Texas Parks & Wildlife Magazine   


Bright Nights

From fireflies to ocelots, many species are adversely affected by ever-increasing levels of artificial lighting.

By Melissa Gaskill

As the sun fades from the sky, porch lights across Texas cast circles of illumination that welcome visitors, guide children home and help pizza-delivery drivers read addresses. The light also attracts insects, which beckon spiders and, in recent years, house geckos. The humble porch light is part of mounting evidence that artificial light creates opportunities for invasive species like geckos. John Davis, a Texas Parks and Wildlife Department urban wildlife biologist, suspects this is only one way that light changes the natural world. Biologists are just beginning to shed light, pun intended, on exactly what those changes might mean.

The insects clearly pay a dear price, in the form of increased predation by the opportunistic spiders and geckos. That may seem like a good thing, until you consider the importance of bugs as food for animals and pollinators for plants. For example, says Mike Salmon, a biologist at Florida Atlantic University, reduced insect populations force many birds to work much harder to find enough insects to feed their young.

And bugs don’t have to die to be affected. On summer evenings, when male fireflies start flashing, they’re doing so in hopes that a female in the vegetation below will blink back and, well, nature will take its course. In today’s typical suburban yard, the poor fellows compete not only with porch lights, but also with street lights and the glow from illuminated malls and car lots to get noticed by the ladies, whose responses may be drowned out as well.

“Some people may not give a [darn] about fireflies,” says Jim Lloyd, professor emeritus in the entomology department at the University of Florida and author of a chapter on the bioluminescent creatures in Ecological Consequences of Artificial Night Lighting, edited by Travis Longcore and Catherine Rich. “But these are clues to what could happen with other species.” Even when sharp-eyed fireflies manage to mate, Lloyd adds, additional troubles await. “Highways, buildings and a lower water table have all combined to reduce the habitat available. When females go to lay eggs, they use signals from other fireflies to choose a spot.” A bright night makes that more difficult, and artificial light could become the last straw. And, not just for fireflies. More and more, evidence indicates that light creates changes in every aspect of the natural world, from animal orientation to navigation, reproduction, interspecies communication, competition for food among related species and predator-prey relationships.


The effects on birds, perhaps the best documented of any species so far, include disruption of annual migrations and ongoing orientation. On clear nights, migrating birds typically use stars for navigation. Under a cloudy sky, they switch to an alternate method that their species developed for just such a contingency, explains Bill Evans, an ornithologist and director of Old Bird, a nonprofit organization that monitors nocturnal bird migration.

One alternative is the earth’s magnetic field, but recent research has shown that certain wavelengths of artificial light can disrupt a bird’s sensitivity to that field. Birds may also switch to visual navigation, which they use during the day, in lighted areas at night. But this can make them reluctant to fly back into the dark, since their eyes need time to adjust when going from light to dark, just as ours do. That results in birds congregating in an urban area when normally they would have moved on, and an area ends up with more birds than it would normally have, or with birds not typically in that area. Birds in both cases are more vulnerable to predation and have difficulty foraging.

John Arvin, research coordinator for Gulf Coast Bird Observatory, cites ongoing studies that show birds are also in danger of collision around lighted structures at night, particularly isolated ones. In what are known as circular events, similar to the trapping effect that gets insects, birds are drawn to the light, then fly around and around in it, where they may eventually strike guy wires, the lighted object or each other — or drop to the ground exhausted. Flocks of birds have died this way. At South Padre Island Convention Center, large numbers died during a spring migration in the 1990s, Evans says. Some 450 migrating species are potentially vulnerable to collisions with lighted structures, with as many as 100 million deaths in North America every year. Since Texas lies directly in major migratory corridors, many of those deaths occur here.


Late at night on Padre Island National Seashore, the black silhouettes of sand dunes rise behind the beach, the expanse of flat water opening in front of it. Hatching sea turtles instinctively use visual orientation to turn away from the former and dash toward the latter, just as they have done for millennia. But on many beaches, turtles no longer encounter that natural order of dark dunes and open, brighter water. Street lights, lighting on houses and condominiums, even a glow in the sky from distant urban areas, can disorient hatchlings. They wander for hours, dying of dehydration or exhaustion, falling victim to predators or being run over. In Florida, thousands of annual hatchling deaths have been documented due to this disorientation.

Thanks to Padre Island’s remoteness, turtles on Texas beaches aren’t directly affected. “But it is safe to say that what happens on any nesting area affects the general turtle population,” says Salmon. “The most heavily used nesting beaches in many places are threatened by development and lights. Everything that interacts with the animal is also affected by the problem.” Fewer turtles means decreased nutrient content on beaches and less healthy sea grass beds.


All 986 species of bats in the world are nocturnal, equipped to do best in low light. Populations in rural areas like Devil’s Sinkhole or Old Tunnel Wildlife Management Area still enjoy those conditions, and lights may actually be beneficial for urban populations, attracting insects for the bats to eat. But bright lights have been known to stop emergence of the bats under Congress Avenue bridge in Austin, says Barbara French, a scientist with Bat Conservation International. And faster-flying bat species that congregate around lights to take advantage of the insect buffet may displace slower-flying species that avoid lights and the increased predation danger they represent.


An estimated 60 ocelots — likely the entire U.S. population — live in the Rio Grande Valley area on the Laguna Atascosa National Wildlife Refuge and protected corridors of the Lower Rio Grande Valley National Wildlife Refuge. (See also “Trapping Ocelots with a Camera” in the June 2005 issue.) “They are strictly nocturnal animals,” says Linda Laack, a former refuge biologist. “Their prey is strictly nocturnal. If you think about how they live, you would suspect that lighting would be bad for them.” There are so few of the animals, and they are so reclusive, that specifics are difficult to document. But Melissa Grigione and Robert Mrykalo at the University of South Florida reviewed literature on how artificial light affects nocturnal rodents, the cat’s primary prey. Light sends rodents scurrying for cover, in your kitchen and the wild, and in conditions as bright as the full moon, rodent activity tends to cease.


Nocturnal frogs suspend normal feeding and reproductive behavior when exposed to light, and individual hoppers may remain motionless long after the light is turned off. Female frogs of at least one species are less selective about a mate in increased levels of light — call it the closing time effect — presumably balancing the need to be choosy against the equally important need to survive. Male tree frogs have been known to stop calling in areas with bright lights, and no calling means no mating, which eventually means no frogs.

Lights Out

Fortunately, this is one problem we have the ability to solve. Some light can simply be eliminated, if not altogether then at least during peak bird migrations or turtle nesting. In Florida’s 2001 season, standard streetlights were replaced with light-emitting diode, or LED, markers in roadways and sodium lamps low on roadsides, and not one hatchling became disoriented due to light sources. White strobe lights on towers do not induce as much congregating of birds as red lights and have not been implicated in mass mortalities.

Lighting’s effect on the environment can be considered in new construction or improvements, already the case at state parks, says Steve Whiston, director of TPWD’s infrastructure division. Efforts include reducing lighting — at Davis Mountains State Park, for example, project manager Laura David determined existing lighting could be reduced by three-fourths — indirect lighting, and simply hitting the off switch. At Government Canyon, a new park outside San Antonio, David says, timers turn lights off when the park closes.

When lighting can’t be eliminated, it can be more thoughtfully designed. Along the Rio Grande, the U.S. Border Patrol greatly increased use of bright lights for Operation Rio Grande, a push in the late 1990s that significantly decreased area illegal crossings and drug trafficking. But in response to concerns about the effect of all that light on brush-loving ocelots and other critters, the agency reduced and redirected lights onto roads and open fields. According to Ernesto Reyes, a U.S. Fish and Wildlife biologist, all parties are pretty satisfied with the compromise.

Fort Bend County, home of Brazos Bend State Park’s George Observatory, passed a low-light ordinance that requires, for example, focused beams that minimize stray light reaching the sky and shielding for some outdoor lighting.

The International Dark Sky Association estimates that one-third of all outdoor lighting illuminates the atmosphere, wasting more than $1 billion in electricity, and creating night skies so bright that some 40 percent of Americans never even adjust to night vision. The TPWD Infrastructure Division follows Dark Sky Association criteria for new buildings and lighting renovations in parks. TPWD also uses low lighting levels and nonpolluting lights with shades and cutoffs to prevent casting light upwards. The stars, long a source of wonder and enjoyment, are often virtually invisible; two-thirds of our population can no longer see the Milky Way. With more thoughtful use of lighting, and the occasional flip of the porch light switch, we can not only protect the wonder of the night sky, but also help protect all the creatures that live under it, too.

Save the Dark

  • Turn off unnecessary lights
  • Reduce wattage to the minimum required for function
  • Redirect and focus lighting so it reaches the ground or areas where needed
  • (proper shielding can redirect lights, for example, onto signage and not up into the sky)
  • Eliminate all upward-directed decorative lighting
  • Use alternative light sources where possible and practical
  • Incorporate latest technology in new construction

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