The effect of the dispersion of rewarding and rewardless flowers on visitation and constancy by bumblebees (&lt;i&gt;Bombus impatiens&lt;/i&gt;)

Emma J Katz,Carla J Essenberg

doi:10.26786/1920-7603(2018)14

Abstract

Deceptive pollination strategies, in which plants rely on animal pollinators but provide no benefits to their pollinators, have evolved many times in angiosperms. However, the conditions that favour deceptive pollination strategies over mutualistic strategies are poorly understood. One factor that may be important for the success of deceptive strategies is the plants’ dispersion in relation to co-flowering, rewarding species. We compared pollinator behaviour across two artificial environments, one in which a rewardless species was mixed with a rewarding species and one in which the two species occurred in contiguous patches. Bumblebees both encountered and visited rewardless flowers more often when they were mixed with rewarding species. However, the rate of switching was also higher under those conditions, which could result in higher rates of interspecific pollen transfer. The environmental conditions most favourable to deceptive pollination strategies may vary depending on the vulnerability of the plant species to interspecific pollen transfer.

Highlights

Deceptive pollination, a strategy in which plants provide no reward to their pollinators but still rely on pollinator visits, has evolved independently many times in the angiosperms, and reversion from rewardlessness to mutualism has occurred multiple times (Renner 2006; Smithson 2009)
Previous studies suggest that the degree of mixing between rewarding and rewardless plant species can affect pollinator visitation rates to the rewardless species, how often pollinators switch between species, and the rewardless species’ reproductive output (Keasar 2000; Internicola et al 2006; Internicola et al 2007; see Hanoteaux et al 2013; Geslin et al 2014; de Waal et al 2015; Bruckman & Campbell 2016, which consider effects of dispersion on pollinator-mediated interactions between rewarding plant species)
Keasar (2000) found that rewardless artificial flowers received more visits and switching between species was less common when rewarding and rewardless flowers were separated in spatially distinct, monospecific patches than when they were mixed within a single, large patch, suggesting that aggregation may sometimes be favourable to rewardless species

Summary

Introduction

A strategy in which plants provide no reward to their pollinators but still rely on pollinator visits, has evolved independently many times in the angiosperms, and reversion from rewardlessness to mutualism has occurred multiple times (Renner 2006; Smithson 2009). The nature of these effects varies across studies, suggesting that multiple aspects of dispersion influence rewardless species’ success (Tab. 1) An experiment manipulating both the degree of mixing and density of the rewardless orchid Dactylorhiza sambucina and a dissimilar rewarding species, Muscari neglectum, found that increasing aggregation of both species negatively influenced the reproductive success of the rewardless plant (Internicola et al 2006). One explanation for the relatively high visitation to rewardless flowers in Keasar’s separated treatment is that the distance between the monospecific patches made switching away from the rewardless species more costly than it was when both species occurred in a single patch, favouring strings of visits to the rewardless species each time a rewardless patch was visited Consistent with this idea are the contrasting results of KATZ & ESSENBERG

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