Butterfly diversity is lower in historically redlined neighborhoods: evidence from 151 US cities

Cities are often seen as concrete jungles, places where nature retreats in the face of buildings and roads. Yet, even within these dense landscapes, pockets of life persist. Birds, trees, and insects still find ways to survive, but they do not thrive equally everywhere. In recent years, ecologists have noticed a pattern often called the "luxury effect": wealthier neighborhoods tend to host more species and more abundant wildlife than poorer ones. This disparity is not just about money; it is about the physical environment. Wealthier areas often have more parks, larger yards, and more trees, while poorer areas frequently suffer from a lack of green space and an abundance of paved surfaces. This raises a difficult question about history and justice: how did these differences come to be, and do they still shape the natural world today?

The answer lies in a decades-old practice known as redlining. In the 1930s, a federal agency called the Home Owners' Loan Corporation drew maps of American cities to guide mortgage lending. They graded neighborhoods from A to D based on perceived risk. The criteria were explicitly racial; areas with Black, immigrant, or working-class residents were marked as hazardous and colored red on the maps. Banks used these maps to deny loans and refuse investment in redlined areas, while pouring money into the highest-graded, predominantly white neighborhoods. Although the practice was outlawed in the 1960s, its physical legacy remains. Redlined neighborhoods still tend to be hotter, have fewer trees, and contain more concrete and asphalt than their higher-graded counterparts. But while we know these areas differ in temperature and green space, it has been unclear whether these environmental disparities have left a lasting mark on the diversity of insects, the most numerous and vital group of animals on Earth.

To find out, a team of researchers from Gettysburg College set out to examine the butterfly populations across the United States. Butterflies are ideal for this kind of study because they are sensitive to their surroundings. As caterpillars, they eat specific plants, and as adults, they need flowers for food. If a neighborhood lacks the right plants, the butterflies cannot survive. The researchers gathered a massive amount of data, pulling nearly 59,000 high-quality observations of butterflies from a citizen science platform called iNaturalist. These records came from people across the country who took photos of butterflies between 2016 and 2025. The team then overlaid these sightings onto digitized versions of the original redlining maps for 151 different cities. They treated each distinct colored zone on the old maps as a separate neighborhood unit, allowing them to compare the butterfly life in areas that were once graded as "Best" against those once marked as "Hazardous."

The results were clear and consistent. Neighborhoods that had been graded as "Best" in the 1930s supported significantly more diverse butterfly communities than those that had been redlined. In the highest-graded areas, the variety of butterfly species was noticeably higher, and the mix of common and rare species was more balanced. In contrast, the redlined zones, which were historically denied investment, hosted fewer species and a less diverse community. This pattern held true even after the researchers accounted for the size of the neighborhood and how many people were looking for butterflies in each area. The difference was not a fluke of data collection; it was a real ecological gap rooted in history.

However, the study went further to ask why this gap exists. The researchers suspected that the type of land cover—the physical makeup of the neighborhood—was the key. They analyzed the current landscape of each zone, looking at how much of it was covered by high-rise buildings, dense housing, open parks, or forests. They found that the redlined zones were dominated by high-intensity development, such as downtown cores and industrial areas, with very little open space. The highest-graded zones, conversely, contained more than five times as much "developed open space," which includes parks, large lawns, and other green areas that can support wildlife. When the researchers included these land cover details in their analysis, the direct link between the old redlining grade and butterfly diversity largely disappeared. This suggests that the historical policy did not harm butterflies directly; rather, it shaped the built environment in a way that made it difficult for butterflies to survive. The lack of green space in redlined areas, a direct result of decades of disinvestment, is what drove the loss of diversity.

The study also looked at forests and found that even small patches of trees within a neighborhood helped support more butterfly species. Forest cover was the only single type of land cover that consistently predicted higher diversity on its own. Yet, the most powerful factor was the presence of open, developed green space. The data showed that the difference in butterfly diversity between rich and poor neighborhoods is not an unchangeable fact of nature, but a consequence of how those neighborhoods were built and maintained. The researchers ruled out the idea that the results were simply due to fewer people looking for butterflies in poorer areas or the presence of a single migratory species like the Monarch. Even when they removed the Monarch records or adjusted for the number of observers, the pattern remained: the redlined zones had less diversity, and this was tied to the lack of green space.

This finding offers a specific path forward for fixing the problem. Because the diversity gap is driven by the lack of open green space, the solution lies in urban greening. Planting native flowers and trees in formerly redlined neighborhoods could restore butterfly diversity and provide the ecosystem services that residents currently lack. This would not only help the insects but also cool the cities, improve air quality, and offer residents of these communities the mental health benefits of nature. The study demonstrates that the ecological scars of social injustice are visible in the wings of a butterfly, but they are also healable. By understanding how history shaped the physical landscape, city planners and communities can now target their efforts to repair the environment where it is needed most, turning the legacy of redlining into an opportunity for ecological renewal.