To monitor the biodiversity of a freshwater habitat, you could camp out by the water and count the rare wildlife. Or you could just scoop up a cup of water. A new Dutch study has found that the DNA traces in a small sample of a body of water can reveal the species that live in it. The work is in the journal Molecular Ecology. As animals swim through a lake, they leave behind traces of DNA. The more individuals of a particular species, the more DNA of that species will be shed. And be available to be measured. The researchers tested about 100 European lakes and streams, comparing the DNA evidence to traditional fauna observations. And the small-sample technique enabled them to correctly identify the species and the sizes of their populations. Measuring biodiversity is an important part of protecting endangered species. This quick and easy method of snagging a water sample, which can be tested at a lab miles away from the site, could refresh the process of species monitoring.