Mechanisms driving species interactions and community structure on the phytochemical landscape

Identifying how variation in abiotic factors and species traits influence the outcome of ecological interactions is fundamental to our understanding of which processes affect community assembly and structure. My goal in this PhD was to describe how nutrient dynamics, plant physical traits, and chemistry affect plant-insect and plant-plant interactions. I pursued this goal in three different systems with natural history observations, comparative field studies, manipulative greenhouse experiments, analytical chemistry, and bioinformatics. I found that investing nitrogen in one tropical passion vine survival strategy did not come at the cost of strategies. Rather, increased soil nitrogen resulted in richer metabolomes, higher leaf toxin concentrations, longer vines, greater biomass, and more leaves with a superior ability to capture sunlight. Next, I investigated how plant traits facilitate the spread of invasive species in Texas, and what consequences their spread has for local communities. I found that lack of variation among native prickly pear nutritional qualities makes them suitable resources to an invasive cactus moth (Cactoblastis cactorum) that recently established in Texas. These findings have serious implications for prickly pear, and their associated food webs that will soon interact with the expanding invasive moth population. I also studied how competition and allelochemical weapons interact to facilitate spread of Guinea grass, a globally distributed invasive pasture grass, in Texas. I found that shading and allelochemistry by Guinea grass interacted to significantly reduce the recruitment and growth of native plants which results in communities with significantly reduced plant diversity. Finally, I compared passion vine phylogenetic, metabolomic, and quantitative traits to describe the drivers of host specialization by sympatric beetles and caterpillars in Costa Rica. I found that passion vine relatedness correlated with host usage in both groups, and different plant characteristics explained how similar each herbivore assemblage was among available hosts. These studies demonstrated the effects of resource availability on plant trait expression and specialized trophic interactions in different environments. Moreover, this dissertation showcased how the mechanisms which govern individual species interactions scale up to influence community structure on a mosaic phytochemical landscape.