POLLINATORS DISCRIMINATE AMONG FLORAL HEIGHTS OF A SEXUALLY DECEPTIVEORCHID - IMPLICATIONS FOR SELECTION

Pollinators have influenced the evolution of many morphological floral traits, although few studies have shown that pollinators have influen ced plant height. Chiloglottis trilabra is one of many Australian orch ids that deceive and attract male pollinators by mimicking the sex phe romones and morphology of females insects. Orchids in this genus have unusually short flowers whose peduncle elongates dramatically after po llination to approximately twice the original height. In a series of c hoice experiments in the field, we show that pollinators of C. trilabr a strongly discriminate among floral heights, preferring flowers prese nted at 15 cm-20 cm over flowers presented at lower and higher positio ns (ranging from 2 cm-100 cm). Our results suggested pollination have the potential to mediate stabilizing selection for floral height when pollination is limiting. However, the natural height range of the orch id (mean = 10 cm, range 5 cm-15 cm) was lower than the experimentally determined optimum for visitation frequency. This difference may indic ate that pollinator-mediated selection does not occur in this species, perhaps because seed set is not sufficiently limited. Alternatively, other life-history factors may counteract pollinator-mediated selectio n, yielding an evolutionary compromise in height.