Wildfires of Varying Intensity Can Be Good for Biodiversity

Many plants depend on fire, said Alexandra Syphard, a senior research associate at the Conservation Biology Institute, and “certain critters depend on those plants, [and] other critters depend on those critters. And so there’s an entire structure of dependency,” she said. “Fire, because it occurred at a certain frequency naturally, is an integral part of that system.” Far from destroying life, she said, wildfires help to stimulate its rebirth.

It’s a major shift from more than a century of U.S. Forest Service policy that made fire suppression a top priority. But as wildfires continue to increase in frequency and severity in the American West and around the world, biologists are beginning to rethink their long-held beliefs about fire. The idea of allowing some wildfires to burn has been popular in conservation for a while, but only recently — because of the wealth of data made available from terrible fires in the past decade or so — have ecologists been able to quantify the effects on biodiversity and to recognize the levels of nuance and complexity at work.

Viewing fire as a positive force in biodiversity and evolution will create a positive feedback loop that not only improves the world around us but also helps to prevent future megafires, said Ryan Burnett, Sierra Nevada group director at Point Blue.

Without increasing the amount of land consumed by fire overall, “we need to push the balance, so we have more low-severity fire than in the mix that we have now,” said Brett Furnas, an ecologist with the California Department of Fish and Wildlife. Prescribed burns could help with attaining that goal.

The Right Level of Catastrophe

We drove northeast on a narrow, twisting country lane, to where Roberts said wildfires had burned in both 2017 and 2020, then pulled over on a quiet residential street winding up a steep hillside. Peeking over bushy madrone, we could see that charred bark on the area’s Douglas firs gives the trees a permanent shadow. Other than that, however, no signs remained of last year’s fire, which burned much of the ground cover but left many old trees intact. On willow trees, bright green shoots emerged against a background of blackened bark. All manner of shrubs had also popped up. Even with a severe drought, the hillside was densely, startlingly green.

“This fire is opening a Las Vegas buffet for woodpeckers,” Roberts said, pointing to a blackened stump of indeterminate origin. As if on cue, a woodpecker swooped in and began to poke its long, narrow beak into the thick trunk.

“These guys probably started showing up while this area was still smoking,” he continued. The dead plant life attracted wood beetles and other insects that dined on the scorched remains of manzanita and laurel. The woodpeckers came for the insects. Roberts focused his binoculars on the feasting bird, then swung them skyward as a red-tailed hawk cried out.

This scene, like the numerous flora and fauna that call this hillside home, was only possible because of wildfire, Roberts said. “This is really healthy. It’s really healthy habitat for all kinds of stuff.”

The rich biodiversity supported by such landscapes is the result of two opposing principles. An environment needs to be stable over a long term for a range of evolving species to have time to fill every niche. The Amazon rainforest, one of the most biodiverse places on Earth, has been a tropical rainforest for millions of years.

But if an area is too stable, life can stagnate. Small-scale disturbances, including disease outbreaks, lightning strikes and tree falls, create opportunities for organisms: The felling of a dead tree allows sunlight to reach an otherwise dark understory. Plants that may have waited decades for this opportunity can sprout skyward. Even events that humans classify as disasters, such as landslides and floods, can create perturbations that let life flourish.

“Provided it’s a relatively localized thing, then you will expect that life will return within a few decades and it will be indistinguishable from the surroundings,” said Euan Furness, who is doing doctoral research in ecology in the department of earth sciences and engineering at Imperial College London.

The idea is known as the patch-mosaic hypothesis: that small-scale disturbances like low-intensity fires can increase biodiversity by subdividing a single homogeneous landscape into a multitude of microhabitats. Using a computer program called REvoSim, Furness and his colleagues showed early this year that disturbance-driven biodiversity climbed highest when habitats showed a combination of large-scale stability and small-scale disturbances.

“Ecosystems are always being disturbed. Generally speaking, there’s an optimal frequency of disturbance for each ecosystem that will give optimum biodiversity,” said Michael-Shawn Fletcher, a biogeographer at the University of Melbourne.

In fire-adapted systems like those in Sonoma County, wildfires can provide the ideal disturbance. Even the most devastating fires don’t burn everything in their path. Instead, they turn the landscape into a patchwork of more- and less-severely burned places. In high-severity patches, at least three-quarters of a forest’s upper canopy is destroyed, often killing large, mature trees with thick bark that can withstand smaller blazes. Low-severity fires burn leaf litter and grasses but leave larger shrubs and trees relatively untouched. Medium-severity fires fall somewhere in between. Even if one part of the ecosystem is destroyed in a high-severity fire, other parts of the landscape remain intact.

Fire-dependent landscapes need fires with a range of severity, a phenomenon scientists call pyrodiversity. It helps create a classic patch-mosaic landscape, allowing varied species to thrive in different parts of the landscape. A 1991 symposium talk and 1992 paper by the ecologists Robert Martin and David Sapsis neatly encapsulated this idea in the phrase “Pyrodiversity begets biodiversity.” The rebirth of these burned areas further amplifies this diversity as plants sprout from buried seeds and animals arrive to make use of the burned remains or new vegetation. By driving biodiversity, fire fuels the evolutionary process.

The problem, said Kira Hoffman, a fire ecologist and postdoctoral fellow at the University of British Columbia, is that much of the world’s fire policy has created conditions that almost ensure large, high-severity wildfires will happen. So much flammable/combustible material has built up that what might otherwise have been a low-severity fire is much more likely to grow very big and very hot.

“Fires are getting to be insuppressible. So bringing back fire and bringing back mitigation techniques is really, really key,” she said. “Indigenous fire practitioners and knowledge holders know very well how these ecosystems are in balance with fire.”

An Ancient Use of Fire

Aboriginal people in Australia have spent tens of thousands of years using fire to shape the their continent. It was one of their most useful tools, explained Fletcher, a descendant of the Wiradjuri people of central New South Wales. When Europeans first began mapping the continent in the 17th century, they perceived the land as untouched by human influence. Nothing, Fletcher said, could be further from the truth.

His own work has focused on the paleoecology of the Southwest Wilderness World Heritage Area of Tasmania (Lutruwita), more than 6,000 square kilometers of some of Australia’s most biodiverse landscapes. Peat deposits in the national park contain remnants of the area’s flora and fauna over millions of years. By taking samples at different depths, Fletcher can watch the climate and the environment change over time; the deeper that he extends his cores, the further back in time he can see.

His work found that during ice ages, most of this rugged corner of Tasmania was dominated by cold climate grasslands. When the climate warmed, the landscape transitioned to dense, temperate rain forests that were intolerant of fire. Twelve thousand years ago, at the end of the last ice age, however, this shift didn’t happen: Fire-dependent plants appeared, and the landscape remained largely open.

However, Fletcher’s samples also contained large amounts of charcoal, indicating that the land’s original inhabitants, the Palawa people, were using fire to keep the area open. This and other examples of ancient fire technologies immediately piqued his interest because, he said, “it was my people, the Aboriginal people, who were doing the burning.”

The fires set by Aboriginal Australians were frequent, low intensity and fostered one of the most highly biodiverse landscapes on Earth. They removed the exquisitely flammable brush and ground litter — key steps for living alongside eucalypts, which are the most fire-prone vegetation on earth.

“A brand-new [plant] community moved in that had never been seen before, with plants that liked the warmer conditions but also can handle the fire. They created a completely novel ecosystem in Tasmania,” Fletcher said. Like much of modern Australia, the Southwest Wilderness is now at risk of high-intensity bushfires because the Indigenous people are no longer around to regularly burn the landscape.

In Northern California, Burnett has seen similar patterns. He began work with Point Blue in 1997, spending his first summer surveying the mid-elevations of the northern Sierra Nevada. But as he began packing up for the season, a wildfire burned much of Burnett’s study area. When he returned in 2001, he braced himself to return to devastation.

What he found was a verdant wonderland. Before the fire, he had surveyed a dense tangle of white fir forest. “It was your prototypical fire-suppressed forest,” Burnett said. But scant months after the smoke cleared, it had transformed into a complex seral (or snag) forest, with stands of regenerating hardwoods and thorny shrubs. The birds were completely different, too — Lewis’s woodpeckers, black-backed woodpeckers and bright, aquamarine lazuli buntings, all of which had been absent before, were suddenly becoming abundant.

As soon as he laid eyes on the new ecosystem, Burnett felt his feelings about fire shift. Fire, he understood, isn’t merely a destructive force (though as someone who nearly lost his home to 2021’s Dixie fire, he knows all too well how it can be). It’s an agent of change and productivity, too. And the spate of wildfires in California, Australia and elsewhere in the world have given scientists a unique opportunity to begin quantifying fire’s impact on biodiversity.

Gauging the Effects of Fire on Diversity

In late 2019, with flames closing in on their rural home, Casey Kirchhoff, an ecology postdoc at the University of New South Wales, and her husband packed up their pets and possessions and evacuated to Sydney. Their home became one of more than 300 burned in the bushfires that blazed through 5,200 square kilometers of the Australian state and that could not be fully contained until February 2020.

On January 7, shortly after officials gave her permission to return, Kirchhoff inspected the damage. Parts of her property were scorched almost beyond recognition. Other patches, however, didn’t look quite so bad.

“It was quite wild to see. You would think that with it being such a high-intensity fire that it would be indiscriminate in taking everything out. But it really wasn’t. It was a mosaic of severity,” Kirchhoff said.

She began to wonder how these bushfires affected biodiversity. As an avid user of the app iNaturalist, which allows users to share and discuss their observations of species around the world, Kirchhoff asked her supervisor at UNSW to launch a citizen science project using the app. The fires affected far too much territory for any one researcher to survey, but maybe the public could help.

“It’s a case of many hands make light work,” she said. If even a single citizen-scientist uploaded a photo of a rare organism that had survived the bushfires, scientists would know that the species hadn’t gone extinct.

Between late January and mid-March 2020, 240 iNaturalist users submitted 3,265 observations of 688 species from across an area of 51 million hectares. They documented how much vegetation had burned (a marker of fire heat and severity), and Kirchhoff found that their estimate closely matched satellite measures of fire temperatures.

Her analysis of this data found an inverse relationship between biodiversity and fire severity. Kirchhoff also noted that plants resprouted more slowly after extremely hot burn sites. She is continuing work to see how bushfire-affected areas recover over the long term.

Other scientists have found biodiversity boosts for animals from low-severity fire. In Northern California, Furnas set nearly 1,500 camera traps to measure the number of carnivores present at different locations between 2009 and 2018. His team detected 15 carnivore species, from skunks and coyotes to fishers and martens, and he found that the strongest predictor of carnivore diversity was low-severity fire in the past decade: More of these less intense burns meant more carnivores.

Furnas’ analysis indicated that the optimal conditions for supporting a diversity of carnivores would be an ecosystem in which on average 1%-2% of the regional landscape burned in low-severity fires each year. “That’s more than an order of magnitude higher than the current amount of fire,” he said.

Bird diversity also has a complex relationship with the effects of fires. Research published last spring by Roberts and Burnett found that very large patches of high-severity burns decreased the diversity of birds in the regenerating landscapes. The severity of the fire wasn’t directly to blame, Burnett said: Rather, because the patches were bigger, birds were confronted with less edge habitat (where one type of landscape meets another) and more homogeneous interior spaces — a change that narrowed the range of ecological niches that species could occupy.

Burnett noted that some research shows high-severity burns, as components of fires with a mixture of severities, can sometimes contribute to increases in subsequent bird diversity. The threat is that with more megafires, the regenerating landscape becomes more uniform and less supportive of diverse birds.

The idea of managed burns intended to kill some trees with the goal of improving biodiversity often makes policymakers anxious — they don’t want to be responsible for incidental damage to human property. But, Burnett said, “the only way we’re going to get ourselves out of this problem is if we use a lot more fire on the landscape. Otherwise, we’re just going to get more extreme fires burning in drought years.” Pyrodiversity, he reiterates, really is key to biodiversity.

But just as pyrodiversity begets biodiversity, biodiversity also begets pyrodiversity. Sonoma’s hillsides, once home to live oaks and coastal redwoods, now contain ordered rows of grapevines, their dark globes of fruit hanging low over the parched dirt as they await harvest. Wine helped put Sonoma on the map, but it has transformed the area into a monoculture. This loss of biodiversity contributed to the severity of the area’s recent wildfires, Roberts said: Because different types of vegetation burn at different speeds and temperatures, a patchy landscape of diverse plants can break up the intensity of fires.

Syphard cautions that just because some landscapes do better with fire doesn’t mean all of them will. “The one generalization you can make is that you can’t make a generalization,” she said. “There’s no one-size-fits-all for appropriate management in terms of how fire fits into the ecosystem and why.”

Getting policymakers to change their thinking about fires might be the biggest challenge, but both Roberts and Burnett say that emphasizing fire’s role in biodiversity is an effective way to get people moving in the right direction.

“This is a rebirth that happens in these systems,” Burnett said. “And they kind of know what to do, if we get out of the way.”