Document: DAN-3-39-12

Population biology of native and alien thistles: Relative importance of competition and herbivory as a function of soil fertility.

GLUESENKAMP, D.*

University of California at Berkeley, Berkeley, CA 94720, USA ¹

Abstract:
While it is generally recognized that plant populations are structured both by competition for resources and by the effect of herbivores, we still have a limited understanding of when one factor is more important than the other. This study examined the population biology of two species of thistle which have been shown to be strongly limited both by competition with background vegetation and by insect herbivory. For each of the two thistle species I assessed the relative importance of competition and herbivory under conditions of low productivity and enhanced productivity by conducting a full factorial manipulation of site fertility (NPK fertilizer added vs. unfertilized), competition (plots clipped to remove background vegetation vs. unclipped) and herbivory (caged and sprayed vs. not). I measured treatment effects on the demography of cohorts of plants emerging from seeds I added (% germination, % survival, % reproduction) and on a per-plant basis (plant size, # herbivores/plant, seed production/plant). Both absolute responses and indices of response were calculated by normalizing for the response in unmanipulated plots. Both species responded similarly, and absolute responses and normalized responses displayed very similar patterns overall. Study plants responded very strongly to NPK addition in the absence of competition and herbivory. Intensity of competition and of herbivory both increased with fertilization. Overall, impact of competition was highly predictable. The impact of herbivores was more variable, among plants within a treatment, among treatments, among plant species, and among guilds of herbivores. While the impact of herbivores increased with fertilization, the impact of competition increased even more, suggesting that herbivory may generally be more important relative to competition at less productive sites and competition more important at productive sites.

Keywords: thistle, cirsium, seed predation, plant invasion

This abstract is being presented at: 11:00 AM in session:
Oral Session #72: Plant Competition.