There is none agent of ecological imperialism more ferocious than the wild pig. Everywhere Europeans have invaded, from America to Australia, their pigs, even those who have fled the countryside to do harm. Beasts uproot native plants and animals, spread disease, destroy crops, and rebuild healthy ecosystems in their wake. There are not as many parasites as the chaos embodied.
Now add climate change to the wild boar destruction curriculum. In their endless search for food, the pigs take root in the fields, shaking the land like a farmer in the fields. Scientists already knew, to some extent, that this releases carbon that is trapped in the ground, but researchers in Australia, New Zealand and the United States have now calculated how much wild pig soil can be disturbing around the world. The authors concluded that the carbon dioxide emissions they produce annually, equal to that of more than one million vehicles.
It’s another piece of an increasingly disturbing puzzle, showing that land change has — in this case, inadvertently — exacerbated climate change. “Every time you disturb the ground, you cause emissions,” says Christopher O’Bryan, an ecologist at the University of Queensland, lead author of a new letter describing the research in the journal Biology of Global Change. “When you cultivate the land for agriculture, for example, or you have a widespread change of land use — urbanization, forest loss.”
Given its dominance of healthy landscapes, pigs avia to be fair, the researchers knew it, but no one had modeled it on the whole world. “We’re beginning to realize that there’s a big gap on a global scale looking at this demand,” O’Bryan adds.
Researchers have landed on their emissions estimate by adding several previous models and data sources. For example, one author had a model that mapped the populations of wild pigs in the world. Another had studied wild pigs in Australia, and had data on how the species disrupts soils. Subsequently, researchers drew estimates made in Switzerland and China of the carbon emissions created by wild pigs rooting there.
This patchwork creates inherent uncertainties. No model can define exactly how many pigs are in a given place at a given time, for example. In addition, different types of soils emit more carbon when disturbed. A material such as peat – composed of dead plant matter that is not completely decomposed – is essentially concentrated coal, so it has more to give up than other soils. The amount of carbon loss also depends on the microbiome of the soil – the bacteria and fungi that feed on that plant material.
Given this wide range of variables, the researchers simulated 10,000 potential density maps of global wild pigs, excluding the native region of the animals in parts of Europe and Asia. (In other words, they modeled only the locations where pigs are an invasive species.) For each of these simulations, they randomly assigned carbon-induced carbon emission values based on data from those previous studies. This allowed them to combine the variables in thousands of ways: Here’s how many pigs could be in a given area, here’s how much land they disturb, and here’s the resulting emissions. From these thousands of attempts, they have been able to generate average emission estimates.
His model showed that, worldwide, invasive wild pigs take root somewhere between 14,000 and 48,000 square miles of land. But they are not spread evenly across the globe. While Oceania – the region that includes Australia and the islands of Polynesia – contains a small part 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 some parts of Oceania – like tropical North Queensland, for example – there’s this substantial amount of coal 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 driven by the eradication of wild pigs.
This estimate, they think, is actually quite conservative. That’s because they haven’t changed emissions from agricultural lands, which are vast, and that wild pigs are known to sack for free food. They understand that, technically, this earth is already disturbed and emits carbon dioxide, so they don’t want to count it twice. In addition, researchers have estimated only where wild pigs can be now, not where they might be coming soon. “This pest is spreading, and it could potentially spread to areas with high carbon reserves,” says O’Bryan.
Research helps to further quantify the rapidly changing carbon cycle on Earth, as humans (and their invasive species) dramatically transform the earth itself. “What this paper highlights is something that field scientists have known for a while – that bioturbation can play this really key role in soil emissions and soil respiration,” says Kathe Todd- Brown, a computational biogeochemist at the University of Florida, who was not involved in the research. “You also see similar effects with the movement of the earthworm – any kind of animal that digs that affects the structure of the soil.”