Sky Island Fireflies
Sky Island Fireflies in the Nature Conservancy’s Davis Mountains Preserve. Image: Stephen Hummel.
Fireflies are not an insect species commonly associated with the desert. They are typically found in humid, dark, wooded environments typically found in the eastern half of the United States and elsewhere. Relatively few fireflies can be found in the dry conditions of the western United States. One exception is the Sky Island Firefly, only known to live in the Davis Mountains of far west Texas.
The Sky Island Firefly, or Photuris Flavicollis, may have once been seen as far as New Mexico according to reports dating back over one hundred years ago. Since that time its range has steadily shrunk due to oil and gas development in the Permian Basin, extended periods of drought, loss of suitable ground cover due to overgrazing, and spread of light pollution. Today they are only known to survive in the thicker vegetation and cooler temperatures the Davis Mountains provides. The combination of numerous threats and reduced range have resulted in the species being listed on the International Union for the Conservation of Nature (IUCN) Red List for Threatened Species. Despite the status, small populations of fireflies can still be found relatively easily in the Davis Mountains along Highway 118 in June through August. The dark conditions and mostly untouched habitat provide a refuge for the fireflies.
Fireflies are one of the insect species most vulnerable to light pollution. While an alarming number of insects worldwide are in sharp decline, with light pollution being a critical factor, fireflies are in a unique group. Many insect species are attracted to artificial light, particularly blue light, to the point that they may become disoriented or even exhaust themselves to death attempting to get closer to the source. Fireflies, however, do not simply fly towards any source of artificial light like many moths do, and seem to largely ignore blue lights or are not able to perceive them. Instead they are only attracted to lights if they blink in a unique pattern and are a green or yellow color, similar to a potential mate. Many firefly species have unique flash patterns to only attract other fireflies of their same species, or even mimic another species to lure them in as prey.
However, the phosphorescent flash of a firefly’s abdomen is quite dim, coming in a just a fraction of a single lumen. By contrast, even a modest night-light found in a bedroom may be 50 or more lumens, and a household security light may emit 1500 or more lumens. It doesn’t take much ambient light to overpower a firefly’s flash and make it difficult for them to detect each other. A recent study in Brazil found that the closer you get to light sources, the fewer fireflies are present. Worse, another study concluded that because most fireflies are nocturnal and only flash at night, if conditions are too bright they may be fooled into thinking it is still daylight and refrain from flashing, or reduce the rate of their flashes.
Fireflies present a unique conservation challenge for dark skies advocates. It has long been known that blue light in particular is a problem from a dark skies perspective because it scatters more in the atmosphere, and affects the circadian rhythms of most mammals and other animals. For these reasons and more, dark skies lighting strategies often focus on reducing or even entirely eliminating blue light. In their place soft-white or amber lights are typically recommended. Unfortunately amber lights have the highest impact on fireflies ability to find each other, perhaps even worse than white, according to one study.
One solution is to use red lights, which a firefly’s vision is not sensitive to, and also is ideal for protecting the night sky. Red lights are already commonly used around observatories and other stargazing areas, but outside of niche applications red is unlikely to be a popular lighting choice in more developed areas. Even amber lights can be a tough sell for those who are used to a bright white glow.
While amber may not be ideal for fireflies, there is more to lighting than color. Shielding lights and aiming them properly, dimming them to appropriate levels, and most importantly turning lights off when not in use all can make a dramatic impact. Red lights or no lights at all are the ideal choice for critical environments such as in the Davis Mountains, within the core of the Greater Big Bend International Dark Sky Reserve.
Sky Island Fireflies cross paths above a pond near McDonald Observatory. Image: Stephen Hummel.
