R. C. Plowright

Flower handling by bumblebees: a comparison of specialists and generalists

Abstract The hypothesis that specialists have evolved behavioural adaptations to handle their preferred food types more efficiently than related generalist species was tested. Naive workers of the specialist bumblebee, Bombus consobrinus, were more efficient than those of generalist species in acquiring flower-handling skills on their specialty plant, Aconitum (monkshood). With no previous foraging experience, B. consobrinus workers began probing in the vicinity of the nectary and quickly located the nectar. Generalists, on the other hand, showed no predisposition toward correct foraging on Aconitum. They probed in the wrong places, and many gave up before finding the nectar.

Competition between hummingbirds and bumble bees for nectar in flowers of Impatiens biflora

SummaryUsing removal experiments and concurrent measurement of resource levels, evidence was obtained for exploitation competition between Ruby-throated hummingbirds and two bumble bee species (Bombus fervidus and B. vagans) foraging for nectar on Impatiens biflora.When all three species were active, flower visitors showed a complex pattern of resource partitioning involving both diel and spatial changes. Hummingbirds foraged almost exclusively from the outermost exposed flowers on plants from which they drained nectar levels beyond the reach of bees over most of the day. In contrast the longtongued bee species (B. fervidus), and the shorter-tongued B. vagans, displayed a preference for the innermost flowers on plants which were protected from hummingbird visitation by surrounding vegetation. The two Bombus spp. began foraging at different times during the day: B. vagans were most active in early morning but were replaced by B. fervidus later in the day.When hummingbirds were rare, only B. fervidus showed evidence of competitive release: an increase in the number of foragers and a broadening of flower choice to include more outer flowers. Workers of B. vagans showed a similar response to temporary removal of B. fervidus and also extended their foraging over the entire day. These responses were consistent with changes in the availability of nectar to different species.Removal experiments demonstrated that individuals of one species can be largely excluded from access to nectar resources as a direct result of exploitation of nectar by foragers of other species with longer tongues. Thus in this system interspecific exploitation is an important mechanism involved in resource partitioning.