CLT Practice Test 7

Science

¹This passage is an original essay written for PrepBanq practice.

²On a warm summer evening, a meadow that seemed empty at sunset can fill with small, drifting lights. Most of them belong to male fireflies, which rise from the grass and flash in patterns specific to their species. A female perched on a blade of grass watches these signals and, if a pattern is right, answers with a flash of her own after a precise delay. The male turns toward her, flashes again, and the exchange continues until the two meet. This courtship is a conversation conducted entirely in light, and like any conversation it depends on the listener being able to hear—or, in this case, to see.

³For most of the history of fireflies, the backdrop for that conversation was darkness broken only by the moon and stars. That backdrop has changed. Streetlights, parking-lot fixtures, illuminated signs, and porch lamps now brighten the night across much of the inhabited world. Biologists group these sources under the term artificial light at night, and its influence is not confined to the area directly beneath a lamp. Skyglow, the diffuse brightening of the sky above towns and cities, can be detected many kilometers away from the nearest light source.

⁴Why should added light trouble an animal that produces its own? Researchers have proposed two main answers. The first concerns contrast: a female must pick out a male’s brief flash against the background, and a bright background makes a faint signal harder to detect, much as a flashlight beam is hard to notice at noon. The second concerns timing. Many fireflies appear to use the fading of daylight as a cue to begin their displays, so a field that never grows fully dark may never signal, in effect, that the evening has begun.

⁵To test these ideas, investigators have taken them into the field. In a typical experiment, lamps are installed in meadows that are otherwise dark, and observers count flashes at increasing distances from each lamp, comparing the totals with those from similar plots left unlit. Several studies of this kind have reported fewer flashes, and fewer completed courtships, close to the lights. The size of the effect, however, differs greatly from one species to another. Species that court at twilight, when the sky is still fairly bright, often seem less disturbed than species that court only in full darkness—a pattern that makes sense if each species’ eyes and signals are tuned to the light levels of its usual display time.

⁶Laboratory work points in the same direction. When females of some species are kept under dim artificial light, they answer male signals less often, and fewer pairs mate. Yet researchers are careful not to treat such results as the whole story. A laboratory arena is a simplified world, without the moonlight, rival males, predators, and shifting weather of a real meadow, and an effect that appears clearly in a container may be weaker, stronger, or simply different outdoors.

⁷Not all light is equal in its effects. Modern light-emitting diodes, or LEDs, are efficient and long-lasting, but many of them emit a large share of blue light. Some studies suggest that amber or reddish lighting interferes less with certain firefly species, while others find that the brightness of a lamp matters more than its color. Because the species differ in which colors their eyes detect best, no single kind of lamp is likely to be harmless to all of them.

⁸This uncertainty has not prevented practical suggestions. Unlike many pressures on wildlife, light pollution is local and immediately reversible: when a lamp is switched off, it leaves no residue behind. Shielded fixtures that aim light downward, motion sensors, and timers that dim lights late at night can all reduce the glow that reaches nearby fields. At the same time, outdoor lighting serves genuine human purposes, from safety on roads to the life of a town after dark. The question facing communities is therefore rarely whether to light the night, but how much light to use, and where, and when.

⁹Fireflies are hardly the only creatures affected by artificial light; moths, migrating birds, and newly hatched sea turtles have all been studied for the same reason. But fireflies offer an unusual advantage to anyone who wishes to observe the problem. Their signals are visible, countable, and beautiful, so that a change in the night’s conversation can be recorded by anyone with patience and a notebook. That is one reason volunteer surveys, in which members of the public count flashes in their own yards and parks, have become a valuable source of evidence for scientists trying to understand how the darkness is changing.