In the genetic age, ecologists’ jobs are made much easier by two things. One is that every organism carries its own chemical identity card, in the form of its genome. The second is that they drop these ID cards everywhere they go. Urine, bits of fur stuck to a hedge, even shed skin cells: all deposit DNA into the environment. Cheap gene sequencing allows scientists to harvest this “environmental DNA" (eDNA) from soil, sand, water and the like, and use it to keep track of which species are living where.