Document Type

Dissertation

Degree

Doctor of Philosophy

Major

Biology

Date of Defense

5-12-2009

Graduate Advisor

Bette A. Loiselle, PhD

Committee

Dr. Joseph L. Polman, Ph.D.

Tiffany Knight

Eugene W. Schupp

Abstract

This study connects data on frugivore activity, post-dispersal seed fate and plant population demography using stage-specific demographic modeling to examine the role of individual dispersers for plant population dynamics of the fleshy-fruited Neotropical tree Guettarda viburnoides (Rubiaceae) in northeastern Bolivian savannas. Detailed information on frugivory and seed dispersal of G. viburnoides reveals that the endocarps of this plant are dispersed mainly by two species of birds: Cyanocorax cyanomelas and Pteroglossus castanotis, which are defined as the quantitatively important dispersers of G. viburnoides. These two species differ in several qualitative aspects of seed dispersal: 1) They select fruits of different sizes; 2) they differ in their fruit handling treatment, which in turn affects the probability of seedling emergence, the temporal pattern of emergence, and the number of emerged seedlings per endocarp and; 3) they disperse the endocarps in different habitats. In turn, the habitats into which seeds are dispersed determine the post-dispersal seed fate of G. viburnoides; specifically, seed predation and seedling emergence. Therefore, habitats vary in their suitability for different plant stages, and ultimately in their overall probability for plant recruitment. There is also evidence, however, of inter-annual variability in the strength of post-dispersal processes among habitats, which leads to a constant shifting of habitat "suitability" from one year to the next; that is, it is context-dependent. Ultimately, demographic modeling reveals that the differences in the foraging behaviors of C. cyanomelas and P. castanotis result in different contributions of these dispersers to the population growth of G. viburnoides. Seed dispersal by C. cyanomelas leads to positive growth, whereas seed dispersal by P. castanotis has a detrimental effect on the population growth of this species. Consequently, C. cyanomelas is a key species for the long-term persistence of G. viburnoides in this landscape, and the loss of this dispersal agent would not be compensated by the dispersal services provided by P. castanotis. The integration of frugivore activity with plant demography are important for plant ecology because they enable us to close the "seed dispersal loop" and gain a better understanding of the demographic consequences of seed dispersal by different dispersal agents.

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Biology Commons

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