Invasive Bumblebees Disrupt Pollination of Senna Flowers

Jim Crocker
12th June, 2024

Invasive Bumblebees Disrupt Pollination of Senna Flowers

Image Source: Natural Science News, 2024

Key Findings

  • The study, conducted by the Jardim Botânico de Belo Horizonte, examined the role of heteranthery in the pollination of Senna arnottiana flowers
  • Native bees preferred short anthers, while the invasive bumblebee Bombus terrestris visited both short and long anthers without distinction
  • B. terrestris contacted the stigma less frequently than native bees, potentially disrupting the plant's reproductive success
Heteranthery, the presence of different types of anthers within a single flower, is a fascinating floral adaptation that helps plants balance the need for pollinators to collect pollen as a food resource while ensuring enough pollen is available for successful pollination. This study, conducted by the Jardim Botânico de Belo Horizonte, investigates the role of heteranthery in the pollination of Senna arnottiana flowers and examines how the behavior of different bee species, particularly the invasive bumblebee Bombus terrestris, affects this process[1]. In this study, researchers measured various characteristics in three populations of S. arnottiana, including the size of three sets of anthers and the style, stigma-anther separation, pollen quantity, and fruit set. They also observed the body size, behavior, and pollination effectiveness of all floral visitors. The study found that different bee species visited S. arnottiana flowers, and their foraging behavior varied significantly. Large-bodied native bees, such as Centris cineraria, Caupolicana sp., and Cadeguala occidentalis, showed a preference for visiting short anthers. In contrast, the exotic bumblebee B. terrestris foraged from both short and long anthers without distinction. Additionally, B. terrestris contacted the stigma at a lower rate compared to large-bodied native bees. Instead of concentrating its pollen-gathering efforts on the feeding anthers, as predicted by the "division of labor" hypothesis, B. terrestris visited both types of anthers similarly. This behavior may disrupt the adaptive significance of heteranthery by mixing the roles of pollination and feeding anthers of S. arnottiana. The findings of this study highlight the potential disruption caused by exotic pollinators like B. terrestris. This invasive species does not adhere to the division of labor that native bees follow, which can ultimately affect the reproductive success of S. arnottiana. The need to consider the impact of exotic pollinators in conservation efforts is evident. This study builds on earlier research by naturalists Fritz and Hermann Müller, who hypothesized that heteranthery often leads to a division of labor into 'feeding' and 'pollinating' stamens[2]. Their hypothesis suggested that the latter stamens are often as long as the pistil to promote successful pollination on the bees' back. However, in many buzz-pollinated species of Senna, including S. arnottiana, the so-called pollinating stamens are short and not level with the stigma, raising questions about how pollen is shed on the bees' back. The mechanism of 'ricochet pollination' explored in earlier studies[2] suggests that pollen from short stamens ricochets off deflector petals to reach the bee's back, promoting a division of labor by involving additional floral organs. Buzz-pollination, a process where bees use vibrations to extract pollen from anthers, is a widespread method of fertilization for thousands of species[3]. This method evolves due to the competition between plants and pollen-consuming floral visitors to control the rate of pollen removal from flowers. The behavior of B. terrestris, which does not discriminate between anther types, may interfere with this delicate balance. Heteranthery is commonly explained as a functional adaptation to alleviate the "pollen dilemma," where pollen serves both as a pollinator food resource and a male reproductive agent[4]. The division of labor hypothesis suggests that one stamen type functions in rewarding pollen-collecting pollinators, while the other focuses on reproduction, minimizing pollen loss. However, this study shows that B. terrestris does not follow this pattern, potentially disrupting the adaptive significance of heteranthery. In conclusion, the study conducted by the Jardim Botânico de Belo Horizonte highlights the potential disruption of the heteranthery relationship by exotic pollinators like B. terrestris. This invasive species does not adhere to the division of labor that native bees follow, which can ultimately affect the reproductive success of S. arnottiana. The need to consider the impact of exotic pollinators in conservation efforts is evident.

WildlifeEcologyPlant Science

References

Main Study

1) The invasive bumblebee Bombus terrestris (Linnaeus, 1758) disrupts the adaptive function of heteranthery by indiscriminately visiting the pollinating and feeding anthers of Senna arnottiana flowers.

Published 11th June, 2024

https://doi.org/10.1111/plb.13673

Related Studies

2) Ricochet pollination in Senna (Fabaceae) - petals deflect pollen jets and promote division of labour among flower structures.

https://doi.org/10.1111/plb.12607

3) What's the 'buzz' about? The ecology and evolutionary significance of buzz-pollination.

https://doi.org/10.1016/j.pbi.2013.05.002

4) Stamen dimorphism in bird-pollinated flowers: Investigating alternative hypotheses on the evolution of heteranthery.

https://doi.org/10.1111/evo.14260


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