Flower constancy of the Cape honey bee pollinator of two co-flowering Erica species from the Cape Floristic Region (South Africa)

Abstract

Erica is the largest genus of the Cape Floristic Region (CFR) of South Africa, and multiple species often coexist. Species that co-flower and share a pollinator may suffer from reproductive interference through heterospecific pollen deposition, especially when such species are closely related. However, little is known about the mechanisms that enable coexistence of co-flowering Erica species that share pollinators. Here we focus on a site where two co-flowering Erica species which are both predominantly pollinated by Cape honey bees grow intermingled, to ask whether this is associated with flower constancy and/or differences in pollen deposition sites. Despite minor differences in sites of pollen deposition on honey bees, most pollen of both Erica species was found on the head and ventral thorax regions. Subsequent visits of honey bees were almost always to the same Erica species from which they departed, and were significantly more constant than expected from random foraging in two consecutive years. Based on differences in pollen types between the two species, we established that pollen loads carried by honey bees were between 67% and 99% species-specific, whereas pollen loads on stigmas were on average over 93% pure. We conclude that despite the superficial similarities of small-flowered Erica species, honeybees can use species-specific cues for constant foraging, which is associated with almost complete absence of heterospecific pollen transfer. Future analyses of pollen carryover rates could help to quantify the benefit of pollinator behaviour for plant fecundity, and inclusion of more study sites sampled throughout the Erica flowering season could shed further light on the importance of flower constancy for facilitating coexistence of Erica species in the CFR.