Darwin Comes to Town: How the Urban Jungle Drives Evolution. Menno Schilthuizen. 2018. Picador: New York, New York.
What strange phenomena we find in a great city, all we need do is stroll about with eyes open. Life swarms with innocent monsters.—Charles Baudelaire, "Miss Scalpel"
Observing wildlife in an urban setting often grabs our attention. It is thought to be something odd or unique. Seeing a swallowtail fluttering down a busy city street or watching Red-tailed Hawks nesting on the ledge of a multi-story concrete and steel structure can bring a smile to the observer. We are looking at something that we believe belongs outside the crowded din of a busy city. Yet here is a little bit of the natural world making a "go" of it in this human, and therefore "unnatural" place. On the other hand, sightings of urban coyotes, bears, or other carnivores can cause a panic, a feeling that nature is creeping in where it doesn't belong, invading "our" space. Even though intellectually we may know better, unconsciously we tend to draw an imagined border between the realm of wild, chaotic nature and the ordered world of humanity where we live.
But cities can be great places for certain plants and animals. Many species of invasive plants, and a few native ones, can find places to grow and propagate in parks and even smaller niche habitats in cities. City parks and graveyards attract numbers of migrant birds, and a smaller number find space to nest there. Some birds like Rock Pigeons and Peregrine Falcons thrive by nesting on city skyscrapers, bridges, and underpasses. Species formerly considered wilderness species have learned to adapt and have become diehard city slickers. In the last few years, Common Ravens have started to nest under railroad overpasses in the city of Worcester.
I live in Worcester and have a small backyard. Yet I have recorded 161 species of birds there in the last decade. That list includes such forest-loving species as Barred Owl, Pileated Woodpecker, and several Wild Turkeys. Red fox, skunks, raccoons, and possums are regular visitors. Black bears have appeared at a friend's house a short distance away, and last year a moose walked down the street a short block away to end up at Salisbury Pond across from Worcester Polytechnic Institute. Whether we realize it or not, that imagined barrier between the natural world and the world of human activity is porous at best.
So what are these animals and plants doing in our cities? Are they just outliers, struggling to survive, barely holding on? Are they just passing through? Accidental trespassers? Or are some species actually thriving in cities? Not long ago, if you were a natural historian, a biologist, or botanist, you typically chose a place to study far from throngs of people: a desert, forest, jungle, or deep under the sea. In recent decades though, a growing number of researchers are choosing urban habitats as their study areas. There are many reasons for this, among them that access is much easier. You can live and work in the same area. But a lot is happening in cities too that may offer clues to how plants and animals are adapting to a future on an overcrowded, climate-changed planet. "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." (p. 5, Darwin Comes to Town)
It's a clear sign that something has changed over recent decades. Rather than using the city as a comfortable base camp from which to explore the wild hinterland beyond the city limits, the city itself has become urbanite naturalists' chief interest. (p. 48, Darwin Comes to Town)
Darwin Comes to Town by Menno Schilthuizen reviews some of the new findings by these urban naturalists and details the authors' belief that cities can actually promote relatively rapid evolutionary changes. It is a book that is both fascinating and controversial. Menno Schilthuizen is a senior research scientist at Naturalis Biodiversity Center in the Netherlands and professor of evolutionary biology at Leiden University. Some of his ideas have garnered angry criticisms because his writings about some exotics fly in the face of what most of us have been taught. Not all invasives are bad and should be expunged on sight.
Darwin Comes to Town begins with the author's observations of "London Underground mosquito," Culex pipiens molestus. Certain populations of this insect now breed only in the vast caverns that are the London Underground, feeding on the commuting passengers. These mosquitoes may be beginning to separate from the wild-breeding Culex pipiens pipiens to become a new species. This begins Schilthuizen's roll call of species that are now calling dense cities home. These include some surprising species like the Australian Brush Turkey, which can be found nesting in small parks and gardens in many Australian east coast cities.
Why are some animals and plants attracted to cities? Schilthuizen offers a few ideas. Many of the species we find in cities are invasive already, and the city is just another place to colonize. But Schilthuizen also points out that many modern cities have a real diversity of habitats. These include obvious green islands, but also smaller personal gardens, playgrounds, and human-made structures that may approximate a species' preferred habitat like that subway nesting mosquito. Schilthuizen sees cities as new vacant niches, waiting to be exploited by the natural world outside the city.
By calling Homo sapiens nature's ultimate ecosystem engineer, I used the word ‘nature' deliberately, because a crowded, noisy, polluted, concrete metropolis is not what we normally think of when we hear the term ‘nature.' (p. 24)
In order to thrive, urban wildlife can develop unique behaviors. Schilthuizen cites many examples, but my favorites are the massive catfish (Silurus glanis) that live in the French city of Albi. They have learned to throw themselves out of the water to grab the feet of city pigeons that have come to drink from the river. Schilthuizen describes this behavior in detail, and it reminds the reader of the films that have been taken of killer whales in southern Argentina throwing themselves on the beaches in order to grab basking seals. Cities are heat islands, offering wildlife a less hostile environment in winter. Schilthuizen cites recent studies of urban populations of the European Blackbird (Turdus merula), a common thrush. Typically, this species is migratory, but in recent decades certain urban populations have become sedentary, taking advantage of the warmth and the feeders found in cities.
Urban gardens have typically been uncounted, but Schilthuizen describes several studies that show that these pocket green areas can be chock full of biodiversity. One such project is called BUGS, Biodiversity in Urban Gardens in Sheffield. Several biodiversity studies of typical small gardens were done in an urban environment. Sixty-one gardens were included with fascinating results. "In these sixty-one garden-size field sites 1,166 different plant species were found. As is to be expected from planted gardens, the majority of those species (70 percent) were exotic. But still, 344 species (a quarter of the entire British flora!) were native species. The 30,000 or so invertebrates they found belonged to roughly 800 species." (p. 60) "And when the team made an ‘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." (p. 60)
But what about vertebrate species? Which animals or birds are most likely to succeed in a city? After citing a number of studies, Schilthuizen concludes that the animals that thrive in cities are creatures with high problem-solving intelligence. This is not surprising when you think about the challenges of finding food and shelter in a human-made environment. These urban species also need to be neophilic, attracted to unknown, unfamiliar objects and situations and willing to explore new spaces. Finally, and perhaps most obviously, urban wildlife need to be fairly tolerant of people.
Typical urban breeding animals or birds should be fearless problem-solvers.
Where Darwin Comes to Town becomes controversial is when Schilthuizen pushes these ideas to posit that cities are becoming engines of evolution. After a thorough crash course in evolutionary genetics in Darwin Comes to Town, Schilthuizen proposes what he ultimately labels HIREC: "Human-induced Rapid Evolutionary Change." He calls humans a "hyperkeystone, eco-system-engineering supertramp species" (p. 246). He believes that that the natural chaos of life in urban habitat will actually accelerate small evolutionary changes in certain species. "As we have seen before, cities are like mad scientists, creating their own crazy ecological concoctions by throwing all kinds of native and foreign elements into the urban melting pot." (p. 158)
The urban loom weaves food webs from weft and warp that are thrown together by chance, linking species in new and exciting patterns. Since such ecological interactions are marriages of convenience, rather than matches made in heaven, the species thus linked may evolve adaptations to deal with their new ecological counterparts. (p. 158)
The reader is left to ponder if what we are witnessing is actual long-term evolution or momentary adaptations by flexible species.
Schilthuizen is fond of invasive species, to a limited extent. He is arguing 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." (p. 8) He believes that invasive species are some of the most successful urban species and are key players in urban ecosystems. He favors building "green buildings," for instance, but he insists that we have to let invasives have their place in those spaces. He is adamant that we should do all we can to preserve the few remaining pristine pockets of wild nature that exist outside our cities. But Schilthuizen believes that in urban habitats, so-called invasives are critical to maintaining a rich urban biodiversity. He states, rightly, that the vast majority of people around the world will never get to visit those saved pockets of truly wild nature. What they will know of nature will be what breeds and grows in cities, and that biodiversity will include invasives. He even ponders whether so-called "wildlife corridors," green areas that connect urban green areas with larger more natural areas, are always a good thing (p. 237). Schilthuizen believes that the "splendid isolation" of urban areas is what fuels the evolution within cities.
Later, he admits that certain invasive species, like the House Crow, are showing up in a growing number of cities around the world. The implications for the future seem to indicate that many cities will have the same roster of urban breeders after some decades. "What all this means is that ecosystems of cities around the world are growing more alike; their communities of plants and animals, fungi, single-celled organisms, and viruses are slowly inching toward a single globalized, multi-purpose urban biodiversity." (p. 224) This will be "a globally homogenized, dispersed ecosystem, inhabited by a dynamic but shared set of organisms that is constantly evolving, exchanging species and genes and innovations to deal with the new technologies with which humans equip their cities." (p. 226)
Because of these controversial sentiments, Darwin Comes to Town is a thought-provoking book. Most of it is an interesting and entertaining survey of what researchers have discovered about species that dwell in urban environments and how that environment can change them. But there are other points in the book in which Schilthuizen seems to relish violating core concepts many of us hold dear about preserving natural spaces. Because of this love of urban ecosystems, he has garnered lots of criticism, but most of it is misplaced.
"The growing band of people who try to generate an appreciation for nature in the urban environment are often 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." (p. 8) If you carefully read this book, you will discover that this just isn't so. Still, there is a lot to think about and argue about in Darwin Comes to Town. We don't often come across books that may change the way we think about nature. This is one of them. It is well worth your time.
Hear the podcast of Mark Lynch's interview with Menno Schilthuizen about Darwin Comes to Town.