For decades, eastern Oregon’s scablands—rocky patches of open terrain—were a refuge for people fighting wildfires in the surrounding forests. The thin soil and sparse vegetation offered little fuel for the flames, creating an oasis from which firefighters could operate and a barrier that could help halt a fire’s spread.

That all changed in 2015. After lightning sparked a fire near a steep-walled canyon, the blaze unexpectedly raced across scablands so quickly that firefighters struggled to catch up. In the end, the Corner Creek Fire scorched more than 11,000 hectares. And Jeff Priest, who has spent more than 2 decades fighting fires in Oregon for the U.S. Forest Service (USFS), realized he had a new problem on his hands: the arrival of an invasive, shin-high grass known as Ventenata dubia. The plant created shaggy golden carpets of dry foliage, transforming once fire-resistant scablands into flame-friendly corridors.

“We knew it was coming,” Priest says about the annual commonly called wiregrass, which is native to countries surrounding the Mediterranean. “But all of a sudden, it was there.”

Ventenata’s spread into the forests of the northwestern United States is just the latest chapter in a phenomenon reshaping ecosystems—and wildfire—around the globe. In northern Australia, invasive gamba grass from Africa fuels intense blazes that rip through eucalyptus groves. In Brazil, molasses grass from Africa turns vast swaths of the savanna known as the Cerrado into fire-prone grassland. In the western United States, two Old World grasses are creating ecological mayhem: Buffelgrass feeds fires in the Sonoran Desert that torch iconic saguaro cacti, while blaze-tolerant cheatgrass crowds out native sagebrush in the high desert known as the Great Basin.

Even as catastrophic wildfires that roar through towering treetops capture the public’s attention, ecologists have been paying increasing attention to this less conspicuous trend: how seemingly modest nonnative grasses are allying with fire to eat away at dry forest and savanna ecosystems.

These invasive grasses can hijack fire to create a self-reinforcing cycle, explains Carla D’Antonio, an ecologist at the University of California, Santa Barbara, who has studied the phenomenon for more than 3 decades in Hawaii and California. Once established, the grasses help fuel blazes that kill and suppress less fire-tolerant native plants, opening up new territory for the invaders to colonize—catalyzing yet more fire. In a short time, land that was once shrubland, savanna, or dry forest is locked into being a grassland. “It’s that trigger of grass and fire that sets the system off in some undesirable direction,” D’Antonio says.