Here Piggy Piggy

Feral hogs study reveals an unexpected consequence of invasive species

We've got way more than 30-50 feral hogs here.

Willd hogs running in a group

On August 4, 2019, Willie McNabb raised a Twitter hellstorm when he tweeted the following response to a post advocating for restrictions on assault rifles:

Legit question for rural Americans - How do I kill the 30-50 feral hogs that run into my yard within 3-5 mins while my small kids play?

Many on Twitter subsequently mocked McNabb for his seemingly serious swine inquiry, turning his tweet into the meme of the year.

But feral hogs (Sus scrofa) are indeed disrupting our lives — though not in the way you might expect, according to research published Monday in the journal Global Change Biology.

The researchers find that this invasive species contributes to growing global greenhouse gas emissions and generally wreaks havoc on the ecosystem. The findings speak to the unexpected consequences that can result from humans unleashing invasive species into non-native environments.

“Wild pigs are just like tractors plowing through fields, turning over soil to find food," Christopher O'Bryan, a co-author on the study and a researcher at The University of Queensland’s School of Earth and Environmental Sciences, said in a press statement.

The soil that these pigs disrupt contains carbon. By rooting through the soil for their next meal, the pigs effectively unleash this greenhouse gas into the atmosphere, contributing to global warming.

Some context — To be clear: humans are the most significant species contributing to the climate crisis. Fossil fuels, not feral pigs, are largely to blame for worsening global warming due to greenhouse gas emissions, which have increased by 90 percent since the 1970s.

According to the US Environmental Protection Agency, the fossil fuel industry made up 78 percent of overall emissions between the 1970s and 2011. Other significant drivers of the climate crisis include:

  • Agriculture
  • Deforestation
  • Other land-use changes

But the last factor, land-use change, gives scientists another reason to look at the emissions of wild pigs. Feral pigs digging up land to secure food disrupts soil ecosystems on all continents except Antarctica.

Scientists hadn’t understood the impact of this prevalent invasive species on the world’s most pressing climate issue. Until now.

How they did it — The scientists generated a statistical model to predict the density of wild pigs in areas outside their native habitat range, which originates in Europe.

From that model, the researchers calculated the size of the land that wild pigs disrupted. The soil that pigs disrupt releases carbon dioxide into the atmosphere, contributing to global warming.

“When soils are disturbed from humans plowing a field or, in this case, from wild animals uprooting, carbon is released into the atmosphere,” O’Bryan says.

The scientists’ model allowed them to predict the total annual carbon dioxide released from the soil that pigs disrupted on a global scale.

According to the research team, the study’s findings provide “a global estimation of affected soil area and the contribution of wild pigs to soil [carbon dioxide] emissions.”

What they found — Ultimately, the researchers learned three key facts about feral pigs’ impact on the global ecosystem.

  1. Wild pigs disrupt 36,214 square kilometers (13982 square miles) of soil an area the size of Taiwan — each year
  2. The soil emits 4.87 million metric tons of carbon dioxide each year, or roughly 0.4 percent of annual greenhouse gas emissions from land use changes
  3. The pigs contributed to greenhouse gas emissions on all continents except Antarctica, though they had the greatest impact on disrupted soil in North American and Oceania. Oceania alone is responsible for more than 60 percent of emissions from the disrupted soil due to its high density of wild pigs.

The emissions are relatively small in the grand scheme of global greenhouse gas emissions, but they speak to the tragic potential of invasive species to disrupt more than just the land.

“Since soil contains nearly three times as much carbon than in the atmosphere, even a small fraction of carbon emitted from soil has the potential to accelerate climate change,” O’Bryan says.

Land uprooted by feral pigs in California.

San Francisco Chronicle/Hearst Newspapers via Getty Images/Hearst Newspapers/Getty Images

Why it matters — Feral pigs are not native to the United States but were introduced by settlers in the 1500s. Domesticated farm pigs that escape into the wild can also quickly become feral in a matter of months.

Today, there are some 6 million feral pigs in the country, but the US Department of Agriculture also notes the population is “rapidly expanding.’

The new study underscores the complex chain reaction that occurs when invasive species are unleashed outside their native habitat. The researchers also suggest that the worst effect of wild pigs may not be their greenhouse gas emissions but their other devastating environmental impacts.

The researchers write: “Although wild pigs contribute to global [carbon dioxide] emissions, their soil disturbance also affects food security, economic development, and biodiversity protection.”

According to the study, feral pigs are “ecosystem engineers” that damage soil and plants by digging around for food, harming wildlife that depend on the soil and plants for sustenance. The USDA describes them as “opportunistic” feeders, which allows them to expand into various non-native habitats, causing mayhem.

In turn, wild hogs can affect the food security of humans if they root through agricultural land. Feral pigs also cause an estimated 1.5 billion dollars of damage to US agricultural land each year.

It’s not just feral pigs — the researchers note that “human-spread animals” like rabbits, pine beetles, and deer are also disturbing the soil and causing severe vegetation loss. Therefore, it’s necessary to examine the environmental consequences of invasive species we’ve unleashed on the planet, the research suggests.

What’s next — These findings might be alarming, but the worst is yet to come, according to the study.

The research team writes: “There is significant likelihood that wild pigs will expand beyond their current non-native distribution,” resulting in soil damage and carbon dioxide emissions “more extensive than what has been estimated in this study.”

“If invasive pigs are allowed to expand into areas with abundant soil carbon, there may be an even greater risk of greenhouse gas emissions in the future,” Nicholas Patton, a co-author on the study and a researcher at the University of Canterbury, said in the press statement.

So, we need to curb the population of wild pigs. But traditional methods, such as using vehicles for hunting or trapping the animals, often result in more greenhouse gas emissions. (Earlier this year, Missouri officials also proposed slaughtering invasive pigs to ease food insecurity in the state, but those plans were quashed due to safety concerns.)

But the researchers also suggest that the tradeoff of slightly increased greenhouse gas emissions might be worth it if we can curb an invasive species that has increasingly devastating impacts on the climate crisis, food production, and wildlife biodiversity. Humans have a responsibility to control the invasive species we’ve unleashed, the research suggests.

“Wild pig control will definitely require cooperation and collaboration across multiple jurisdictions, and our work is but one piece of the puzzle,” Patton says.

Abstract: Most of Earth's terrestrial carbon is stored in the soil and can be released as carbon dioxide (CO2) when disturbed. Although humans are known to exacerbate soil CO2 emissions through land-use change, we know little about the global carbon footprint of invasive species. We predict the soil area disturbed and resulting CO2 emissions from wild pigs (Sus scrofa), a pervasive human-spread vertebrate that uproots soil. We do this using models of wild pig population density, soil damage, and their effect on soil carbon emissions. Our models suggest that wild pigs are uprooting a median area of 36,214 km2 (mean of 123,517 km2) in their non-native range, with a 95% prediction interval (PI) of 14,208 km2–634,238 km2. This soil disturbance results in median emissions of 4.9 million metric tonnes (MMT) CO2 per year (equivalent to 1.1 million passenger vehicles or 0.4% of annual emissions from land use, land-use change, and forestry; mean of 16.7 MMT) but that it is highly uncertain (95% PI, 0.3–9 MMT CO2) due to variability in wild pig density and soil dynamics. This uncertainty points to an urgent need for more research on the contribution of wild pigs to soil damage, not only for the reduction of anthropogenically related carbon emissions, but also for co-benefits to biodiversity and food security that are crucial for sustainable development.
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