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.
Looking for light in all the wrong places
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.
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.
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.
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.
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 com-
petes 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-wave-length 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 re-
search 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 in-
volved in ongoing education to promote
light management for sea turtles through
publications, workshops and direct assis-
tance 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
