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Abstract
Eastern US salt marshes are productive ecosystems providing numerous benefits to coastal communities. Within marshes, the snail, Littoraria irrorata, is an abundant grazer feeding on the dominant marsh plant, Spartina alterniflora. Previous work has demonstrated that the sign and strength of Littoraria’s effect on Spartina depends on both the abundance and size structure of Littoraria populations. Both of these attributes have the potential to be influenced by processes acting at multiple levels of organization and across spatial and temporal scales. This dissertation aims to quantify several sources of intraspecific variability across Littoraria populations with the ultimate aspiration of informing variation in consumer effects across the entirety of this species’ geographic range.
By first understanding latitudinal variation in consumer population traits, we may be better able to explain coarse intraspecific patterns in consumer effects and the conditions under which consumer populations stimulate vs. suppress resource biomass. Across southeastern and mid-Atlantic US salt marshes, we quantified variation in Littoraria’s population density, size structure, resulting population biomass, and its effects on aboveground Spartina biomass over two growing seasons. Results from this study point towards the importance of regional intraspecific trait variation within the broader context of macroecological clines in species interactions. Furthermore, the large amount of unexplained variance in Littoraria density, biomass, and consumer effects partitioned among our experimental and survey plots within individual sites suggest that there are valuable sources of variation to consider at even smaller spatial scales. Lastly, we show that variation in Littoraria thermal traits interact with size-structure of the Littoraria population to generate divergent population-level (cross site) responses to temperature. For example, when site-specific thermal response parameters are applied to populations with varying size-structures, striking and unexpected patterns emerged in among-site performance.
These results collectively highlight the importance of considering variation in population size structure, distribution, and physiological traits when scaling up from individual consumers to cross-population consumer-resource interactions. Future work should strive to more cohesively integrate and resolve the relative importance of these sources of intraspecific variation in such a way that consumer effects can be more seamlessly explained.
By first understanding latitudinal variation in consumer population traits, we may be better able to explain coarse intraspecific patterns in consumer effects and the conditions under which consumer populations stimulate vs. suppress resource biomass. Across southeastern and mid-Atlantic US salt marshes, we quantified variation in Littoraria’s population density, size structure, resulting population biomass, and its effects on aboveground Spartina biomass over two growing seasons. Results from this study point towards the importance of regional intraspecific trait variation within the broader context of macroecological clines in species interactions. Furthermore, the large amount of unexplained variance in Littoraria density, biomass, and consumer effects partitioned among our experimental and survey plots within individual sites suggest that there are valuable sources of variation to consider at even smaller spatial scales. Lastly, we show that variation in Littoraria thermal traits interact with size-structure of the Littoraria population to generate divergent population-level (cross site) responses to temperature. For example, when site-specific thermal response parameters are applied to populations with varying size-structures, striking and unexpected patterns emerged in among-site performance.
These results collectively highlight the importance of considering variation in population size structure, distribution, and physiological traits when scaling up from individual consumers to cross-population consumer-resource interactions. Future work should strive to more cohesively integrate and resolve the relative importance of these sources of intraspecific variation in such a way that consumer effects can be more seamlessly explained.