Functional Biogeography: Leveraging Trait-Based Methodologies to Study Species Form and Function Across Scales

Abstract

Plant-herbivore interactions are crucial components of ecosystem structure and function, yet these interactions are susceptible to environmental variation across space and time. Therefore, understanding the spatial and temporal drivers of patterns in plant-herbivore systems is key to understanding controls on ecosystem structure and function. Here, I use a combination of observational and experimental approaches to study relationships between environment, plant traits, and herbivores in a model coastal ecosystem. I do so at fine spatial scales (within a single marsh) and broad spatial scales (hundreds of km of latitude). Specifically, I investigate the relationships between environmental gradients (the marsh elevational gradient and the latitudinal gradient), traits of smooth cordgrass (Spartina alterniflora), traits of the salt marsh periwinkle snail (Littoraria irrorata), and herbivory. My findings indicate that plant traits are shaped by both bottom-up (environmental) and top-down (consumer) forces, and that these forces interact. For example, latitudinal gradients in temperature are associated with variation in plant traits (e.g., toughness, C:N ratio), which in turn influence herbivore abundance and herbivory intensity. At the local scale, consumer pressure is greatest in the middle of the marsh elevational gradient, where snail density is highest. This variation in consumer pressure drives variation in plant traits across the marsh elevational gradient. Several of the relationships between variables observed in these higher-resolution small scale results match those found at lower-resolution across large spatial scales and provide unusual congruence across broad comparative spectrums. Further, I synthesize disparate bodies of literature within ecology – plant functional ecology, biogeography, and plant defense theory – in order to develop a synthetic theoretical framework for understanding how climate, plant traits, and herbivory interact. The framework focuses on temperature and solar irradiance as a key climate variables, and leaf mass per area (LMA) and leaf nitrogen (N) as central plant traits. These traits are not only directly influenced by temperature, but also indirectly through temperature-mediated changes in herbivore pressure. Furthermore, leaf traits themselves can influence herbivore pressure, thus creating a feedback loop within this interaction. My research highlights the importance of considering both direct and indirect pathways when predicting how global change drivers will alter plant traits, particularly in the context of invertebrate herbivory. By integrating across scales, this research contributes to a more holistic understanding of the effects of global change on the structure and function of ecosystems.