Species assemblages and their interactions vary through space, generating diversity patterns at different spatial scales. Here, we study the local-scale spatial variation of a cavity-nesting bee and wasp community (hosts), their nest associates (parasitoids), and the resulting antagonistic network over a continuous and homogeneous habitat. To obtain bee/wasp nests we placed trap-nests at 25 sites over a 32 km² area. We obtained 1541 nests (4954 cells) belonging to 40 host species and containing 27 parasitoid species. The most abundant host species tended to have higher parasitism rate. Community composition dissimilarity was relatively high for both hosts and parasitoids, and the main component of this variability was species turnover, with a very minor contribution of ordered species loss (nestedness). That is, local species richness tended to be similar across the study area and community composition tended to differ between sites. Interestingly, the spatial matching between host and parasitoid composition was low. Host b-diversity was weakly (positively) but significantly related to geographic distance. On the other hand, parasitoid and host-parasitoid interaction b-diversities were not significantly related to geographic distance. Interaction β-diversity was even higher than host and parasitoid b-diversity, and mostly due to species turnover. Interaction rewiring between plots and between local webs and the regional metaweb was very low. In sum, species composition was rather idiosyncratic to each site causing a relevant mismatch between hosts and parasitoid composition. However, pairs of host and parasitoid species tended to interact similarly wherever they co-occurred. Our results additionally show that interaction β-diversity is better explained by parasitoid than by host β-diversity. We discuss the importance of identifying the sources of variation to understand the drivers of the observed heterogeneity.

Data were collected in 25 plots distributed across the Garraf Natural Park (Barcelona, NE Spain). In each plot we placed a trap-nesting station facing SE. Each station contained seven drilled wood blocks with inserted paper tubes. Each wood block accommodated 25 tubes of one of the following diameters (2, 3, 4, 5, 6, 7 and 8 mm), resulting in 175 nesting cavities per station. Paper tube length was 5 cm for the 2 and 3 mm diameters and 15 cm for the rest. Nesting stations were checked every 2 weeks and tubes containing occupied nests were pulled out, taken to the laboratory and replaced with empty ones, so that there were nesting cavities of all diameters available at all times. Nesting stations remained in the field from February to October, in 2011 and 2013. Data of the two years are pooled together. Nests were kept in a temperature chamber simulating monthly ambient temperatures of the study site. Upon one year from collection, nests were dissected. The nests of solitary bees and wasps contain a variable number of cells, some of which may be parasitized. We recorded the contents of each cell (identity of the host or the parasitoid).