The Color of Biodiversity: How Segregated Cities Altered Urban Wildlife

In a vast wilderness, a sparkling stream trickles down a steep hill surrounded by lush trees. As the forest becomes less dense, you realize that you are not in the wild at all. The local community gathers on the exedra while children sell cookies nearby to raise funds for an animal shelter. In this gem of California nature, Piedmont Park, it is easy to forget you are in a city – even though Downtown Oakland is a twelve-minute drive away. The houses surrounding Piedmont Park are equally gorgeous. Stately single-family homes with mini botanical gardens in their front yards sit on quiet tree-lined streets away from the bustle of the city.

Only two miles down the road in Eastshore Park, things could not be more different. The greenery thins as you approach the park via pot-hole ridden streets. The park itself is small, flat, and has a fraction of the tree coverage seen in Piedmont Park. Children climb a jungle gym bordered by the 580 Freeway and two busy four-lane streets. Moving further south, the houses start to change too. Dense swaths of apartments with no space for front yards dominate. The streets have few trees to provide shade or refuge for wildlife.

This contrast in parks and homes is not unique to Oakland. Most Californians will say that Los Angeles, Oakland, San Diego, and San Francisco have nothing in common. But in reality, they are very similar. If you drive though any of these cities you will notice drastic variations in the houses, roads, tree coverage, and quality of the parks, divided by income level and racial and ethnic background. These disparities were established almost a century ago by a practice called redlining — a “state-sponsored system of segregation,” according to Richard Rothstein, author of “The Color of Law: A Forgotten History of How Our Government Segregated America.”

The legacy of structural inequality has had a lasting impact on the lives of people who call these areas home. But until recently, whether these effects extend to the wild residents of cities was left unexplored. In 2024, Cesar Estien, an urban ecologist at urban-planning consulting agency Second Nature Ecology + Design, set out to better understand these differences as part of his PhD thesis at the University of California, Berkeley.

Now, Estien has revealed that formerly redlined neighborhoods in major cities across California have worse environmental quality and less diverse wildlife than other parts of the city. “[As] someone who grew up in a low-income neighborhood, these are things that we talk about or know already,” Estien says. “Unfortunately, [if] issues like this are not published or documented, it’s hard to do anything about it.”

This finding is a part of a growing field of urban ecology that is starting to reckon with how racist practices have altered the environments of more than 200 cities across the country — and learn how urban wildlife has developed differently in response.

Today, the thousands of people who live in formerly redlined areas of Los Angeles, Oakland, San Diego, and San Francisco — the majority of whom are low-to-middle income and people of color — cannot experience the physical and mental health benefits of living in a thriving, biodiverse neighborhood. Access to nature can promote recreation, mental health, and reduce the risk of disease.

The biodiversity of an area shapes ecosystem resilience, the services that wildlife can offer, and natural experiences for humans who live in their cities, Estien says. “A more diverse wildlife pool also enriches what people are interacting with.”

 

Redlining 101

Estien is intimately familiar with these disparities. He recalls how his experiences growing up in Tampa, FL mirror what he sees today in Oakland. He loved the parks in his neighborhood but realized that outdoor spaces, and homes, were vastly different in wealthier parts of town. “Not only is your house bigger, but your parks are different and better quality,” in the more affluent neighborhoods, he says.   

For 239 cities across the country, including eight of California’s largest cities, these differences can largely be credited to the government sponsored Home Owners Loan Corporation.  

In 1933, President Franklin Delano Roosevelt created the Home Owners Loan Corporation (HOLC) as part of his New Deal, a series of government reforms and projects with the goal of aiding the American people. The HOLC bought mortgages from banks, then refinanced them at lower interest rates over longer periods of time, making home ownership more affordable for American citizens left struggling after the economic turmoil of the Great Depression. Ultimately, HOLC also served to further disenfranchise many more people, as it determined that immigrants and Black, Latino, and Asian Americans were untrustworthy loan holders.  

To help banks and mortgage lenders determine the value of residential properties, the HOLC created Residential Security maps of cities with populations over 40,000. Residential areas were given one of four classifications, with a corresponding color: green for best, blue for still desirable, yellow for definitely declining, and red for hazardous. These classifications determined if prospective homebuyers would be eligible for mortgage loans from banks or other mortgage lenders.    

The HOLC determined the value of neighborhoods based on quality of housing, recent history of sale and rent values, and residents’ race, ethnicity and class. Some of their decisions were based solely on the housing stock of the area. For example, HOLC classified the area around Eastshore Park in Oakland as yellow due to being zoned for apartment complexes (this was before the 580 Freeway was built). Further south in the Oakland flats, however, the language becomes less objective. One area “splendidly situated for a ‘slum clearance’ project” was zoned red due to industrial odors and “infiltration of Negroes and Orientals.” Throughout Oakland, “colored residents” and “undesirables” were listed as “detrimental influences” that gave an area a yellow or red grade. In contrast, “favorable influences,” like being “restricted to Caucasians,” earned the Piedmont area a green grade. As a result of this program, the predominantly racial and ethnic minorities looking to buy homes were denied loans, as the majority of them lived in red neighborhoods. This discriminatory practice, which came to be known as redlining, occurred throughout California and the US. As a result, these groups could not own their homes and build generational wealth.     

The Fair Housing Act made discriminatory loan practices illegal in 1968, but to this day, an undeniable relationship between HOLC grades and patterns of inequality exist. Redlined areas are still highly segregated by race and ethnicity and have lower household income than greenlined ones. Living in a formerly redlined neighborhood now means being surrounded by environmental hazards, such as factories and freeways, that negatively impact residents’ health and well-being. Studies have found that residents of these formerly redlined areas are more likely to have asthma, more frequent preterm births, less access to healthy food, and are more likely to die younger.  

The environment has been negatively impacted as well. Redlined areas are more polluted, have fewer trees, and are hotter than greenlined areas. These impacts have been widely explored and well documented. But up until the early 2020’s, few ecologists were looking at the impacts on the biological diversity of the creatures who also call these cities home.   

Christopher Schell was one of the first to propose that the racist legacy of redlining might also have a negative impact on the biodiversity of cities. In 2020, Schell, an urban ecologist now at UC Berkeley, published the review paper, “The ecological and evolutionary consequences of systemic racism in urban environments,” in Science. He argued that few others in his field had considered the impact of structural inequalities on ecological variation in cities, instead focusing on social variables like household and neighborhood wealth without considering the underlying reasons behind these disparities. He urged others to take an interdisciplinary approach by drawing on social and political science research to better understand the ecology of urban environments.   

In this paper, he proposed that racial segregation via redlining could have played an important role in shaping the biodiversity of cities. 

 

Biodiversity in California Cities

As an urban ecologist, Estien’s work looks different from other ecologists’ studies. Rather than chasing polar bears in the Arctic or searching for rare birds in the Amazon, Estien roams city parks and streets.  

I bought all these special clothes when I first started, but I never used them. I just go out in my Vans and some jeans, he said.  

Walking around Oakland in a blue hoodie, black pants, and, of course, Vans sneakers, Estien blends right in. But his view of the city is unlike anyone else’s. Driving down Grand Avenue towards Piedmont, he points to the tree-lined sidewalk saying, you can see that the tree canopy gets denser here; each parcel has two to three trees on it now. 

Estien began his scientific career studying how species, such as sea urchins, are responding to climate change. Having considered that the impacts of climate change are driven by human activity, he became more interested in studying the areas where humans are concentrated — cities. As a new graduate student in Schell’s lab, he aimed to discover how exactly redlining had altered the biodiversity of California neighborhoods.  

“At that point in time, this is all theory. It’s like, we did a review, we think these things are happening, we haven’t tested it yet, but someone should,” Estien recalls. “The main gap I was trying to fill was putting empirical support behind these theoretical linkages.”  

With help from engaged citizen scientists all over California, who upload observations of creatures onto the biodiversity sharing platform iNaturalist every day, Estien found that empirical support. He gathered research-grade observations of mammals, birds, reptiles, amphibians, insects, and arachnids from iNaturalist data in Los Angeles, Oakland, San Diego, and San Francisco between January 2017 and January 2022. (The other four California cities with HOLC maps — Fresno, Sacramento, San Jose, and Stockton — were not included in this study because they did not have enough data.) He then used the digitized HOLC maps from the Mapping Inequality project to see how biodiversity differed among former HOLC classifications.   

The data confirmed Schell’s original theories. He found that in all four cities combined, greenlined areas had an average species richness, or number of unique species, of 20, compared to formerly redlined areas, which had an average species richness of six. Redlined areas in each individual city had the lowest species richness out of all HOLC grades, though the difference compared to greenlined areas varied in each city. Having more unique species helps the ecosystem be more resilient to disturbances, such as natural disasters.  

As Estien explains, if an area has only one species of tree that’s vulnerable to a pest, and a pest outbreak occurs, then that species could be lost. But if that place had ten species of trees, and only three were vulnerable, there would still be seven species left over.   

Species richness is just one aspect of biodiversity. Another aspect to consider is community composition, which refers to the specific types of species that make up an ecosystem. Two areas can have the same number of species but have completely different kinds of species. Different community compositions can help the ecosystem in different ways. Community composition also varied based on HOLC grade, with green and red neighborhoods having different compositions in all four cities examined.  

Greenlined neighborhoods might have more pollinators and pest regulators, which are critical for ecosystem health and could help urban farming initiatives. In contrast, redlined neighborhoods might have species like racoons, pigeons, and rats that can better cope in dense urban environments. While those species are important to that ecosystem, Estien does note that some might consider them as “boring” or a “nuisance” and might therefore feel less connected to them and their environment. Other studies show that when people experience nature in their daily life, they feel more connected to their environment and are more likely to engage in activities that protect biodiversity.   

How frequently people interact with wildlife varies across the city. People in greenlined areas tend to observe unique species more easily than those in redlined areas. While one might assume there are simply more species in these areas, this is not the case. As Estien explains, there are actually fewer total observations in the greenlined areas, yet they still have a higher species richness. “Yes, there’s less species in the [greenlined] neighborhood, but you would encounter [them] faster,” says Estien. He explains that if you were to go on a ten-minute walk in a greenlined area, there is a higher chance of finding more unique species than if you walked around a redlined area for that same amount of time.   

For those who live in the redlined areas, this also affects the sense of connections they feel to the environment around them. If they want to experience nature, they have to venture farther from their homes.  

“It’s exciting to walk in your neighborhood and see different kinds of birds, pollinating flowers, and butterflies—it makes a space feel more alive,” says Estien. “If you think about the fact that most people live in cities and will never venture out of cities, the access to nature you have is important. That’s why parks and how diverse those communities are matters.”   

“[Urban ecologists are] all trying to understand how we can develop our cities to be beneficial for people and biodiversity. We have to have both of them if we’re going to get this right.”

Access to green spaces, tree-lined streets, and even private gardens can affect the psychological well-being of a community. Urban residents facing socioeconomic hardship report higher levels of mental well-being and show healthier cortisol levels, a marker of stress, when they have better access to green spaces.   

Eric Wood, an associate professor of avian and urban ecology at California State University, Los Angeles, says that this study was strong, highlighting its breadth in covering multiple cities and aspects of urban biodiversity. However, he notes that more work needs to be done before we can fully understand how these differences in the environment affect people.  

“Really understanding how ecology and people come together [is] still not really approached. I think that’s what the future needs to be,” Wood says. “[Urban ecologists are] all trying to understand how we can develop our cities to be beneficial for people and biodiversity. We have to have both of them if we’re going to get this right.”  

 

The Growing Field of Urban Ecology

Although ecologists began to turn their sights from far-flung wilderness to the urban jungles that they live in around the 1970s, research dedicated to urban ecology remained niche.   

In 2021, Erica Spotswood, an urban ecologist at Second Nature Ecology + Design, a private consultancy in Berkeley, CA, proposed the “biological deserts fallacy” in a paper published in BioScience to explain why she believes cities have been largely overlooked in ecology research, and why cities matter to biodiversity. She argued that ecologists do not give enough attention to the benefits cities provide to wildlife. She refers to a map created by the Conservation Lands Network 2.0 that showed Bay Area cities as “grayed out blobs” surrounded by areas that drew the focus of conservation networks. While she notes that her paper, “The Biological Deserts Fallacy: Cities in Their Landscapes Contribute More than We Think to Regional Biodiversity,” was only a part of a growing shift towards considering cities as important ecological environments, she wanted to make the case that urban environments are valuable areas of biodiversity.   

“I think the consequences of not intervening to do a better job of providing nature access in cities is both a conservation missed opportunity and a public health problem,” says Spotswood.   

Due to this increased interest in urban biodiversity, and papers like Schell’s, more researchers have started to explore the link between the environment and redlining in cities across the US.   

Wood’s 2023 paper showed that bird biodiversity varies across former HOLC grades in Los Angeles. Greenlined neighborhoods house more “charismatic” birds, such as Ruby-crowned Kinglets, while redlined areas have more “boring” city birds, such as pigeons. But a year later, research in Washington D.C. found that there is similar mammal diversity across the city, despite systemic differences in neighborhoods. This result suggests that the 239 cities surveyed in the 1930s have developed differently, and more city-specific studies will be able to unpack the legacy of redlining on biodiversity across the US.    

“As urban ecology grows, this conversation becomes richer, more nuanced, and more complex in ways that are very productive for the field,” says Spotswood. “It’s not a field that’s ever ignored people as a core dimension. And that is to its benefit.”  

 

Uncertain Futures for People and Wildlife

As climate change wreaks havoc on the planet, more people are starting to consider the roles that cities play in biodiversity and conservation efforts. Currently, 81% of the American population lives in cities, according to the US Census Bureau. Marginalized communities bear the brunt of disparities in urban environments. Estien hopes papers like his can inform policies to ensure that urban greening efforts are equitable and that historically disadvantaged communities are centered. However, transforming this data into actionable policies proves difficult.  

Oakland has an exhaustive Equitable Climate Action Plan, created with input from the community, that centers their most vulnerable populations. However, they lack the funding to fully realize their goals of equitable environmental protection. Under the Biden Administration, the White House launched the Justice 40 initiative with the goal of investing in environmental justice policies that benefit disadvantaged communities. With a new administration that aims to open national parks for oil and mining industries and eradicate Diversity, Equity, and Inclusion initiatives, there are new challenges for equitably improving environmental quality.  

“We didn’t expect the federal government to fund everything, but we expected, because this was the inclination of the Biden administration, that there would be resources for [Oakland’s Equitable Climate Action Plan]. And now this is being dismantled in Washington, D.C.,” Darlene Flynn, the Executive Director of Oakland’s Department of Race and Equity explains.  

The legacies of systemic racism are embedded in the fabric of American society and policy. Undoing the impacts will be a long and difficult journey. To Flynn, it is one worth taking. “Oakland has wonderful plans to close racial disparities in the city. We also are not a rich city, so we have to find another way,” says Flynn. “We’ll be here fighting the good fight.”  

Estien now works at Second Nature Ecology + Design, a consulting firm that helps businesses, urban planners, designers, and communities implement nature-focused strategies in cities. He uses his knowledge of how redlining has impacted urban biodiversity to help improve equitable access to nature. “When I moved here, I really wanted to improve the way that the city or other orgs are thinking about improving access to nature and biodiversity,” he says. “Seeing that my research will be useful is exciting and heartwarming.” 

 

© 2025 Jasmin Galvan / UC Santa Cruz Science Communication Program