By Marisa Buxbaum
Once white and covered with lichens, the birch trees of British forests underwent a radical transformation during the Industrial Revolution. As factories burned coal and spewed pollution into the air, the lichens died, and pale bark turned black with soot. The change resulted in one of the most concrete examples of natural selection known to date. Local peppered moths – typically white with some black, birch-like patterning – no longer camouflaged with the trees on which they landed. Since the white-bodied moths were more visible to preying birds, a genetic mutation that produced dark, sooty-colored moths became widespread. As air quality has improved, the dark morph is consequently becoming less common in British moth populations.^
The story of the peppered moths tells us that urban environments can exert evolutionary pressures on species, and an increasing number of ecologists are trying to uncover which metropolitan creatures may be undergoing evolutionary change. Dr. Jason Munshi-South, in partnership with the New York City Parks and Recreation Department, is studying how city life is affecting the biology of white-footed mice.^ Since the rodents arrived in the New York area 12,000 years ago, their wooded environment has been mostly replaced by skyscrapers and concrete. Now confined to isolated patches of forest, populations of white-footed mice in each New York City park are genetically distinct from those in others, and the genetic diversity observed in a single borough rivals that across the entire Southeastern United States. In fact, the genetic differences between mice populations in parks separated by just a few kilometers trumped those between the prehistorically isolated boroughs – Bronx, Queens, and Manhattan. Munshi-South believes that this is evidence of urbanity’s power in driving rapid evolution.^
White-footed mouse. Photo by Bill Hubick www.billhubick.com
Street medians are another city locale that sports surprising diversity, although you might have to squint to see it. Ants can serve as important scavengers and cleaners of human waste in urban environments, and medians on Broadway, Park Avenue, and the West Side Highway in New York boast quite the variety of these insects.^ Thieteen species, ten native and three introduced, were identified as coexisting within these small, isolated ecosystems – a high species richness even when compared with natural habitats. The reason for this diversity remains undetermined, but the researchers note that previously studied urban stink ants have different life history traits than their rural counterparts. Perhaps New York City ant species, like stink ants, are genetically adapting to city living.
A New York City median on Broadway, home to 13 different ant species. biking-in-manhattan.com
For a fish species exposed to PCBs in the Hudson River, it took only decades to develop a resistance to city waste.^ General Electric dumped 1.3 million pounds of PCBs into the Hudson from 1947 to 1976, a concentration that killed most fish – except the Atlantic tomcod. These bottom-feeders were saved by a genetic mutation, usually uncommon in tomcod populations, that prevents PCBs from binding to a certain protein and causing heart defects. Those fish with the mutation fared better, and so the genetic resistance spread quickly. We usually think of evolution as occurring over long time scales, but human industrialization is showing itself to be a strong selective force.
More thoroughly studied than mice, ants, and fish are the behavioral changes of songbirds in response to urban noise pollution, a phenomenon that has been documented not just in New York, but in cities across the globe.^ In order to communicate above the ruckus, some city-dwelling species have altered their vocalization patterns – either singing during quieter times of day, singing loudly and more slowly, or at frequencies that don’t compete with the growling of vehicles. In Australia, Melbourne University scientists have found that urban silvereye birds are not just modifying their enunciation and pitch, but the content of the songs themselves.^ Since songbirds have species-specific calls, ecologists are wondering whether noise pollution can also elicit change at the genetic level.
Evidence of urban adaptation may seem encouraging, but densely populated cities are still inhospitable to most native species. Flora and fauna that manage to persist are exceptions, and many that flourish are invasive non-natives – introduced species that out-compete and displace the locals.^ The value of public parks and green spaces in maintaining urban biodiversity cannot be understated: besides providing habitat and ecosystem services (air and water purification, wind and noise buffering, and local climate stabilization), there is evidence that access to these spaces has a positive impact on human health and perceptions of nature.^
Despite the global expansion of cities and the need to understand their environmental and evolutionary repercussions, urban ecology is an uncommon pursuit for scientists. In the past five years, only .4% of papers published in nine major ecological journals examined urban ecosystems – that’s 25 out of 6157.^ Since traditional ecological models do not incorporate human activity as an integral factor, investigating urban influence on species is a difficult, complex process. Yet as urbanity spreads, the importance of studying affected species becomes increasingly evident. Understanding what genetic traits enable urban survival is vital to conservation efforts and the development of more sustainable cities.
2007 Independent article, “Moth study backs classic ‘test case’ for evolution.” www.independent.co.uk
2007 report from the European Molecular Biology Association, “The human impact on biological diversity. How species adapt to urban challenges sheds light on evolution and provides clues about conservation.” www.ncbi.nlm.nih.gov