Eco-evolutionary shifts in plant-insect-microbe interactions over community succession

Abstract

Plant defenses are typically predicted to increase over ecological succession, paralleling increases in the abundance of herbivores. While this successional escalation of defenses has been widely observed on an interspecific level, little is known about how the defense phenotypes of plant populations change over succession and succession additionally presents a dynamic environment to study the factors that influence herbivore resistance within species. Here I use tall goldenrod, Solidago altissima (Asteraceae), as a model system to examine intraspecific patterns of plant defense over succession. Chapter 1 examines changes in the resistance of S. altissima plants to insect herbivores over a gradient of oldfield succession in a large-scale field experiment and assesses the roles of microevolution and phenotypic plasticity in mediating these shifts in resistance. We find that both genotypic shifts in plant populations and soil environmental changes affect plant herbivore resistance. Chapter 2 reviews how anthropogenic disturbances, such as agriculture, can alter plant-microbe interactions and discusses how advances in methodology, particularly soil microbiome transplant experiments, can be used to assess the impacts of these disturbances on important plant ecological interactions. Chapter 3 applies these methods to characterize changes in S. altissima plants’ interactions with soil bacteria and fungi over oldfield succession in the field and assess the potential of these successional microbial shifts to affect plant herbivore resistance. We find that the microbial communities that colonize S. altissima shift over oldfield succession and that experimentally inoculating S. altissima plants with later succession microbiomes confers greater resistance to their main specialist herbivore, Trirhabda virgata, paralleling the patterns of herbivory observed in the field. Chapter 4 explores the potential of this microbe-mediated herbivore resistance that we have observed in S. altissima plants inoculated with late succession soil microbiomes to improve the pest resistance of crop species. We find that while microbiomes from different stages of succession differentially affect both plant growth and resistance, the effects are species-specific. Collectively, these studies indicate that microbiomes can play an influential role in determining the defense phenotypes of their host plants and that these microbiome effects can interact with other factors such as host genotype and species.