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Pollination by animals is critical to sexual reproduction of most angiosperms. However, little is known about variation in pollination service to single plant species. We report results of a long-term study of Ipomopsis aggregata, a semelparous montane herb whose flowers are visited by hummingbird and insect pollinators as well as “floral larcenists.” We censused flower visitors over seven summers at permanent study sites separated by several hundred meters, and counted pollen delivered to flowers on a subset of plants observed for visitation. The species composition of the community of visitors varied significantly across years and within the flowering season; sites varied significantly only in the magnitude of parallel annual changes in the visitor community. Rates of flower visitation fluctuated over an order of magnitude or more. Variation in mean stigma pollen load among plants flowering in the same site and year was explained by a causal path model in which visitation rates by pollinators and larcenists had linear positive and negative effects, respectively. A simplified model including only pollinators explained almost as much variance as did the full model. However, qualitatively different parameter estimates were produced by an analogous causal model based on population means across site–year combinations. Discrepant results from within- and between-population levels of analysis suggest that pollen receipt is influenced by environmental factors that vary among sites and years, as well as by pollinator visit rates. We present a heuristic causal model that includes such factors, and we note its implications for ecological and evolutionary studies of pollination.
Price et al. (Mon,) studied this question.