Nectar robbing by the invasive bumblebee Bombus terrestris (Apidae) changes the behavior of native flower visitors of Fuchsia magellanica Lam. (Onagraceae) but not seed set

TL;DR

Invasive bumblebees steal nectar from Fuchsia magellanica, altering native pollinator behavior but not affecting the plant's seed production.

Contribution

The study reveals that nectar robbing by invasive bumblebees changes pollinator behavior without reducing plant reproduction.

Findings

Nectar robbing reduced nectar production rates by 4 times and standing crop by 2.9 times.

Increased nectar robbing led to fewer pollinating visits by hummingbirds and more robbing visits by invasive bumblebees.

Despite changes in pollinator behavior, seed set was not significantly affected.

Abstract

Mutualisms between plants and pollinators may be threatened by nectar robbers, as these animals typically consume nectar without providing the essential pollination service. In the temperate forests of Chile, the shrub Fuchsia magellanica is primarily pollinated by the hummingbird Sephanoides sephaniodes and the native bumblebee Bombus dahlbomii. However, some populations are also affected by nectar robbing from the invasive bumblebee Bombus terrestris. In a study evaluating 24 populations of F. magellanica, we assessed the effects of nectar robbing on nectar production, the foraging behavior of key pollinators, and the reproductive success of the plant. We measured the nectar production rate and standing crop in flowers that were either protected from visitors or exposed to them. We also observed flower visitation, the foraging behavior of pollinators, and seed sets in experimentally manipulated flowers. Results indicated that nectar robbing reduced nectar production rates by a factor of 4, and the standing crop of nectar by 2.9 times. With an increased percentage of flowers pierced by B. terrestris, the hummingbird S. sephaniodes appeared to reduce its pollinating visits, while B. terrestris increased its robbing visits. Despite these changes in pollinator behavior, there was no significant effect on the seed set. Ultimately, our findings suggest that for F. magellanica, B. terrestris does not pose a significant threat to reproduction, despite its disruptive impact on pollinator behavior.

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