Why Some Animals Thrive in Cities

Why does some wildlife thrive in the city? Figuring this out is the first step to boosting urban biodiversity. And that's good for everyone.

Eat almost anything. Sleep almost anywhere. These, it seems, are the secrets to surviving in the city as a wild animal. Among the species that dominate urban spaces—pigeons, cockroaches, rats, foxes—these are the most obvious characteristics successful city dwellers have.

But they aren’t the only tactics for urban survival. A new study has uncovered four very different sets of traits that animals use to prosper in the city. “There isn’t one-size-fits-all for how different species or different taxa respond to urbanization,” says Amy Hahs of the Green Infrastructure Research Group at the University of Melbourne, who led the research. Understanding how different types of animals adapt to the city in different ways, and what drives these changes, could help us improve urban biodiversity, and with it the overall health of our urban environment.

Biodiversity studies in cities tend to focus on which species dominate, not how they manage to do so. So the study’s research team set out to change this. Specifically, their ambition was to answer two questions: Is eating anything and sleeping anywhere the only way to succeed as an animal urbanite? And how does this vary across the globe?

The researchers looked at four animal characteristics—diet, body size, mobility, and reproductive strategy—that can vary according to what a city has to offer and how flexible a species can be. By reaching out to experts who had previously published research on the traits of urban animals, and drawing together these researchers’ data sets, the team then built a bespoke mega-database to compare these four characteristics across more than 5,000 species found in nearly 400 cities around the world. The team was able to gather data for six groups of animals: amphibians, bats, bees, birds, carabid beetles, and reptiles.

Unsurprisingly, they found flexibility is useful—the ability to move throughout large areas, eating a broad diet and keeping an open mind about nesting and resting places. They labeled animals in this group “mobile generalists,” with urban bats and carabid beetles tending to profit from adopting these traits. But it wasn’t the only strategy for success they found.

In contrast, urban birds and bees often succeed by becoming “central place foragers.” These creatures have a fixed place to nest and rest, but they compensate for this site fidelity by broadening their diets. The next time you see a pigeon pecking at a scrap of food waste on a downtown street, you’ll be witnessing this in action.

Reptiles and amphibians adopt a different strategy again: Faced with scarcer food, higher vulnerability to predators, road accidents, and pollution, they respond to urbanization by specializing their diets, moving around smaller areas, and reducing the size of their clutches. It makes sense: If the shelves are stacked with fewer but constant varieties of food, eating only one of them reduces competition with other species, while having fewer offspring means enough food for them all to grow well and be fitter. Known as “site specialists,” these species run the risk of ending up trapped. Because they don’t move around, if their food or habitat disappears, so do they.

The team also hypothesized that there could be a fourth category: “mobile specialists”—animals that eat a very specific diet, and are able to easily travel to wherever they need to get it. They’d seen such animals in other locations, for instance waterbirds living on wetlands, but didn’t encounter any in their urban study.

Overall, the research looked at data from 72,086 plots in 379 cities in 48 countries, covering 5,302 species. Working at this global scale was important, for two reasons. First, studies about animals and urbanization usually only look at the evolution of one particular species, mostly plants or birds, in one specific location, and this doesn’t allow for comparisons across multiple groups of animals in multiple locations. Yet, Hahs explains, “biodiversity is diverse, and what has been observed in one context may not necessarily translate to another.” To make reliable assessments of how animals behave, the team needed to include multiple groups of animals that might adapt to metropolitan life in different ways. This required working with experts on many species.

Secondly, research on urban biodiversity has traditionally focused on cities from the global north and Australia. Yet cities in the global south are also critical biodiversity hot spots, and they are expected to expand significantly in the coming decades. As much as 90 percent of the increase in urban populations between now and 2050 will take place in Asia and Africa, amounting to billions of additional people living in urban areas in these regions. Such a large amount of urban expansion could mean key habitat and species losses; a better understanding of urban biodiversity in these places will be needed if losses like these are to be stopped. Knowledge from papers like this could help.

“Organisms live or die based on environmental conditions like habitat availability, food, lethal threats,” says Loren Byrne, a professor of biology and environmental science at Roger Williams University in Rhode Island, who was not involved in the research. “This paper provides some fascinating new perspectives about how to think about this filtering process.”

If you look at the traits animals are adopting to survive in urban environments, you can see how cities could be modified to become more habitable to a wider variety of species. For example, to encourage a wider variety of birds and bees, you could increase the number of potential nesting sites. And to help reptiles and amphibians avoid ecological traps, city planners could introduce more connections between waterways to allow them to move around wider areas. But more research is needed to see what specific changes certain species would need to thrive. “This research does not provide the specific information about species that is actually needed for implementing good conservation plans,” says Byrne. “So there’s more work to be done in that regard.”

Would this work be worth it? Does having a richer, more diverse array of wildlife in cities, as opposed to fewer, more dominant species, really make a difference? The answer, according to ecologists WIRED spoke to who weren’t involved in the research, is a resounding yes. “Wildlife can help mitigate against the impact of climate change in cities,” says Nathalie Pettorelli of the Zoological Society of London. Greater biodiversity provides knock-on benefits, what are known as “ecosystem services.”

“Macroinvertebrates that live in the soil keep the soil alive and well,” says Pedro Pinho from the Centre for Ecology, Evolution, and Environmental Changes at the University of Lisbon. And healthy soil is really important in cities, Pinho adds, because it can absorb a lot of water. This can help to avoid flash flooding during heavy rains and protect against drought. A more vibrant urban ecosystem also helps plant life thrive and suck more CO2 down from the air. “We can get more carbon stored in soils when the insects and their predators are present than when they’re absent,” says Oswald Schmitz, a professor of population and community ecology at the Yale School of the Environment.

Having more animal life in cities can also protect human health. One effect of climate change is that it can increase the spread of where disease-carrying insects, such as mosquitoes, can thrive, raising their populations in cities. A more diverse set of predators can keep these insects in check. “Those can be animal species, like birds or bats,” says Pinho.

“We can’t forget that a lot of organisms in the city are fun to watch, like birds and butterflies,” says Byrne. “People derive educational value, psychological and spiritual value from living alongside other organisms.” An important fact, given that more than two-thirds of the world’s population is projected to live in cities by 2050.

Falling biodiversity is a global problem, and cities are already responding to the UN’s call to “be part of the solution” by investing in green infrastructure—parks, green belts, urban forests. London has invested almost £30 million ($37 million) since 2016, and New York a huge $3.5 billion on its waterways since 2012. In 2021, 31 mayors from cities around the world pledged to cover up to 40 percent of their urban areas with green or blue infrastructure. Armed with knowledge from research like this, these sorts of investments can become better and better at improving urban biodiversity in the future—and make city wildlife about much more than pigeons, rats, and foxes.

“Fundamentally, biodiversity underpins our world, and the sustainability and resilience of our systems,” says Hahs. “If we want to have sustainable and resilient urban areas, we need biodiversity.”

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