Making environmental DNA count

The arc of reception for a new technology or method--like the reception of new information itself--can pass through predictable stages, with audiences' responses evolving from 'I don't believe it', through 'well, maybe' to 'yes, everyone knows that' to, finally, 'old news'. The idea that one can sample a volume of water, sequence DNA out of it, and report what species are living nearby has experienced roughly this series of responses among biologists, beginning with the microbial biologists who developed genetic techniques to reveal the unseen microbiome. 'Macrobial' biologists and ecologists--those accustomed to dealing with species they can see and count--have been slower to adopt such molecular survey techniques, in part because of the uncertain relationship between the number of recovered DNA sequences and the abundance of whole organisms in the sampled environment. In this issue of Molecular Ecology Resources, Evans et al. (2015) quantify this relationship for a suite of nine vertebrate species consisting of eight fish and one amphibian. Having detected all of the species present with a molecular toolbox of six primer sets, they consistently find DNA abundances are associated with species' biomasses. The strength and slope of this association vary for each species and each primer set--further evidence that there is no universal parameter linking recovered DNA to species abundance--but Evans and colleagues take a significant step towards being able to answer the next question audiences tend to ask: 'Yes, but how many are there?'