a highrise or a yacht harbor, chances are you’ve come across some of the feral pigs that are increasing their range throughout the Islands, even moving into urban areas.

(Image: Feral pig with native ferns. Credit Hawaii Volcanoes National Park.)

People in the Hawai`i suburbs are waking to find their lawns chewed up. Gardens are at constant risk. Pastures are torn brown by hogs looking for worms and grubs.

In the forests, pigs create vast mudholes where native understory used to grow—a double threat, since not only are the native species destroyed, but it creates open ground for aggressive invasive alien plants to set root.

But what are these pigs? Is there something special about them?

A team of researchers late last year published a study on the genetic makeup of Hawaiian feral hogs in the journal Royal Society Open Science. The team, led by Anna Linderholm of Oxford and Texas A&M, took genetic samples from dozens of feral pigs from across the state.

They found that there’s still a lot of Polynesian pig in the genetic mix, but also evidence of multiple introductions of other porcine species, including the Eurasian boar, which—it is argued—made them more aggressive and invasive in the environment.

“Understanding the degree to which modern feral pigs retain their Polynesian ancestry and whether pigs introduced by Europeans have replaced those originally introduced will lead to more informed debates regarding the management of Hawaiian pigs,” wrote Linderholm and her group.

Pigs got to the Islands with Polynesian voyagers about 1200 A.D., the paper says. And then came Westerners, who also carried pigs on their vessels.

“Western explorers, like the Polynesians before them, traveled with and introduced domesticated plants and animals across the Pacific. In many cases, Westerners came into contact with local cultures that already possessed domesticated varieties of the same taxa that led to gene flow and possibly replacement,” the paper said.

In Tahiti, the local pig population got larger quickly when crossed with European stock—in as little as three years. The crossing has been extensive in Hawai`i as well, they wrote.

“The genetic evidence presented here indicates that the current Hawaiian feral pig population is a mixture of those brought to Hawaii by the Polynesians and pigs of European (and possibly Asian) origin introduced to the islands much later,” they wrote.

All but a couple of the pigs they sampled had some Polynesian pig genes. That couple was pure European. But it is clear that some of the traditional Polynesian genetics is still in most Hawaiian feral pigs: "The predominance of the Pacific clade haplotypes ... suggests that the original Polynesian lineages have not been completely replaced by more recent introductions."

The authors are careful not to suggest that its only breeding that is making pigs more common and widespread. The Hawaiian environment of the pre-European period might not have been as conducive to pig survival. But the arrival of a lot of fleshy fruits (banana poka, strawberry guava and the like), and the appearance of earthworms—which are not native to the Islands—helped make the Hawaiian forest a lot tastier, they said.

“And though the issue of whether the first pigs on Hawaii became feral prior to the arrival of Europeans remains contentious, extensive damage to native habitats by feral pigs appears to be recent. In fact, it was probably not until the twentieth century, with the introduction of new sources of protein such as earthworms and invasive fleshy-fruited plants that pigs were able to thrive in the forests, thus becoming a significant problem to the native flora and fauna,” they wrote.

Feral pigs are a problem in many parts of the world. They’re increasing their range on the Mainland as well, where they can be a major threat to established agriculture—chewing up corn fields and other crops. Pigs are Eurasian, originally, and didn’t appear in the Americas until the mid-1500s. Here’s a report on problems in Virginia. 

A team led by Pamela Scheffler wrote in 2012 about pig density in Hawaiian forests here. They found a direct correlation between increasing numbers of pigs and decreasing levels of native plants. The explanation in the paper is pretty damning.

“In Hawai‘i, feral pigs can be considered ecosystem engineers due to the changes they catalyze in Hawaiian ecosystems. They root and trample soils, disrupting soil microarthropod communities, leading to potential seedling mortality, and to reduced plant species richness. Feral pigs also eat or otherwise destroy native vegetation; cause changes in soil; act as dispersal agents and create habitat for exotic plants. They also create mosquito breeding habitat by knocking over and hollowing out troughs in native tree ferns and making rain-filled wallows.”

© Jan TenBruggencate 2017