Specialist pollinators deplete pollen in the spring ephemeral wildflower Claytonia virginica

Abstract

Pollinators that collect pollen – and specifically, pollen‐specialist bees – are often considered to be the best pollinators of a (host) plant. Although pollen collectors and pollen specialists often benefit host plants, especially in the pollen that they deliver (their pollination “effectiveness”), they can also exact substantial costs because they are motivated to collect as much pollen as possible, reducing the proportion of pollen removed that is subsequently delivered to stigmas (their pollination “efficiency”). From the plant perspective, pollen grains that do not pollinate conspecific stigmas are “wasted”, and potentially costly. We measured costs and benefits of nectar‐collecting, pollen‐collecting, and pollen‐specialist pollinator visitation to the spring ephemeral Claytonia virginica. Visits by the pollen‐specialist bee Andrena erigeniae depleted pollen quickly and thoroughly. Although all pollinators delivered roughly the same number of grains, the pollen specialist contributed most to C. virginica pollen delivery because of high visitation rates. However, the pollen specialist also removed a large number of grains; this removal may be especially costly because it resulted in the depletion of pollen grains in C. virginica populations. While C. virginica appears to rely on pollen transfer by the pollen specialist in these populations, nectar‐collecting visitors could provide the same benefit at a lower cost if their visitation rates increased. Pollen depletion affects a pollinator's value to plants, but is frequently overlooked. If they lower the effectiveness of future floral visitors, visits by A. erigeniae females to C. virginica may be more detrimental than beneficial compared to other pollinators and may, in some circumstances, reduce plant fitness rather than increase it. Therefore, A. erigeniae and C. virginica may vary in their degree of mutualism depending on the ecological context.

Using niche breadth theory to explain generalization in mutualisms

“…Similarly, trait variation within species may stabilize population density over time (Bolnick et al 2011). Often, when a high-quality pollinator is unavailable, plants benefit from visits by low-quality pollinators (e.g., Thomson 2003, Parker et al 2016, meaning that retaining the capacity to interact with both highand low-quality pollinators can produce more consistent pollination. In the Albrecht et al (2012) pollinator study, temporal niche complementarity of pollinators may also have increased plant seed set.…”

Geographic patterns and pollination ecotypes inClaytonia virginica

“…() demonstrated that A. erigeniae remove more pollen (61% per single visit) than B. major (23.7%) in this system (Parker et al. ), and simulation models have demonstrated that these empirical removal rates by A. erigeniae would deplete pollen faster than B. major (Parker ). In fact, simple bookkeeping shows that if a male‐phase C. virginica flower containing 2764 pollen grains (the mean pollen production, as measured by Parker et al.…”

“…Parker et al. () demonstrated that A. erigeniae remove more pollen (61% per single visit) than B. major (23.7%) in this system (Parker et al. ), and simulation models have demonstrated that these empirical removal rates by A. erigeniae would deplete pollen faster than B. major (Parker ).…”

“…() demonstrated that A. erigeniae remove more pollen (61% per single visit) than B. major (23.7%) and other bees (20.31%) in this system (Parker et al. ).…”

On the ecological significance of pollen color: a case study in American trout lily (Erythronium americanum)

“…, Sahli and Conner , Parker et al. ), the observed site‐ and class‐specific pollinator preferences could lead to selection on pollen color. If pollinator preferences do lead to selection, among‐site variation in pollinator preferences could maintain local pollen color variation via migration–selection balance.…”