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.
The Quiet, Intentional Fires of Northern CaliforniaThe 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.