Variation in composition of two bumble bee species across communities affects nectar robbing but maintains pollinator visitation rate to an alpine plant, Salvia przewalskii

Abstract

1. In many flowering plants, bumble bees may forage as both pollinators and nectar robbers. This mixed foraging behaviour may be influenced by community context and consequently, potentially affect pollination of the focal plant.2. Salvia przewalskii is both pollinated and robbed exclusively by bumble bees. In the present study area, it was legitimately visited by two species of bumble bees with different tongue length, Bombus friseanus and Bombus religiosus, but it was only robbed by Bombus friseanus, the shorter‐tongued bumble bee. The intensity of nectar robbing and pollinator visitation rate to the plant were investigated across 26 communities in the Hengduan Mountains in East Himalaya during a 2‐year project. For each of these communities, the floral diversity, and the population size and floral resource of S. przewalskii were quantified. The abundances of the two bumble bee species were also recorded.3. Both nectar robbing and pollinator visitation rate were influenced by floral diversity. However, pollinator visitation rate was not affected by nectar robbing. The results revealed that relative abundance of the two bumble bee species significantly influenced the incidence of nectar robbing but not the pollinator visitation rate. Increased abundance of B. religiosus, the legitimate visitors, exacerbated nectar robbing, possibly by causing B. friseanus to shift to robbing; however, pollinator visitation remained at a relatively high level.4. The results may help to explain the persistence of both nectar robbing and pollination, and suggest that, in comparison to pollination, nectar robbing is a more unstable event in a community.