FFor a long time, ecology has tended to ignore people. He mainly focused on beautiful places far from large-scale human activity: dense tropical forests or untouched grasslands. Then, in the late 1990s, in the desert city of Phoenix, Arizona, scientists turned their gaze closer to home.
A team of ecologists traveled to their area to map the distribution of urban plants in one of the first studies of its kind. Armed with tape measures and clipboards, they documented the trees and bushes, sometimes crawling through the bushes on all fours as local residents watched curiously.
“We had a lot of fun. The little kids loved our measuring wheel,” says Professor Anne Kinzig of Arizona State University. “It made me look at cities differently.”
Existing ecological theories did not explain the distribution of plants in cities. In the wild, plant placement is typically influenced by factors such as geography and weather: rockiness of the soil, rainfall levels, and exposure to light and shade.
Scientists wondered whether a new theory was needed. “And the answer was yes,” says Kinzig.
The city data kept telling them the same thing: the most significant influence on plant distribution was not altitude, proximity to bodies of water, proximity to rural areas, or soil type. It was the wealth of the people living nearby. In 2003, her team published first article on the “luxury effect”.
This phenomenon describes how wealthy parts of cities have more wildlife and greater diversity – a finding that could have a big impact. serious consequences for human health and well-being. In Phoenix, researchers wondered if heat was the culprit. It is a desert where water is scarce and temperatures can be harsh.
As prosperity grew in the Phoenix area, people tended to plant more vegetation and water it. Increase in median household income at the Phoenix study site by US$10,000 (£7,500). resulted in a decrease of 0.3°C (0.54F). by average surface temperature.
But since the initial studies in Phoenix, researchers have found the luxury effect in other cities with other species, where heat was less of a concern. According to the data, black-tailed deer in British Columbia are found in wealthier parts of the city with more gardens, golf courses and parks. 2023 study.
Luxury effect was found on plants in North America, Burundi, China and Australia. It was also recorded in birds in North America, Europe and New Zealand. Birds depend on plant species for food and shelter, so it is possible that they follow plants into lush areas. Even some bats gravitate towards wealthier areas of the city. Back in Ancient Egypt, places where workers lived There were different types of insects compared to richer regions.
“I was surprised—not that human activity could shape patterns of biodiversity in unexpected ways, but that the luxury effect persisted so well,” Kinzig says. “In other words, you might… expect different types of people to create landscapes that support different types of other species, but you wouldn't necessarily expect the rich to almost always have higher biodiversity.” (Fewer studies of the luxury effect have been conducted in tropical regions – one study from Burundi found a positive relationship between plant diversity and richness, but another study in Puerto Rico, no correlation was found, perhaps because there is a smaller income gap between areas.)
In urban environments, “people control everything,” says Jeff Ackley, a U.S. government biologist who discovered the influence of luxury on Phoenix's “rich lizards” and found that household income was one of the best predictors of nearby lizard diversity. “When people have the time and resources to do this, they can control what their backyards look like. And for the most part, people like more varied vegetation.”
But the luxury effect is not limited to gardens – it seems to be happening indoors too. Richer houses have more insects and spiders. according to research from the USA.
The researchers randomly selected 50 homes in Raleigh, North Carolina, across the income spectrum. According to the study, wealthier homes had an average of 100 species of arthropods, twice as many as less wealthy homes.
The results were controversial because people tend to think that richer people have cleaner homes, highlighting “how much we still have to learn about indoor ecology,” the researchers wrote.
No research has examined how the luxury effect might affect the millions of species of microbes living in cities, but research suggests that poorer people tend to have more inflammatory and immune diseases, which may be linked to a lack of exposure to microbial life. Contact with biodiverse environments, soils and animals is associated with many positive health effects.
“Almost all immune system diseases are on the rise,” says Dr James Kinross, consultant surgeon at Imperial College London and author of the book Dark Matter: The New Science of the Microbiome. “There is something about the way we build and design our urban systems that separates us from nature and therefore from the kind of microscopic ecosystems that we need to maintain our health.”
Biodiversity loss is recognized as a pressing global problem, but it usually refers to the extinction and decline of species in our environment rather than what is happening in the human body. However, a growing collection of studies shows that they are related. Last year, The article introduced the concept of “lack of nature.”keeping in mind how natural losses in the human body affect health.
To reverse these health inequities, scientists point to the outside world, where conserving and enhancing biodiversity can make a difference. A study conducted in Finland showed that teenagers living in biodiverse gardens, for example, have fewer problems with allergies and autoimmune diseases.
“The luxury effect is more than just an interesting ecological pattern,” writes a team of researchers studying the phenomenon. “[It is] and another layer of social and structural injustice present in cities.”
Find more Age of Extinction coverage hereand follow biodiversity reporters Phoebe Weston And Patrick Greenfield on the Guardian app to find out more about nature
