The phenomenon known as the 'luxury effect' describes how wealthier parts of cities tend to support more wildlife and greater biodiversity. The term emerged from research in Phoenix, Arizona, in the late 1990s, when ecologists began studying urban plant distribution.
A team led by Prof Ann Kinzig of Arizona State University found that household wealth was the most significant factor in plant distribution, surpassing geographical and climatic influences. In Phoenix, a $10,000 increase in median household income corresponded to a 0.3C drop in mean surface temperature, reflecting the cooling effect of more vegetation.
Subsequent studies have documented the effect across the globe and among various species. Black-tailed deer in British Columbia, plants in North America, Burundi, China and Australia, birds across several continents, and even some bats have been found to favour affluent areas. The pattern may date back to ancient Egypt, where labourers' quarters hosted different insects.
Prof Kinzig said she was surprised that the effect held so consistently, noting that while different people create different landscapes, the rich almost always experienced higher biodiversity. However, not all studies agree: research in Puerto Rico found no correlation, possibly due to a smaller income gap. Jeff Ackley, a regulatory biologist, found that household income was one of the best predictors of lizard diversity in Phoenix.
The effect extends indoors. A study of 50 homes in Raleigh, North Carolina, found that wealthier homes contained an average of 100 types of arthropods, including insects and spiders. The findings carry implications for human wellbeing, as exposure to nature is linked to health benefits.
