Testing competing hypotheses about the behavior of floral visitors in mixed-flower stands.

Plants occur in mixed stands and flower simultaneously. Most hypotheses about how flowers affect the visitation rates of conspecific or heterospecific neighbors involve attractiveness. Attractive flowers are thought to monopolize visitors, but also to act as magnets increasing visits to unattractive neighbors. Attractiveness may be density-dependent: visitors could profit from focusing on abundant species, sometimes forming search images that make rare flowers less detectable. Four hypotheses was tested using data on visitation rates by honeybees (Apis melifera) and bee flies (Bombyliidae) to 1072 stands comprising six plant species in different densities. Hypotheses were compared through multi-model inference using the disk equation (a functional-response model) modified for co-flowering species and density-dependent attractiveness.