Along the edge of each garden, they placed three “pitfall traps”: white plastic coffee cups dug into the ground, aimed at trapping insects and other arthropods that blunder into them and cannot get out. To make sure, the team filled the cups with alcohol. Normally, a more toxic chemical, ethylene glycol, is used for this, but, as the team wrote, ethanol was used instead because of “the risk of being found by pets or children.” They then dug up bags full of dead leaves and soil to look for other invertebrates, and, as if all that weren’t enough, they also built a Malaise trap to catch flying insects (the same kind of contraption that Denis Owen used in his Leicester garden, as we saw in the previous chapter). Despite their scorched-earth approach to urban garden ecology, the team reports that in most gardens they still were offered tea and biscuits.
In these sixty-one garden-size field sites they found 1,166 different plant species. As is to be expected from planted gardens, the majority of those species (70 percent) were exotic. But still, 344 species (that’s a quarter of the entire British flora!) were native species. The 30,000 or so invertebrates they found belonged to roughly 800 species. No numbers to be sniffed at, but also not huge, compared with what Denis Owen found in a single garden. But what is more important is not the sheer numbers of species, but rather the changes from one garden to the next. About half of all the species of insects and spiders that they found were living in just a single garden. And when the team made a so-called “accumulation curve,” which showed how the overall tally increased with every new garden added to the list, the curve showed no signs of leveling off. In other words, every garden has an almost completely different flora and fauna.
So that is just sixty-one gardens, a vanishingly small fraction of all Sheffield’s gardens, which, in turn, is just a sliver of the entire garden area in the whole of the UK. Just imagine the combined biodiversity that all those yards, gardens, and plots hold. And imagine the same for all those other forgotten bits of habitat in the city: neglected gutters, roadside verges, mossy roofs …
Of course cities are challenging in their own way, but at least for animals and plants that can disperse over long distances and are able to survive in those small, isolated pockets of habitat, the city is a surprisingly varied, mosaic landscape, offering more microhabitats to a great array of species. Add to that the other three reasons for urban biological richness (exotic species, pre-existing biodiversity hotspots, sanctuary from persecution), and we begin to understand why the species lists that the urban naturalists of the previous chapter are compiling are so lengthy.
Lengthy, perhaps, but not random. Not every kind of animal or plant can live its life in the city. Some species have properties that make them less suited for life as a city slicker, whereas other species are cut out for the job. Those properties, and how they evolve, form the core of this book. So let’s move a little bit closer to that core by looking at the fascinating phenomenon of urban “pre-adaptation.”
6
IF I CAN MAKE IT THERE
We are pacing in front of the central station in Leiden, my hometown. Like most Dutch railway stations, it overlooks an ocean of parked bicycles. A two-story open-air bicycle parking garage stretches on either side of the main entrance, and is crammed with bikes, their thousands of chrome handlebars glistening in the morning sun like ripples on a calm inland sea. While I can survey this tangle of metal spokes, springs, tubes, frames, cogwheels, and chains, my companion, famed biologist Geerat Vermeij of the University of California at Davis (and a regular visitor), cannot. Blind since the age of three, Vermeij has built a career as a palaeontologist, ecologist, evolutionary biologist, and best-selling author using his other delicate faculties of fingertips, sensitive ears, and powerful brain.
That Vermeij can still detect the mass of bicycles in front of us is thanks to its being the habitat of one of the most endearing, yet underappreciated, urban birds: the house sparrow (Passer domesticus). Hopping about on the ground among the wheels, taking dust baths in the sandy patches between tiles, perched on spokes and flitting from saddle to rear carrier, the troupes of the brownish-gray birds are creating a continuous rustle of wing flutters and chatty chirps. Vermeij smiles, and sparkles of tender creases appear in the corners of his unseeing eyes. “Yes,” he says. “You are right: they are everywhere.”
I brought Vermeij to this bicycle-rack-inhabiting population of sparrows to discuss the power of pre-adaptation.
Pre-adaptation is a bit of a mysterious and controversial term in evolutionary biology. After all, evolution is nature’s hindsight: today’s adaptations are caused by yesterday’s natural selection. So how can an animal or plant be “pre-adapted,” when evolution cannot look into the future and cannot prepare any organism for things to come?
Let’s go back to those sparrows. The habitat they are occupying in front of Leiden’s train station is not a habitat the species ever evolved to occupy. In its evolutionary past, it never encountered any bicycle racks. And yet, as Vermeij and I observe these birds with our ears and eyes, respectively, they seem perfectly suited for a life among the spokes. Their short wings are ideal for taking brief flights from pedal to saddle. They scurry about in groups and in the dense tangle of metal keep in touch with one another with continuous brief chirps. They have a habit of taking off en masse and then, at the least cause for alarm, dissolve among the parked bikes. The reason they seem so at home here probably is that the house sparrow’s natural habitat is thickets of thorny trees and bushes. To them, the vast expanse of metal rods of varying thickness, density, inclination, and curvature perfectly resembles the brushwood of their original home.
Not that we really know what that natural home is. Like the house crow, the house sparrow is one of those birds whose life has become so entwined with human habitation, that it no longer occurs in the wild. Its ancestors in pre-human times probably were specialists of half-open bush-land in dry areas, groupwise nesting in shrubs, feeding on seeds and insects, and retreating into their spiny shelter whenever a sparrowhawk would show itself on the horizon. Then humans and their agriculture appeared and the house sparrow became one of those species that deserted its natural habitat in favor of human presence, feeding on discarded grains, shacking up with them in roofs of houses and stables. And, eventually, bicycle racks.
In other words, Passer domesticus has become an urban species because it was already adapted to a lifestyle that, purely by accident, prepared it for the niches that we have created in cities. The urban environment offers conditions that happen to resemble one or more aspects of a species’ way of living in pre-urban times. And it is those species that are pre-adapted to the novel niches in the city. They are the first to move in.
Besides house sparrows, there are other pre-adapted birds that live in and around the Leiden train station. The city pigeons perched over the grand clock at the main entrance are descendants of the wild rock pigeon (Columba livia), a species native to Europe and North Africa, where it naturally occurs only in areas with rocky cliffs—for roosting and nesting. Obviously, the swampy lowlands of the Netherlands, devoid of any accidentation higher than a mole hill, let alone cliffs, were never part of its natural range. Until, that is, humans began erecting artificial cliff faces: brick and concrete buildings with ledges and windowsills ideal for these birds to perch on—even if we try to discourage that with rows of plastic needles.
Likewise, the sooty, sickle-winged swifts (Apus apus) that, screeching, are criss-crossing the sky above us, are also typical cliff-hangers. They have made Leiden their own thanks to the gaps under the zinc gutters in 1970s housing estates, the openings beneath the roof tiles of the seventeenth-century church and between the bricks of the old windmill—ideal nesting sites for these birds of rocky terrain. The black-and-white oystercatchers (Haematopus ostralegus), with their long bright red bills that are pacing across the lawns behind the station as if they were at home, are originally coastal birds, nesting on beaches and using their strong bills to draw clams from the mud. In Leid
en, they have traded mudflats for lawns, clams for earthworms, and pebbly beaches for the flat roofs of the Leiden University Medical Center. Each of these bird species—the house sparrows, rock doves, swifts, and oystercatchers—was in one way or another pre-adapted to life in the city. They were the chosen ones that the urban environment has selected from all available avifauna.
It’s fairly obvious why the birds flying around the Leiden central station may be pre-adapted (or predisposed, as Vermeij prefers to call it—to dispel the false notion of evolution thinking ahead). The link between the character of their original habitat and that of the inner city is plain to see. In a similar vein, many of the tiny arthropods that live in our houses are species that originally dwelled in caves—and some may even have been moving buddies of our ancestors as they decided to stop being cavemen and become housemen instead. The closest relatives of bed bugs (Cimex lectularius) are parasites of cave bats—which indicates that that was also bed bugs’ original niche. The cellar spider (Pholcus phalangioides), which lives in houses all over world, simply likes stony, dank, enclosed spaces. It naturally occurs in caves and caverns; the hollow spaces we occupy inside brick and concrete shells (“houses”) are, to this spider, no better or worse than its underground habitat in the wild.
But some other pre-adaptations are a bit more elusive. The relentless “predation” by traffic, for example. “We can see and hear traffic approaching,” says Vermeij, “but roadkill shows that some birds cannot. And why do some bird species fly into glass windows, but others don’t? I find that an interesting question. Also, some kinds of bird, like crows, seem to be global champions at surviving in cities and suburbs. And why is the American Robin an urban bird in North America, but none of the other members of its family?”
One way to get a handle on such less than obvious pre-adaptation is by looking for common patterns across different cities. Ecologists Carmen Paz Silva and Olga Barbosa of the Universidad Austral de Chile, for example, used such an approach in three medium-sized cities in Southern Chile: Temuco, Valdivia, and Osorno. These cities (each with 100,000 to 350,000 human inhabitants) lie in an area rich in biodiversity, the so-called Valdivian Rain Forest Ecoregion. Silva, Barbosa, and their colleagues first drew a grid of 250 by 250 yards mesh width over each city and its surroundings. Then, they randomly selected 110 of those grid cells in each city, and 50 in the rural vicinity of each city (so, 480 cells in all), and went to each to see which birds occurred there. They did this simply by picking a spot in the center of the grid cell, and standing there for six minutes in the morning time to record all the birds seen or heard during that time.
This work, which spanned the whole of the 2012 breeding season, showed that the birds inside these cities were not a random selection from the avifauna in the surrounding countryside. In each city, a similar set of urban birds dominated, composed of species like the Chilean swallow (Tachycineta meyeni) and the Chimango caracara (Milvago chimango), as well as, again, the cosmopolitan house sparrow and rock pigeon. Conversely, birds such as the robin-like chucao tapaculo (Scelorchilus rubecula), though very common in the Chilean countryside, would never venture into town, and neither would the fire-eyed diucon (Xolmis pyrope) and the stunning Patagonian sierra-finch (Phrygilus patagonicus). Still, for many birds the differences were not that stark: they would occur both in and outside the city, but at slightly different proportions.
To figure out what were the crucial urban pre-adaptations, Silva and Barbosa first classed each bird species as either carrion, fruit, seed, insect, or nectar feeding, carnivore, or omnivore. Then, they recorded what their natural habitat was: forest, open terrain, water/wetland, or ubiquitous. Finally, they performed a series of statistical tests to see what the urban birdlife should look like if a random selection had been made from the available bird types, either regarding habitat or food preferences. This analysis told them that the urban bird composition was anything but random. For urban birds in Southern Chilean cities, it definitely helps to be an omnivore or a seed-eater, and to be not too picky about the habitat you live in. This makes sense: as we saw in the previous chapter, cities are mosaics of different types of habitats, so a species that, outside of cities, is adapted to variable, unpredictable environments (for example, dynamic, unstable places like forest gaps or flood plains) is well honed for the life of the city. But why seed-eaters? This is because humans are also primarily seed-eaters. Much of our diet is based on grains, so most of our food-scraps (crusts of bread, discarded cooked rice from the bottom of the pan, but also half-eaten crackers and crumbs of biscuits) fit perfectly into the diet of birds that normally eat seeds and nuts.
So city birds include those that like rocky or tangled substratum, and ones that either share our food preferences or are not too picky about their living quarters. But there is more to being a city bird than food and flexibility. Think of communication: most birds communicate with one another via sound. How to do that against the combined din of traffic, sirens, alarms, shouting people, and power tools? Silva and Barbosa’s colleague Clinton Francis at the University of Colorado set out to see which birds are better able to deal with such human noises. Surprisingly, to study this he did not set up an experiment in a big city. Instead, he headed for the desert of northern New Mexico.
Here, in the deserted Rattlesnake Canyon, there is no urban development to speak of. But there is human-generated noise. The area, one of the nation’s most productive fossil fuel sites, is dotted with some 20,000 oil and gas wells. Some of those gas wells are equipped with noisy compressors: pumps that force gas out of the ground and into pipes day and night. Other gas wells make do without a compressor and are blissfully quiet. Francis realized that here was an ideal “natural laboratory” to study the effect of noise on birds, without any of the problems of comparing city and countryside. After all, in cities, noise is accompanied by all of the other changes in the environment that we have come across before. So if we find that, say, mockingbirds are less common in cities than outside of cities, there is no way to tell for sure that this is due to noise. It could be due to any of the other factors that characterize the urban habitat. But out in the desert, the environment was piñon-juniper woodland all around, either with or without a painfully loud compressor groaning away in the background. An experimenter’s dreamland.
What Francis and his team did was similar to Silva and Barbosa’s study in that they picked a set of noisy and a set of silent gas wells and spent seven minutes looking and listening for birds. At the noisy well pads, they persuaded the oil company’s managers to turn off the compressor for that duration, because the noise would also have hampered their attempts at detecting the birds. The results were clear-cut: birds with low-pitched calls and songs, such as the mourning dove (Zenaida macroura), were absent from sites with compressors. The sound of the machines was such that these birds simply could not make themselves heard any more and therefore had left. Birds with high-pitched voices, on the other hand, did not seem to care where they lived: the soprano calls and songs of species like the chipping sparrow (Spizella passerina) carried well above the baritone of the gas pumps. There were even some birds, for example the black-chinned hummingbird (Archilochus alexandri), that actually preferred to build their nests near the compressors—the closer the better. Francis thinks that this is because their predators, Woodhouse’s scrub jays (Aphelacoma woodhouseii), cannot tolerate the noise. So for the hummingbirds, the noise actually affords protection.
Surprise, surprise: in cities, where the noise is also mostly low-frequency, the commonest birds seem to be the ones with relatively high-pitched voices. But it took research out in the desert to prove the link between noise-pollution and pre-adaptation of the Mariah Careys among birds.
So pre-adaptation is crucial for urban ecosystems. It determines which species are filtered by the sieves of concrete and cars, garbage and grime, and get to hang their hats on the city street signs. The urban flora and fauna are largely made up of native and exotic species th
at, coincidentally, have evolved to deal with one or more challenges similar to the ones posed by cities.
Let’s go back to myrmecophiles for a minute; those animals that have evolved to live inside ant societies—we met them at the beginning of this book. They, too, are not a random selection of insects and other invertebrates. In an article in the journal Myrmecological News, Joe Parker of the California Institute of Technology claims that pre-adaptation lies at the root of the compendium of myrmecophily as well. Many of them are clown beetles, he says. These beetles have sturdy wing covers, which make them resemble armored vehicles, and this protects them against ant attack. It has made them succeed in penetrating ants’ nests where lesser insects have failed. Similarly, a lot of myrmecophiles belong to the pselaphine rove beetles, which have internal reinforcements of their body, allowing them to be bitten and squeezed by an irate ant without much ill effect. And some other myrmecophiles are aleocharine rove beetles, which all have a gland at their back-end that helps them to wage chemical warfare on ants.
So, what is happening in our cities is perhaps similar to what happened millions of years ago, when small soil animals had the audacity to penetrate the first ant colonies. Those species that were pre-adapted to the toils of life inside an ant nest were the ones that evolution then further improved and molded into expert myrmecophiles. Compared with the long evolution of myrmecophiles in ant societies, pre-adapted animals and plants have only just begun setting themselves up in human cities. But that does not mean that the initial stages of further evolutionary improvement of their city streak may not be going on.
Darwin Comes to Town Page 6