Effects of Stand Composition on Insect Visitation in Two-Species Mixtures of Hieracium

Abstract

The effects of host plant distribution were investigated on the host plant choice of flower-visiting insects in mixed stands of the hawkweeds H. aurantiacum L. and H. florentinum All. The way in which pollinators divide their visits among simultaneously blooming plant species determines the outcome of plant competition for pollinators, which may affect community structure. Although the hawkweeds are apomictic, these stands are simple communities that embody many of the assumptions of published models concerning the effects of stand composition on insect foraging. Existing theory suggests that visitation on 1 of 2 competing spp. should increase with its proportion in the stand. Visitation on H. aurantiacum follows this relationship, but H. florentinum receives more visits with H. aurantiacum present than when it blooms alone. Extension of this result to other, non-apomictic species implies that a species may occasionally be benefitted by the presence of an apparent competitor. A tentative explanation based on pollinator inconstancy and foraging habitat selection is proposed. Some possible effects of these phenomena on community structure are suggested.