Functional flower traits
The outstanding diversity of morphological and coloration traits in animal-pollinated flowers is one of the most recognized examples for niche differentiation in animal communities in ecology. Hence, functional diversity of flowers is considered to positively affect pollinator diversity and vice versa (e.g. Fontaine et al. 2006, Fornoff et al. 2017). However, we found no evidence that neither functional flower diversity nor species richness of flowering plants positively affected both pollinator guilds. This is indeed surprising, since we investigated a strong gradient from one to 22 flowering plants in our study sites. Similarly, to our study, Fornoff et al. (2017) neither found strong positive effects of functional flower diversity on pollinator species richness in experimental plant communities of the size 1m². These plant communities were set even in the same landscape context and therefore local effects should appear more clearly, compared to our study. Possibly, the flower diversity at a rather small scale (in our case study 10m diameter circle) may play a minor role for pollinator diversity, as they are highly mobile and may search nectar resources over large distances of several hundred meters (see above). A valid estimation of functional flower diversity at such scales is hard to achieve, but may be indicated by the positive effect of landscape heterogeneity on both pollinator guilds in our study (see above). Alternatively, diversity of flower traits is negligible in our system, as species are less specialised on particular flower traits than expected. Only six out of the 80 caught wild bee species are listed as oligolectic in our data set (Westrich 2019). While a previous study within our study area observed a much higher share of oligolectic species (Saure and Berger 2006: 28 out of 161), our pollinator community seems to become less specialized as a consequence of possible fragmentation and land-use intensification (Jauker et al. 2019). Hereby, the observed negative effects of functional flower diversity on pollinator abundance may indicate that particular flower types are not accessible for the present pollinator species, as specialized species disappeared. Under such circumstances, the quantity of few plant species with high floral rewards rather than the diversity of flowers may maintain pollinator diversity (Bergamo et al. 2020), as indicated by the positive effect of CWMflower height on pollinator abundance, since large plants produce more flowers and are more attractive for pollinators (Donnelly et al. 1998).