Urbanization and Evolution: How Cities Alter Ecosystems

By Michelle Ding

Imagine this. It’s a late afternoon on some random Tuesday. As you make your long trek back from class, something intriguing catches your eye. Against the gum-stained sidewalks, there perches something equal parts bumbling and beautiful. Hues of emerald green and violet glimmer in the sunlight. Quiet coos murmur against the cacophony of the city. Steel gray feathers ruffle gently as you pass; a pair of inquisitive eyes meet yours and for a brief moment, you wonder how you’ve never stopped to admire them before.

Humble avian. Noble messenger. Forgotten companion. Evolutionary masterpiece?

The domestic pigeon can seem dowdy and petulant to some, but in the face of modern urbanization and industrialization, the species Columba livia is just one example of how human activity and habitats have spurred adaptation and even evolution for a multitude of species. The ability to adapt and live alongside such densely concentrated human populations has cultivated a highly specialized yet often overlooked ecosystem unique to major urban centers such as New York City. Thus, it’s only fair that we do our part as good neighbors to try to understand a bit more about the organisms with whom we share our space.

Historical Context
Though our impact can be traced back much farther, the industrial revolution of the 18th century brought forth some of the most dramatic effects on and destruction of once natural habitats. As of 2025, modern cities account for roughly 70% of global greenhouse gas emissions (International Energy Agency, 2023). Widespread water pollution resulting from improper disposal and degradation of urban and industrial waste has contaminated roughly 73% of water bodies and critically depleted aquatic habitats, affecting at least 267 animal species (Bashir et al., 2020; US EPA, 2015). In a study of 30,393 terrestrial vertebrae species, urban land expansion has been a contributing driver of habitat loss for approximately one third of the species (Simkin et al., 2022).

Unsurprisingly, all such factors have resulted in dramatic biodiversity loss. Modern cities lose an average of one third of their native biodiversity (Elmqvist et al., 2016). In regions home to endemic species, urban growth in just 10 percent of all ecoregions would account for almost 80 percent of the expected loss in species (Simkin et al., 2022). Yet in spite of the expansive destruction, some organisms have persevered and even adapted to the hostile environment with remarkable speed. In fact, increasing amounts of evidence suggest that urban evolutionary change is becoming an increasingly prominent and important process in shaping contemporary evolution (Miles et al., 2020).

Cities serve as a unique anthropogenic disturbance and incubator for speciation and evolution. Many environmental factors such as elevated temperatures, pollution, and habitat fragmentation due to the presence of buildings and roads, are consistent for thousands of cities across the globe (Miles et al., 2020). As the fastest growing ecosystems on the planet, cities also create the urgent necessity for species to evolve in order to survive alongside rapid human activity. Cities are also unique in that they represent novel ecosystems with “no natural analog” (Johnson & Munshi-South, 2017). Evolution has long been thought to be too slow to study on scales relevant to urbanization, yet it is now recognized, in large part due to studies of urban evolutionary ecology, that observable evolutionary changes can occur in as little as two generations (Johnson & Munshi-South, 2017). Thus, urban environments provide valuable insight on behavioral plasticity and demonstrate how populations evolve under the unique environmental pressures presented by human civilizations.

Case Studies
Perhaps the most famous case of urban development spurring signs of evolution is that of the English peppered moth. As pollution from coal darkened the trees of England throughout the 19th century, urban peppered moth populations gradually darkened as well (Magazine, 2022). Because darker moths were better able to camouflage with trees and avoid predation, natural selection favored naturally occurring dark phenotypes over their white counterparts. The phenomenon of directional color change is an example of industrial melanism. When industrial pollution was later reduced in response to clean air initiatives and trees returned to their original coloration, lighter phenotypes once again had the advantage of camouflage and became predominant. 

Habitat fragmentation can also be a major cause of speciation. Not only do roadways physically split neighborhoods within cities, variations in neighborhood culture amongst humans can also act as a barrier. A Fordham University study analyzing the genomes of various rats in New York City found that in Manhattan alone, rat populations from different neighborhoods exhibited distinct genomic profiles. Previous studies have shown that only a very small percent of urban rats move between study sites and that Middletown Manhattan, with its relatively non-residential atmosphere and stricter pest control, acts as a barrier between rats of Upper and Lower Manhattan (Combs et al., 2017). In short, reduced gene flow around Middletown Manhattan has created a clear split between major uptown and downtown genetic clusters and thus resulted in genetic drift between neighborhoods.

Variations in cranial and mandible shapes of New York City Brown Rats across the past century also suggest that their anatomy may be evolving due to environmental demands of cities. Contemporary rats were found to have smaller hindbrain cases, shorter snouts, shorter upper molar teeth rows, and shifted ear canal positions (Puckett et al., 2020). Limited need for travel within cities may contribute to more stable living conditions for rats, thereby decreasing risk, need for problem-solving skills, and food insecurity. Longer teeth rows are usually associated with chewing low-quality and harder foods such as bark. The decrease in row length could be  attributed to the abundance of calorie dense, soft foods from waste in residential neighborhoods. 

In urban areas, many species are either generalists or able to take up highly specific, free ecological niches created by cities (Luniak, 2004). Rats and pigeons stand out as generalists who thrive in cities because their needs are non-specific, but the urban developments and correlated habitat destruction also create ecological vacuums that can attract populations previously not found in the areas. Despite having once been strictly forest species, European blackbirds, for example, first began to colonize urban parks in 19th century Germany (Luniak, 2004). The absence of competition and most woodland predators allowed for rapid population growth. Wintering birds traveling through the area tended to stay for breeding, resulting in a trend of decreased migration rates. Breeding seasons for many species, such as the Dark-eyed Junco, increased in urban populations and the increase in resource availability has also been linked to reduced competition for mates and increased parental investment (Francis & Chadwick, 2012). Some studies have even demonstrated birds changing the frequencies of their calls in response to urban noise.

Adaptations are also not limited to animals. White clover plants–Trifolium repens–are able to produce small amounts of hydrogen cyanide to deter predation from herbivores. However, hydrogen cyanide also makes the plants more susceptible to freezing in cold weather. Samples of the white clover population in Toronto show that the closer to the city center a population is, the less cyanide it produces (World Health Organization, 2022). It has been hypothesized that the adaptation comes from lessened snow coverage and protection in city centers. White clovers also have fewer natural predators in urban areas, making the decreased cyanide production a smaller trade-off.

Finally, pigeons present an especially interesting example of urban adaptation. Despite having once been domestic companions, modern pigeons have endured habitat destruction, loss of prey, predation from pets, and much more. However, unlike the genetic isolation of brown rats, urban pigeons display widespread genetic connectivity and ample gene flow (Carlen & Munshi‐South, 2020). The relative cohesiveness across cities provided a continuous habitat for pigeons, thus facilitating gene flow across a larger area, such as across the northeast and bolstering resistance to disease. Pigeons have also responded to habitat loss via habitat replacement. Females frequently build nests on urban infrastructures that mimic the height and shelter of cliffsides and other rock formations (Kai Ning Lim et al., 2023). Flight initiation distance—the distance in which an animal flees when approached by humans—has also decreased with increased human activity (Carlen, 2021). In city centers lacking natural food sources, pigeons feed on food waste. City cores with high human populations are correlated with higher pigeon populations as well, likely due to the concentration of resources (Carlen & Munshi‐South, 2020). Even against an onslaught of eradication efforts, urban pigeons seem to prefer more densely populated areas. 

Implications
Ultimately, pigeons and other adaptive organisms are just as much a part of the urban environment as skyscrapers, city streets, and human beings. Apart from just “being better neighbors,” understanding evolution and speciation in urban areas can improve human health. Though some organisms have adapted specifically for urban environments, cities generally still severely lack biodiversity. Observing responses to urban life can also aid in conservation efforts and urban planning with the goal of maintaining functioning ecosystems. Robust ecosystems are often more resilient to natural disasters and environmental changes, improve air and water quality, manage predator-prey populations, and have even been shown to substantially benefit psychological well-being (World Health Organization, 2025).

Pest-control efforts may benefit especially from insight on organism behavior in cities. Rats, cockroaches, and many other unwanted city roommates frequently adapt to pesticides and evade control efforts. Increased understanding of population dynamics, size, and movement could be key to effective pest mitigation (Johnson & Munshi-South, 2017). Implications of improved pest control include reduction in disease and improved sanitation.

Though it is a great tragedy that such means have become necessary for the survival of so many species in the presence of humans, the possibility of adaptation and evolution also serves as a fascinating reminder of resilience. Human activity has undoubtedly influenced the behavior of other organisms. In the midst of discussions about whether the next major extinction event is on the horizon, it is worth noting that adaptation, speciation, and possibly even evolution are also happening as a result of human activity. The future of how species will fare in response to human activity is still largely unknown. The persistence of a few of our urban neighbors in particular reminds us that even in the heart of human activity, we are not the only ones navigating the world around us.

References
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