|Abstract or Summary
- I experimentally test the hypothesis that the potential for selfing
increases with plant size in Mertensia ciliata (Boraginaceae), a self-compatible,
profusely-flowering perennial. This follows from the
premises that 1) pollen dispersal by pollinators between flowers is
limited, and 2) individual pollinators, in this study bumblebees, will
visit more flowers per visit on large than on small plants thereby
promoting intra- rather than interplant dispersal of pollen. I show
that while M. ciliata is self-compatible, outcrossing results in greater
reproductive output (seed numbers and seed weight) than selfing. Thus,
under the hypothesis above, the reproductive output of flowers should
decline with increasing plant size.
I demonstrate, through pollen carryover experiments, that pollen
transfer by bumblebees is extensive. Observations of pollinator
foraging behavior show that individual bumblebees visit only a few more
flowers and stems, and indeed encounter a smaller proportion of a
plant's flowers and stems on visits to large than to small plants.
Large plants attract more pollinators per minute. I use these results to predict that flowers on all plants should receive equal amounts of
outcrossed and total (self + outcrossed) pollen, and that selfing rates
should not differ among plants. This is supported by direct
measurements of pollen receipt by flowers, and of the reproductive
output of flowers on large and small plants in the field. No
differences were found among plants in outcrossed and total pollen
receipt, and in seed-set per flower and seed weight.
I examine the pattern of insect visitation in more detail to show
that individual bumblebees encounter only a small number and proportion
of flowers and stems per visit on all plants, and encounter a smaller
proportion on large than on small plants. Individual bees, then,
exploit large plants less intensely per visit than small plants. Bees
move predominantly between neighboring plants and fly randomly with
respect to direction. Many insect visitors are nectar robbers. I
propose four factors to explain the short visits of pollinators and the
less intense exploitation of large plants by individual bumblebees.
These include the complex architecture of the flowering display, the
circular geometry and density of stems in plants, the close proximity of
plants in the population, and the variance among flowers of plants in
nectar reward caused by visits of nectar robbers.