In 2018, the Dunn Lab at North Carolina State University started the iNaturalist project “Never Home Alone: The Wild Life of Homes.” The goal of our project was to collect observations of organisms seen indoors from all over the world.
Despite the fact that most people (including biologists) spend the majority of their lives inside, we know remarkably little about the species that share our indoor spaces. In one of the few studies to date of indoor biodiversity, a 2016 journal article found more than 1,000 species in a few dozen homes in Raleigh, North Carolina– a veritable rain forest of life right under our noses, and a higher arthropod diversity than some natural ecosystems!
Now, thanks to the work of thousands of iNaturalist users from around the world, we are able to take a closer look at the overlooked denizens of our shower drains and closet floors. Our team has completed an initial analysis of the data and we are excited to share these findings with the iNaturalist community! Note that this is just a preliminary analysis and some of these findings may change as more data are collected. Speaking of which, please continue to share your indoor observations, and classify observations submitted to the project!
As of December, 2023, iNaturalist users have submitted nearly 44,000 observations to Never Home Alone. Approximately 42% of these observations are Research-Grade. While some of these organisms are transients (i.e., they don’t want to be inside; a dragonfly buzzing at a window, or a sparrow trapped in a garage), we believe that the most common indoor species have adapted to exploit the unique advantages of a life spent in or near human dwellings. See below for just a few of our favorite observations.
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| House Mouse observed by @carrieseltzer | European Mantis observed by @gianni_del_bufalo | Varied Carpet Beetle observed by @gillessanmartin |
| Colombian Weasel observed by @sultana | Barn Owl observed by @chartuso | Asian Lady Beetle observed by @pajeu |
| Curculio elephas observed by @osoandino | Carolina Wren observed by [at]apgarm | Micropholcus fauroti observed by @franzanth |
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| Christmas Island Red Crabs observed by @sea-kangaroo | American Alligator observed by [at]akkhalil | Pantropical Huntsman Spider observed by [at]jersey_john |
These observations are concentrated in the United States, which accounts for 47% of all observations, but there are also observations from 126 other countries and every continent except Antarctica (we would love some Antarctica observations, by the way). India has the second-most observations (n = 2,080), followed by Thailand (n = 1,830), Mexico (n = 1,424), Canada (n = 1,418), and South Africa (n = 1,387).
Number of observations submitted to Never Home Alone from around the world as of December, 2023.
Approximately 63% of all observations are insects, 27% are arachnids, 3% are another class of arthropod, 5% are vertebrates, and the remaining 2% are another taxa (molluscs, fungi, etc.). The insect observations are primarily butterflies/moths (15% of all observations), beetles (11%), flies (11%), bees/ants (8%), and true bugs (7%). The vast majority of arachnid observations are spiders (25% of all observations). Observations submitted to Never Home Alone are far more likely to be arthropods (93% of observations) than typical iNaturalist observations (30% of observations). A few arthropod families appeared to be particularly over-represented in the Never Home Alone data; for instance, moth and sand flies (Psychodidae) are the most common fly family in the Never Home Alone data (13% of fly observations) yet this family makes up just 1% of all fly records in the entire iNaturalist dataset (in fact, approximately 2% of all iNaturalist Psychodidae observations have been submitted to Never Home Alone!).
A: All iNaturalist data
B: Never Home Alone data
Abundance of different taxonomic groups in all iNaturalist data (A) and iNaturalist observations submitted to Never Home Alone (B).
The most commonly observed organisms identified to species are Long-Bodied Cellar Spiders, Asian Lady Beetles, Bathroom Moth Flies, Brown Marmorated Stink Bugs, and House Centipedes. Perhaps it should come as no surprise that many of these species have house-related words in their English common names! If you speak another language, let us know in the comments if this holds true for common names in other languages.
| Species | Number of Observations | Proportion of Total Observations |
|---|---|---|
| Long-Bodied Cellar Spider (Pholcus phalangioides) | 620 | 0.01 |
| Unknown Lepidoptera | 615 | 0.01 |
| Asian Lady Beetle (Harmonia axyridis) | 498 | 0.01 |
| Bathroom Moth Fly (Clogmia albipunctata) | 490 | 0.01 |
| Brown Marmorated Stink Bug (Halyomorpha halys) | 462 | 0.01 |
| Unknown Araneae | 416 | 0.01 |
| House Centipede (Scutigera coleoptrata) | 385 | 0.01 |
| Varied Carpet Beetle (Anthrenus verbasci) | 364 | 0.01 |
| Triangulate Cobweb Spider (Steatoda triangulosa) | 340 | 0.01 |
| Unknown Pholcidae | 311 | 0.01 |
| Indian-Meal Moth (Plodia interpunctella) | 280 | 0.01 |
| American Cockroach (Periplaneta americana) | 234 | 0.01 |
| Long-Tailed Silverfish (Ctenolepisma longicaudatum) | 218 | <0.01 |
| Unknown Theridiidae | 203 | <0.01 |
| False Widow (Steatoda grossa) | 201 | <0.01 |
The most commonly-observed taxa submitted to Never Home Alone as of December, 2023.
Our next goal was to determine how indoor biodiversity is distributed around the globe. One hypothesis we had is that the indoor organisms of different regions are all pretty similar (i.e., a handful of species adapted to live in homes early in the history of civilization and those species followed us as we spread around the world). A different hypothesis is that a region’s indoor biodiversity is shaped by its biogeography. In this case, two regions’ indoor species would become more and more dissimilar as those regions get further apart and more climatically dissimilar.
So far, we have found evidence for both hypotheses. In support of the first hypothesis, certain species appear to be globally common in homes. For example, the Bathroom Moth Fly is among the ten most common indoor species reported in North America, South America, Southern Asia, Northern Eurasia, Central America, Northern Africa and the Middle East, and China and Japan. In support of the second hypothesis, other species are locally common but with restricted ranges. For example, the Golden-Brown Jumping Spider is the most common indoor organism in New Zealand, but has not been reported from any other country.
To better elucidate the degree to which different regions host different types of indoor life, we next calculated what countries were most similar in terms of the diversity and abundance of their indoor organisms (for those interested: we did this using Chao’s “adjusted-Sørensen index” and Ward’s Hierarchical Clustering algorithm). We found that the countries sorted into two primary clusters, with the first cluster composed almost entirely of countries from Southeast Asia and the other cluster composed of all other countries. We also saw some weaker clustering among observations from Western Europe, the Mediterranean, and the Americas, depending on how hard you squint.
Similarity of countries in terms of the diversity and abundance of their indoor species.
Given the high concentration of observations from the United States, we repeated this analysis with observations from different US states. Again we found two primary clusters, with the first cluster composed mostly of western states and the second cluster composed mostly of eastern states.
Similarity of US states in terms of the diversity and abundance of their indoor species.
As iNaturalist users submit more data and classify more observations to research-grade, we’ll be able to conduct more complicated analyses, such as calculating the rarity of different indoor species around the globe, which will help us construct a more precise inventory of the distribution of indoor life.
Thanks for reading our update! Let us know if you have any questions or suggestions in the comments. As I mentioned above, we are definitely still collecting data for this project. As we get more data, and as more observations are identified to research-grade, we will be able to learn even more about the life of the great indoors. We are truly “never home alone,” in which case we might as well get more acquainted with our six and eight-legged tenants. There is a whole ecosystem of creatures chasing prey, building homes, and raising young under our laundry baskets and sofas, and many of them are poorly understood or wholly unknown to science. In an era with no more blank spots on the map, you can still be an explorer of these wild landscape without even leaving the living room. And you just might find something amazing.
Please join us! Special shoutout to our top contributors, some of whom have contributed the majority of observations from an entire country.
While we’re still waiting on a Planet Earth series documenting the drama taking place in our cellars and closets, I made a short promotional video for the project while in graduate school. You can check it out here, or below.
Some tips for contributing:
- Observations must be of organisms seen inside homes, offices, warehouses and similar spaces. Observations in gardens, on streets, or in other outdoor spaces, even if those spaces are used heavily by humans, should not be submitted to the project.
- Most organisms you’ll see indoors are quite small. To take high quality photos of these organisms on your smartphone camera, consider purchasing a clip-on macro lens (these are recommended by the New York Times Wirecutter column).
- We are not interested in captive/cultivated observations (potted plants, pets, etc.).