Niche differentiation in nectar-collecting stingless bees: the influence of morphology, floral choice and interference competition

1. Dozens of social bee species, most of them stingless bees, occur sympatr ically in the tropics. The proximate mechanisms through which they partitio n their resources are, apart from aggressive interactions between Trigona s pecies, not well studied. In the work reported here, niche differentiation at patch level was studied, using two species of Melipona that occur sympat rically in the Central Pacific part of Costa Rica. Foragers of Melipona bee cheii are known to collect more concentrated nectar than do Melipona fascia ta foragers, even from the same plant species. This observation raises the question of what mechanism leads to such partitioning of nectar resources? To address this question, the roles of bee morphology, floral preferences, and interference competition in partitioning of nectar sources were studied . 2. It was shown experimentally that the feeding rate of both species was hi ghest at 60% nectar concentration. Melipona fasciata preferred 60-70% conce ntrations to less concentrated solutions, whereas M. beecheii ignored 20% s olutions and visited the other solutions equally often. Both species prefer red sucrose to glucose and fructose. Melipona beecheii, with a yellowish co loured body, preferred sunny patches, whereas M. fasciata, with a dark brow n body, preferred shady patches. Interference competition between the speci es occurred when they visited the same sugar-water feeder: nit fasciata was dominant over EA. beecheii. 3. The nectar foraging strategies of the two species can be summarised as f ollows: M. beecheii is able to Visit sunlit patches, due to its lighter bod y colour, and is thus able to collect nectar of optimal or near optimal sug ar concentration (40-65%). Individuals do not actively choose nectar with a high concentration of sugar, but the average sugar concentration in bee-co llected nectar will typically be high in sunlit patches. Melipona fasciata, on the other hand, avoids sunlit patches, but actively chooses the richest nectar in shady patches and tries to dominate such patches. Thus, niche di fferentiation occurs according to the radiation regime at flower patches, a nd interference competition will occur rarely under natural conditions. The role of similar mechanisms in other closely related stingless bees is disc ussed.