Causes of variation in damage by folivores: The roles of ecosystem productivity and habitat complexity

Abstract

Understanding the causes of variation in herbivore damage among individual plants is important because it 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 addressed four 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 and abundance of insect orders? Two sites with four 40 m X 40 m plots each were chosen for ecosystem productivity manipulations and individual plants within these large plots were used for habitat complexity manipulations. Nitrogen was added to experimentally increase ecosystem productivity. Plant litter and neighbor plant species richness were manipulated for habitat complexity. I found no significant effects of ecosystem productivity and habitat complexity on folivore damage, measured as the proportion of leaves damaged over >50% of their area per tall thistle rosette. Only seasonal variation was observed for folivore damage to tall thistle rosettes with more damage in August and September in 2007 and 2008, respectively. No significant effects of ecosystem productivity and habitat complexity were observed on rosette growth in 2008, but in 2007 a marginally significant effect of litter was observed on rosette growth and there was a significant effect of litter upon survival. No significant interaction effect of ecosystem productivity and habitat complexity was seen on herbivore damage to tall thistle rosettes. Only a vi i few significant effects of ecosystem productivity were observed on biomass and abundance of insect orders, including Orthoptera, Hymenoptera, Heteroptera, Coleoptera. There were weak interaction effects of ecosystem productivity and litter upon insect biomass for the orders, Orthoptera, Heteroptera, Coleoptera and strong effects upon Hymenoptera. Two novel contributions of my research are large-scale manipulations of ecosystem productivity and examining effects of litter manipulation on plants at post-seedling life stages.