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Thursday, February 22, 2024

Hungry Animals Can Change How Severely a Landscape Burns

As California has descended into wildfire hell, with ever bigger blazes burning ever more intensely over the past 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 cataloged 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 eventually rebounds.

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

firefighter surrounded by smoke in a forest

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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. In the United States, there are 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 fewer 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 burn.

That doesn’t mean that all bugs are bad for forests. In fact, insects play a critical role in breaking down the leaf litter to make forests less flammable. The fewer insects, the more that leaf litter is going to pile up. And the prognosis doesn't look good here: One review published last year, by researchers in Australia and China, estimated that 40 percent of insect species are in decline, and a third are endangered.

Making matters worse, invasive predators are wiping out native species that play their own part in redistributing vegetation—small mammals, for instance, that pull vegetation underground for use in their nests. In Australia, this is a particular problem, as foxes and feral cats hunt native species like the malleefowl. This bird rakes soil and leaves into mounds, where it then deposits its eggs. “So they're not doing all those things like turning over leaf litter and burying it,” says Foster, which means there is more dry material left on the surface to burn.

When it comes to fire risk, the most problematic actor of all is humans. As a city-building, farming species, we have long set small burns to lower the risk of future big ones and to encourage the growth of new vegetation. We’ve also used it on grasslands to herd prey for easier slaughter. But as our species transitioned away from the hunter-gatherer lifestyle, we started seeing fire as a threat, not a tool.

“Humans have been setting fires in grasslands for thousands of years, and then it kind of went out of style,” says UC Davis ecologist Truman Young, who wasn’t involved in this new work. “There's a sense that you don't burn your resource, right? If grass is what cattle eat, you burn it, you lose it.” The long-term reality, though, is that fire is a natural phenomenon. Smaller, periodic fires mean fewer out-of-control blazes, and the vegetation that does bounce back is more nutritious for grazers, because it’s growing in the nutrient-rich ashes of the previous flora. It’s a sort of hard reboot for the landscape.

In California, where we’ve been obsessively putting out fires for decades, this is now a full-blown crisis, as mountains of dead brush have piled up, turning whole landscapes to tinder. Traditionally, these landscapes would burn regularly and more mildly. Now, supercharged blazes are pretty much leveling ecosystems. Climate change is a major factor, too. Simply put, a warmer, drier world is built to burn catastrophically; we’re now living in what fire historian Steve Pyne is calling the Pyrocene, or the age of flames. Nowhere was this more dramatic than in Australia over the past few months. Those bushfires weren’t just unprecedented—models didn’t even predict they could happen for another 80 years.

In this new age of flames, there’s a lot we can do to support the native fauna, which have naturally worked to attenuate wildfires for ages, and to make sure that invasive species don’t eat them. “By getting these animals back out into landscapes, we’re potentially reducing the fire risk in some of those systems,” says Foster. Plus, we’d be saving species, which is nice.

Goats may be adorable and all, but they won’t be getting us out of this mess.

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