Nectar robbing rather than pollinator availability constrains reproduction of a bee?flowered plant at high elevations

Abiotic factors are generally assumed to determine whether species can exist at the extreme ends of environmental gradients, for example, high elevations, whereas role biotic interactions is less clear. On temperate mountains, insect-pollinated plant with bilaterally symmetrical flowers exhibit a parallel elevational decline in richness and abundance bees. This suggests that lack mutualistic interaction partners sets range limits plants via reduction reproductive success. We used bee-pollinated mountain Clinopodium alpinum (Lamiaceae), which blooms along continuous 1000-m gradient has flowers, as model test predicted flower visitation seed production. Although community visitors changed elevation, rate by most frequent visitors, bumble bees (33.8% legitimate visits), overall potential pollinators did not vary significantly elevation. However, we discovered nectar robbing theft ants, two potentially negative effects on sharply increased Seed set depended across elevations followed weak hump-shaped pattern, peaking mid-elevations decreasing about 20% toward both edges. Considering mid- variation production could be explained bee rates but was inversely correlated frequency robbing. Our observations challenge hypothesis decrease availability bee-flowered suggest an increase (nectar larceny) constrains