Understanding the causes of variation in herbivore damage among individual plants is important in that this understanding may lead to strategies for increasing damage to invasive weeds, thereby, reducing their population growth. My research focuses on variation in insect folivore damage to tall thistle (Cirsium altissimum) rosettes under differing levels of ecosystem productivity and habitat complexity. I address four specific questions. First, does ecosystem productivity affect the amount of tissue loss to insect herbivory? Second, does habitat complexity, in terms of litter and local species richness, affect tissue loss to insect herbivory? Third, is there an interaction effect between ecosystem productivity and habitat complexity on plant tissue loss to insect herbivores? Fourth, do ecosystem productivity and habitat complexity affect the fresh biomass of insect carnivores and herbivores? Two sites with four 40 m X 40 m plots each were chosen for ecosystem productivity manipulations and subplots within the 40 m X 40 m plots are used for manipulations of habitat complexity. Nitrogen was added to experimentally increase ecosystem productivity and litter (dead plant parts that are lying down) and species richness were manipulated for habitat complexity.

Results/Conclusions

In the first year of field work there was pronounced seasonal variation in leaf tissue removal from tall thistle rosettes, with highest levels of removal in August.  The mean (± standard error) proportions of leaves damaged severely (>50% leaf area damaged) in control plots were 0.06 ± 0.01 in May, 0.25±0.01 in August and 0.10±0.07 in October. Preliminary analyses of nitrogen and neighboring litter effects, suggest that neither ecosystem productivity nor local habitat complexity in the form of litter had strong effects upon magnitude of damage.  Although nitrogen addition increased plant standing crop by 16.7%, differences in proportion of leaves damaged severely between control and N-addition plots were not statistically significant at any of the three months.  Litter manipulation only appeared to affect damage to tall thistle rosettes in August in plots without added nitrogen where there was a trend toward higher levels of damage where litter was removed or added than where litter was unmanipulated. No evidence was found for an effect of local species richness manipulation on damage to tall thistle rosettes.