Bumblebee Response to Variation in Nectar Availability

Abstract

I examined the response of bumblebees to two kinds of spatial variation and two kinds of temporal variation in nectar levels. The spatial variation involved differences in reward value among plant species and differences in nectar availability among patches of flowers of a single species. The temporal variation involved changes in nectar availability over a season and between years. In all cases the response of bees was measured by differences in bee density or abundance. The response of bees of different plant species was determined by the density of foraging bees on the flowers of each species at a given time (bee:flower ratio = B/F). I examined several groups of plant species whose members overlapped in their flowering periods and in their bumblebee visitors. Within a group, the relative magnitudes of the B/F's for species indicate the degree to which their flowers are preferred by bees. If bees are optimal foragers, preference should reflect the per—flower reward value of species. I characterized the potential reward value of species by their nectar production rates (NPR's). The bees' preference for species was found to be very similar to the relative magnitudes of their NPR's. I also considered whether including the time and energy costs of foraging on the different species would produce a better predictor of preference. While these factors did affect the absolute reward value of species they did not influence their reward value relative to one another. Foraging costs are most likely to have no effect on preference when species have similar floral morphologies. To assess the response of bees to differences in patch quality I bagged some of the inflorescenes in an experimental patch. Following this reduction in the number of flowers, the number of bees in the patch decreased proportionately, producing a B/F very similar to that of a control patch. When the bagging was removed the number of bees increased such that the B/F was again similar to that of the control patch. In general these results provide further support for optimal foraging in bumblebees. I examined the response of bees to temporal variation in nectar levels to determine whether resource limitation for bumblebees change over time. Seasonal changes in the abundances of three bumblebee species were compared with seasonal changes in the total nectar production by the plants each bee visited. Preference factors were used to convert floral production to nectar production. In general, seasonal production and utilization were closely matched, indicating no major change in nectar limitation. The cumulative numbers of bees seen during each of two summer seasons (1974 and 1975) were very similar. This indicates that the resource base, which had not changed between years, was capable of supporting only a limited number of bees. During 1975 a natural species removal experiment occurred which allowed an assessment of competition among bees. Apis mellifera, which had been well represented in 1974, was absent. The bumblebees of short and medium tongue length with which honeybees overlapped in resource used exhibited competitive release. Their abundances increased in 1975 resulting in near—perfect density compensation.