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).