Dissertation Defense: Jewel Tomasula
Candidate: Jewel Tomasula
Major: Biology
Thesis Advisor: Gina Wimp, Ph.D.
Title: Effects of Environmental Factors on Intraspecific Genetic Diversity of a Foundation Plant Species and the Extended Ecological Consequences
Global change pressures increase environmental stress on natural ecosystems and lead to biodiversity loss. In Chapter 1, I reviewed how different global change pressures impact marine coastal ecosystems (e.g., salt marshes), focusing on effects on insects and their predators. While many pressures directly impact insect herbivores and their predators, anthropogenic nutrient inputs into coastal ecosystems impact them indirectly through the foundation plant species population. Then, in a field experiment (Chapter 2), I investigated which conditions of nutrient enrichment (pulse or press input; low, medium, or high amount) lead to salt marsh ecological collapse (i.e., salt marsh dieback). Stem-boring insect larvae responded to the nutritional quality of the host plant, Spartina alterniflora. Chronic, high nutrient enrichment increased larval density to levels that caused salt marsh dieback at the plot scale. Because S. alterniflora is a foundation plant species, patterns and levels of its genetic diversity may explain variation in salt marsh ecosystem responses to nutrient enrichment. To discover how S. alterniflora genetic diversity varies spatially along its natural environmental gradient (Chapter 3), I used microsatellite DNA markers to genotype plants collected from transects that spanned the inland habitat edge and tidal creek edge. I found natural environmental factors did not affect S. alterniflora spatial genetic patterns, and instead neutral mechanisms more likely explain them. In Chapter 4, I conducted a greenhouse experiment to test how S. alterniflora responds to varying levels of genetic diversity and nutrient enrichment. Importantly, belowground biomass significantly decreased under conditions of low genetic diversity and high nutrient inputs, which may help explain variation in salt marsh dieback found in previous studies. Finally, I tested arthropod food web responses to both nutrient enrichment and S. alterniflora genetic diversity by setting up fifty common garden plots in a two-year field experiment (Chapter 5). My findings revealed that both nutrient enrichment and plant genetic diversity structured arthropod community composition after one year of nutrient addition (pulse), but after two years (press) only the effect of nutrient enrichment persisted. Anthropogenic nutrient inputs into marine coastal ecosystems are an ongoing environmental problem. Through my dissertation research I determined which S. alterniflora responses to nutrient enrichment are mediated by genetic diversity, and which are not, and the consequences of these varying responses for the salt marsh ecosystem. Broadly, my work demonstrates how natural levels of intraspecific genetic diversity within a foundation plant species population have specific influence on ecological responses to global change pressures.