Year of Award

2009

Document Type

Dissertation

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 L. Maron

Commitee Members

Ragan M. Callaway, Elizabeth E. Crone, Lila Fishman, Anna Sala

Keywords

Lithospermum ruderale, Lupinus sericeus, plant-consumer interactions, population dynamics, demography, seed predation

Publisher

University of Montana

Abstract

Although post-dispersal seed predators are common and often reduce seed density, their influence on plant abundance remains unclear. We examined the impact of seed predation by small mammals, primarily deer mice (Peromyscus maniculatus), on seedling recruitment and plant establishment of two perennial grassland forbs: Lupinus sericeus (Fabaceae) and Lithospermum ruderale (Boraginaceae), using experimental seed addition and rodent exclusion treatments. For both species, small mammal exclusion increased the total number of seedlings that emerged, and these effects were still significant three years after seed addition, resulting in greater numbers of established plants inside exclosures than in control plots. To shed light on how these relatively short-term rodent-driven reductions in seed abundance and recruitment might influence longer-term patterns of L. ruderale population growth, we combined experimental results with demographic data in stage-based population models. Model outputs revealed that rodent seed predation had a significant impact on L. ruderale population growth rate. These results demonstrate that rodent granivory can be a potent force limiting the abundance of a perennial forb.

In the third chapter, we examined the effects of multiple consumers on L. sericeus populations. We combined the experimental evidence of rodent-driven reduction in seedling recruitment in with the impacts of two other consumers, folivorous ground squirrels (Spermophilus columbianus), and herbivorous insects (Lepidopteran and Coleopteran larvae) on L. sericeus fecundity, with stage-based matrix models. We examined how these consumers, individually and in concert, influence the population growth of L. sericeus at three sites. Because consumers sequentially attack flowers, pre-dispersed seeds and then post-dispersed seeds, the opportunities for any given species to influence the population growth rate of L. sericeus is contingent on the impacts of the preceding consumer. We found that release from all consumers caused significant, and sometimes dramatic, increases in the population growth rate. These results suggest that despite high rates of asexual reproduction, consumers impose strong limits to the population growth and therefore the abundance of this long-lived forb. The final chapter reflects outreach to the public school community, bringing scientific content and methods to an elementary school classroom in an inquiry demonstrating the importance of seed dispersal in plants.

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© Copyright 2009 Mary Carolyn Bricker