Metabarcoding of trap nests reveals differential impact of urbanization on cavity-nesting bee and wasp communities

Urbanization is affecting arthropod communities worldwide, for example by changing the availability of food resources. However, the strength and direction of a community’s response are species-specific and depend on the species’ trophic level. Here, we investigated interacting species at different trophic levels in nests of cavity-nesting bees and wasps along two urbanization gradients in four German cities using trap nests. We analyzed bee and wasp diversity and their trophic interaction partners by metabarcoding the DNA of bee pollen and preyed arthropods found in wasp nests. We found that the pollen richness increased with increasing distance from city centers and at sites characterized by a high percentage of impervious and developed surfaces, while the richness of pollinators was unaffected by urbanization. In contrast, species richness of wasps, but not their arthropod prey, was highest at sites with low levels of urbanization. However, the community structure of wasp prey changed..., Arthropod and pollen samples were collected from trap nests at 18 locations situated in the cities of Berlin, Cologne, Frankfurt, and Hamburg and were selected to represent an urban-rural gradient. Traps consisted of 20 cm long bundles of reed and bamboo in PVC tubes (diameter 11 cm; reed cavity diameter ranged 1-16mm). 
Arthropod remains (faeces, undigested prey tissue) and pollen of a maximum of three brood cells was extracted and stored individually in 2 mL microcentrifuge tubes at -18°C until further processing. Living larva, pupa, and imago were reared (8 weeks at 6°C, subsequent weeks at 20°C until development was completed) allowing later morphological species identification.
Arthropod and pollen samples were subjected to DNA-extraction followed by two-stage PCR protocol to build the DNA libraries for next-generation sequencing. Primer pair mlCOIintF/jgHCO2198 (Leray et al., 2013, Geller, Meyer, Parker, & Hawk, 2013) were used for amplification of athropod prey, primer pair I...,