Linking pollinator visitation rate and pollen receipt

Abstract

The majority of flowering plants require animals for pollination, a critical ecosystem service in natural and agricultural systems. However, quantifying useful estimates of pollinator visitation rates can be nearly impossible when pollinator visitation is infrequent. We examined the utility of an indirect measure of pollinator visitation, namely pollen receipt by flowers, using the hummingbirdpollinated plant, Ipomopsis aggregata (Polemoniaceae). Our a priori hypothesis was that increased pollinator visitation should result in increased pollen receipt by stigmas. However, the relationship between pollinator visitation rate and pollen receipt may be misleading if pollen receipt is a function of both the number of pollinator visits and variation in pollinator efficiency at depositing pollen, especially in the context of variable floral morphology. Therefore, we measured floral and plant characters known to be important to pollinator visitation and/or pollen receipt in I. aggregata (corolla length and width and plant height) and used path analysis to dissect and compare the effect of pollinator visitation rate vs. pollinator efficiency on pollen receipt. Of the characters we measured, pollinator visitation rate (number of times plants were visited multiplied by the mean percentage of flowers probed per visit) had the strongest direct positive effect on pollen receipt, explaining 36% of the variation in pollen receipt. Plant height had a direct positive effect on pollinator visitation rate and an indirect positive effect on pollen receipt. Despite the supposition that floral characters would directly affect pollen receipt as a result of changes in pollinator efficiency, corolla length and width only weakly affected pollen receipt. These results suggest a direct positive link between pollinator visitation rate and pollen receipt across naturally varying floral morphology in I. aggregata. Understanding the relationship between pollinator visitation rate and pollen receipt may be of critical importance in systems where pollinator visitation is difficult to quantify.

Pollinator Interactions with Yellow Starthistle (Centaurea solstitialis) across Urban, Agricultural, and Natural Landscapes

“…In agricultural and natural landscapes, a positive correlation between pollinator visitation and seed set is typical [50]. Surprisingly in our system, in human-altered landscapes, higher total observed bee visitation did not result in higher proportions of seed set, as would be expected.…”

Sex‐specific floral attraction traits in a sequentially hermaphroditic species

“…Observational measures of pollinator visitation often do not directly correlate with the deposition of pollen grains on stigmas (Alarcón, 2010;King, Ballantyne, & Willmer, 2013) because not all floral visitors are effective pollinators (Alarcón, 2010;Engel & Irwin, 2003;King et al, 2013). Consequently, estimating pollinator visitation using the number of pollen grains deposited on individual, receptive stigmas is a more realistic assessment of the pollination services received.…”

The incidence and selection of multiple mating in plants

“…with sequential visitations each depositing a mixed-donor pollen load [22,23]. Because pollinators tend to visit more than one flower and may be abundant at a site, individual flowers will often receive multiple visits by animals that have visited more than one flower in their foraging bouts [24]; mixed-donor pollination is therefore likely to be very common in animal-pollinated plants [25]. But polyandry should be common in wind-pollinated plants too, because stigmas often receive more than one pollen grain, haling from different donors, particularly in dense populations [26,27].…”