Early morning sunlight and strong cooling winds hit the roof of a 52-story Chicago high-rise, where three wooden structures that, in another setting, might look like unfinished filing cabinets sit in a row. They are tucked up against an overhang that buffers them from lake breezes in the winter. But inside of these structures, you won’t find files. Instead, they hold tens of thousands of honey bees, whose honey serves as a perk for the building’s tenants. The bees drink from the rooftop garden water lines, and the golden-yellow flowers of sedum plants are a convenient source of pollen and nectar. Flowers in parkways and median planters, weeds in abandoned lots, and plants in other rooftop gardens also attract the foraging bees. “The city is their garden,” says Sarah Long, the lead beekeeper.

Long works for Best Bees Company, which provides beekeeping services to clients across the country who want to start honey bee programs. They also collect data on their hives and the honey the bees produce, with the aim of contributing to the overall movement to “save the bees.” Each hive they maintain represents a data point that supports research on improving pollinator health. Long believes in the benefits of rooftop programs. “Studies have shown that urban settings have more plant diversity than rural settings,” Long says. “Just like humans, more diverse diets are thought to provide better nutrition and be better for bee health.”

While it may seem strange that these insects live more than 50 stories up in the middle of the Windy City, Corky Schnadt, president of the Illinois State Beekeepers Association, notes that a Chicago rooftop is a surprisingly good habitat for beekeeping. “Honey bees travel up to three-and-a-half miles to find forage, so an elevation of 695 feet doesn’t add too much to the distance,” says Schnadt. “Chicago, at least historically, uses less pesticides, and there is a lot more greenery than you would imagine in the city’s parkways.”

In addition to some famous hives like the ones atop the White House and the Colorado Convention Center, more than 2.98 million honey bee colonies are registered across the US, according to the US Department of Agriculture. In 2019, 4,922 beekeepers registered more than 6,000 apiaries with 34,255 active honey bee colonies in Illinois, according to the state’s own agriculture department. State-specific statistics probably reflect low-end estimates, as hive registration is not required in many states, and backyard or urban beekeepers with fewer than five hives are not counted nationally.

But the growing interest in hobbyist beekeeping has some ecologists worried. The European honey bee, as its name might suggest, is not native to North America and was brought over in the 17th century for agricultural and economic purposes. While honey bees are a managed pollinator species, about 4,000 species of native bees also call the US home, including its urban areas. One group of researchers observed dozens of wild species across several Chicago neighborhoods, while another nature organization recorded more than 200 species in New York City. Now, some ecologists are concerned that with so much human help, the newcomers might outcompete their wild cousins, causing an ecological ripple effect that would threaten both the bees and the plants that depend on them.

In August, Monika Egerer, a postdoctoral fellow at the Technical University of Berlin, published a paper in the journal Trends in Ecology & Evolution about the “Gordian Knot” of urban beekeeping, borrowing a metaphor from Greek mythology representing a problem with little or no solution. “Urban beekeeping is a tricky, complex problem tied up like a knot, and many people want a simple way to untie it,” says Egerer, who will be heading to the Department of Ecology and Ecosystem Management at the Technical University of Munich this October. “But there’s no one way to solve this issue, and it really is city context dependent.”