Darwin Comes to Town Read online

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  for Iva

  CITY PORTAL

  It’s perfectly formed. A miracle of micro-engineering ready for its short visit to the world. Gossamer wings, still unfrayed, folded carefully over its imperceptibly breathing abdomen. Six nimble legs, delicately placed on the dusty wall, in mint condition—each with a complete set of nine segments, not yet diminished in number by sharp connections with ventilation fans or jumping spiders’ forelegs. Its golden-bristled thorax, a power nugget encasing the balled-up energy of flight muscles, so massive that it almost hides from view the serene face behind which a miniaturized brain coordinates input and output channels of antennae, palps, all-seeing eyes, and the eight interlocking sheaths of its parasitic proboscis.

  I am standing in a hot and crowded pedestrian tunnel of London Underground’s Liverpool Street station, spectacles in one hand, nose pressed against the tiled wall, admiring this fine specimen of the house mosquito, Culex molestus, freshly emerged from its pupa. But I am slowly waking up from my entomological reverie. Not only because of the rushed passersby that avoid knocking into me with a last-second swerve and a muttered “excuse me” that is more accusatory than apologetic, but also because I am becoming uncomfortably aware of the CCTV camera suspended from the ceiling and Transport for London’s repeatedly broadcast advice to its passengers to report any suspicious behavior to a member of staff.

  For a biologist, the inner city is an unlikely place for any professional activities. The unwritten rule among biologists is that, when prompted, one should answer gruffly that cities are only necessary evils where a true biologist spends as little time as possible. The real world lies outside the urban realm, in forests, dales, and fields. Where the wild things are.

  But if I am honest I must admit I secretly like cities. Not so much the organized, slick, well-oiled parts of them, but rather the grimy, organic fabric of the city, revealed in forgotten corners where the threadbare carpet of culture gives way—the city’s underbelly where the artificial and the natural meet and engage in ecological relations. To my biologist’s eye, the inner city, for all its hustle and bustle and thoroughly unnatural appearance, becomes a constellation of miniature ecosystems. Even in these seemingly sterile, thoroughly brick-and-concrete-clad streets of Bishopsgate, I spot life-forms that cling on in stubborn defiance. Here, a snapdragon growing in wild profusion from some invisible crack in the plastered wall of a flyover. There, the unspeakable chemistry between cement and leaked sewage from which glassy off-white icicles are born, which in turn serve as anchor points for the soot-dusted webs of common orb-weaver spiders. The emerald veins of moss sprouting from the slits between cracked reinforced glass and its frame, fighting for supremacy with the rust bubbles working their way through the red lead paint. Feral rock pigeons with diseased feet balance among the plastic spines on a ledge. (Somebody has put up a sticker next to it showing an enraged pigeon with balled wing-fists, saying: “Plastic needles represent a cynical, oppressive restriction of our right to free assembly. This fight is not over!”) And a mosquito on the wall of a train station underpass.

  It is not just any mosquito. Culex molestus is also known as the London Underground mosquito. It gained this name first because of the havoc it wreaked on Londoners sheltering on the platforms and tracks of the Central line at Liverpool Street station during the German bombing raids of the city in 1940. And then, in the 1990s, because of the interest taken in them by University of London geneticist Katharine Byrne. Byrne joined maintenance crews on their daily expeditions into the bowels of the city’s tube network. They went into the deepest parts of the tunnels where the brick walls supporting tangles of wrist-thick electric cables are blackened by dust from the trains’ brake shoes, and where the only place indicators are mysterious codes in chalk, spray paint, and ancient enameled plates. Here, the London Underground mosquitoes live and breed. They steal the blood of commuters, and lay their eggs in flooded sumps and shafts, which is where Byrne collected their larvae.

  She took samples of water-with-larvae from seven places on the Central, Victoria, and Bakerloo lines, brought them to her lab, waited for the larvae to develop into adult mosquitoes (like the one I saw on that tunnel wall) and then extracted their proteins for genetic analysis. Twenty years ago, I saw her present her results at a conference in Edinburgh. Even though her audience consisted of seasoned evolutionary biologists, she managed to thrill us all. First, the underground mosquitoes in the three tube lines were genetically different from one another. This was, Byrne told us, because the tube lines form nearly separate worlds, with the clouds of mosquitoes in each line stirred and mixed by the constant piston-like action of trains moving around in snugly-fitting tunnels. The only way for the mosquitoes in the Central, Bakerloo and Victoria lines to become genetically mixed, she pointed out, would be “for all of them to change trains at Oxford Circus station.” But not only were the mosquitoes in separate underground tubes different from each other. They were also different from their above-ground relatives. Not just in their proteins, but also in their way of life. Up on London’s streets, the mosquitoes feed on bird, not human, blood. They need a blood meal before they can lay their eggs, they mate in large swarms, and they hibernate. Down in the tube, the mosquitoes suck commuters’ blood and lay eggs before feeding; they don’t form mating swarms but seek their sexual pleasures in confined spaces, and are active the whole year round.

  Since Byrne’s work, it has become clear that the Underground mosquito is not unique to London. It lives in cellars, basements and subways all over the world, and it has adapted its ways to its human-sculpted environment. Thanks to mosquitoes that get trapped in cars and planes, its genes spread from city to city, but at the same time it also cross-breeds with local above-ground mosquitoes, absorbing genes from that source as well. And it has also become clear that all this has happened very, very recently—probably only since humans began constructing underground buildings, did Culex molestus evolve.

  As I take a last good look at my very own London Underground mosquito, in that crowded passageway in Liverpool Street station, I imagine the invisible modifications that evolution has accomplished inside that tiny, fragile body. Proteins in its antennae have changed shape so that our human odors, rather than bird smells, elicit a response. Genes that regulate its biological clock have been reset or turned off, to prevent it from going into hibernation, since there is always human blood underground and it never gets very cold. And think of the complex diversifications that have been needed for the change in sexual behavior! From a species where the males swarm in large clouds that females dart in and out of to seek fertilization, to one that mates by simple one-on-one pairing in the small spaces where the sparsely distributed underground mosquitoes happen to run into each other.

  The evolution of the London Underground mosquito speaks to our collective imagination. Why are we so tickled by it and why do I so vividly remember Katharine Byrne�
�s presentation from all those years ago? First, we have been taught that evolution is a slow process, imperceptibly whittling species over millions of years—not something that could take place within the short timespan of human urban history. It drives home the fact that evolution is not only the stuff of dinosaurs and geological epochs. It can actually be observed here and now! Secondly, the notion that our impact on the environment is so great that “wild” animals and plants are actually adapting to habitats that were originally created by humans for humans, makes us aware that some of the changes we are enforcing on the earth are irreversible.

  The third reason why we prick up our ears when hearing about the London Underground mosquito is because it seems such a cute addition to evolution’s standard portfolio. We all know about evolution perfecting the plumage of birds of paradise in faraway jungles or the shape of orchid flowers on lofty mountaintops. But apparently, the process is so mundane that it is not above plying its trade below our feet, among the grimy power cables of the city’s metro system. Such a nice, unique, close-to-home example! The sort of thing you’d expect to find in a biology textbook.

  But what if it is not an exception anymore? What if the Underground mosquito is representative of all flora and fauna that come into contact with humans and the human-crafted environment? What if our grip on the earth’s ecosystems has become so firm that life on earth is in the process of evolving ways to adapt to a thoroughly urban planet? These are the questions we will be tackling in this book.

  None too soon, either. In 2007, the world passed a crucial benchmark: in that year, for the first time in history, the number of people living in urban areas outnumbered those living in rural areas. Since then, that statistic has been rising rapidly. By mid-twenty-first century, two-thirds of the world’s estimated 9.3 billion will be in cities. Mind you: that’s for the entire world. In western Europe, more people have lived in cities than in the countryside since 1870, and in the US that tipping point was reached in 1915. Areas like Europe and North America have been firmly on the way to becoming urban continents for more than a century. A recent study in the US showed that each year, the average distance between a given point on the map and the nearest forest, increases by about 1.5 percent.

  Never before in the history of our planet has a single life form been so dominant. “Well, what about the dinosaurs?” you might ask. But the dinosaurs were an entire class of animals, probably thousands of species. Comparing the thousands of species of dinosaurs with the single species of Homo sapiens would be like comparing all the world’s sole-proprietor greengrocers with Tesco’s. No, in ecological terms the world has never before seen the situation that we find ourselves in today: a single large animal species completely and utterly blanketing the planet and turning it to its advantage. At the moment, our species appropriates fully one-quarter of the food that all of the world’s plants produce and half of all the world’s freshwater run-off. Again, something that has never happened before: no other species that evolution has produced has ever been able to play such a central ecological role on such a global scale.

  So, our world is becoming thoroughly human-dominated. By 2030, nearly 10 percent of the landmass of the planet will be urbanized, and much of the rest covered by human-shaped farms, pasture, and plantations. Altogether a set of entirely new habitats, the likes of which nature has not seen before. And yet, when we talk about ecology and evolution, about ecosystems and nature, we are stubbornly factoring out humans, myopically focusing our attention on that diminishing fraction of habitats where human influence is still negligible. Either that, or we are trying to quarantine nature, as much as possible, from the harmful impacts of the human, implicitly non-natural, world.

  Such an attitude can no longer be maintained. It’s time to own up to the fact that human actions are the world’s single most influential ecological force. Whether we like it or not, we have become fully integrated with everything that goes on on this planet. Only in our flights of fancy can we still keep nature divorced from the human environment. Out in the real world, our tentacles firmly entwine nature’s fabric. We build cities full of novel structures made of glass and steel. We irrigate, pollute, and dam waterways; mow, spray, and fertilize fields. We pump greenhouse gases into the air that alter the climate; we release non-native plants and animals, and harvest fish, game, and trees for our food and other needs. Every non-human life form on earth will come across humans, either directly or indirectly. And, mostly, such encounters are not inconsequential for the organism in question. They may threaten its survival and way of life. But they may also create new opportunities, new niches. Like they did for the ancestors of Culex molestus.

  So what does nature do when it meets challenges and opportunities? It evolves. If at all possible, it changes and adapts. The greater the pressure, the faster and more pervasive it does so. As the neck-tied traders who rush past me in that Liverpool Street station tunnel know all too well, in cities there is great opportunity, but also great competition. Every second counts if you want to survive. In this book, I will show that nature is doing just that. While we all have been focusing on the vanishing quantity of unspoiled nature, urban ecosystems have been evolving behind our backs, right in the cities that we have been turning up our naturalist noses at. While we have been trying to save the world’s crumbling pre-urban ecosystem, we have been ignoring the fact that nature has already been putting up the scaffolds to build novel, urban ecosystems for the future.

  I will reveal the myriad ways in which urban ecosystems are assembling themselves and how they might, one day, be the chief form of nature on our urbanized planet. But before we take off, there is something I need to get off my chest.

  The growing band of people who try to generate an appreciation for nature in the urban environment often get accused of providing excuses for developers to destroy wild nature—or even of getting into bed with the enemy, and stabbing nature conservation in the back. Several years ago, with my colleague Jef Huisman of the University of Amsterdam, I wrote an opinion article in the Dutch national newspaper De Volkskrant, in which we argued that nature is dynamic, constantly in flux, and we should not try to preserve Dutch ecosystems in exactly the same shape and composition as we see them in landscape paintings of several centuries ago. We argued for a more pragmatic approach to conservation in which there is a place for exotic species, urban nature, and more attention to the smooth running of the ecosystem, rather than to the exact species therein.

  That did not go down too well with some people. We received angry emails from colleagues who accused us of playing into the hands of right-wing politicians who would seize on the flimsiest of excuses to continue their rampage over the natural world. Other irate readers advised us to “tell that to the people of Australia and New Zealand, who see their nature overrun by cane-toads and rabbits.”

  Such attacks hurt me deeply. I grew up as a bug-collecting, bird-watching boy, spending days on end by myself in the fields surrounding my hometown, armed with a pair of binoculars, a plant guide, or a jar for collecting beetles. Today, the fields where I photographed nesting godwits, stepped through carpets of early marsh-orchids, and caught my first great silver water beetle, have been absorbed by the urban sprawl of the Rotterdam conurbation. I looked on in furious, tearful impotence, with balled fists, as the first bulldozers began leveling my playground and swore to avenge the nature that had been lost forever. Later, as a tropical ecologist living and working in Borneo, I watched powerlessly as mangroves were converted into parking lots and pristine rainforests to oil palm monocultures.

  But that same love and care for nature also gave me an understanding of the power of evolution and the relentless adaptability of the living world. The expansion of the human population is a given. Barring global disaster or dictatorial birth control, humans will be smothering the earth with their cities and urbanized environments before the century is up. For that reason, we must conserve as many unspoiled wilderness areas as possible, and this book shoul
d not be misconstrued to devalue such efforts. However, at the same time, we must realize that outside of pristine areas, traditional conservation practices (eradicating exotic species, vilifying “weeds” and “pests”) may in fact be destroying the very ecosystems that are going to sustain humankind in the future. Instead, I argue in this book that we must embrace and harness the evolutionary forces that are shaping novel ecosystems right here, right now, and work toward allowing nature to grow in the hearts of our cities.

  I.

  CITY LIFE

  Numberless crowded streets, high growths of iron, slender,

  strong, light, splendidly uprising toward clear skies

  WALT WHITMAN, “MANNAHATTA” (Leaves of Grass, 1855)

  1

  NATURE’S ULTIMATE ECOSYSTEM ENGINEER

  Some 20 miles west of the city of Rotterdam lie the coastal sand dunes of Voorne—an extensive area (at least, by Dutch diminutive standards) of rolling, vegetated dunes, though increasingly consumed from the north by Rotterdam’s expanding port. You can sit there, with your buttocks on a carpet of mosses and lichens, eating a sandwich among the rare yellow-wort and marsh helleborines, while in the distance gigantic heaps of iron ore and coal are shifted around, the cling and clang wafting in and out on the incessant wind.

  It is here that I spent almost every Saturday as a schoolboy, hunting beetles for my expanding collection. My juvenile-naturalist friends and I, sometimes accompanied by our indefatigable biology teacher, would cycle along the Meuse, take the ferry across the river, zigzag among the oil-storage tanks and daunting chemical installations of the refineries, and then spend a whole day in the dunes, botanizing and entomologizing. Sundays would then be devoted to sorting, pinning, and identifying the booty, and conscientiously penciling everything into notebooks, an oasis of bliss before the dreary school week began again on Monday morning.