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

We studied the pollination ecology and assemblage structure of 31 spec ies 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 a nd bombyliid flies. Within and among species there is significant vari ation in nectar-tube length (and therefore in the insects that visit t he 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 withi n and among species, thereby causing variation in site of pollen depos ition. 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 no nrandom assemblage structure we compared our observation with the outc ome of null models. We developed three null models to cover the most l ikely structuring processes: that communities are organized by (1) eco logical sorting, (2) evolution of plant phenotypes, or (3) both proces ses. We concluded that it was unlikely (P = .055-.002) that so few ove rlaps in pollination niches would occur by chance. We developed anothe r null model to test whether chance could have created the apparent pa ttern 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 stu dy is one of the dearest demonstrations to date of reproductive intera ctions generating assemblage structure and character displacement in p lants.