Abstract
A vast majority of angiosperms are pollinated by animals, and a decline in the number and diversity of insects often affects plant reproduction through pollen limitation. This phenomenon may be particularly severe in rare plant species, whose populations are shrinking. Here, we examined the variability in factors shaping reproductive success and pollen limitation in red-listed Polemonium caeruleum L. During a 5-year study in several populations of P. caeruleum (7–15, depending on year), we assessed the degree of pollen limitation based on differences in seed set between open-pollinated (control) and hand-pollinated flowers. We analysed the effects of flower visitors, population size, and meteorological data on plant reproductive success and pollen limitation. Our study showed that pollen limitation rarely affected P. caeruleum populations, and was present mainly in small populations. Pollen limitation index was negatively affected by the size of population, visitation frequency of all insects, and when considering the visitation frequency of individual groups, also by honeybee visits. Seed production in control treatment was positively influenced by the population size, average monthly precipitation in June and visits of hoverflies, while visits of honeybees, average monthly temperature in September, and average monthly precipitation in August influenced seed production negatively. As generalist plant P. caeruleum can be pollinated by diverse insect groups, however, in small populations their main visitors, the honeybees and bumblebees, may be less attracted, eventually leading to the disappearance of these populations. In pollination of P. caeruleum managed honeybees may play a dual role: while they are the most frequent and efficient flower visitors, their presence decreases seed set in open-pollinated flowers, which is most probably related to efficient pollen collection by these insects.
Highlights
Mutualistic interactions between plants and their pollinators play pivotal roles in the functioning of terrestrial ecosystems and global food production
The present study demonstrated that pollen limitation, seed production, insect visitation frequency, and insect assemblage composition exhibit spatiotemporal variations among P. caeruleum populations
Pollen limitation was rarely detected in P. caeruleum populations in the Polish range, and when present, it was enhanced by the small size of the population and the low frequency of pollinator visitation
Summary
Mutualistic interactions between plants and their pollinators play pivotal roles in the functioning of terrestrial ecosystems and global food production. Recent decline in the number and diversity of insects has been severe (Hallmann et al, 2017; Lister and Garcia, 2018; Wagner et al, 2021; Warren et al, 2021), and it has negatively affected many other groups of organisms, destabilising ecosystem functioning (Thomas et al, 2019; van der Sluijs, 2020). Another causes of disturbances in ecosystem functioning and interactions among its components are habitat loss and climate change. High temperature and water deficit may lead to plant wilting and low fertility, such as through seed, fruit, or anther abortion and effects on pollinator interactions (Young et al, 2004; Liu et al, 2012; Gallagher and Campbell, 2017; Borghi et al, 2019)
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