Passive partner choice through exploitation barriers

Abstract

Floral features that affect the efficiency with which pollinators can harvest their resources, or the profitability they obtain from them, affect the foraging decisions of pollinators. Foraging choices of pollinators, in turn, affect pollen flow: increases in flower constancy lead to more efficient pollen transport. It follows that exploitation barriersflower traits that differentially affect net intake rates of potential visitors-will promote resource partitioning and enhance pollen export. In this paper we first generalise foraging models to show that exploitation barriers can lead to partial resource partitioning even when flowers are randomly distributed in space. Then we develop a model to study how the foraging rules of pollinators, pollen removal and pollen deposition, affect pollen flow. The model shows that resource partitioning, even incomplete, can substantially increase the efficiency of pollen flow. Finally, we use computer simulations to demonstrate that exploitation barriers promoting partial resource partitioning can evolve. Many of the flower traits associated with pollination syndromes have small but consistent effects on the efficiency with which different taxonomic groups exploit flowers, and can be considered exploitation barriers. Even if these barriers are not strong enough to promote strict specialisation, and may have little effect on the female component of fitness when pollinators are not a limiting resource, they are likely to be selected because they enhance the male component of fitness.

“Hummingbird” floral traits interact synergistically to discourage visitation by bumble bee foragers

“…Regardless, our results indicate that M. cardinalis color, orientation, and reward traits interact synergistically to discourage bumble bee visitation and force individuals to adopt a highly specialist foraging strategy. 4A, B), support the long-standing hypothesis that the red coloration typical of classic bird syndrome flowers functions in bee deterrence rather than bird attraction (Schemske and Bradshaw 1999, Lunau et al 2011, Rodriguez-Girones et al 2015. Why do M. cardinalis trait combinations function as an effective deterrent to bumble bee visitation?…”

“…These findings, combined with the effects of red flower coloration on reward preference and foraging rate found in Experiment 2 (Fig. 4A, B), support the long-standing hypothesis that the red coloration typical of classic bird syndrome flowers functions in bee deterrence rather than bird attraction (Schemske and Bradshaw 1999, Lunau et al 2011, Rodriguez-Girones et al 2015. However, our results do not support the pervasive mechanistic view that red flower color by itself generates aversive responses in bees.…”

“…In this view, bees adopt flower avoidance as a foraging strategy to maximize their rate of energetic gain. In addition, signaling traits of bird flowers such as red coloration, horizontal orientation, and reduced display size may increase foraging costs to bees by decreasing the speed and accuracy of foraging decisions (Rodriguez-Girones and Santamaria 2004, Burns and Dyer 2008, Rodriguez-Girones et al 2015, and potentially interact with one another to further increase the magnitude of such costs (Gegear and Laverty 2005, Raguso and Willis 2005, Gegear and Burns 2007, Campbell 2009, Leonard and Masek 2014. For instance, the reflexed lower petal typical of bird flowers may increase the amount of time required for bees to extract nectar (Castellanos et al 2004, Zung et al 2015.…”

“…For instance, the reflexed lower petal typical of bird flowers may increase the amount of time required for bees to extract nectar (Castellanos et al 2004, Zung et al 2015. In addition, signaling traits of bird flowers such as red coloration, horizontal orientation, and reduced display size may increase foraging costs to bees by decreasing the speed and accuracy of foraging decisions (Rodriguez-Girones and Santamaria 2004, Burns and Dyer 2008, Rodriguez-Girones et al 2015, and potentially interact with one another to further increase the magnitude of such costs (Gegear and Laverty 2005, Raguso and Willis 2005, Gegear and Burns 2007, Campbell 2009, Leonard and Masek 2014. Nectar traits of bird flowers such as high volume and low concentration may also function to deter bee visitors by reducing sugar intake rates (Heinrich 1975, Harder 1986, Cnaani et al 2006 or increasing the amount of energy expended to produce honey in the colony (Bolten and Feinsinger 1978).…”

Learning in two butterfly species when using flowers of the tropical milkweed Asclepias curassavica: No benefits for pollination

“…Through this competitive advantage, such visitors maximize their intake rate by preferentially using complex flowers, leading to (partial) flower constancy (Rodríguez‐Gironés and Santamaria, 2005, 2006). Plants benefit from flower constancy because it minimizes heterospecific pollen transfer (Waser, 1986; Rodríguez‐Gironés et al, 2015). While the effects of learning on flower constancy are well known, a second aspect has so far been ignored: visitors change the way they manipulate flowers as they become experienced (Laverty, 1980; Russell et al, 2016), and this could affect the probability per visit of pollen transfer between flower and pollinator.…”