There’s no agent of ecological imperialism fiercer than the wild pig. Wherever Europeans invaded, from the Americas to Australia, their pigs did the same, many of which fled into the countryside to wreak havoc. Beasts tear apart native plants and animals, spread disease, destroy crops, and rebuild entire ecosystems in their wake. They are not so much parasites as embodied chaos.
Now add climate change to the wild pig destruction summary. In their relentless quest for food, pigs take root in the ground, stirring up the earth like a farmer plows his fields. Scientists already knew, to some extent, that this releases the carbon that is locked in the soil, but researchers in Australia, New Zealand and the United States have now calculated how wild pigs in the soil can disrupt the world. The carbon dioxide emissions they produce each year, the authors concluded, equal those of more than a million cars.
This is yet another piece of an increasingly disturbing puzzle, showing how the earth modification has – in this case, inadvertently –exacerbated climate change. “Every time you disturb the soil, you cause emissions,” says Christopher O’Bryan, University of Queensland environmentalist and lead author of a study new paper describe the research in the journal Biology of global change. “When you plow the ground for agriculture, for example, or you have a widespread change in land use – urbanization, loss of forests. “
Given their dominance over entire landscapes, pigs had to make matters worse, the researchers knew it, but no one had modeled it around the world. “We have started to realize that there is a significant gap globally when it comes to this issue,” adds O’Bryan.
The researchers landed on their estimate of emissions by aggregating several previous models and data sources. For example, one author had a model that mapped populations of feral pigs around the world. Another had studied wild pigs in Australia and had data on soil disturbance by the species. The researchers then extracted estimates made in Switzerland and China of the carbon emissions created by wild pigs that take root in the surrounding area.
This patchwork creates inherent uncertainties. No model can determine exactly how many pigs are in a given location at any given time, for example. In addition, different types of soils emit more carbon when disturbed. A material like peat, made up of dead plant material that has not fully decomposed, is essentially concentrated carbon, so it has more to give up than other soils. The amount of carbon loss also depends on the soil microbiome, bacteria and fungi that feed on this plant material.
Given this wide range of variables, the researchers simulated 10,000 maps of potential global densities of feral pigs, excluding the animal’s native ranges in parts of Europe and Asia. (In other words, they only modeled places where pigs are an invasive species.) For each of these simulations, they randomly assigned values for pig-induced soil carbon emissions on the database of these previous studies. This allowed them to combine the variables in a thousand ways: here’s how many pigs could be in a given area, here’s how much land they would disturb and here’s the resulting emissions. From these thousands of attempts, they were able to generate average emission estimates.
Their model showed that, all over the world, invasive feral pigs are rooting somewhere between 14,000 and 48,000 square miles of land. But they are not distributed evenly around the world. While Oceania, the region that includes Australia and the islands of Polynesia, makes up only a tiny fraction of the world’s land surface, it has a large number of pigs. At the same time, the tropics are home to much of the world’s peat. “In parts of Oceania, like tropical northern Queensland, for example, there is this substantial amount of carbon stores,” says O’Bryan. The combination of the two means that, according to the team’s model, Oceania accounts for 60 percent of total global emissions from rooting feral pigs.
This estimate, they think, is actually quite conservative. That’s because they haven’t modeled emissions from farmland, which is vast, and wild pigs are known to plunder for free food. They figured that, technically, this earth was already disturbed and emitting carbon dioxide, so they didn’t want to count it twice. In addition, the researchers only estimated where feral pigs could be now, not where they could be soon. “This pest is expanding and could potentially spread into areas with high carbon reserves,” says O’Bryan.
Research is helping to further quantify the rapidly changing carbon cycle on Earth, as humans (and their invasive species) dramatically transform the Earth itself. “What this article highlights is something that soil scientists have known for some time: bioturbation can play this really key role in soil emissions and respiration,” says Kathe Todd-Brown, computational biogeochemist at the University of Florida, which was not involved in the research. “You also see similar effects with the movement of earthworms – any type of burrowing animal that disrupts the structure of the soil.”