Temporal activity patterns in a flower visitor community of Dictamnus albus in relation to some biotic and abiotic factors

Several biotic and abiotic factors affect the seasonal and daily activities of flower-visiting insects, which, ultimately, influence the composition and abundance of a pollinator community and the extent of their pollination service. The aim of this four-year study was to assess the effect of some abiotic and biotic factors on the abundance and activity pattern of flower-visiting insects throughout the blooming of a locally rare plant species, Dictamnus albus L. (Rutaceae). We found a large inter-annual variation in the abundance and activity of flower visitors, with up to a six-fold variation in the total number of individual insects observed. Moreover, a similar fluctuation among years was also highlighted by biodiversity indices and by the changes in the relative frequency of each flower-visiting taxon. Annual, seasonal and daily variations were explained by a certain level of “response diversity” between flower-visiting taxa in relation to different environmental conditions during D. albus blooming. The foraging activity was related to ambient temperature in the tribe of Halictini and to relative humidity in four flower-visiting taxa (positive relation in the tribe Anthophorini; negative relation in the taxa Megachilini, Halictini and Syrphidae). The climate conditions during the winter might have also affected the phenology and the abundance of flower-visiting taxa with a large number of insects observed in warmer years. Among the biotic factors, flower availability did not affect the abundance of insects observed in the patch. Moreover, we did not find any significant negative associations (e.g. possible competition) in the co-occurrence of different flowervisiting taxa, instead we found six significant positive associations probably related to similar preferences towards specific environmental conditions. In conclusion, our results show that the flower visitor community of D. albus is quite variable among years, seasons and daily hours, but the relative high visitor diversity and the different patterns of activity related to different environmental responses might guarantee a stable pollination service of this plant species in years with extreme environmental conditions.