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Observing floral visitors in a hybrid zone.
Floral Morphology and Pollination


Kimball, S. 2008. Links between floral morphology and floral visitors along an elevational gradient in a Penstemon hybrid zone. Oikos 117:1064-1074.

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Penstemon Research Topics:
Dissertation Summary


Reproductive Isolation

Links to Additional Research Topics:

Traits influencing plant community composition

Restoration ecology at Carrizo Plain National Monument

Local ecology and geographic range limits



Hybrid zones can form when two species of plants hybridize along an environmental gradient. Changes in floral morphology across such a gradient and the degree of reproductive isolation can be influenced by pollinator behavior, but little is known about pollinator visitation to hybridizing species that receive visits from a diverse array of animals. I studied floral morphology, nectar rewards, and pollinator visitation for Penstemon newberryi, P. davidsonii, and their hybrids along an elevational gradient in the Sierra Nevada of California.

Sixty-two floral visitor species were observed visiting plants, making this system highly generalized compared to previous studies of visitation in hybrid zones. Morphological measurements were used to construct a plant hybrid index and examine the correlation with elevation and with floral visitors. Using observations of floral visitors along the gradient, I performed an ordination to determine whether the pollinator community changed along with plant morphology.

Plants varied in a clinal pattern along the elevational gradient. The visitor community varied incrementally with altitude, although visitors to Penstemon davidsonii were separated from those to P. newberryi and hybrids along the main ordination axis. Hummingbirds were only found at low and middle elevations, and small Halictid bees were relatively more abundant at high elevations. Although there is some potential for ecological isolation in the pollinator community, 11 common pollinator species visited both parent species and could be contributing to hybrid formation.

Elevational ranges based on visitor censuses for visitor species found in more than 10 censuses. The thickness of each bar is proportional to the average abundance of each visitor. Black dots indicate the average elevation of censuses in which each visitor was found.

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