Armed with kneepads, headlamps and glass vials, the research team collected samples of arthropods — think insects and spiders — from 50 urban homes in Raleigh, N.C. They also took notes on the homes' sizes, nearby vegetation and locations.
Back in the lab, they used microscopes to identify each kind of bug and note its species. They identified each dwelling's mean neighborhood income using publicly available information, then ran the numbers to see whether they could tease out possible biological connections between the factors.
The team, led by California Academy of Sciences postdoctoral researcher Misha Leong, thought that arthropod biodiversity — the number of types of species found within a given dwelling — would increase based on a home's square footage and the density and diversity of plants outside. But another factor ended up being almost as predictive of a large number of bug species types: neighborhood income.
The signal was so strong that Leong ran the numbers again and again just to be sure. "We wound up analyzing the data a bunch of different ways," she says. "I kept asking myself, 'Is this a fluke?' " But even homes with relatively light ground cover outdoors had more diverse arthropods inside when they were in wealthy neighborhoods.
The finding seems to point to what is known in biodiversity circles as "the luxury effect" — the more affluent the area, the more diverse its animals. (Last year, for example, biologists found that the higher a household's income, the higher the number of lizard species that could be found there.)
"We've known about this effect," Leong says. But both Leong and Michelle D. Trautwein, a curator of entomology at the academy and co-author of the paper, say they were surprised at just how strongly luxury and diversity correlate when it comes to arthropods.
Though Leong admits that more thorough studies of high-density households in lower-income neighborhoods are needed to complete the picture of indoor bug diversity, her findings fly in the face of a lingering perception of poorer homes as places with bug problems. Many existing studies on income and bugs focus only on pests such as cockroaches or arthropods as disease vectors. That's important work, Leong says, but it can skew the public's perception about bugs.
"It's a bit of a humbling fact that no matter how impervious you think your house is to the outdoors, it's very easy for tiny things to find their way in," she says. That's especially true for wealthy homes and houses in ritzy neighborhoods filled with parks, flowers, trees and other havens for nature-loving arthropods. Those habitats are the direct result of the investment and political clout of the wealthy, who can turn their attention to green space instead of focusing on the bare basics.
"There are untold benefits of wealth that aren't necessarily noticed but that somehow translate to enhanced environments and ecosystems," Trautwein says. "Even if we can't really pinpoint what those benefits are, we can see that bugs and plants and birds and plants and lizards all respond to affluence." Even bugs, it seems, like a bit of luxury.