From genes to landscapes: direct and indirect interactions across ecological scales

Abstract

Interactions within and between species profoundly shape ecosystems, influencing community structure, ecosystem function, and evolutionary trajectories. This dissertation examines how direct and indirect interactions vary across spatial, temporal, and ecological scales using a multitrophic plant-herbivore network consisting of the invasive plant Russian knapweed (Rhaponticum repens), and its two biological control insects: the gall wasp Aulacidea acroptilonica and gall midge Jaapiella ivannikovi. I address three central questions: (I) What mechanisms drive plant-mediated interactions between insect herbivores? (II) Does R. repens ecotype selection influence its interactions with herbivores? and (III) How do climate variables and global change influence contemporary and future interactions between specialist plant-insect networks? To address these questions, I: (I) perform network analysis of plant-insect communities; (II) perform reciprocal transplants and phylogenetic analysis of R. repens populations; and (III) examine plant-insect-climate interactions across spatial and temporal scales. My findings emphasize the interconnectedness of ecological systems, revealing how plant-mediated effects drive indirect interactions between herbivores across ecological scales. Density-dependent interactions and gall community composition emerge as key drivers of herbivore performance, suggesting plant defense activation as the primary mechanism mediating asymmetric interactions between species. Rhaponticum repens exhibits evidence for rapid climate driven evolution in its invasive range resulting in dramatically increased resistance to herbivory. Climate modeling reveals complex responses to environmental change, including context-dependent herbivore facilitation under stress and unexpected species niche convergence under severe climate scenarios.