In animal-pollinated plants, the opportunity for selection and the strength of pollinator-mediated selection are expected to increase with the degree of pollen limitation. However, whether differences in pollen limitation can explain variation in pollinator-mediated and net selection among animal-pollinated species is poorly understood. In the present study, we quantified pollen limitation, variance in relative fitness and pollinator-mediated selection on five traits important for pollinator attraction (flowering start, plant height, flower number, flower size) and pollination efficiency (spur length) in natural populations of 12 orchid species. Pollinator-mediated selection was quantified by subtracting estimates of selection gradients for plants receiving supplemental hand-pollination from estimates obtained for open-pollinated control plants. Mean pollen limitation ranged from zero to 0.96. Opportunity for selection, pollinator-mediated selection and net selection were all positively related to pollen limitation, whereas nonpollinator-mediated selection was not. Opportunity for selection varied five-fold, strength of pollinator-mediated selection varied three-fold and net selection varied 1.5-fold among species. Supplemental hand-pollination reduced both opportunity for selection and selection on floral traits. The results show that the intensity of biotic interactions is an important determinant of the selection regime, and indicate that the potential for pollinator-mediated selection and divergence in floral traits is particularly high in species that are strongly pollen-limited.

Trunschke et al._NewPhytologist_2017_strength of selection_DRYAD_raw dataThe data file provides row data collect in summer 2013 (Mai-August) on five phenotypic traits (flowering start, plant height, flower number, flower size, spur length) and measurements of female reproductive success (number of fruits, fruit mass) for one population of each of twelve different orchid species (Anacamptis morio (L.) R.M.Bateman, Pridgeon & M.W.Chase, Dactylorhiza incarnata (L.) Soó, D. sambucina (L.) Soó, Orchis mascula (L.) L., O. militaris (L.), and Neotinea ustulata (L.) R.M.Bateman, Pridgeon & M.W.Chase, and of the rewarding species Dactylorhiza viridis (L.) R.M.Bateman, Pridgeon & M.W.Chase, Epipactis palustris (L.) Crantz, Gymnadenia conopsea (L.) R.Br., Neottia ovata (L.) Bluff & Fingerh., Platanthera bifolia (L.) Rich., and P. chlorantha (Custer) Rchb.). These data were used calculate mean±SE of phenotypic traits and female fitness. In addition, phenotypic traits are provided as standardized values to a mean of 0 and a variance of 1, and relative fitness as individual fitness divided by mean fitness. Standardisation is done by population and treatment. These data are used to calculate selection gradients by population and treatment, whose absolute values were then used for all higher-level analysis relating strength of selection and degree of pollen limitation.