Abstract
Nectar robbers, which affect plant fitness (directly or indirectly) in different degrees and in different ways, potentially constitute a significant part of mutualistic relationships. While the negative effects of nectar robbing on plant reproductive success have been widely reported, the positive effects remain unknown. The target of our study was to evaluate the effects of nectar robbers on the reproductive success of Symphytum officinale (Boraginaceae). We observed the behavior, species and times of visitors in the field, and we assessed the effect of nectar robbers on corolla abscission rate and time. To test the fitness of corolla abscission, we detected the changes in stigma receptivity, pollen viability, pollen amount and appendage opening size along with the time of flower blossom. The flowering dynamics and floral structure were observed to reveal the mechanism of self-pollination. Finally, pollen deposition seed set rate and fruit set rate were determined to estimate the effect of nectar robbers on reproduction success. We observed 14 species of visitors and 2539 visits in 50 h of observation; 91.7% of them were nectar robbers. The pressure and nectar removal of nectar robbers significantly promoted corolla abscission during a period when pollen grains are viable and the stigma is receptive. In addition, corolla abscission significantly increased the pollen deposition and seed setting rate. Our results demonstrate that nectar robbing contributes to enhancing seed production and positively and indirectly impacts the reproductive success of S. officinale. This mechanism involved the movement of anthers and indirect participation by nectar robbers, which was rarely investigated. Considering the multiple consequences of nectar robbing, understanding the impact of nectar robbers on plant reproduction is essential to comprehend the evolutionary importance of relationships between plants and their visitors.
Highlights
Mutualistic interaction between plants and pollinators is an important part of ecosystem function
We investigated the effect of nectar robbing on plant reproductive fitness in Symphytum officinale L
We observed a total of 2539 individual floral visits and 14 insect species in 50 h of observation on S. officinale flowers (Table 1)
Summary
Mutualistic interaction between plants and pollinators is an important part of ecosystem function. The mutualistic interaction between plants and pollinators evolved from adaptive speciation interaction by animals and plants [1,2,3]. Sometimes plants and pollinators may experience conflict because they have different interests. It is in the best interest of the pollinator to maximize the collection of the floral reward for food [7], relative to handling costs, while it is in the best interest of the plant to maximize pollen receipt and transfer [8]. Nectar robbers steal nectar without contributing to pollination [2]
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