Influence of geographical distribution and floral traits on species richness of bees (Hymenoptera: Apoidea) visiting Echium species (Boraginaceae) of the Canary Islands

Echium (Boraginaceae) is one of the largest evolutionary radiations in the Canary Islands and exhibits considerable variation in geographical distribution patterns and floral characters. Flower visitors are mainly bees (Hymenoptera: Apoidea). We investigated factors determining the level of generalization for pollination in 20 endemic and one introduced species of Echium in the archipelago. For each Echium species, we analyzed variation in species richness of bee visitors in relation to geographical distribution of the plants (area and altitude), floral traits ( morphology and color), and sampling effort (observation time and number of study populations). Geographical distribution range was the only good predictor of species richness of bees. In our study, widespread species of Echium were visited by a larger fraction of Echium-visiting bee taxa than were rare species. Thus, plant commonness per se may act as a strong attractant of bees. Alternatively, if widespread species interact with different flower visitor faunas across their distribution range, they can accumulate flower visitor taxa across populations. The influence of floral traits on the level of generalization appeared to be negligible in the Canarian Echium species. However, floral traits cannot be completely dismissed because of the confounding effect of phylogeny. The introduced E. plantagineum was more widespread in the Canary Islands than any of the congeneric endemic species. Further, it had larger flowers and attracted most of the Echium-visiting bee taxa. This species may become a serious competitor for pollinators for the endemic Echium species.