Floral nectar provides crucial energy sources for diverse animals, many flowering species produce nectar to reward flower visitors, which provide pollination services for flowering plants. In addition to sugars and amino acids, the nectar of many plant species often contains secondary compounds such as alkaloids and phenolics, which are associated with plant defense against herbivores. The composition of nectar may influence an entire assemblage of local visitors and animal-pollinated plants, but the basis for nectar resource selection is complex. Species co-existence and community assembly mechanisms depend on links between consumers and resources. A previous feeding trial showed that a sugar solution can motivate honey bees to use nectar containing plant defense chemicals (toxic nectar). Toxic-nectar plants are thus expected to be tightly linked with other non-toxic or less toxic floral resources for sharing pollinators, while general impacts of toxic nectar on plant communities have been little investigated. Most previous studies of the ecological and evolutionary significance of nectar secondary compounds have focused on the benefits to the plants that produce these compounds. For example, feeding experiments have shown that honey bees prefer to collect nectar containing secondary compounds. However, few studies have attempted to evaluate the effects of nectar secondary compounds on other co-flowering species in a natural community, so their impact on other synchronously flowering species is still unclear. We conducted a field trial with the Chinese honeybee species Apis cerana in a permanent plot at Xishuangbanna from March to May 2009. We compared the flowering species visited, at the entire colony and individual foraging bee levels, between colonies fed 30% sucrose syrup (hereafter ‘S-fed’) and the same syrup containing 0.01% quercetin (hereafter ‘P-fed’). The nectar sugar concentration was also compared between the two treatments. The main results were as follows:
(1)At the colony level, although the number of flowering species visited by an entire colony did not differ between the two treatments, the flowering species visited was significantly different between the treatments. Of the 96 species visited, 43 were visited by P-fed bees, whereas 17 were visited by S-fed bees. Thirty-six species (about 30% of the total number) were visited by both P- and S-fed bees.
(2)At the individual bee level, the number of bees from the P-fed colonies that collected pollen from more than two flowering species was significantly higher than that for bees from S-fed colonies. Thus the number of flowering species selected by individual bees also differed between the treatments. P-fed bees tended to visit more than one species in a single foraging trip, whereas S-fed bees usually foraged on one species only.
(3)Compared to S-fed bees, P-fed bees tended to visit the species with nectar containing high concentrations of sugar.
Collectively, these results indicate nectar phenolic compounds may reduce pollination efficiency through an indirect impact on bees' specialization on a given flowering species in a single foraging trip, and phenolic alter bee foraging; bee niche breadth increases dramatically, pollination of rare species may increase, interspecific pollen transfer at flowers may also be affected.