It’s no secret that Florida has a snake problem. The Burmese python, which can reach up to 200 pounds and stretch to more than 20 feet, first became common in the Everglades in the late 1990s, likely as escaped pets. The snake quickly settled into its new home, breeding and taking down rabbits, bobcats, and other native animals in its path.

Biologists thought the Arthur R. Marshall Loxahatchee National Wildlife Refuge was one place that was safe. But from 2014 through 2016, scientists combed the waters in and around the refuge for environmental DNA (eDNA) — the trail of DNA left behind by an organism in sources such as feces, mucus, gametes, and shed skin or hair. The results suggested that the python’s DNA was, in fact, widespread throughout the refuge.

Caren Goldberg.

But interpreting eDNA results can be tricky. Tiny amounts of cross contamination in the field and lab could result in positive detections where animals aren’t present. “You can get these low signals that are either critically important or not reflecting the truth,” says ecologist Caren Goldberg of Washington State University, whose team has developed eDNA tests to monitor for a wide range of amphibians, including the endangered Sierra Nevada yellow-legged frog. In those cases, the eDNA is there, Goldberg says, but the interpretation of what that means can be wrong. Goldberg, for example, cannot control for moose that carry water in their coats from one pond to another, potentially transferring DNA of fish and other species.

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