Brilliant_ The Evolution of Artificial Light - Jane Brox [115]
It is not only light itself but the duration of the light that affects birds. They, too, are exhausted by sixteen-hour days. Artificial light triggers their dawn response and leads them to sing after sunset, sometimes to sing all night. The artificially extended day affects their migration and breeding patterns as well.
In the animal world, even what goes on under one streetlight after dark has complex and far-reaching consequences, for a single light is capable of changing the equilibrium of an ecosystem. Moths and insects gather around a streetlight; bats and toads come in to pick at easy prey. Notes one scientist, "The habit of feeding at artificial lights is now so common and widespread among bats that it must be considered part of the normal life habitat of many species." This not only increases the stress on insect populations; it also changes the relations between different bat species, since not all species use lights for feeding, though they may feed on similar insects. The presence of streetlights gives species that use lights a competitive advantage over other species. The non-light-using species may decline because they have lost their competitive edge. By altering habitat and spurring adaptations that might eventually become encoded in the future lives of insects, mammals, birds, and reptiles, "humans are changing the evolutionary trajectories of those affected species, causing them to adapt to new sets of conditions," notes biologist Bryant Buchanan. "Simply conserving species richness or population sizes does not conserve the evolutionary and behavioral diversity contained in those taxa."
Sometimes artificial light becomes an evolutionary trap as the age-old biological imperatives of a species, which helped it survive for eons, turn into liabilities. The most well-known example of such a trap is the predicament of the loggerhead turtle, which can live for more than 130 years. It inhabited coastal waters long before humans existed on earth, trolling the shallows, feeding on sand dollars, whelks, and conchs. The female, year after year, crawls out of the surf and onto sand beaches to nest. She has always preferred the cover of darkness for safety, and now the bright lights of shoreside developments often drive her away from prime nesting sites. When she does settle on a nesting place, she digs a pit along a sandy shore with her flippers, then deposits a clutch of eggs, falling, "as they have fallen for a hundred million years," writes David Ehrenfeld, "with the same slow cadence, always shielded from the rain or stars by the same massive bulk with the beaked head and the same large, myopic eyes rimmed with crusts of sand washed out by tears."
She then covers the nest and returns to the sea. The eggs take months to develop, during which time, if the female has not been able to nest in the best of places, they are all the more vulnerable to extreme high tides, storms, and predators. If they survive their incubation period, the hatchlings then extricate themselves and dig their way—en masse—to the surface. If the surface sand is hot, they know it to be daylight, and they burrow back down and wait until the sand cools after sunset. Then they begin their trek to the sea. They are keyed to move toward the lightest horizon, and for thousands of years this meant they crawled away from dark dunes and vegetation and toward the ocean, whose surface, glinting and sparkling with reflecting starlight and moonlight, was brighter than the interior land. In a dark landscape, the baby turtles usually have no more than a two-minute trip to the beach.
But on developed beachsides, lit with condominiums, streetlights, and commercial districts, the turtles are confused by the brilliance of the built landscape at night. They crawl toward high land instead of the sea; crawl into roadways, where they are killed by cars; or crawl