FLORAL CHARACTER DISPLACEMENT GENERATES ASSEMBLAGE STRUCTURE OF WESTERN-AUSTRALIAN TRIGGERPLANTS (STYLIDIUM)

We studied the pollination ecology and assemblage structure of 31 species of Stylidium (Stylidiaceae) at 25 sites in Western Australia. The number of species per study site varied between two and six. Stylidium species are pollinated by a variety of nectar-seeking solitary bees and bombyliid flies. Within and among species there is significant variation in nectar-tube length (and therefore in the insects that visit the flowers) and in pollen placement on pollinators. Pollen is placed ''explosively'' on the insect by a motile column of fused staminate and pistillate tissues; the position and reach of the column varies within and among species, thereby causing variation in site of pollen deposition. When discrete pollination niches were defined for all species, only one niche overlap was observed across the 86 interacting pairs of Stylidium species at the 25 sites. To determine whether this was a nonrandom assemblage structure we compared our observation with the outcome of null models. We developed three null models to cover the most likely structuring processes: that communities are organized by (1) ecological sorting, (2) evolution of plant phenotypes, or (3) both processes. We concluded that it was unlikely (P = .055-.002) that so few overlaps in pollination niches would occur by chance. We developed another null model to test whether chance could have created the apparent pattern of character displacement in pollination niches exhibited by the nine species showing intraspecific variation. The analysis indicated that character displacement has probably occurred (P = .014). This study is one of the dearest demonstrations to date of reproductive interactions generating assemblage structure and character displacement in plants.