From five to forty species of arthropods might call your place home. Are you surprised? That’s more than I expected for my apartment, at least. But don’t start looking too sideways at your corners and cabinets. Hear me out: not all of these creatures are creepy. (Though some, I’m sorry to say, might make you sneeze).
Arthropods come in an astounding array of forms, from ants to butterflies, crabs to scorpions, millipedes to spiders (the phylum Arthropoda includes all invertebrate animals with an exoskeleton, a segmented body, and jointed appendages – a motley bunch with motley bodies).
Biologists have cataloged over a million species of arthropods, and estimate that five to ten million species crawl the earth. Like you’d expect from such crazy diversity, arthropods live in almost every animal-friendly niche on the planet. In the past, researchers cataloged this diversity in grasslands and forests. But as anyone with ladybugs crashing their windowsills in the winter knows, arthropods can also live in environments created by humans. Until recently, we didn’t know how many lived with us, or how those species were spread across the United States.
To find out, postdoctoral researcher Anne Madden used a sequencing-based method to catalog and map the arthropods in our homes, published in “The diversity of arthropods in homes across the United States as determined by environmental DNA analyses”. She completed the work in Noah Fierer’s lab here at CU Boulder and then in Rob Dunn’s lab at North Carolina State University.
Making a list of all the arthropods in even a single house takes hours using traditional methods: catch as many critters as possible, then bop them down on a microscope for a trained entomologist to identify. That hands-on approach might be fun for one house, but makes it impossible to map the arthropod diversity in houses from all 50 states.
Instead, CU Boulder and North Carolina State researchers deputized homeowners curious about the invisible life sharing their space as part of the “Wild Life of Our Homes” project. These citizen scientists – over a thousand of them – collected dust from their houses, one swipe from the top molding near the living room, another from just outside the front door. Originally they sent in their dust swabs for a study on bacteria and fungi.
CU Boulder researchers amplified and sequenced DNA from cells stuck to the swabs in order to catalog the household microorganisms that lived above the doorframes – think scanning a barcode rather than poring through a wildlife guidebook or a microscope.
But dust holds clues to our other roommates – at least, those that leave behind DNA. Researchers can identify plants from DNA found in pollen and arthropods from DNA shed in exoskeleton flakes or in “frass” (a genteel term for insect feces). Anne Madden saw the swabs already kicking around in the Dunn and Fierer labs as “the perfect opportunity to look at what arthropods could be in these samples.” Madden studies the microorganisms that exist in and on insects, so she used this project as “a platform to investigate just how arthropods may be contributing to the microorganisms in our homes.”
To begin that investigation, Madden and her colleagues amplified arthropod-specific DNA from over 730 dust samples, then sequenced it to identify which arthropods lived in each house.
Of the houses tested, 651 were home to arthropods: 600 genera of them between all the houses studied. Some of the most common insects were familiar, like dust mites and carpet beetles. But this project also found moths, aphids, mosquitoes, and parasitic wasps. Rural houses and houses with basements or pets were more likely to have diverse arthropod ecosystems.
The quick but essential work of citizen scientists made it possible to map the range and frequency of interesting arthropod species across the United States. The researchers focused on our most annoying roommates, such as the crustaceans and dust mites that cause allergies. These maps make it possible to study who gets exposed to arthropod allergens and how that exposure impacts health. The results also reveal continent-scale ecology, such as the geographical range of invasive insects or disease-carrying mosquitos such as Aedes species, which can spread Zika virus in subtropical climates. According to Matthew Gebert, a research technician in the Fierer lab, public participation in science is the only way to collect the number and range of samples required to answer these broad ecological questions.
Madden describes her current work as a mix of the ecological and the practical: “Broadly, as a scientist I’m interested in understanding how wild microorganisms can help solve our human problems, from bland tasting food to novel therapeutics.” Though she now works in the Dunn lab at North Carolina State University, her research still has Colorado flair. She takes a new approach to investigating the microbial diversity associated with arthropods by culturing brewing yeast from bees. And she’s looking for home cooks and curious students to help her learn about the ecology of sourdough starters.
Citizen science continues in both the Dunn and Fierer labs. The Dunn lab is looking for participants in “the life of pants” project (yes, pants) to find out what microorganisms live in different types of fabric, and the “school of ants” project to map urban ant populations. And both groups aim to further uncover the ecology of our homes with the Showerhead Microbiome project. By getting involved in citizen science, anyone can give researchers data and ideas. So if you want to contribute to science – or if you just want to know what other organisms in your place should contribute to the rent – sign up!
By Bridget Menasche