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Hungry Animals Can Change How Badly a Landscape Burns

As California has descended into wildfire hell, with ever bigger blazes burning ever more intensely over the last few years, an unlikely firefighting hero has emerged: the goat. Officials in mountain cities in particular have been hiring herds to hoover up overgrown vegetation, creating fire breaks around the edges of towns. It’s what these ungulates—and…

As California has descended into wildfire hell, with ever bigger blazes burning ever more intensely over the last few years, an unlikely firefighting hero has emerged: the goat. Officials in mountain cities in particular have been hiring herds to hoover up overgrown vegetation, creating fire breaks around the edges of towns. It’s what these ungulates—and their brethren the world over—are born to do. Grazers like deer and sheep play an important role in wildfire ecology, mowing down plants and reducing the severity of conflagrations.

But all is not so cut and dried. The interactions between grazers, plants, and wildfires turns out to be wildly complex and surprising, as catalogued in a new review paper in the journal Trends in Ecology & Evolution by researchers in Australia. It turns out that in their interactions with vegetation, some animal species can at times make wildfires worse. And to complicate matters even more, grazers can not only transform the physical structure of an ecosystem—by avoiding shrubs in favor of eating grasses, for example—but its chemistry as well. That all has big implications for how humans can manage wildfires on a rapidly-warming planet.

Imagine, if you will, a landscape of grass and shrubs. If you’re in Africa, you might see antelope leisurely grazing. If you’re in Australia, instead imagine kangaroos bounding around while munching on grass. Everything looks to be in its natural balance, as it’s been for millennia; after the grazers finish chowing down, they move along and the vegetation will eventually rebound.

But, of course, few ecosystems on Earth are actually still in balance. Many of these landscapes are now home to newcomer species that also want to graze there. In addition to there now being an overpopulated with kangaroos, today Australia is also home to domesticated grazers like sheep and cows. All of these extra vegetarians prefer the greenest plants because they’re more nutritious, and they may leave behind the brownest plants, which then can accumulate as dangerous fuels for wildfires.

firefighter surrounded by smoke in a forest

The grazers might also prefer grasses to shrubs, which changes the vertical structure of the vegetation, further increasing the fire risk. A landscape dominated by taller shrubs burns a lot differently than a landscape dominated by shorter grasses. So while the grazers are doing a helpful job of eating up some potential tinder, they’re leaving behind vegetation that is extra-flammable—which is a mixed bag, in terms of wildfire prevention. “So from changing a grassland into a shrubland, you might actually reduce some of the total biomass of fuel,” says Australian National University ecologist Claire Foster, lead author on the new paper. “But the structure of fuel is very different: The fuel is elevated and aerated and you get really hot, fast-traveling fires in shrubland.”

Livestock like cows are also changing the fire risk in forested areas, which are normally grazed more sporadically by herbivores like deer. Here in the United States, we have mixed conifer deciduous forests. Conifers include evergreen, fluffy-up-and-down trees like firs, while deciduous trees shed their leaves annually and tend to be top-heavy with barer trunks. But the balance between the two types of trees tends to fall apart when livestock infiltrate these forests, because they gravitate toward eating grasses and the deciduous seedlings. In the process, they leave behind the conifers that are more likely to lend themselves to a major conflagration, species that become abundant because with less deciduous trees, there’s less competition for water, nutrients, and light.

“In the long term, you get more and more conifers,” says Foster. These trees tend to spawn supercharged wildfires because of the way they’re shaped. “If you think about the shape of a deciduous tree compared to a conifer, the conifer has fuel that goes from the ground all the way up to the canopy, whereas a deciduous tree has a gap,” she says. Although historically wildfires may have burned close to the ground, not reaching the tops of all the trees, in a conifer-heavy forest, blazes can rapidly ascend to become explosive crown fires that burn through the canopy.

The study also considered other critters, specifically insects, that are raising the risk of fires thanks to their eating habits. When invasive species like bark beetles attack vegetation, the plants produce defensive compounds—like the organic polymer lignin—to make themselves less tasty. But the side effect is that they may also make themselves more flammable. If a pest kills a tree outright, it becomes tinder. But now the extra-flammable tree debris also falls to the ground along with it, creating a bed of yet more burnable material. Still more problematic, a previous study from a separate group of researchers in Minnesota found that when lace bugs attack bur oak, the increased lignin content cuts decomposition rates of leaves by a quarter, meaning that tinder stays on the ground just asking to bu
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