A report on the study — led by a team from California Academy of Sciences in San Francisco — was published recently in the journal Scientific Reports.
Bugs have been cohabiting with humans ever since they started living indoors some 20,000 years ago. And, while the idea may not appeal to us, the presence of these uninvited residents may have an important, if indirect, effect on our health.
"A growing body of evidence suggests some modern ailments are connected with our lack of exposure to wider biological diversity, particularly microorganisms," notes senior study author Dr. Michelle Trautwein, of the California Academy of Sciences. She adds that "insects may play a role in hosting and spreading that microbial diversity indoors."
In the United States, people spend around 87 percent of their time inside buildings. They share this environment with thousands of species of bacteria, fungi, and other microbes, as well as hundreds of species of arthropods, or "joint-legged" creatures, such as insects, spiders, millipedes, and mites.
We are only just beginning to fathom this vast indoor ecosystem and the extent to which it reflects the outdoor ecosystem, the features of the home or building, and the habits of the humans inside.
With a view to discovering implications for health and disease, so far, the research has focused mainly on the microbial communities.
Outdoor access had strong influence
To widen the research on "indoor biomes" to include the next most abundant group of organisms, Dr. Trautwein and her colleagues conducted a study of arthropods in 50 homes in southeastern U.S.
They compared the abundance and diversity of species of arthropods in the homes with certain structural and functional features, such as room types and access to the outdoors.
They also looked at how the species of bugs varied with the behavior of the human residents, and the physical features of their homes, such as the number of windows and doors (a measure of access to the outdoors) and the types of room.
The researchers found that the diversity of indoor arthropod species was "strongly influenced by access to the outdoors," and that "carpeted rooms hosted more types of arthropods than non-carpeted rooms."
The results showed that species diversity in homes as a whole tended to mirror that of the outdoor surroundings.
The team say that the diversity of indoor arthropods — made up largely of "outdoor vagrants and infrequently collected families" — reminded them of the tent-like "Malaise traps" that are used to capture insects in the field.
"We're beginning to see how houses can be a passive go-between for insects traveling through the surrounding landscape. The more numerous the entry points of windows and doors, the more diverse the community that thrives inside."
Dr. Michelle Trautwein
Within homes, the type and location of rooms had an effect on the spread of bug species that lived in it. For instance, rooms with more access to the outdoors — such as those on the ground floor with more windows and doors — had the greatest diversity of species.
Basements were 'like caves'
The study's results showed that basements contained arthropod communities that resembled the ecosystems of caves. Their damp, dark spaces were favored by spiders, millipedes, ground beetles, and mites.
In contrast, the researchers found that the habits of the human residents of a home — such as use of pesticides, tidiness of homes, and ownership of pets — had no "significant influence" on the composition of its arthropod communities.
However, they note that the small sample size might have limited this part of the analysis, and that studies in larger populations might show different results.
Each room in the home contained a complex ecosystem with its own distinct pattern of predators and prey. This included scavengers, stray species from outdoors, and "transient go-betweens," each occupying its own key slot in the food chain.
In their conclusions, the scientists note that knowing more of the complexity and dynamics of these indoor communities should put us in a better position to reduce health hazards and control pests. It may also lead to a "cultural acceptance of the nature that surrounds us," they add.