Document: NIC-3-44-4

Pollination success and plant population size: How strong are the links?

WASER, N.M.*, M.V.PRICE, A.K.BRODY and D.R.CAMPBELL

Rocky Mountain Biological Laboratory, Crested Butte, CO, U. S. A. ¹

Abstract:
Pollination services to a given plant species are far from constant. For example, 4 summers of observation at 3 permanent sites in the Rocky Mountains separated by 200-400 m from each other indicates more than a 3-fold range in the rate of visitation to flowers of Ipomopsis aggregata (scarlet gilia), and occasional shifts from hummingbirds to bumblebees as major pollinators. Spatial and temporal variation in pollination does directly translate into seed set in these populations, but density-dependent seedling germination or growth of juveniles might subsequently dampen the signal of variable fecundity before the next generation of adults is reached. This link between fecundity and performance of the subsequent generation has rarely been explored. To explore it experimentally, we seeded 6 replicate 8 m² plots with 750 seeds, and 6 with 1500 seeds, representing average vs. full pollination. A second, single set of plots was seeded with a much wider range of 300-3000 seeds. Over both experiments we planted 22,500 seeds. In neither experiment was there detectable density-dependence in seedling emergence, which averaged ca. 6.5% in the 750- and 1500-seed plots and ca. 10% in the 300-3000 seed plots. Survival to age 1 yr averaged 39% in 750-seed plots vs. 35% in 1500-seed plots, again not significantly different. These results do not exclude the possibility of later density-dependence, which our continued censuses of all individually-mapped experimental individuals should detect. Empirical evidence for density-dependence, or lack thereof, in the transition from pollination and seed production to the next generation of juveniles and adults will be of interest for "pure" questions in plant population dynamics and phenotypic selection analysis, and for the management of endangered plant populations.

Keywords: Annual variation, Demography, Density-dependence, Experiment, Pollination

This abstract is being presented at: 10:15 AM in session:
Oral Session #6: Pollination Ecology.