Year of Award


Document Type


Degree Type

Doctor of Philosophy (PhD)

Other Degree Name/Area of Focus

Biology, Ecology

Department or School/College

Division of Biological Sciences

Committee Chair

John Maron

Commitee Members

Ragan M. Callaway, Winsor Lowe, Elizabeth Crone, Monica Geber


Clarkia pulchella, Osmia lignaria, plant invasion, pollination, solitary bees, toxic pollen


The University of Montana


Flowering plants and bees are fundamentally linked. Bees rely wholly on floral resources for food and many native plants rely on bees to assist in reproduction. Despite this fundamental connection, how plants and their bee pollinators influence one another's abundance remains unclear. Also unclear is how invasion by exotic flowering plants disrupts bee- native plant relationships. I examined how the invasive plant, Euphorbia esula (Euphorbiaceae) influenced the native annual forb, Clarkia pulchella (Onagraceae) by altering pollinator visitation (i.e., indirect effects) and through direct resource competition. I observed pollinator visitation and estimated the extent to which plant reproduction was limited by pollen receipt (i.e., pollinator visits) in invaded and native-dominated sites. Though pollinator visitation was strongly reduced in invaded sites, plants were only weakly pollen limited. In contrast, E. esula removal experiments indicated that the effect of resource competition on C. pulchella fitness was very strong. Seed addition experiments indicated that changes in fecundity resulting from ameliorating competition for pollinators could influence future plant abundance, however these effects were dwarfed by the effects of resource competition. To determine if strong reductions in pollinator visitation in invaded sites was mirrored by changes in entire bee communities, I quantified bee communities in sites dominated by E. esula and native-dominated sites. Bee abundance, richness and diversity were reduced in invaded sites. Thus, though native bees foraged on E. esula flowers, the net effect of invasion on bee communities was negative. However, because bees are highly mobile, patterns of forager abundance in relation to floral abundance may be misleading. Such patterns may be the result of bees moving to high resource patches rather than any fundamental change in bee abundance based on altered demographic processes. To examine this issue, I quantified the influence of floral resources and plant invasion on the demography of the native solitary bee, Osmia lignaria (Megachilidae). I placed nest blocks and preemergent cocoons in 27 sites that varied along a floral resource gradient and in extent of invasion, and found that nesting and fecundity were positively correlated with the abundance and richness of the native forb community and that nesting decreased with increasing exotic forb species richness. Despite increased parasitism in high-resource native-dominated sites, a marginally positive relationship between native forb species richness and bee population growth rates remained, suggesting that floral resources can positively influence bee demography. Finally, via a literature review, I examined whether excessive harvesting of pollen by bees has led to the evolution of chemical defense of pollen. While bees are generally perceived as mutualists, they also act as herbivores, collecting substantial quantities of pollen to rear larvae, and the loss of pollen to consumption by bees has the potential to decrease plant fitness. Many morphological features of flowers are interpreted to serve in defense of pollen from excessive harvesting, and there is growing evidence that pollen can contain secondary compounds that may be an additional means of pollen defense.



© Copyright 2013 Jennifer Palladini