Trait-based   analyses that elaborate the biological mechanisms underlying species   interactions are crucial for predicting the structure of community plants and   animals. Flower nectar, a sugar-rich solution, is fundamental to linking   flowers and animal visitors for most plants. However, there is a paucity of   knowledge regarding the influence of nectar micro-components, such as amino   acids (AAs), and their relative composition on foraging decisions of flower   visitors within a community context.We quantitatively calculated the relative   composition of 21 AAs of nectars from 40 species and recorded the visitation   frequency of their 207 species of insect visitors (15 functional groups from   five orders) in a species-rich alpine meadow. Phylogenetically generalized   mixed models with Bayesian estimation were used to detect the influence of   nectar AAs on the visitation frequency of flower visitors from the different   groups.Our results indicated that the increase of gamma-aminobutyric acid   (GABA) and methionine in nectar enhanced the visitation of social bees, the   dominant flower visitors in this meadow. Furthermore, the AAs in nectar that   affect visitation frequency varied among different functional groups of   Dipteran, e.g., hoverfly, mucoid fly, meat fly, and horsefly. Their different   diet sources and life history strategies (e.g., feeding larvae or not) may   explain the preferring or disliking of nectar AAs.Our findings suggest a way   that flower reward may structure plant-pollinator interactions in communities   with diverse species, which enriches the understanding of mechanisms for   biodiversity maintenance.