Chatelain’s pigeon studies might mean that city pigeons are evolving toward a darker plumage thanks to the detox properties of melanin-laden feathers. But perhaps the story is more complex, because the genes that produce melanin are also involved in stress hormone regulation and the immune system. So the birds’ hue and their environment’s heavy metal content might not be a simple one-to-one relation, but part of a more complex system: after all, the immune and stress-response systems of urban animals are also taxed differently than those of rural birds. We’ll return to this in a later chapter.
So, rather amazingly, some animals and plants can evolve to cope with the most foul and horrid stuff we humans dump in their environment. As we will see later, this is not true for all species, for many fail to adapt, and perish. Also the ones that do adapt often pay a high price for it. Nonetheless, it is a testament to the power of rapid urban evolution that there are at least some species that manage to keep pace with the chemical pollution that befalls their environment.
12
BRIGHT LIGHTS, BIG CITY
Each year, New York City commemorates the lives lost during the terrorist attacks of September 11 with the “Tribute in Light.” Eighty-eight xenon light beams, 8,000 watts each, directed straight up into the night sky, create two translucent pale blue towers that rise high above this permanently scarred area in downtown Manhattan. It’s a stunning reminder of the havoc wreaked on that fateful day in 2001. The installation’s producer, the late Michael James Ahern, once said of it: “It triggers a whole host of feelings and memories and the things you aspire to, that are without conflict and without aggravation.”
And yet, like almost every major undertaking by humans, even a seemingly pure and ethereal display like this wreaks a havoc of its own. For each year, some tens of thousands of migrating songbirds, which usually fly at night, are trapped in the cage of luminescence. Mid-September being the height of the autumn migration, and the tip of Manhattan a geographical feature that funnels the southbound flyways of multiple species of warblers, each September 11 the memorial display is marred by clouds of confused warblers fluttering from light beam to light beam and uttering a cacophony of alarm calls. Volunteers of the Audubon Society are on site to recover exhausted American Redstarts, Ovenbirds, Black-and-white Warblers, and Northern Parulas, and to advise the display’s operators on when to turn the lights off momentarily to allow the birds to regain their bearings and continue their southward migration. Nonetheless, the light display surely leads to bird deaths or, at the very least, to stress and exhaustion on an already taxing trip down south.
A similarly massive, but emotionally very different event of light pollution and its effect on animal mass migration was the finals of the 2016 UEFA European Championship. The football match, between France and Portugal, was staged in the gigantic Stade de France in Paris, on the hot summer night of July 10, 2016. The previous night, for security reasons, ground staff had left the stadium’s lights on. Attracted by the gigantic floodlights, thousands upon thousands of moths, mostly the silver Y moth, Autographa gamma (named for the bright white Y or γ on its dark-gray speckled forewings), had descended into the empty, cup-shaped stadium. The silver Y is a migratory moth. Each spring, hundreds of millions of them fly north from southern Europe, cruising at altitudes of a few hundred yards above the ground, to benefit from the delayed northern-clime growth spurt in the fields of cabbages, potatoes, and other crops. In some years, there are additional mid-summer migrations across western and northern Europe and, as moths are wont to, one of these flocks had been lured in by the stadium’s lights. Thousands of them were killed by the heat of the lamps, but the rest, dazzled and confused, eventually ended up among the grass of the playing surface where, after the lights were turned off in the morning, they hid throughout the day of the big match.
Then, when evening fell, 80,000 spectators took their seats and the lights were turned back on, causing the sleeping moths to stir. The players’ warm-up was already interrupted by clouds of the moths fluttering low across the pitch and by the time of kick-off at 9 p.m., thousands of the insects were zigzagging among the players. Photographs taken that night show annoyed UEFA officials picking moths off each other’s dark blue suits, moths blocking the lenses of TV cameras or bunches of them hanging from the goal posts, workers desperately wielding vacuum cleaners to keep the lines on the soccer pitch clear, and, the highlight, Cristiano Ronaldo weeping on the grass after a knee injury in the 24th minute, while a lone Autographa gamma sips his teardrops away.
The clouds of birds caught in the Tribute in Light and the masses of moths descending on the Euro 2016 final are just two rather prominent examples of nocturnal animals attracted by artificial lights. The same, in fact, goes on all the time, everywhere, whenever people switch on their incandescent lights, LED displays, gas discharge lamps, or any of the other types of illumination that we have invented to keep out the night-time darkness. For as long as people have done this, this has had the unintended knock-on effect of interfering with the behavior and body clocks of nocturnal animals and even plants.
Moths and other night-active insects are best known for falling prey to light-induced confusion. Light a candle on your porch on a warm summer night and the bugs come flying in from far and wide, circle the flame, searing their wings and eventually doing a kamikaze dive head-first into the boiling-hot candlewax. Scientists still are not completely sure why they do this. Obviously, for the millions of years that insects have been evolving, artificial light wasn’t around, so their attraction to light bulbs has got to be the side-effect of some primal behavior, triggered by natural lights. One popular theory posits that night-flying animals use the moon and the stars for navigation. Since these are so far away from earth and move across the sky so slowly, their positions appear static to a cruising insect, and this would allow them to fly in a straight line simply by maintaining a fixed angle with the moon or a bright star. The first insects to come across an artificial light would have treated these the same way. However, ground-bound artificial lights cannot be used as fixed beacons in the same way that astronomical objects can, because they are too close. So, maintaining a fixed angle to a light bulb or a candle flame in fact means drawing closer to it in ever decreasing circles until you snuff in a puff of smoke. In Shakespeare’s words, “Thus hath the candle singed the moth.”
Whatever the reason, for as long as we humans have lit up the nocturnal environment with our fires, torches, candles, whale-oil lights, and electric lamps, insects have been dying at them by numbers. They die because they burn themselves at close range in the emitted heat or because they are targeted by bats, owls or geckos who have learned that easy pickings are to be had at lampposts. And even if they do not burn or get eaten, simply the time wasted sitting and gazing at the light that they should have spent searching for mates or food could make a hypnotized bug taste defeat in the struggle for life.
If you pay attention to the sheer numbers of insects spinning around street lanterns, caught in the beams of floodlights or forming the accumulated debris caught in the frosted covers of porch lights, you cannot help but wonder exactly how large an impact artificial lighting has on all kinds of animals (insects, mammals, migratory birds, turtles, fishes, snails, amphibians, even plants). They all perform part of their activities under cover of darkness so similarly get confused.
Until recently, science was mute on this question, but for some anecdotal data: 50,000 birds were killed at Warner Robins Air Force Base, Georgia, when they followed landing lights straight into the ground in 1954, and one night in 1981, more than 10,000 birds slammed into floodlit chimneys at an industrial plant near Kingston, Ontario. As for insects, two English entomologists caught more than 50,000 moths at one lamp on the night of August 20, 1949, and once, an estimated 1.5 million mayflies were found dead under the lights on a single German bridge.
For the past fifteen years or so, several scientists have been trying to put more reliable numbers on the impact of
“ALAN” (or Artificial Light At Night—light pollution has landed its own acronym in the ecological literature). The German researcher Gerhard Eisenbeis, of the Johannes Gutenberg University in Mainz, for example, has been on the heels of what he calls the “vacuum cleaner effect.” As soon as ALAN hits a previously dark area, he writes, “insects are sucked out of habitat areas as if by a vacuum, depleting local populations.” For example, the vacuum cleaner effect is probably responsible for the fact that floodlit petrol stations built at remote locations along highways initially attract large numbers of insects, but then, after the first two years or so, the insect numbers visiting the fuel station quickly drop off. By extrapolating from the numbers of insects killed by different types of ALAN, on both moonlit and moonless nights and in different kinds of urban environment, Eisenbeis has come up with an estimate of 100 billion insects killed by ALAN each summer in the whole of Germany—a staggering number, but of the same order of magnitude as the numbers of insects thought to be squished by road traffic.
Though birds are watched much more closely than insects, even avian ALAN-related deaths are hard to gauge. One of the few sets of hard data comes from the Long Point Bird Observatory, on the shore of Lake Erie in Canada. For decades, the observatory has made daily counts of the birds found dead on the lights of the nearby lighthouse out at the very tip of the 15-mile-long Long Point Peninsula. Throughout the 1960s, 1970s, and 1980s, each year there were some 400 kills on the fall migration and about half as many on the spring migration, with occasional peaks of up to 2,000 birds on a single night. But when weaker lights with a much narrower beam were installed in 1989, deaths dropped dramatically to only a few percent of what they had been before.
Kevin Gaston, the urban ecologist whom we already came across in Chapter 5, studying the biodiversity of urban gardens, is another scientist who has taken up the gauntlet and begun series of experiments on the impact of ALAN. I watch him deliver a guest lecture at Leiden University—a friendly but imposing figure, tanned, with the solid features that seem to fit a New York fireman better than an academic ecologist. “People have introduced artificial lighting into the environment on a massive scale and into places and times and forms where it did not occur before,” he says. Moreover, he reminds us, “we are moving from narrow light spectra, such as sodium lighting, to a much wider spectrum such as LEDs. It starts to overlap with a whole range of biological sensitivities. It ramifies throughout almost everything.”
Given the massive onslaught that ALAN causes, the vacuum cleaner effect, and its rapidly increasing pervasiveness, I put it to Gaston that ALAN may be forcing organisms to evolve some sort of resistance against attraction by light. But Gaston is dubious. “This is something these organisms have never seen before, messing up with those classic daylight cycles of theirs. This has happened very quickly. I’m not convinced that adaptation is very easy; some of these light-triggered systems are evolutionarily very deep-rooted and it may not be possible to adapt to that.” But, he adds, it’s not something that has been studied much yet.
He is right there. In fact, all told there are just two articles in the whole of the accumulated urban biology literature that deal with evolution in response to ALAN. Quite amazing, really, given how easy it is to think up an experiment. All you need to do is select a species of animal which you know is attracted to light, then catch some individuals of that species in a rural, dark area and also some in a built-up area with a lot of ALAN, see if you find differences in how strongly they orient toward light, and presto! You have your evolutionary experiment.
In fact, that’s exactly how Swiss researcher Florian Altermatt from the University of Zurich approached it. Altermatt, an expert in freshwater biodiversity, is, in his spare time, also a passionate lepidopterist. “My pleasures are—as Vladimir Nabokov said once—the most intense known to man: writing and butterfly hunting (with the camera;-),” he proclaims on his website. He had been using portable mixed mercury vapor lights to attract and study moths all over Central Europe since he was a kid at high school, and had long been intrigued by the irresistible lure of the light on a moth’s brain.
So he devised a simple experiment. As his target, he chose the small ermine moth (Yponomeuta cagnagella), so named because of the regularly spaced black dots on its otherwise pure white wings—like the multitude of stoats’ tail tips on a regal ermine robe. A sensible choice, because its caterpillars make communal nests in the European spindle tree, and are very easy to find. Wherever spindle grows, you can spot the silken nests chock full of their caterpillars. So it was not hard for Altermatt to travel around the city of Basel and also across the border into France, and get fistfuls of baby caterpillars from ten different places. He took care to choose half of those places in urban areas with a lot of artificial light, and the other half in rural places where it was still properly dark at night.
He then put all the caterpillars from the same place in a plastic box with plenty of spindle leaves and left all ten boxes in his lab for the larvae to pupate and develop into moths. When the moths emerged, each received a mark to distinguish dark-sky from urban moths and then he released all of them, 320 rural moths and 728 urban moths, in one go into a dark room with a fluorescent light trap at one end. The idea, of course, was to see how many of each type would end up at the light. The results, published in 2016 in the journal Biology Letters, showed a clear signature of urban evolution: whereas 40 percent of the country moths flew straight to the light, only about a quarter of the city moths did so—the rest staying put where they had been released.
This simple experiment, on a random moth, shows exactly what would be expected if ALAN had been purging flight-to-light genes from the urban population. Is this a common situation in more insects? Are all urban insects perhaps evolving the ability to withstand the lure of the lamp? We won’t know until Altermatt’s experiments are repeated with other species and on a considerably larger scale.
Elsewhere in Central Europe, a further twist to urban evolution in response to light pollution has been revealed. In the late 1990s, Astrid Heiling, an arachnologist at the University of Vienna, studied the urban spider Larinioides sclopetarius. Commonly known as the bridge spider, you can find it building its webs over water (and, indeed, often on bridges) in both urban and rural settings around the globe on the northern hemisphere. But Heiling did not study them all over the northern hemisphere. Instead, she focused on a single 60-yard-long pedestrian bridge across the Danube Canal in the heart of Vienna.
Here, the spiders built their webs in the open spaces above the handrails. There were four handrails along the bridge, Heiling explains in the article she published in Behavioral Ecology and Sociobiology. Two were lit with fluorescent tubes, the other two were dark. For an entire summer, Heiling walked up and down this footbridge, doing daily censuses of the bridge spiders at the handrails. Ignoring the curious glances of passersby, she accumulated notes that revealed to her that the spiders predominantly built their webs near the lights: in early autumn, the illuminated handrails supported almost 1,500 fat spiders, on average four per square yard, their webs sometimes even overlapping. The dark handrails, on the other hand, housed only a few hundred. And what’s more, the spiders in the artificially lit “habitat” caught up to four times more prey in their webs than the ones that stayed in the dark—not surprising, given insects’ propensity to fly to light.
Unlike insects, spiders normally are not lured in by light at all. Quite the contrary: they tend to flee from artificial light and hide in dark corners instead. So what Heiling wanted to know was: how did the spiders manage to seek out the best visited web sites? Did they just move around until they found a place with good prey traffic, that is, near the lights? Or had they perhaps evolved an attraction to the light? To put this idea to the test, she devised an experiment. She caught spiders and, in her lab, placed them in tanks with a dark end and a lit end. To filter out any effects of the spiders learning, rather than evolving, the blessings of light, she did the
same with adult spiders that she had hand-reared in the dark in the lab. Nearly all the spiders, the ones that came straight from the lit handrails, as well as the inexperienced ones that had grown up in the lab and had never seen a light bulb, went straight for the lit end of their tanks and constructed their webs there.
Unfortunately, Heiling never carried out the same experiments with bridge spiders from a non-urban place without any light pollution. Still, while not completely water-tight, her results give the impression that the spiders have evolved their attraction to light to exploit the droves of winged insect-snacks attracted to artificial lighting.
The genetically-based ALAN attraction and repulsion, respectively, that Heiling and Altermatt have uncovered, are crying out for follow-up studies. With artificial lights making such astronomical numbers of victims among nocturnal animals, and probably more subtle effects on the daily lives of all organisms, it seems that resistance to the innate lure of the light must be evolving all over the place. And yet, until now only very few biologists seem to have been interested in revealing the workings of this kind of urban evolution. It’s time for more researchers to see the light!
13
BUT IS IT REALLY EVOLUTION?
In 2016, The New York Times asked me to write an article about urban evolution. After the piece came out, I received dozens of emails from intrigued readers, many of whom shared with me their own observations of urban wildlife. A Mr. Spanier, who lived for many years in Santiago, Chile, wrote about the stray dogs there and how an out-of-town visitor, riding as a passenger in his car, suddenly exclaimed, “That dog back there looked both ways before stepping into the road!” Could that also be down to evolution, he wondered.
Darwin Comes to Town Page 12