Abstract
Urban ecosystems and their nonhuman constituents remain a frontier for ecological, biological and biogeographical research and teaching. As spatio-temporally complex systems, they are a challenge to investigate and analyse, although the field of urban ecology is developing fast (Wu, 2014). Interestingly, much of the teaching on urban ecology (in the UK at least) seems to be housed within the environmental sciences and geography rather than biology (e.g. Francis, 2018), although understanding of urban systems necessitates pan-disciplinary learning. This is why books such as Menno Schilthuizen’s Darwin Comes to Town: How the Urban Jungle Drives Evolution are so important. In an engaging, accessible text, Schilthuizen presents the latest knowledge on how species adapt to the urban environment, and how selection processes can drive evolution within cities. It is a fascinating topic, deftly handled.
The book starts on the London Underground, with the story of the emergence of the London Underground mosquito (Culex molestus), which I have always been intrigued by, using this as an exemplar of the novel nature of the urban environment and its evolutionary forces. This is a technique utilised throughout the book, with the author presenting an interesting species or observation, often with a personal anecdote, before going into more detail of the fundamental science or evidence behind it. It works well. After establishing the author’s intentions that we should ‘embrace and harness the evolutionary forces that are shaping novel ecosystems…and work towards allowing nature to grow in the hearts of our cities’ (p.9) – something which may still be considered controversial in some areas of biological conservation – the book launches into Section I: City Life.
This section consists of six short chapters that first of all outline the principle of ecosystem engineering, drawing on ants and the somewhat amazing myrmecophiles that have adapted to live alongside them, and using this to draw an analogy between the anthill and the city: the environment we have engineered for ourselves, and the evolutionary opportunities we have created. This is followed by a short history of urbanisation (including some discussion of my all-time-favourite lecture break-out activity ‘what is a city?’), before examining some central components of urban ecology, including how species disperse and establish, a bit of island biogeography/landscape ecology, habitat fragmentation and, of course, urban wildlife. Some of the key urban wildlife examples are introduced here, such as non-native house crows in Singapore (and in Rotterdam!) and how people respond to them in different ways: ‘Save the House Crow’ indeed. This is partly due to the proliferation of natural historians and natural history societies in cities, which are given an overview (especially with a nod to Sukopp’s seminal Berlin work and Denis Owen’s Leicester malaise trapping efforts).
Schilthuizen notes the importance of non-natives for urban communities and biodiversity, and how cities may in many cases be more biodiverse than the surrounding agricultural countryside, even for some of the larger mammals. This is due to the spatial heterogeneity of habitat found in the city, which can support a surprising array of species (as Schilthuizen demonstrates, drawing on Kevin Gaston’s ‘Biodiversity of Urban Gardens in Sheffield’ work). Of course, some species are naturally pre-adapted to the urban environment, and Schilthuizen uses the examples of house sparrows (Passer domesticus), pigeons (Columba livia) and swifts (Apus apus) to explore this, as well as some of the behavioural adaptations that some birds have exhibited in cities – those that can, at least.
Overall then, this section highlights what is unique and interesting about cities and their biodiversity and begins to explore why some species tend to the urban, as a preface for the rest of the book. It has also made me determined to visit the Rotterdam Natural History Museum and its underwhelmingly titled Dead Animal Tales exhibit.
Section II (Cityscapes) is where Schilthuizen, having set the stage, gets into evolution in more depth. The section starts by noting that, contrary to Darwin’s original supposition that it may only occur very slowly, evolution can take place rapidly, in only a few hundred generations. This is introduced through the 19th century example of the Annulet moth (Charissa obscurata) in the UK, and then further explored in a re-telling of the peppered moth (Biston betularia) tale – the classic example of industrial melanism – including its (unfounded) controversy. These establish the principle of Human-induced Rapid Evolutionary Change (HIREC), which is then examined in greater depth drawing on urban research on bird wing evolution, plant seed morphology and lizard limbs. There is then a fascinating exploration of the phylogeography of some urban species (ring-necked parakeets, bobcats and white footed mice) and how spatial metapopulation dynamics are influenced by urban geography, such as the availability of green space and street trees.
The book subsequently moves to consider evolutionary adaptations to urban pollution, including mummichogs swimming in polychlorinated biphenyls (PCBs) and polycyclic aromatic hydrocarbons (PAHs), water fleas exposed to higher salinity from road salting and how both plants and pigeons cope with higher heavy metal loadings. The chapter on light pollution in this section was a highlight of the book for me, including the suggestion that urban populations of moths can lose their flight-to-light genes in recognition of the harm that urban lighting can do. (And, not being much of a football fan, I was unaware of the light-induced mothscapade at the Euro 2016 final, which I then had a marvellous time looking up.) The section ends with some clarification as to the nature of evolution, and whether these examples count as ‘soft’ or ‘hard’ selection.
Section III (Close urban encounters) mines interactions in more depth, looking at how urban species may adapt in tandem with a changing environment, and with other species (e.g. predator–prey relationships). The chapters here examine some of these fascinating and complex relationships and interactions (I was particularly taken with house sparrows and house finches using cigarette butts in their nests to get rid of mites!), and are very thought-provoking. Birds and their formidable capacity for learning form an important part of this section, and one has to be impressed at their ingenuity. The old case of tits snatching cream from the tops of milk bottles in England was a welcome example here, illuminating as it does the swiftly moving battle between humans and tits for their milk, including milk-mad tits raiding the delivery carts. Those were the days.
These chapters highlight some of the key traits of successful urban species – the ability to learn, neophilia (attraction to new objects, such as milk bottles) and a tolerance of humans; attributes that can be selected for. There then follows a discussion of sexual selection in urban species, couched within an exploration of the urban soundscape and how this can influence the amorous intentions of city birds (who have to sing at a higher pitch, when they can, to be heard over the background urban drone). In some cases this is a behavioural response as the birds in question can simply adjust their pitch in response to background noise, but other, less plastic, species have displayed similar changes, so that their response is probably part evolution and part plasticity. It is a wonderful example of the complexity that novel urban ecosystems present. This then feeds into sexual selection – with a salient lesson being that what makes an attractive mate in the countryside may well differ in the city. The thought of the poor jewel beetles (Julodimorpha bakewelli) endlessly trying to mate with confusingly attractive ‘stubby’ beer bottles in Australia is somehow poignantly farcical.
Section IV (Darwin city) consists of two short chapters that highlight the ways in which cities, and their communities, are becoming more similar: similar technologies (construction materials, lighting, etc.) that create broadly comparable conditions; and homogenisation of ecological communities facilitated by the spread of non-native species. However, this is interesting, as it creates locations of intense human–nonhuman interactions, as well as rapid environmental change, which will ultimately drive selection. The possibilities are endless. In addition, these are further developed by efforts in ecological engineering and urban greening that are creating biophilic architecture and facilitating more effective planning for urban nature, alongside the conservation of remnant green spaces. Schilthuizen’s final call for an Urban EvoScope, an urban citizen science programme that may help to chart urban evolution in action, is something I could definitely get behind.
The book is at heart popular science, and an excellent example of such, delivered with scholarly authority and wit and meticulously researched with notes for each chapter and a comprehensive bibliography. There are lots of amusing anecdotes (I love the idea of the Rotterdam house crow hunter searching for the crows in disguise and changing his car so that the crows do not recognise him!) that really bring the text to life. The text is not aimed at biologists, but anyone – and anyone with an interest in urban ecosystems, regardless of background, would do well to read it. The book touches on many of the elements of urban ecology that I teach to undergraduates and postgraduates. If I had specifically commissioned an entertaining, informed and readable book on urban evolution for my urban ecology classes, it could not have been any better than this.