April 2014 Featured Paper
"Building Twilight 'Light Sensors' To Study the Effects of Light Pollution on Fireflies" by Anchana Thancharoen, Marc A. Branham, and James E. Lloyd. The American Biology Teacher, Online Publication, February 2008
Read the Original Paper
Light pollution—the artificial light from man-made sources—affects many nocturnal animals, including mammals, birds, reptiles, amphibians, fish, and arthropods. Many scientists believe that light pollution is partially responsible for the decline in firefly numbers, but as yet there is no experimental data to support this view. The paper published in The American Biology Teacher online magazine addresses this lack of data. The authors also offer a simple experiment that will allow students to test the effect of light pollution on firefly mating behavior for themselves while learning something about biodiversity and ecological conservation.
The firefly flash is best known as a mating signal. Males flash while flying in search of females. An interested female responds with a flash from her perch on vegetation or ground. The male seeing this signal flies towards her, lands, and mates.
Each species of firefly has a nightly window of activity. For some species, this window begins at twilight, before full darkness of night; the ambient light of dusk may be the cue to begin their mating ritual. Light pollution in these areas may disrupt the communication between males and females.
The authors’ study of light pollution on Photinus collustrans, a common species in Florida and southern Georgia and one of the more thoroughly studied firefly species, can serve as a model for the study of other fireflies. P. collustrans is active at twilight and inhabits open grassy areas free from shadows cast by surrounding trees or structures. This habitat allows for relatively unobstructed viewing of the low-flying males searching for their mates.
This paper offers students instructions on how to build sensors of their own to measure light intensity. Using the light sensors they build themselves, they can perform two experiments to demonstrate the effect of artificial light on fireflies.
Building the Light Sensors
Light sensors are constructed using readily available photocells, wire, ping-pong balls, PVC pipe, and an ohmmeter. A series of nine light sensors are laid out along a 27-meter path with 3 meters between each sensor. Light pollution is introduced using a 60-watt bulb on a tripod at a height of 3 meters placed at one end of the string of sensors.
Experiment 1: Are Fireflies Deterred by the Presence of Artificial Light?
In this experiment, the number of males flying over each section of the light sensor (the three-meter space between light sensors) in one minute is counted and recorded. The light level at each sensor is also recorded.
The experiment is repeated twice—once with the 60-watt bulb off and once with the bulb lit—to represent artificial light pollution in the field. Results are graphed and the data analyzed.
In their trials, the researchers found that the presence of artificial light did negatively impact the number of males recorded.
Experiment 2: Do males have more difficulty finding females in regions of artificial light?
To demonstrate the effect of light pollution on male and female courtship signals, three small light-emitting diodes (LEDs) are spread out along the path of the sensors. These are flashed at the proper intervals to imitate female response to a male’s flash.
Males flying past the LED-simulation females are observed for any reaction to the perceived female flash. Responses varied: some only swerved slightly in flight while others approached the LED more closely. The number of responding males, their response level, and the light intensity is recorded, graphed, and analyzed.
The researchers’ data indicated that male fireflies responded less often to the simulated females in areas of greater light intensity than in areas with a lower light level.
Students performing these experiments may afterwards be asked for ideas to reduce light pollution, such as:
- Limit usage of external lights. Use low-light bulbs or install a dimmer switch for lights that must remain on.
- Limit the area illuminated by using fixtures that direct light downward.
- When possible, use low-pressure sodium light sources instead of broad spectrum bulbs.
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