Does hardness make flower love less promiscuous? Effect of biomechanical floral traits on visitation rates and pollination assemblages

Abstract

Visitation rates and assemblage composition of pollinators have often been related to environmental, ecological and phenotypic variables. However, the interaction between flowers and pollinators has not been evaluated in a biomechanical context. Floral rewards in keel flowers (Fabaceae, Faboideae) are concealed behind four joined petals, the keel-wing unit, and are accessible only if pollinators open this unit by exerting force on it. Force needed to open the flower is expected to affect the interaction with pollinators because pollinators must invest time and energy to open the keels. Consequently, plants with stiff flowers should be expected to experience diminished visitation frequency, particularly by weak visitors. To test this expectation of diminished visitation rates and of assemblage composition biased by pollinator strength, we measured the force needed to open the keel flowers of five co-occurring legume species and, using a canonical correspondence analysis (CCA), we tested their association with pollinator visitation rates and assemblage composition. We additionally included a size flag variable in CCA to test the effect of attractiveness on pollinator visits. There was no association between flower stiffness and visitation frequency. According to the CCA, pollinator assemblage compositions were associated with the force needed to open the keel and not flag size. As a general pattern, weak flowers are pollinated by an uneven assemblage of weak bees while the stiffest flowers are pollinated by an even assemblage of large and strong bees. These results supports the idea that force has an effect in controlling pollinator assemblage composition.