| Summary: | In mixed stands of the annual Impatiens capensis and I. pallida, bumblebees visit I. pallida almost exclusively even when the nutritional reward of I. pallida is removed. This phenomenon contradicts certain models proposed for foraging behavior in simultaneously flowering species and those dealing with optimal foraging. Such foraging behavior could have potentially devastating effects on the nonpreferred species, especially if the plant is an annual.
Both Impatiens species share the same primary pollinators when they grow allopatrically, but when they grow sympatrically I. pallida continues to attract bumblebees while I. capensis depends on hummingbirds and small bees for pollination. Thus competition for pollinators between these Impatiens represents an important plant-plant and plant·pollinator interaction with potential consequences. When I. capensis is less frequented by bumblebees when sympatric with I. pallida, I. capensis exhibits reduced fecundity.
Mechanisms of interference through improper pollen transfer are described for the Impatiens species. Pollen of I. pallida adheres to and germinates on the stigmas of I. capensis and the pollen tubes reach the ovules. In contrast, pollen of I. capensis adheres poorly to I. pallida stigmas and fails to germinate. No hybridization occurs between the species. Scanning electron micrographs of pollen and stigma surfaces of the two species revealed no morphological differences.
Among the floral characteristics examined, only flower color appeared to influence bumblebee preference for I. pallida. Differences between the species in ultraviolet absorbance patterns, perianth size and shape, odor, and nectar quantity and quality did not affect the pollinator preference of bumblebees. Differences in nectar characteristics and production rate were found among Impatiens populations. However, nectar quantity, quality, and production rates did not differ within allopatric or sympatric populations, but both species had higher nectar volumes with lower solute concentrations in the morning, and lower nectar volumes with higher solute concentrations in the late afternoon. Differences in nectar volumes among populations appeared to be associated with environmental factors affecting population sites.
I. capensis avoids competition and maintains itself in sympatric populations with I. pallida by depending on hummingbirds and insects other than bumblebees as pollinators, and has the ability to produce cleistogamous flowers and their subsequent progeny to ensure sexual reproduction. However, inbred progeny of I. capensis are competitively inferior to outcrossed stock. Nevertheless, I. capensis does persist in mixed stands of I. pallida despite the absence of bumblebee pollination and the presence of lower seed set. === Ph. D. === incomplete_metadata
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