The effect of predator identity on post-feeding prey DNA detection success in soil-dwelling macro-invertebrates

Soils harbour an extraordinary diversity of animals which are functionally connected by their feeding interactions. Determining these feeding links, however, is difficult due to the opaque habitat the animals live in and because many prey remains found in predator gut contents cannot be identified morphologically. DNA-based gut content analysis overcomes these hurdles. This approach opens new possibilities to empirically examine below-ground feeding links by measuring prey DNA detection frequencies in various predators. Comparing prey DNA detection rates between different predator taxa, however, is not straightforward as digestion rates and thus post-feeding prey DNA detection intervals can be affected by predator identity. In the present study we examined the effect of predator identity for a range of soil-dwelling adult and larval beetles as well as lithobiid and geophilid centipedes. These predators were fed with white grubs (scarabaeid beetle larvae) and their post-feeding prey DNA detection intervals were established for small, medium and large prey DNA fragments. In geophilids the 50% detection probability for the medium prey DNA fragment was 98 h post-feeding whereas it ranged between 22 and 34 h in beetles. Among the different beetle species, the magnitude of the differences in detection intervals was strongly affected by the size of the prey DNA fragment targeted: the intervals showed the highest similarity in the medium-sized prey DNA fragment which was most frequently detected. Predator biomass and meal size rarely affected post-feeding prey DNA detection intervals. Our findings suggest that prey DNA detection success is similar within taxonomically related species of soil-dwelling macro-invertebrates such as beetles and centipedes. Between these groups prey detection can be adjusted according to taxon-specific digestion rates for better comparisons. Moreover, the magnitude of the predator identity effect is modulated by assay sensitivity: highly sensitive assays which target small- to medium-sized prey DNA fragments are recommended as these minimize the interspecific differences in post-feeding prey DNA detection intervals.