Study area
The field survey was conducted in an alpine meadow located in Shangri-La County (27°37’40” N, 99°47’32” E; 3379 m), Northwest of Yunnan Province, China. The study area was about 12 hectares, which was a small valley basin surrounded by mountains. For the pollinators, the study site could be regarded as an isolated area because the mountains were mainly covered by coniferous forests and the nearest meadows were 2 km away (straight-line distance) and separated by the forests. More than 120 flowering plant species grow in the meadow, including more than 55 plant species visited by bumble bees. The flowering season extends from May to September. The most dominant plant is Polygonum macrophyllum D. Don., which contributes to no less than 40% of the coverage for this meadow. The next most dominant species areSanguisorba filiformis (Hook. f.) Hand. -Mazz., Anemone obtusiloba D. Don., Pedicularis densispica Franch.,Phlomis atropurpurea Dunn. etc.