From rats to foxes: How gentrification transforms cities’ wildlife populations

Low-income urban residents get rats and pigeons. But when the wealthy move in, they get species like rabbits and flying squirrels.

As gentrification displaces lower-income people from American neighborhoods, the animal populations in the areas they’re leaving behind are shifting toward local species less typically associated with city environments, a new study has found. 

The findings, published Monday in the Proceedings of the National Academy of Sciences (PNAS), shed new light on the complex impacts that human demographics exert on the wildlife of American cities.

In particular, the researchers focused on what happens as neighborhoods gentrify — or experience an influx of whiter, wealthier and more educated inhabitants.

The study draws from the rich tradition of urban ecology, which seeks to understand how the structure of cities — which are home to 56 percent of the world’s people — influences the growing populations of wild plants and animals that live within them.

One focus of that research is the linked effects that human development policies can have on people and animals alike, lead author Mason Fidino, an ecologist at the Urban Wildlife Institute of Chicago’s Lincoln Park Zoo, told The Hill.

The extent to which those policies can push minority communities “out of places where they’ve been living for decades and also shift mammal communities is absolutely astounding,” Fidino said. 

“At the end of the day, there are some aspects of nature that are chronically inaccessible” to millions of Americans, he added. 

The implementation of development policies leads to rising property values and more expensive rents, often pricing out neighborhoods’ original residents in what can become a cascade of complete demographic transition — as has occurred in many now-trendy zones of cities like Los Angeles, New York or Chicago that were once bastions of middle- and working-class minority communities. 

At the same time, gentrification has led to more investment in parks and greenspaces, in many cities giving rise to dramatic shifts in mammal populations that in some cases have driven the creation of new and distinctive habitats in the heart of those urban landscapes. 

That change comes with a bitter irony for those priced out, Fidino noted. “The people that have been moved out of those areas don’t get those environmental benefits that may come along with them.”

In many cities, this transition has led to an environmental inequality that parallels the social kind: Americans moving into once low-income neighborhoods are far more likely than the areas’ original inhabitants to catch a sudden glimpse of an interesting mammal.

By contrast, lower-income residents would have been more likely to spot a species that is highly adapted to life in the urban jungle, like a rat or pigeon.

Fidino emphasized that the problem wasn’t just that lower-income people were seeing less interesting animals. 

“I don’t necessarily want to say that if you live next to a raccoon, you’re benefiting from that,” he said. 

“But if you live in a species-rich area, you’re having the opportunity to have positive interactions — and the potential for those interactions is something that people are being excluded from.”

Even for those who don’t love coyotes, armadillos or possums — or who just don’t want to live next to them — the change in the community of wild mammals in a neighborhood points to larger transitions and more troubling inequities when it comes to access to nature.

A robust line of study over the past decade has established that access to nature makes people healthier both mentally and physically. For example, a 2017 study in the British Journal of Psychiatry found that access to green spaces significantly improved the mental health — and reduced the stress — of those living nearby. 

The clinical benefits are notable. A February study in PNAS found that Texas communities with more access to green space had lower rates of people seeking treatment for mental health concerns — even once researchers controlled for socioeconomic status.

But these benefits are not shared equally. In what is sometimes called “the luxury effect,” urban animal biodiversity across the developed world tends to correspond to wealth — an idea that holds even in the middle of relatively dense cities. (For example, a 2011 study found that in Phoenix wild birds were more common in richer areas of the city.)

That inequality isn’t an accident. It in large part stems from racist land-use and zoning practices, according to a 2020 paper in the journal Science by Christopher Schell, one of the coauthors on Fidino’s PNAS paper.

Schell and his fellow researchers found that neighborhoods with histories of segregation or redlining — a discriminatory housing practice that included a lack of loans or municipal spending on green space in certain areas — still have significantly less biodiversity than wealthier, whiter neighborhoods. 

In a striking metric of the long shadow of American discrimination, Schell’s team found that the evolution of urban animals was at the time of their research largely driven by factors rooted in racism. Animals in lower-income areas tended to have greater levels of inbreeding — a result of more cramped habitats with less tree cover and connections to other population groups — and more exposure to heat and pollution.

“Because structural inequalities form the foundation of city infrastructure,” the paper found, “inequality among humans defines the ecological setting and evolutionary trajectories for all urban organisms.”

To Fidino, Schell’s research suggested a new line of investigation. Over the past two decades, a return of investment and development to once-neglected neighborhoods has meant a significant increase in spending on restoring parks, planting trees and converting power and sewer easements into publicly accessible greenspaces. 

That trend — sometimes called “green gentrification” — tended to raise property values, helping to price out many neighborhoods’ original inhabitants. That led to an obvious question: What had those changes done to local animal populations, and what might that say about the changing dynamics of how nature functions in American cities?

This required a staggeringly complicated analysis. The PNAS study is based on a vast and diverse array of data: nearly 200,000 days of camera trap surveillance, taken over three years across almost 1,000 sites in 23 U.S. cities each with a unique mammal population, pattern of urban development and interaction between the two.

Putting the study together required many different steps. The dozens of researchers involved in the project had to come up with a scientifically rigorous definition of gentrification that could apply across nearly two-dozen cities; build out and parse the data from a national network of camera traps; and create a mathematical framework that could sift the broader signal out of all that noise.

The team looked at terrestrial mammals for two reasons. First because unlike birds, they have to move on foot directly through the city, which makes them particularly responsive to changes in ground cover.

Second, the Lincoln Park Zoo’s Urban Wildlife Information Network — a national network of camera traps, which triggered when animals walked in front of them — had begun to yield significant data on the mammals found in parks and green spaces across the country. 

Finally, the presence of mammals was indicative of many of the same features that make landscapes congenial to people. Since mammals must get all their needs met by the natural features in a neighborhood, their prevalence in an area can demonstrate how good those features are at providing shade, shelter, clean water and food — which also appeal to people, but can be hard to study directly.

Across the U.S., mid-sized predators like coyotes and racoons, for example, tend to be more likely to colonize and stick around wealthy white neighborhoods, according to a 2016 study in Animal Conservation — the kind of trend that Fidino suggested indicates those areas also have food for such animals. 

“If you have a predator living in the area, you already know that there’s prey, right?” he said.

“We couldn’t go out and survey all the different yards across 25 different cities — but we can use these patterns that we’re observing in mammal communities and relate them back to changes in the landscape.”

One finding came as little surprise. The biggest element linked to mammal prevalence in American neighborhoods was not whether the area was gentrifying, but the extent to which it was covered in impervious surfaces like asphalt and concrete. 

A heavily gentrified neighborhood with a lot of impervious cover still had far lower mammal biodiversity than a nongentrified neighborhood with a lot of green space — although gentrification did moderate the still-negative impacts of a lot of asphalt and concrete.

Other findings were more striking. Across most of the U.S., gentrification correlated to what biologists call “alpha diversity,” or healthier populations of the same animals found elsewhere in the region.

But in a few cities — largely on the West Coast — gentrification instead correlated to what biologists call “beta diversity,” a different species mix being found in green spaces than in the surrounding areas.

Why? “Everyone has their pet theory,” Fidino said. Maybe East Coast cities are centuries older, so their wildlife has been made more homogenous by centuries of pressure from European settlement. Maybe they are denser, so mammal species have a harder time finding their way across the concrete from the isolated park ecosystems in which they’re trapped, even if those parks are more welcoming than they used to be.

But Fidino cautioned against leaning too hard on those explanations.

In experimental science, the practice of coming up with stories to explain patterns in the data after the fact is called HARKing, for “hypothesizing after the results are known” — which leaves researchers at risk of being seduced by an intuitive-seeming explanation that they haven’t actually tested. 

The findings also point to another big question: Is it possible to have the habitat-restoring benefits of green gentrification without the accompanying displacement of human residents? 

It’s a thorny problem, Fidino said, because investment in green spaces — a neighborhood amenity whose presence drives up rents, home prices and property taxes — is so tied to the processes by which gentrification happens.

“Creating more green spaces is normally viewed as an economic development strategy, right? And that’s kind of the thing that needs to be combated for the most part,” Fidino said.

“But that’s difficult, because how do you disentangle urban green space development from Western capitalism? That’s what would need to happen for this to be much more equitable: for cities to view urban green space as a human right, versus the way that it’s typically done now.”