IMPACT OF NECTAR ROBBING ON ESTIMATES OF POLLEN FLOW: CONCEPTUAL PREDICTIONS AND EMPIRICAL OUTCOMES

Abstract

Pollen movement within and among plants affects levels of inbreeding and plant fitness as well as the spatial scale of genetic differentiation. Pollen movement has primarily been studied as a function of the direct relationships between plants and pollinators; however, nonpollinating floral visitors, such as some nectar robbers, may have indirect effects on pollen dispersal by altering plant–pollinator interactions. Theory predicts that nectar robbing should have indirect positive effects on plants by increasing pollen dispersal distance and reducing self-pollen transfer (geitonogamy) and inbreeding through changes in pollinator behavior associated with reduced nectar rewards. Here I experimentally tested the indirect effects of the nectar-robbing bumble bee, Bombus occidentalis, on the distance of pollen movement among plants and levels of pollen movement within plants (estimated using powdered fluorescent dyes as pollen analogues) using the self-incompatible, hummingbird-pollinated host, Ipomopsis aggregata. Empirical results did not fully support the theoretical predictions. Heavy levels of nectar (>80% of available flowers robbed) had no effect on the mean and mean-squared distance of dye dispersal within populations. However, hummingbird pollinators were 50% more likely to depart from a population after visiting a plant with low robbing relative to a plant with high robbing, suggesting that pollen dispersal among populations might be reduced in plants with high robbing relative to plants with low robbing. Heavy nectar robbing reduced levels of self-dye transfer by one-half. However, when I manipulated robbing levels and access to self-pollen, the costs associated with increased robbing and reduced pollinator visitation (e.g., reduced seed production) outweighed any advantages associated with reduced geitonogamy. Taken together, these results suggest that the negative indirect effects of robbing on plant reproduction through decreased pollinator visitation and changes in pollinator departure from plants far exceed any positive effects associated with decreased geitonogamy. Given the widespread nature of plant attack by nectar robbers, these indirect effects are likely common phenomena structuring natural plant populations. Corresponding Editor: T.-L. Ashman