Date of this Version Insect herbivory is common, but the
conditions under which it reduces the fitness and population
size of plants remain poorly understood. We quantified
populationlevel impacts of floral herbivory by specialized
insects on cobweb thistle (Cirsium occidentale var. occidentale)
in a California coastal dune ecosystem, and then compared these
demographic effects to those published for Platte thistle
(Cirsium canescens) in similarly designed exclusion experiments
in the continental sand dune ecosystem in the Great Plains. As a
separate test of the strength of the seed-to-seedling linkage,
we quantified seedling establishment rates in seed addition
plots and compared these results to those obtained in herbivore
exclusion experiments. This is the first test of direct vs.
indirect methods of evaluating the potential impact of seed loss
in plant dynamics. Floral herbivory on cobweb thistle in coastal
dunes substantially reduced key components of plant fitness.
Reduction of insect feeding within flo