The effect of inflorescence size on pollinator visitation of Delphinium nelsonii and Aconitum columbianum

Abstract

Two factors have been suggested to play a role in determin ing the limit to inflorescence size within a species: energy limitation and diminishing pollination returns for larger inflorescence sizes. In an effort to assess the significance or pollination limitation we examined the effect or inflorescence size on pol1inator visitation patterns for 2 species, Aconitum columbianum and Delphinium nelsonii. These species are similar in their pollination biology, and both have a racemose inflorescence, but they differ markedly in inflorescence size (A. columbianum has from 1-26 open flowers while D. nelsonii has 1-6 open flowers). For each species the following parameters were examined as a function of inflorescence size: visits per inflorescence, flowers visited per visit, and visits per flower. For D. nelsonii all 3 parameters increased with increasing inflorescence size although for large inflorescences the rate of increase slowed slightly for flowers per visit and visits per flower. For A. columbianum all 3 parameters also generally increased with increasing inflorescence size but for the largest sizes there was no further increase in visits per inflorescence or flowers per visit and there was a decrease in visits per flower. The pattern of a smaller increase in attractiveness with progressively larger inflorescences corresponds to what would be expected if inflorescence attractiveness were based on the concept of just noticeable difference. The observed pattern of a smaller increase in flowers per visit with increasing inflorescence size can be described accurately by a model in which there is, on average, a fixed probability of moving from one flower to another on an inflorescence. The number of visits per flower is simply the consequence of the other 2 parameters. The lack of any significant decrease in pollinator visitation with increasing inflorescence size for D. nelsonii suggests that energy is probably the factor limiting inflorescence size. For A. columbianum, the largest Inflorescences have a reduced number of visits per flower and the total number of visits per inflorescence for the very largest inflorescences is reduced. This, in conjunction with a possible reduction in visit quality for large inflorescences, raises the possibility that inflorescence size in A. columbianum may be pollination limited.