IDEAL FREE DISTRIBUTIONS OF NECTAR FORAGING BUMBLEBEES

Optimal foraging theory predicts that foragers should distribute themselves on a variable resource so that individual food intake rate is the same at all local areas or patches. The number of nectar foraging bumblebees on plants varying in size (Viscaria vulgaris) was proportional to the number of open flowers per plant, and on plants varying in rate of nectar production per flower (Anchusa officinalis) proportional to the nectar production per plant. These distributions results in a nectar gain per flower which is independent of plant size and rate of nectar production per flower. A computer simulation showed that the distribution of the bees and the equalization of the gain per flower can partly be explained by the bees' systematic searching for nectar, and partly by a non-random plant choice. The bees search systematically by using an optimal rejection threshold gain to avoid recently depleted flowers. The non-random plant choices consist in preferences for large plants and high nectar producers. At stands varying in plant density (Viscaria vulgaris), bee and flower densities were proportional, which also results in a constant gain per flower. This distribution is explained by a preference for foraging on dense stands and maybe by a systematic searching among the plants.