To understand factors that shape emergent evolutionary radiations, community assembly, and the maintenance of diversity in changing landscapes, my work combines molecular and ecological population assessments of a novel plant-herbivore interaction. In this dissertation I examine how deterministic and stochastic factors shape ecological interactions and population structure in the Chinese tallow tree, Triadica sebifera, and a newly discovered herbivore in the invasive North American range, Caloptilia triadicae. Much is known about evolved variation in invasive Triadica populations, but the extent to which stochastic and deterministic factors shape post-invasion species interactions with Caloptilia, the only known specialist in North America, remains unclear in this and many other systems involving invasive species. To characterize how Triadica-Caloptila associations vary according to geography and host genetics, I conducted an observational field study, incorporating a range of genetically variable Triadica populations (intrinsic deterministic factors), climate (extrinsic deterministic factors), and isolation by distance (stochastic factors). I found that geography and genotypic variability in the tree explained the most variation in leaf toughness and caterpillar abundance. To confirm the role of host genetics on this interaction, I also conducted a common garden experiment to assess the role of geographically structured genetic variation of Triadica in shaping interactions with Caloptilia, while controlling for confounding climatic gradients and distance related stochastic processes. The common garden study reinforced the conclusion that intrinsic deterministic factors are important drivers shaping this interaction, but also revealed that seasonal changes mediate the relative impact of host genetics and microhabitat. Using measures of genetic variation, I also conducted molecular analyses to determine if the ecological patterns I observed in my field and common garden studies emerged as patterns of genetic structure between host and herbivore. The results of this study confirmed the primary role of geographic isolation, and a secondary role for host genetic identity, as drivers of herbivore population structure. Overall, this dissertation informs our current understanding of the role of host genetics, geographic isolation, microclimate and seasonality in shaping mosaics of species interactions. Additionally, this work expands our understanding of how these factors influence the applied conservation of habitats threatened by invasive plants and outbreaking novel herbivores. / 1 / Rebecca Frances Hazen

This dissertation addresses the question of how leaf litter from trees affects animals that live in aquatic environments, with an emphasis on the effect of Chinese tallow (Triadica sebifera) leaf litter on anuran larvae (i.e., frog tadpoles). This question is important to our understanding of how allochthonous inputs to aquatic habitats drive biodiversity in wetlands. It also addresses a timely conservation concern in southeastern Louisiana where invasion by Chinese tallow trees (Triadica sebifera) is displacing native trees. The invasion process is homogenizing forest composition and changing the quantity and quality of litter inputs to ponds from those produced by a mixture of native species to that of a single invasive species. This change in litter quality may have important effects on aquatic animals because leaf litter that falls into ponds is an important source of nutrients and energy in wetland foodwebs. Leaf litter also affects water quality via effects on dissolved oxygen and leaching of defensive compounds, which may subsequently affect the diversity and performance of aquatic animals. Herein I address these issues by presenting a series of studies in which tadpole and aquatic invertebrate responses were tested using leaf litter from Chinese tallow leaves and three native tree species. The major findings of this research are: (1) Leaf litter has a direct effect on water quality (2) Chinese tallow can cause differential survival and performance of tadpoles (3) Differences in water quality due to leaf litter can cause changes in tadpole behavior (4) Chinese tallow leaf litter breaks down much faster than litter from native trees (5) Difference in litter breakdown rates influence aquatic community composition.

Global climate change is predicted to affect timing and severity of disturbance events (e.g., fire, drought, hurricanes, wind storms, and inundation), but the extent of these disturbance events and their impacts on natural ecosystems may vary regionally. Rising sea level, increased frequency and intensity of tropical storms, and altered inundation regimes are likely to create changing environmental conditions in low-lying coastal ecosystems. These large scale disturbances may increase resource availability and regeneration spaces, reduce competition, and possibly increase community vulnerability to invasion. Shifting disturbance regimes and invasion together are predicted to drive long-term shifts in coastal plant community structure and ecosystem processes. However, impacts of altered environmental conditions on native and invasive plant species and the species responses to changed environmental conditions are poorly understood. The aims of this study were: (i) to assess the probability of occurrence of juveniles of the invasive exotic Triadica sebifera and co-occurring native species, Baccharis halimifolia, Ilex vomitoria, and Morella cerifera in the field in relation to surrounding environmental factors, (ii) to assess the effects of elevated salinity across a typical coastal transition on germination of T. sebifera, B. halimifolia and M. cerifera, using controlled growth chamber and greenhouse experiments, (iii) to assess the effects of climate change and shifting inundation and tropical storms regimes on T. sebifera, B. halimifolia and M. cerifera, and (iv) to evaluate the role of vesicular arbuscular mycorrhizae (VAM) on spread of invasive T. sebifera in coastal transition ecosystems at the Grand Bay National Estuarine Research Reserve (GBNERR), Coastal Mississippi, southeastern USA. Results from assessing the probability of occurrence of juveniles of invasive and co-occurring native species showed that soil water conductivity (i.e., salinity) was the major factor related to the occurrence of invasive T. sebifera and native B. halimifolia, I. vomitoria and M. cerifera along the coastal transitions. Probability of occurrence of the invasive T. sebifera was significantly related to landscape factors and occurrence was highest in close proximity to roads, trails, power lines, and recreational sites, and water bodies. These results imply that future increases in salinity will negatively impact I. vomitoria, M. cerifera, and T. sebifera, leading to range contraction of these species away from the coast. However, natural and anthropogenic disturbances that often increase resource pulses and reduce competition, likely increase the dominance of T. sebifera in already invaded areas. Positive effects of landscape structures on T. sebifera occupancy highlight the role of landscape variables in promoting new invasions in coastal forests of the southeastern USA. Controlled growth chamber and greenhouse germination experiments demonstrated that germination of all species (i.e., T. sebifera, B. halimifolia, and M. cerifera) decreased with elevated salinity and that the reduction was most pronounced in soils from the most seaward zones along the coastal transition. Although native B. halimifolia was least sensitive to elevated salinity, invasive T. sebifera displayed plasticity of germination trait across different salinity levels in most inland soils. These results suggest that the phenotypic plasticity may facilitate spread of Triadica sebifera under some degree of salinity stress in more inland section of the coastal transition. A manipulative greenhouse experiment demonstrated that simulated canopy damage from intense hurricane winds and associated storm surge produced differential effects on survival and growth of native (B. halimifolia and M. cerifera) and invasive (T. sebifera) species at simulated different forest conditions common in the GBNERR. Invasive T. sebifera was by far the most shade tolerant of the three species and seedling survival under highly shaded conditions may provide it with a competitive edge over native species during community reassembly following tropical storms. T. sebifera may better utilize post-hurricane conditions (e.g., resource-rich empty spaces) and potentially increase its dominance in coastal forested ecosystems. The last experimental study revealed that invasive T. sebifera had higher VAM colonization compared to co-occurring native species both in controlled greenhouse and field experiments, and that the higher colonization leads to significant increases in aboveground biomass, supporting the hypothesis that VAM fungi strongly benefit the invasive species. These results suggest that the VAM colonization may be necessary for the initial establishment of T. sebifera along the coastal transitions. Furthermore, my research also suggested that T. sebifera was not allelopathic and did not interfere with growth of native species. Overall, the findings of this research provide insight into the impacts of climate change related shifts on performance of invasive and co-occurring native species across coastal transitions of the southeastern USA. Variation in invasive and co-occurring native species' performances under changed environmental conditions (e.g., elevated salinity and increased light availability) and improved mutualistic association between invasive T. sebifera and VAM fungi may drive increased invasion with frequent community reassembly of low-lying coastal ecosystems undergoing rapid climate change.

Understanding the ecology of invasive species is vital to curb the homogenizing of ecosystems, yet the pollination mechanisms of the Chinese tallow tree (Triadica sebifera) in its introduced habitat remain ambiguous. This study examines self-pollination, wind pollination, and flower-visiting insects of tallow in a bottomland hardwood forest and Longleaf pine savannah in the U.S. Gulf South. These data suggest that self-pollination and airborne pollination are possible, but likely rare occurrences, although the possibility of apoxisis was not investigated. Seed production in exclusion experiments was significantly less than in open-pollinated flowers, and wind dispersal of tallow pollen dropped to essentially zero 8 meters from the source. Results show that tallow is primarily bee pollinated, with external pollen loads of Apis, Melissodes, and halictids visiting at similar rates, and Xylocopa species visiting less frequently. The researchers believe that to date, this is the first study of the pollination mechanisms of T. sebifera in its introduced range and recommend further study to understand the ecology of this destructive invasive species.

Knowledge of sex allocation trade-offs with tree growth in insect-pollinated woody plants is limited, particularly in invasive plants. This study examined patterns of growth and reproductive investment in a North American invasive plant species, Triadica sebifera, I hypothesized that the energy limitations of smaller trees may result in the production of more male reproductive structures that are energetically less costly. Diameter at breast height was a significant predictor of seed and catkin mass and regression can describe these relationships across sites. Seed and catkin mass were positively correlated across sites. The relationship between the seed mass:catkin mass ratio and DBH was not significant, nor was seed mass:catkin mass and total investment. Results showed a significant positive relationship between total reproductive investment and tree size across sites. Seed mass:catkin mass ratio and reproduction investment showed substantial variation among individual trees of similar size within sites.

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