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Looking for light in all the wrong places

Photonics Spectra
Mar 2009
Caren B. Les, News Editor,

Creatures in the wild that follow natural light cues to negotiate their environment are sometimes misguided by man-made polarized light sources, such as asphalt roads, oil spills, black gravestones, glass panes on buildings, dark-colored paint work on automobiles and black plastic sheets used in agriculture, according to a study done by a group of ecologists, biologists and biophysicists.

A marine turtle hatchling’s orientation to the sea is guided by light cues. Cues from man-made lighting can lead them away from the water. Photo courtesy of Blair E. Witherington.

These miscues lead the animals away from their natural habitat and into environments where they sometimes become permanently trapped, highly susceptible to predators and sometimes unable to feed, migrate or reproduce successfully. The failure of any species to thrive is known to affect the wider ecosystem.

At least 300 species of dragonflies, mayflies, caddis flies, tabanids, diving beetles and other aquatic insects orient themselves using horizontally polarized light sources to find water in which they can feed and breed, according to the report, which was published in the journal Frontiers in Ecology and the Environment.

A stonefly lays eggs on granite. Photo courtesy of György Kriska.

Artificially polarized light can be even more attractive to them, diverting them from their natural habitats into areas such as waste-oil pools, where they are likely to become trapped and die. Dragonflies and mayflies lay eggs on surfaces including shiny cement, glass panes and black plastic sheets, which, as with water, reflect horizontally polarized light. Male dragonflies often establish their territories on car antennas, and females lay their eggs on the shiny surface of the hood, where hatching fails.

Female sea turtles can be disoriented by artificial beach lighting. Photo courtesy of Blair Witherington.

According to the report, humans can lessen the effects of these ecological perils in a variety of ways, including painting white hatch marks on asphalt, using rough instead of shiny building materials, using lighter-colored building materials, and minimizing night lighting and/or directing it away from buildings, asphalt and cars near aquatic areas. The report also notes that photovoltaic solar panels are a possible source of polarized light pollution and that increasing numbers of them may be installed in response to rising energy costs.

A water beetle is attracted to a shiny red car. Photo courtesy of György Kriska.

Ambient lighting for sea turtles

The Florida Fish and Wildlife Conservation Commission (FWC) is working to protect sea turtles from the hazards of artificial lights on or near beaches. These lights are the source of mortality for many thousands of sea turtles each year in Florida and represent an important conservation challenge for the endangered species, according to Dr. Blair E. Witherington, a scientist with the FWC’s Wildlife Research Institute.

Female sea turtles come ashore to lay their eggs on the beaches at night. They are sensitive to artificial lighting visible from the beach and tend to shun the brightest beaches, said Witherington, who is an expert on sea turtles and lighting. He added that, when the hatchlings emerge from their nests at night, they have their orientation to the sea guided by light cues. When artificial light sources are visible from the beach, hatchlings become disoriented or are led away from the sea into the dune, where they often die from exhaustion, dehydration, predation or crushing by cars in parking lots and roadways.

Hatchling sea turtles orient toward the center of the brightest horizon as if brightness were measured by a detector with a broad, flat acceptance cone and a sensitivity to wavelengths between the near-UV and the yellow-green, according to Witherington. He added that, because a hatchling’s assessment of “brightness” is spatially comparative, highly anisotropic light fields – such as those produced by artificial light sources – create an overwhelmingly bright direction that more than competes with natural light cues from the night sky over the open sea.

Experiments have shown that sea turtles show no preference for vertically, horizontally or unpolarized light sources of equal radiance, Witherington said.

Roadway, security, parking lot and balcony/porch lighting all cause problems for the turtles, as does interior lighting through windows. Both direct and indirect illumination – e.g., sky glow – also can be hazardous. Witherington said that lamp types with the greatest harmful effects include broad-spectrum sources such as mercury vapor, metal halide, fluorescent, incandescent and high-pressure sodium vapor. Lighting that is more turtle-friendly includes near-monochromatic long-wavelength sources such as low-pressure sodium vapor, amber and red LEDs, and some sources to reduce insect attraction.

Not only sea turtles but also other types of beach wildlife suffer the consequences of artificial lighting, Witherington said, adding that a species of tiger beetle that lives only on beaches and is drawn to lighting has been eliminated from artificially bright beaches. Endangered beach mice, sensitive to lighting, restrict their foraging in artificially bright dune areas. Shorebird nesting and migration can be disrupted by unnatural light as well.

One state’s efforts to lessen light

The state of Florida has engaged in research to learn more about how artificial lighting negatively affects sea turtles and how shielding and filtering of light sources and how choices of light-managed fixtures can reduce the problem. It is involved in ongoing education to promote light management for sea turtles through publications, workshops and direct assistance with lighting plans.

Florida requires approval of coastal lighting plans for new development and maintains records of turtle hatchling and adult orientation events. It also conducts site visits to detect lighting changes following permitted lighting plans. Witherington said the state has promulgated a model lighting ordinance that specifies how lighting near beaches should be managed to protect sea turtles. The majority of coastal counties and municipalities with sea turtle nesting have adopted ordinances similar to this model, he noted.

Companies involved in the development of light fixtures and lamps are showing an interest in providing options that provide minimal harm to marine turtles and other wildlife while still contributing to the safety and needs of the human population. The FWC works with light manufacturers to provide them with information on the appropriate types of fixtures and wavelengths as well as on correct siting to minimize the impact on coastal turtle nesting habitats. The staff then can provide coastal property owners and local governments with information on these fixtures and how to use them properly.

Beachfront development

Despite the continuing and extensive development of Florida’s beaches, individual beaches have achieved striking reductions in lighting effects on sea turtles following light-management efforts, and overall rates of hatchling disorientation have not increased, Witherington said. Statewide success in these efforts is difficult to assess, he added.

The solutions to these lighting problems are not complex and do not require new levels of understanding or technology, Witherington said, noting that continued vigilance and effort will lead to improvements today and in the future.

Partners in lighting

Conservation of threatened and endangered species such as marine turtles depends on cooperative partnerships that consider and address the needs of both humans and wildlife, Witherington said. He added that Florida’s efforts to offer lighting options to coastal communities that create a safe, attractive environment for both beachfront properties and marine turtle nesting habitats can succeed only through partnerships such as between the biologists who study these species and the engineers and architects who provide appropriate lighting solutions.

1. A device designed to convert the energy of incident radiation into another form for the determination of the presence of the radiation. The device may function by electrical, photographic or visual means. 2. A device that provides an electric output that is a useful measure of the radiation that is incident on the device.
The general term for the application of light to a subject. It should not be used in place of the specific quantity illuminance.
ConsumerdefensedetectorenergyGreenLightilluminationpolarized light sourcesSensors & Detectors

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