Darwin Comes to Town: How the Urban Jungle Drives Evolution by Menno Schilthuizen 2018. Quercus Editions Ltd., London. 352pp.
The book deals with urban ecology, a subject that interest me. After all, Singapore’s harsh concrete jungle is pleasantly toned down by lush greenery – wayside trees lining roads and pockets of gardens and parks everywhere. Plants are even grown on roof tops and the façade of high-rise buildings. Yet we still have patches of mangroves and rainforests set aside as Nature Reserves. And yes, Singapore is a perfect example of the urban jungle driving evolution, except we have yet to see any urban ecologists studying the actual impact of evolution – see Chapter 3: Downtown Ecology.
Darwin believed that evolution is a slow process that cannot be observed in one’s lifetime. This may be so during Darwin’s time but Memo Schilthuizen’s book has proved Darwin wrong. Rapid urbanisation and the emergence of mega cities the world over had all along been exerting a strong selection pressure on urban plants and animals that struggled to survive in this harsh envirionment. As large swaths of natural areas gave way to the development of urban cities as well as to the planting of crops to feed the urban population, many of the displaced plant and animal species moved into urban areas. Those that could not adapt faded away and only those that could, survived. These, plus the many exotic species that had always found refuge in urban areas form the new biodiversity. And with time – decades, centuries… we would see more and more urban flora and fauna evolve. Yes, we are beginning to be able to witness evolution within the short span of a human’s life.
A classic example of urban evolution is the peppered moth (Bistom betularia) of 1770-1850 Manchester (above). That was a period when the coal-powered textile industry polluted the air with black soot and turned tree trunks black. Suddenly these pale-winged moths became conspicuous targets to birds when they rested on tree trunks. Soon the population of these moths declined. But then a black-winged mutant appeared and proliferated. Their darker wings provided better camouflage against the soot covered trunks. This is an example of “human-induced, rapid evolution change” with birds as the selecting agent. With effective pollution control in subsequent decades, the soot-covered tree trunks got cleaned up and the pale-winged moth population increased as the black-winged ones became conspicuous and their numbers declined.
Another example but less dramatic is the urban blackbirds (Turdus merula) of twentieth century Europe. Because of abundant food, safety from hunters, predators and parasites, they proliferated in the cities. These blackbirds probably arrived to winter in the cities. Then a few remained until spring to eventually breed with one another. They gave up migrating, to turn into resident city-birds. Researchers were convinced that these urban blackbirds were evolving into separate species from their forest counterparts. The consistent different morphological features like bill shape, the former with shorter and stubbier bills than forest blackbirds, provided the necessary proof. Their songs were different. Urban songs were sung in a higher pitch a few hours before dawn, before the urban noise drowned their communication. Their forest cousins on the other hand sang only at dawn. Urban blackbirds also started breeding earlier in the year as their biological clock advanced by more than a month. The forest blackbirds being migratory, needed to fly south to avoid the cold of winter and the scarcity of food. They only started breeding when they returned to their home ground. All these differences were found to be genetically controlled. The author concluded that city blackbirds were on their way to becoming an entirely new species, that could be named Turdus urbanicus.
In the French city of Albi, flocks of feral pigeons gather daily to bathe along the River Tarn. Lurking in the water are European catfish (Silurus glanis), a large freshwater fish that can grow up to 1.5-2 metres long (above). As the pigeons frolic on the gravel beach, a catfish would launch at a pigeon, grabbing it by a foot and retreating back into the water, swallowing the prey whole. The catfish’s normal diet is fish and aquatic invertebrates, not birds. Yet it “evolved” to feed on birds.
The Japanese carrion crows (Corvus corone) wait near a traffic light until it turns red. It then places the nuts of the Japanese walnut in front of wheels of oncoming cars to crack them (above). Mexican birds use cigarette filter ends to line their nests. The tobacco in the filter ends no doubt reduce mites in their nests. After all, birds are known to incorporate leaves of medicinal plants in their nests for the same purpose. In many urban cities, birds are forced to raise the pitch of their songs, frogs to croak louder and crickets to similarly chirp louder to in order to communicate. And there are many more examples of urban adaptations.
Urban evolutionary biology is becoming an exciting field of study. With animals and plants literally in our backyards, there are plenty of exiting examples that can be unearthed right here in Singapore. Most days, many local birdwatchers and photographers are out in the field – birdwatchers looking for new species to fill their checklists and photographers to get their perfect images of the birds. The Bird Ecology Study Group (BESG) has been persuading these nature enthusiasts since 2005 to see “beyond the feathers” with some success. Many have documented birds incorporating wires, styrofoam, tissue papers and plastic bags in their nests. Others have written about the many birds that crashed onto glass panels and migratory birds roosting in trees planted along roads fronting high-rise homes and causing noise pollution. These citizen scientists should now include in their focus, urban adaptations and their evolutionary potentials… like changing behaviour in feeding, nesting, etc.
A note about the author: Menno Schilthuizen is a Dutch evolutionary ecologist attached to the Naturalis Biodiveristy Centre in Leiden and the Leiden University, the Netherlands. He is also the co-founder of Taxon Expeditions that organises scientific expeditions for laymen.
27th February 2018