Can fireflies flee forest fires?

Following a summer of scorching drought, wildfires have ravaged parts of the southeastern United States. Last month, a devastating fire burned out of control through Gatlinburg, Tennessee.

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Chimney 2 forest fire burns in the Smoky Mountains (Nov 2016)

From its starting point in the Great Smokies National Park, the fire exploded into an inferno fanned by strong winds. For days, hundreds of firefighters battled the blaze. The Sevier County mayor called the combination of wind and dry tinder a “once-in-a-lifetime event…a perfect storm.” By the time they finally extinguished the fire, the charred landscape stretched across 17,500 acres. Fourteen people lost their lives, and nearly 200 were injured. Tens of thousands were forced to evacuate, and over 2400 homes and businesses were destroyed.

Why do such out-of-control wildfires seem to be happening more often? Earth’s climate is changing, and many places are getting hotter and drier. Lots of dead vegetation accumulates naturally in forests, but it dries out much more quickly under such drought conditions. As a result, fires are burning more intensely, spreading more rapidly, and getting harder to control.

Every summer thousands of people flock to Gatlinburg to view the Firefly Light Show (video below). For two weeks, the forest blossomed with the magical synchronous display put on by the Smokies fireflies. So I found myself wondering – could any fireflies have survived these fires?

You’re probably thinking – wait, fireflies are out in summertime, so of course they won’t be harmed by a forest fire in December! Yet these forests are also a cradle for baby fireflies. Although we don’t often see them, fireflies need to live underground in a grub-like juvenile stage for up to two years before they can emerge as adults. During the fall and spring these larvae crawl slowly through the soil, foraging under leaf litter on earthworms and other soft-bodied prey.

While most birds and animals can flee a forest fire, small animals like firefly larvae can’t escape. Forest fires typically burn at around 1100˚F, so it gets pretty hot underground. And studies show that when the soil is dry, heat penetrates more deeply. In one study with dry soil, the temperature reached 140˚F as far down as 4 inches. Any creatures in the top soil layer would certainly die from the heat.

Lynn Faust, the leading expert on the Smokies fireflies, thinks the preceding drought might have caused some firefly larvae to have burrowed deeper underground, where they maybe escaped the heat of the fire. But she saw another problem when she visited her study sites after the burn. All the leaf litter that normally covers the forest floor was gone, consumed by fire. Starting next spring, this is where any surviving firefly larvae would be foraging for earthworms and other prey. The fires’ destruction of this rich layer of decaying organic matter could be problematic, as it could reduce firefly foraging success and survival.

Gatlinburg will take a long time to recover. Our thoughts and prayers go out to the many people who suffered such unimaginable loss of loved ones, homes, and livelihoods. With its devastating human toll, this Smoky Mountains forest fire will long be remembered. As for next summer’s fireflies, we will just have to wait and see.

Update: This article  from the Knoxville News Sentinel provides a sobering retrospective on the Gatlinburg fires.

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