Species have complex and contextual relationships with their environment; both the relative contributions of life-history stages to population growth and the effect of environmental factors on each stage can be different among co-existing species. Timing and extent of reproduction, survival, and mortality determine population growth, species distributions, and assemblage patterns. I evaluate the role of habitat (intact, degraded) and microsite (shrub, leaf litter, bare sand) on population dynamics of Florida scrub herbs. Isolated overgrown shrubs and extensive bare sand areas in degraded scrub were expected to decrease seed predation, reduce competition of herbs with shrubs, and provide larger habitat for recruitment. I provide evidence that habitat and microsite variation influenced demography of five endemic and two common native species through effects on seed removal, emergence, and establishment. Habitat and species affected seed removal: endemic species with large seeds were removed in higher frequency in degraded habitat, likely by vertebrates, while species with small seeds were removed in higher frequency in intact habitat, by invertebrates. There was no evidence of differences in individual seed production between habitats for the two common species, C. fasciculata and B. angustifolia. Invertebrates were primarily responsible for seed removal of both species, although peak season of removal and microsite varied with species. Removal of seeds, emergence, and establishment increased with seed density. Matrix modeling indicated that population growth of C. fasciculata was greater in degraded habitat and greatest in litter microsites, and population growth of B. angustifolia was similar between habitats and greatest in bare sand. Contrasting responses among species to environmental factors in intact and degraded scrub indicated that natural disturbances are not ecologically equivalent to anthropogenic iv disturbances. Idiosyncratic species dynamics in common environments suggest that understanding relationships between life-history traits and environmental conditions will be required to facilitate restoration

Master of Science / Department of Entomology / Brian P. McCornack / Winter canola production in the south-central US is commonly threatened by a complex of aphid species that can cause up to 70% in yield loss. Aphid species vary in their life-history traits, performance (sequestration/excretion of secondary compounds; glucosinolates), vertical distribution within the plant, and temporal dynamics across the growing season. Colonizing behavior of these aphids may be affected by intrinsic characteristics of the host plant (bottom-up effects), such as nutritional value, secondary compounds, or plant architecture. Understanding bottom-up effects may enable the evaluation of plant-level interactions that are influencing predator-prey dynamics. The goal of my research project is to understand aphid population dynamics in different canola plant structures, assess whether aphid quality (sequestration/ excretion of glucosinolates) is influenced by feeding location on the canola plant, and if so, assess the impact on the existing predator communities, specifically the development and fitness of immature and adult Hippodamia convergens. A combination of filed and greenhouse experiments provided novel contributions that will help shape our understanding of key factors regulating aphid population growth in canola fields, which will lead to more judicious use of insecticides and better sampling strategies.

In this thesis, I examine variation in occurrence and performance of glabrous and trichome-producing plants of the perennial herb Arabidopsis lyrata. I combine field studies and genetic analysis to (1) examine the function of trichomes as a resistance character in natural populations, (2) compare the magnitude of population differentiation in trichome-production and at putatively neutral marker loci, (3) examine the molecular genetic basis of trichome-production, and (4) quantify the effects of herbivore removal on population growth and relative performance of glabrous and trichome-producing plants. In a survey of 30 populations of A. lyrata in Norway and Sweden, I documented spatiotemporal variation in damage from insect herbivores. With few exceptions, glabrous plants were more damaged by herbivorous insects than trichome-producing plants in polymorphic populations. Damage levels varied substantially among populations and among years. The intensity of herbivory quantified as mean leaf removal to glabrous plants was higher in polymorphic populations than in monomorphic glabrous populations. Within the Swedish range, populations were more strongly differentiated at the locus coding for glabrousness than at eight putatively neutral isozyme loci. This is consistent with the hypothesis that trichome production is subject to divergent selection. A study of the genetic basis of trichome production showed that glabrousness was associated with mutations in an orthologue to GLABROUS1, a regulatory gene known to cause glabrousness in A. thaliana. Comparative data indicate that the genetic basis of glabrousness varies among populations. Experimental removal of insect herbivores in a natural A. lyrata population increased population growth rate and the relative fitness of the glabrous morph. The results suggest that insect herbivory may influence both population dynamics and selection on trichome production in A. lyrata.