Using patterns in prey DNA digestion rates to quantify predator diets

Dietary metabarcoding – the process of taxonomic identification of food
species from DNA in consumer guts or feces – has been rapidly adopted by
ecologists to gain insights into biocontrol, invasive species, and the
structure of food webs. However, an outstanding issue with metabarcoding
is the semi-quantitative nature of the data it provides: because
metabarcoding is likely to produce false positives for some prey more
often than for other prey, we cannot infer relative frequencies of prey in
the diet. To correct for this, we can adjust detected prey frequencies
using DNA detectability half-lives unique to each predator-prey
combination. Because the feeding experiments required to deduce these
half-lives are time- and resource-intensive, our ability to weight the
frequency of observations using their detectability has thus far been
limited to systems with just a few prey. Here, we present a meta-analysis
of 24 spider prey DNA half-lives and show that these half-lives are
predictable given predator and prey mass, predator family, temperature,
and DNA amplicon length. We further provide a new technique for weighting
observations with half-lives, which allows not just for the ranking of
prey in the diet, but reveals the proportion of the diet each prey
comprises. Lastly, we apply this method to published dietary metabarcoding
data to calculate half-lives and proportion of the predator’s diet for 35
prey families, demonstrating that this technique can generate improved
understanding of diets in real, diverse systems.