Pollen transfer in fragmented plant populations: insight from the pollen loads of pollinators and stigmas in a mass‐flowering species

Abstract

Pollinator and/or mate scarcity affects pollen transfer, with important ecological and evolutionary consequences for plant reproduction. However, the way in which the pollen loads transported by pollinators and deposited on stigmas are affected by pollination context has been little studied. We investigated the impacts of plant mate and visiting insect availabilities on pollen transport and receipt in a mass‐flowering and facultative autogamous shrub (Rhododendron ferrugineum). First, we recorded insect visits to R. ferrugineum in plant patches of diverse densities and sizes. Second, we analyzed the pollen loads transported by R. ferrugineum pollinators and deposited on stigmas of emasculated and intact flowers, in the same patches. Overall, pollinators (bumblebees) transported much larger pollen loads than the ones found on stigmas, and the pollen deposited on stigmas included a high proportion of conspecific pollen. However, comparing pollen loads of emasculated and intact flowers indicated that pollinators contributed only half the conspecific pollen present on the stigma. At low plant density, we found the highest visitation rate and the lowest proportion of conspecific pollen transported and deposited by pollinators. By contrast, at higher plant density and lower visitation rate, pollinators deposited larger proportion of conspecific pollen, although still far from sufficient to ensure that all the ovules were fertilized. Finally, self‐pollen completely buffered the detrimental effects on pollination of patch fragmentation and pollinator failure. Our results indicate that pollen loads from pollinators and emasculated flowers should be quantified for an accurate understanding of the relative impacts of pollinator and mate limitation on pollen transfer in facultative autogamous species.

Pollinator specialization increases with a decrease in a mass‐flowering plant in networks inferred from DNA metabarcoding

“…In patches with low R. ferrugineum density and limited alternative resources, pollinators could compete for less available flower resources (Table 1). Delmas et al (2016) observed more frequent visits to R. ferrugineum and smaller quantities and proportions of its pollen in insect loads in low-density patches than in high-density…”

“…In patches with low R. ferrugineum density and limited alternative resources, pollinators could compete for less available flower resources (Table ). Delmas et al () observed more frequent visits to R. ferrugineum and smaller quantities and proportions of its pollen in insect loads in low‐density patches than in high‐density patches, suggesting that pollinators had to share fewer resources among more individuals in low‐density patches. We hypothesized that in response to global resource depletion, individuals would either diversify their diet (H2: higher individual generalization) or specialize in different floral resources (H3: higher individual specialization) with different consequences for individual and species networks.…”

“…Despite these limitations, in a simulation study, Deagle et al (2018) showed that even with methodological biases are included, sequence counts can provide a more reliable description of species diet in many scenarios than link occurrence alone. Moreover, since pollen grains may K E Y W O R D S DNA metabarcoding, food niche breadth, individual pollination network, mass-flowering species, pollinator generalization, pollinator specialization, species pollination networks accumulate on insect bodies during successive visits (Delmas, Fort, Escaravage, & Pornon, 2016;Jacobs et al, 2010), the proportion of either pollen grains or sequences of a given plant species in an insect pollen load may reflect the proportion of visits to the species Stanley & Stout, 2014).…”

Pollinator‐mediated mechanisms for increased reproductive success in early flowering plants

“…Floral constancy, or fidelity, refers to when a pollinator consistently prefers one plant species to another; the less preferred species may experience reduced fitness (Campbell and Motten 1985). Pollinators often exhibit more constancy (Kunin 1997) and carry more conspecific pollen (Delmas et al 2016) in high-density populations. Consequently, we might expect that generalist pollinators exhibit the highest fidelity to a plant species during peak flowering of that species, when conspecific plant density is highest.…”

Pollinator Response to Patch Distribution and Reproductive Consequences in Lasthenia californica

“…More attractive patches are predicted to receive a greater quantity of pollen by optimal foraging theory, and as a result, more pollen deposition yields a positive correlation between individual patch size with fecundity and seed viability (Jennersten and Nilsson, 1993). Similar principles can also be applied at a population scale as well; optimal foraging theory predicts that groups of large patches will be favored and that small neighboring patches may also prosper from local pollinator activity (Delmas et al, 2016).…”