A new study by University of Hawaii researchers shows that the removal of feral pigs may not be enough to protect plants native to the islands from being overrun by nonnative species.

The study, published online last month in Biotropica: The Journal of Tropical Biology and Conservation and slated to appear in a forthcoming print issue, examined the effects of feral pig removal on native and nonnative plant species in Hawaii Volcanoes National Park. In Hawaii, nonnative hooved animals, such as feral pigs, are thought to be especially harmful because native plant species evolved without developing defenses against them. 

In the UH study, after feral pigs were removed from a selected area, not only did the number of native of plants increase, but so did the number of nonnative, or invasive, plants including the highly invasive strawberry guava.

These noteworthy results come amid the decades-old, heated controversy surrounding the feral pig, which is thought by scientists to be the most serious threat (even more than goats and cows), besides humans, to native plant species. To see Americans’ fascination with showdowns between humans and wild pigs, one need look no further than Discovery Channel’s popular reality show “Hogs Gone Wild,” which premiered in January 2011. A recent post by Steven Rinella (warning: very graphic), host of the Travel Channel’s “The Wild Within,” on the Huffington Post’s video blog is further testament to this obsession. The feral pig, presented in both, is rarely shown in a good light.

Native plant species first came into contact with pigs when the animals were brought to Hawaii by early Polynesian settlers more than 1,000 years ago. Then in 1778, Captain Cook released European pigs, among other grazing animals, on the Hawaiian Islands in order to ensure a food source for crews of future voyages. Conservationists say the pigs damage the environment by eating and destroying native vegetation, by wallowing and rooting, and contaminating watersheds.

Though no pig population studies have been conducted within the UH study site, pig population densities for sites of similar climate and vegetation are estimated at about 12 pigs per square kilometer on the Big Island and at between roughly five and 30 pigs per square kilometer on Maui. The feral pig population on all the Hawaiian Islands is protected by regulations on game mammal hunting established by the Hawaii Department of Land and Natural Resources. Mary Ikagawa, a master’s student in the Botany Department at University of Hawaii at Manoa and a former botanist for the Hawaii Department of Land and Natural Resources, Division of Forestry and Wildlife (DOFAW), maintains the website rarehawaii.org. She urges Hawaii to look to the example set by the parks and wildlife departments of other states that do not consider pigs game animals, meaning hunting regulations on game animals do not apply to them.

“I think the big thing is that other states are very clear with their residents about the need to control feral pigs, and even engage the public to help,” says Ikagawa. “Official websites for Texas, Florida, Missouri and several other states provide FAQ pages about feral pig damage, control, and state policies that encourage public participation in control (e.g., trapping information, unlimited hunting, prohibiting transport and release).” These proposed solutions sound like something any hunter would want to hear.

However, as shown by last month’s protest of a proposed hunting ban on 4,800 acres near Hilo, the relationship between conservationists and hunters is a complicated, volatile one. The ban is part of a proposed plan to protect the area, located in the Puu Makaala Natural Area, from invasion of nonnative species. Ikagawa says the State of Hawaii is “alienating hunters from conservationists when the two should be tight, working together to protect the land.”

The new UH study was conducted to take advantage of the field site at Hawaii Volcanoes National Park, which was inhabited by feral pigs 16 years ago. This is the first study of its kind with such a large lapse since the time of pig removal. (A 2011 study by UH Manoa researchers measuring soil runoff—an increase in which is often attributed to rooting— between pig-present and recently pig-free sites was less conclusive possibly as a result of the relatively small amount of time between pig eradication and the time of soil sampling.) Also making this site ideal is that it has never undergone any sort of invasive plant management.

So, does getting rid of feral pigs help control invasive plant species? Yes, but, according to conservationists, it’s not enough. Dr. Creighton Litton, assistant professor of Forest Ecology at University of Hawaii at Manoa, one of the researchers who conducted the study, stresses that while feral pig removal in the case of the UH study caused an increase in native and nonnative plant varieties, “this does not suggest that pigs are a viable management tool for controlling nonnative plant invasions.” Rather, the coexistence of invasive and native plant species is a direct result of seed dispersal by the pigs themselves. Pig removal, the researchers say, is only the first step in an effective plan for conservation of native plants. Special attention must also be paid to the management of invasive plant species.

With so many differing interests, the feral pig debate in Hawaii is unlikely to be resolved any time soon. However, further studies such as this one investigating the long-term effects of pigs on native and nonnative plants in Hawaii may provide the key to conservation programs that could be successful for years to come.