Environmental DNA surveys for rare species often result in low, inconsistent values that are difficult to interpret. Communications about these results have sometimes led to frustration and misinterpretation, which has contributed to skepticism about the utility of eDNA data.
Samples with a small amount of target species eDNA may indicate the presence of a rare species, or the target species eDNA may be from allochthonous sources, such as predator feces, or slight contamination. Samples fitting this description may be described by the proportion of replicates for a sample that test positive (e.g., 1 out of 3 replicates) or by the average amount of eDNA quantified in each sample across all replicates. These are sometimes called ‘slight positives,’ which can create a lot of confusion.
It is very important for everyone involved in interpreting eDNA data to understand that a low-level positive does not necessarily indicate that the species is present, and that we can use the consistency of results across replicates, space, and time, as well as ecological knowledge and field surveys to interpret the level of evidence for species presence. Developing a communication plan before eDNA data are generated, or at least having a conversation about possible outcomes, can help prevent confusion.
-Caren Goldberg, Washington State University