Outside her cabin in northern Belize, Elizabeth Clare, a biodiversity scientist at York University, walks along a path. Everywhere she looks, it’s teeming with life.

“There’s hibiscus flowers over there,” she says. “One of my favorite things to find in this part of the world is leaf-cutter ants.” She points out the kingfisher birds that can be spotted flying overhead and the iguanas populating the trees.

“You can look at a couple square feet of ground here and never possibly describe all the things that are in it,” Clare says. “This is the problem of biodiversity. How do you describe this? I mean, I don’t know. No one knows.”

It’s a question that’s become ever more urgent to answer given how many species around the world are in danger of winking out due to habitat loss, climate change, and other disruptions.

“We don’t know what lives on planet Earth,” says Clare. “Most things in the world have never been recognized by science. So we wanted to see whether we could measure biodiversity on the scale of an entire country — actually monitor it over and over again to tell us how things are changing.”

In a preprint article published on bioRxiv that hasn’t yet been peer reviewed, she and her colleagues say they’ve done just that — by pulling DNA out of the air.

The parts of ourselves we can’t help but leave behind

To help explain the approach, Nina Garrett, a biology PhD student at York University, approaches a colossal guanacaste tree erupting out of the ground not too far from that path Clare was pacing. A good half of it is in the grip of a strangler fig tree. But at the base of the trunk is a hole that Garrett can just peek inside.

“Now you can hear them chittering,” says Garrett, referring to a group of common vampire bats that she knows reside in this tree. In fact, she spots a baby on the inner back wall of the trunk. “It’s not uncommon to see a pup without a mom inside the roost,” she says.

But Garrett is curious if there might also be white-winged vampire bats inside — a different species. “It’s never been caught here physically,” she says, “but it’s always been suspected to be in the area just based on habitat type and range maps.”

The problem is that bats are elusive and skittish. Most techniques would likely spook any animals roosting here. So how can Garrett tell what’s inside this tree?

It turns out that even if the bats themselves are out of reach, they can’t conceal themselves completely. That’s because they cast small fragments of their DNA into the environment. Clare says all creatures big and small are forever “losing little bits of themselves. It’s what we do by being alive.” She says to think of it like a footprint that all of life leaves behind.

“They are shedding hair, could be little skin cells, it could be saliva,” says Garrett. “Anything that they are putting out into the environment — even when they breathe out.”

Garrett wants to collect this environmental DNA, or eDNA, from inside the tree to deduce who’s here. She lays a piece of filter paper atop a small fan, places the apparatus inside the tree, and flips the switch.

The fan draws the internal air across the filter, trapping free-floating DNA, which Garrett can analyze later for the presence of not just white-winged vampire bats, but any mammal. If she wanted, she could survey the genetic material for different species of plants and fungi too. This is the power of the technique: It can catalog the breadth of life crammed into a place like this little tree hollow.