Is urban nature (basically) all the same?
Biodiversity in urban areas is more complex that we give it credit for
Urbanization is widely recognized as one of the greatest threats to global biodiversity, because it replaces natural ecosystems with habitats built for people. Buildings and roads crowd out forests and grasslands. Water systems are rerouted. Natural plant communities are replaced with landscaping.
But what about the biodiversity of cities themselves? As the world urbanizes, is the living world becoming more the same?
Yes, according to a theory that emerged twenty years ago in urban ecology, called the urban biotic homogenization hypothesis. The idea, as articulated by urban ecologist Michael McKinney, is that cities “homogenize the physical environment because they are built to meet the relatively narrow needs of just one species, our own.” Urbanization replaces native biodiversity with species chosen by or adapted to people and their built environment, leading to similar communities of species, both within cities and across different cities. On a larger scale, the homogenization of suburban and urban landscapes across the US can even create areas that have similar climates and habitat characteristics.
But a recent paper argues that the theory has been oversold, and may actually be unhelpful for understanding how urban environments work.
The urban biotic homogenization hypothesis is simple and makes intuitive sense. It’s easy to see evidence that cities are similar to each other. Turfgrass—the largest irrigated crop in the U.S.—collectively blankets about 2% of its land surface with monocultures of relatively few species. The garden department at your local Home Depot does not offer the rich diversity of a natural ecosystem. Moreover, Home Depots will carry many of the same plants whether they are in Kansas City or Seattle.
I’ve written about this idea in the past, and it made sense when I was working on my book about the natural history of the street pigeon, a bird that populates urban areas across the world. Just like we see the same building types and chain stores in different cities, urbanization—like globalization—could lead to more homogeneous flora and fauna.
But a 2022 assessment of more that 200 studies testing the hypothesis in plants, birds, and other species found that little more than half of studies supported the hypothesis. One problem is that there are many ways of studying and measuring biodiversity. Different species react to urbanization differently. And it is logistically hard to prove such an overarching hypothesis given the time and scale limitations of individual studies.
But in this new paper, researchers Aaron Sexton and Monika Egerer make a more fundamental argument: Urban areas are more complex than the hypothesis gives them credit for, and cities aren’t necessarily more homogeneous than other kinds of ecosystems, like a forest or a lake. You can see a presentation on this work by Sexton, a researcher at Cornell University, here. The paper points out three aspects of urban environments that actually create a lot of diversity.
First, urbanization fragments the environment. Look at urban areas from an airplane, and what do you notice? Grayness, of course, but also structure: roads, blocks, fences, yards. Property ownership divides land into plots, while roads and other infrastructure separates plots from one another. This means that urban habitats are broken up into highly fragmented patches. Highly mobile species like birds can stitch a habitat together from several patches, while others may be confined to their own patch.
Second, urban management is diverse. In my neighborhood of Boston, there’s one house that always has newly-planted bulbs every spring neatly arranged by type and color. This yard is managed to maximize sociality; it’s the one that makes you feel safe and content, like you live in a “good” neighborhood. My neighbor across the street (hi, Betsy!) has had her expansive yard planted in a permaculture style with trees and grasses and native shrubs. It’s also well tended, but designed to maximize natural habitat (it’s also the yard that my daughter most wants to play in). Then there’s my yard, which was planted with showy landscaping by a previous owner but that I’ve gradually filled in with random bulbs and native shrubs, and allowed to be taken over by pollinator-friendly migrants. It’s a kind of palimpsest of strategic planting, natural intrusions, and neglect.
Not only are urban areas patchy, but those patches are little fiefdoms managed with very different ideas, practices, values, and resources. These different ways of managing land have consequences. Urban fiefdoms often lead to high diversity between different sample sites in the same city (called beta diversity). Every time my husband waits an extra week (or two) to mow our lawn, it creates different micro-habitat for bees and other pollinators, which looks very different from the professionally managed condos next door. Overlooked areas in cities—vacant lots, rights-of-way, large sidewalk cracks—can also harbor surprisingly diverse populations of plants and insects.
Finally, urban areas are often disturbed. They are subject to floods, fires, drought, and other kinds of natural disturbances, but also human-caused disturbances. Land is constantly changing hands and being redeveloped. Every time a building is flipped in my neighborhood, the landscaping is ripped out and replaced. Changing policies and social values can shift the way people manage landscapes over time, like bringing more pollinator gardens and xeriscaping into yards.
These three forces — fragmentation, diverse management, and disturbance — act as biodiversity generators within cities that counteract some of the other homogenizing forces. Different cities also have different social histories and land use strategies that create distinct biodiversity dynamics. A city with large areas of parkland like Central Park will generate different types of diversity than one with smaller, evenly-distributed parks.
“In sum,” the paper says, “cities are more than uniform landscapes of turf grass lawns, ornamental plantings, or invasive species.”
In fact, when you start thinking about all the ways that humans and other species interact in urban areas, urban ecology becomes mindbogglingly complex. It is all the dynamics of non-human nature unfolding in a fragmented landscape shaped by human psychology, economics, law, and social life.
This complexity and downright weirdness is missing from the typical depiction of urban nature as a sad shadow of capital-N Nature. Yes, the human footprint poses a huge and overwhelming threat to biodiversity on a global scale by encroaching on ecosystems that can’t coexist with a built environment. And I don’t discount the many studies that have indeed found evidence that urbanization produces similar communities. But the nature we make is also incredibly interesting and more dynamic than we think. Homogenization may be an aspect of cities but not a destiny. We can intentionally support unique patterns of urban biodiversity over sameness.