Spring 2006

Darkness Visible: An Interview with Travis Longcore and Catherine Rich

On the ecological consequences of artificial lighting

Courtney Stephens, Travis Longcore, and Catherine Rich

Though the social impact of electric lighting has long been studied, in recent years increasing numbers of scientists and activists have turned their attention to examining other, often highly nuanced effects that light has on the biosphere. Prominent groups like the Dark Sky Association, an organization formed to advocate for the reduction of the phenomenon that astronomers call “sky glow”—the surplus of ambient electric light that reduces visibility for both individual stargazers and large telescopes—have lobbied for efficient bulbs, sensor-based lighting, capped fixtures, and other solutions with some success. Yet these aesthetically inspired adjustments rarely address the non-human world, where lighting plays a critical role in the regulation of both plant and animal physiology. Ecosystems and individual species rely on light-and-dark patterns as cues for sleeping, foraging, mating, hibernating, and hunting. When human light alters these rhythms, the whole choreography of biological life is jeopardized. Travis Longcore and Catherine Rich are the co-directors of the Urban Wildlands Group, and the editors of Ecological Consequences of Artificial Night Lighting (Island Press, 2006). They spoke to Courtney Stephens by phone from their home in Los Angeles.

Cabinet: In what ways can we think about man-made light as a pollutant?

Travis Longcore: Well, a pollutant is only a pollutant because we call it such. Carbon dioxide, of course, has been produced on earth for billions of years, but we now consider it a pollutant because it’s more abundant than it would be naturally. In the same way, we can think of light as polluting when it’s more prevalent than it would be naturally.

It seems that in the popular vocabulary, “light pollution” refers to light that is going up, diluting the stars. What you’re concerned with is actually a separate issue.

Catherine Rich: And more difficult to remedy. In our work on the effect of light on animals and plants, it has become clear that it’s helpful to draw a distinction between ecological effects and the concerns of the dark sky community about the effects of light on astronomical observation…

TL: …and aesthetics. In our work, we felt we needed to distinguish between when light affects astronomical observation and when it has ecological effects.[1] In a Venn diagram, they overlap in the middle, but there are some things that have the potential to cause ecological disruption that aren’t necessarily interfering with our views of the night sky. The ecological issues are far more complex to deal with—much more of a case-by-case basis, and location- and species-specific.

CR: To clarify, one could protect night skies by installing full cut-off lighting fixtures, which prevent direct light from escaping above the horizontal plane. This is a typical dark sky solution. But these lights may shine right into a wetland.

Which are the most vulnerable species?

TL: The groups we’ve maybe understood the longest in terms of disorientation and death by light are birds, and before that moths, which brings us back to Aristotle and his observation that some moths are attracted to light. With the use of coastal flare fires and lighthouses, even back at the end of the nineteenth century, there are records of large kills of migrating birds that are caught by the light.[2]

Migrating birds are attracted to or disoriented by the lights of tall television and radio towers and end up flying in circles around them. There are mass mortalities of birds running into the guy lines or running into each other and being killed.[3] Buildings not only attract birds at night but then, during the day, the birds remain stranded in these labyrinths of glass and light. The other well-known example, which gained concern in the 1960s, was the misorientation or disorientation of sea turtle hatchlings on beaches. When there is light present, the hatchlings are unable to properly orient themselves and make their way into the ocean as they need to do. The misconception was that sea turtle hatchlings are attracted to the moon and therefore attracted to the light. They will indeed gather under the lights but the attraction is actually an orientation away from the darkest horizon, which in natural circumstances is the dark dune vegetation, and towards the brighter horizon, which naturally would be the reflective ocean. When you have artificial lights present, the turtles will get reversed and go in the wrong direction, are eaten, crushed by cars, dry out in the sun, and what not.[4] So those are the two best-known examples, plus the millions of insects that are killed at lights. An estimate for the number of insects killed by streetlights in Germany each summer, for example, is something on the order of one hundred billion.[5]

When we talk about light, particularly as other animals experience it, is brightness the applicable measurement?

TL: There are two major ways of measuring light. The first is brightness—how bright something looks against a background depending on the conditions. If you look at a match in a field at twenty yards during the day, you may not even see the match burning, but at night it will seem quite bright. The measure of brightness is luminance. The other measure is illumination, which is how much light, in photons per meter squared, is incident on, and thus reflecting off, some object. So the spatial orientation of organisms can, first of all, be affected by the luminance of glare from looking directly at a light. This can affect spatial orientation—attraction, repulsion. The comparable measure of illumination is lux, a measurement that is weighted by the sensitivity of the human eye. Using that term biases the measurement to the human visible spectrum, but there are also ways of measuring the intensity of light by how many photons are streaming in and what the wavelengths of those photons are. So all these things in addition to the rapidity of lighting changes will combine and have their particular effect on an organism.

Spectrum is also an issue. The light that people have thought of as having the least effect is low-pressure sodium, which produces a single-wavelength yellow light. It is best for many groups of organisms, but it is not perfect. It doesn’t particularly attract insects or sea turtles but it does, for example, disrupt the orientation of some salamander species.[6] Similarly, birds under red-only light don’t seem to be able to orient with their magnetic compasses.[7]

So there isn’t an overall “best” fixture or bulb, but rather there are better practices. Maybe it’s a combination of things; using a specific wavelength in one area; using motion detectors so that light goes off at a certain time; other operational controls for focusing or locating lights closer to the subject to minimize spillover light; using lower wattages, and so on. There are things that are definitely worse than others, the foremost being ultraviolet light. Mercury vapor and metal halide lights will emit a lot of light in the ultraviolet spectrum and will attract a lot of insects.

Many of the examples are of species that are attracted to light, but many are similarly repelled.

TL: In the simple sense, yes. Species behave in different ways under different lighting conditions. Some species tend to do things in darkness and others in the light. So species which, under normal circumstances, would avoid the light of day or the full moon are similarly going to avoid the light of a house or a city. Just to take one example that can be easily extrapolated: rodents such as kangaroo rats or beach mice will do most of their foraging during the new moon when it’s darkest and do the least amount during the full moon when it’s brightest. This is a pattern that’s gone on for a very long time. In experiments with artificial lights, they will forage less.[8] This mimics their behavior under a full moon. The presumed reason for this is the risk of predation. Generally, more light tends to help the predator. There is an exception with communal species like a flock of crows for whom the light helps them watch for owls, or a school of fish that has a communal predator response, but in general more light is better for the predator, and so species that are avoiding predation will be less active when artificial light is around.

So there are organisms that stand to gain in certain situations as others stand to lose?

TL: I think you’ve asked the question in exactly the right way. In the natural world, and perhaps also in the human world, it’s dangerous to talk about benefit in the simple sense of more being better. There are certainly species that exploit artificial light to their benefit, species that extend activities from daytime into the night and forage under the light—bats, for instance, forage under streetlights; spiders will weave webs under lights; geckos and lizards forage under lights. This gets a name in the herpetological literature, which is the “night-light niche.” So in a simple sense, it’s good for those species, but of course, what happens to the species they prey on? Or to their overall physiology? Another example is swans in Slimbridge, Great Britain, where there’s enough light to let them forage all night. The swans that were able to forage day and night laid down fat faster and were ready to leave and migrate to their breeding grounds faster than they would have normally, and you think great![9] More is better! But the consequence of that is that they leave and get to where they need to go and it’s the wrong time of year. The weather is wrong, or whatever the situation. There are whole webs of species altering their cycles to feed on each other. The species that can do this are the ones who get along pretty well with humans, the ones who can live around urbanization, the more weedy species. There are preferences for illumination; some species are perfectly suited and active at extremely low illumination—the darkest of the dark of night. Others are more active during the crepuscular period. We find the biggest declines in nocturnal species that really need the dark of night, which are being sacrificed for the ones who are benefiting.

CR: It’s a matter of resource partitioning. So if you take out the darkest part of the night, you lose some of the complexity of the system itself and diminish its diversity.

You’ve talked mostly about behavioral adaptation. Is there any evidence of physiologically selective adaptations, in the evolutionary sense?

TL: Yes, we have limited examples of that. There’s a paper on spiders that build their webs underneath streetlights.[10] Lab studies found that this is not simply a behavioral response, where spiders learned that there will be more insects near lights. In the lab, they still would choose the lighted area. The conclusion was that this was an actual, inherited trait that couldn’t have been found in the species prior to streetlights existing.

CR: This is something you can see in an organism that has a brief lifespan. A desert tortoise that is going to live how many years won’t give you the opportunity to observe the evolutionary adaptations that have occurred over the incredibly short timeframe that we’ve lit up this world. There’s this heartbreaking contrast between the enormous amount of detailed work that goes into studying these organisms and the macro change in lighting over the Earth that is destroying any opportunity we have to know them.

TL: Right. One hundred, one hundred-fifty years is too short for long-lived species to be showing evolutionary changes. And the dangerous thing is that once people learn that a species is able to exploit light and make changes, their human thinking is, “Oh, one species was able to exploit light, why don’t the birds evolve so that they don’t run into the towers?” It’s a little preposterous, but the actual answer is that not all species can evolve in these directions. There are constraints on the actual physiology of the organism.

CR: There was a lawyer from the communications industry, with no biology background, who filed a document with the Federal Communications Commission arguing that birds running into towers was aberrant behavior, as in, “What’s wrong with the birds?” People have an idea that any behavior can be learned in a very short period.

TL: Scientifically, we’re very interested in these evolutionary adaptations but it’s perilous to talk about because of the misunderstandings we get from the general public about how evolution works.

How is reproductive behavior affected?

TL: The amount of light will influence whether the female of certain frogs, for example, will spend a lot of time choosing their mate or if they’ll choose one quickly in order to get out of the light, where they are vulnerable.[11] So choosiness is one thing. It’s subtle, but female mate choice, which is all about fitness and picking the best mate, can influence the genetic result; the mating is still going to occur but it’s going to be a different genetic outcome than it would be otherwise.

Are there other examples with underwater environments and light?

TL: Well, first of all, research says that although it may be very faint, light from cities can actually disrupt underwater patterns that are tied to the moon, to lunar cycles. There’s some German research on marine polychaetes, worms that have very intricate mating behaviors that are synchronized to lunar cycles; behaviors that break down in the presence of light pollution.[12] In general, light is something that structures underwater environments perhaps even more than it does terrestrial environments. Dr. Marianne Moore at Wellesley looks at the effects of light pollution on the zooplankton Daphnia, which graze on algae. They will move up and down in the water column over the course of the day and night. They move to the surface in the greatest numbers during the darkest times and will sink to the depths to avoid predation when it gets light. She finds in her research a suppression of this migration to the surface to eat algae in places that have a lot of artificial light.[13] There’s also research on undersea submersibles. There are acoustic studies with lights on and off and many fish completely scatter with the light on.[14] But in the really deep ocean simply being exposed to light can actually seriously harm organisms.

What about some of the implications for human health? Recently there has been a lot of press on lifetime increases in melatonin (one effect of elevated night lighting) being linked to breast cancer, and a few years ago there was concern that nightlights caused shortsightedness.

CR: Yes, those are very real and compelling pieces of information.[15] In what we do, we deliberately try to engage the non-human world, which is so overlooked.

TL: That said, it’s a big deal.

  1. Travis Longcore and Catherine Rich, “Ecological Light Pollution,” Frontiers in Ecology and the Environment, vol. 2, no. 4 (2004), pp. 191–198; Longcore and Rich, eds., Ecological Consequences of Artificial Night Lighting (Washington, DC: Island Press, 2006).
  2. See Sidney A. Gauthreaux Jr. and Carroll G. Belser, “Effects of Artificial Night Lighting on Migrating Birds,” in Ecological Consequences of Artificial Night Lighting, op. cit., pp. 67–93.
  3. Lesley J. Evans Ogden, Collision Course: The Hazards of Lighted Structures and Windows to Migrating Birds (Toronto, Canada: World Wildlife Fund Canada and the Fatal Light Awareness Program, 1996).
  4. Michael Salmon, “Protecting Sea Turtles from Artificial Night Lighting at Florida’s Oceanic Beaches,” in Ecological Consequences of Artificial Night Lighting, op. cit., pp. 141–168.
  5. Gerhard Eisenbeis, “Artificial Night Lighting and Insects: Attraction of Insects to Streetlamps in a Rural Setting in Germany,” in Ecological Consequences of Artificial Night Lighting, op. cit., pp. 281–304.
  6. See Sharon Wise and Bryant Buchanan, “Influence of Artificial Illumination on the Nocturnal Behavior and Physiology of Salamanders,” in Ecological Consequences of Artificial Night Lighting, op. cit., pp. 221–251.
  7. Wolfgang Wiltschko and Roswitha Wiltschko, “Magnetic Compass Orientation in Birds and Its Physiological Basis,” Naturwissenschaften, vol. 89 (2002), pp. 445–452.
  8. Burt P. Kotler, “Risk of Predation and the Structure of Desert Rodent Communities,” Ecology, vol. 65 (1984), pp. 689–701.
  9. Eileen C. Rees, “The Effect of Photoperiod on the Timing of Spring Migration in the Bewick’s Swan,” Wildfowl, vol. 33 (1982), pp.119–132.
  10. Astrid M. Heiling, “Why do Nocturnal Orb-Web Spiders (Araneidae) Search for Light?,” Behavioral Ecology and Sociobiology, vol. 46 (1999), pp. 43–49.
  11. A. Stanley Rand, Maria Elena Bridarolli, et al. “Light Levels Influence Female Choice in Túngara Frogs: Predation Risk Assessment?,” Copeia (1997), pp. 447–450.
  12. Matthew G. Bentley, Peter J. W. Olive, et al. “Sexual Satellites, Moonlight and the Nuptial Dances of Worms: The Influence of the Moon on the Reproduction of Marine Animals,” Earth, Moon and Planets, vol. 85–86 (2001), pp. 67–84.
  13. Marianne V. Moore, Stephanie M. Pierce, et al. “Urban Light Pollution Alters the Diel Vertical Migration of Daphnia,” Verhandlungen der Internationalen Vereinigung für Theoretische und Angewandte Limnologie, vol. 27 (2001), pp. 779–782.
  14. Phillipe Laval and Thierry Baussant, “Effect of the Lights from an Approaching Submersible on the 15 kHz Deep Scattering Layer in the Ligurian Sea (Mediterranean),” Comptes rendus de l’Académie des Science Paris, vol. 311, series III (1990), pp. 181–186.
  15. See, for example, David E. Blask, et al. “Melatonin-depleted Blood from Premenopausal Women Exposed to Light at Night Stimulates Growth of Human Breast Cancer Xenografts in Nude Rats,” Cancer Research, vol. 65, issue 24 (2005), pp.11174–11184.

Catherine Rich holds degrees in psychology, geography, and law.

Travis Longcore is research assistant professor of geography at the University of Southern California and lectures regularly at University of California Los Angeles. They co-founded and run the Urban Wildlands Group, a Los Angeles–based conservation nonprofit, and co-edited the book Ecological Consequences of Artificial Night Lighting (Island Press, 2006).

Courtney Stephens is an assistant editor of Cabinet.

If you’ve enjoyed the free articles that we offer on our site, please consider subscribing to our nonprofit magazine. You get twelve online issues and unlimited access to all our archives.