Conneely, David M. (1997)
The Effect of Competition and Defoliation on Resource Allocation and Survivorship in the Annual Species, Brassica nigra.
Bachelor of Arts, Pomona College, Biology.
Advisor: Frances Hanzawa.
Herbivory is a powerful evolutionary force. It may reduce plant fitness by terminating reproduction, increasing susceptibility to pathogens, and reducing competitive ability. Plants have evolved two main strategies to cope with herbivory – defense and regrowth. Plants defend themselves by producing constitutive and inducible secondary chemicals. They also regrow tissue to replace biomass lost to herbivores by increasing photosynthetic rates in undamaged leaves, translocating resources from roots, and transporting appropriate hormones to induce vegetative and reproductive growth. The ability of a plant to compensate for herbivore damage and maintain reproductive output depends on its life history and its environment. Annuals are fast growing plants that tend to mitigate the effects of herbivory by allocating resources for regrowth. The timing of herbivory and competition for limited resources can have strong impacts on resource allocation patterns. A number of studies have examined the impact of competition and timing of herbivory separately, but no studies have examined them together. In this investigation, I grew an annual plant Brassica nigra (Brassicaceae) at two population densities, and simulated defoliation at an immature stage. I was unable to examine the influence of timing of defoliation because B. nigra did not flower. As a result, I only examined dry weight and mortality. Total biomass measurements indicate that plants in both low and high density treatments nearly compensated for 25 and 50% defoliation. The main effects of competition and defoliation on total biomass was non-significant, and there was no interactive effect. Root:shoot ratio data indicate that plants in the high density, 50% defoliation, treatment had a smaller root: shoot ratio than plants in the low density, 50% defoliation treatment. Although the main effects of competition and defoliation on root:shoot ratios were non-significant, there was an interactive effect, indicating that high density plants at 50% defoliation regrew less biomass than low density plants at 50% defoliation. The main effects competition and density, and the interactive effect of both on mortality were non-significant.