Doctor of Philosophy (Ph.D.)
Virginia Institute of Marine Science
David S Johnson
Matthew L Kirwan
James E Perry
Plants can alter their chemical, structural (‘resistance strategy’), or morphological traits (‘tolerance strategy’) to deter or mitigate herbivore damage. Developed in terrestrial ecosystems, plant defense theory provides a testable framework for evaluating drivers of plant trait variation and defense strategy selection. Yet, it has rarely been tested in coastal vegetated ecosystems, where intense grazing can denude large spatial areas and disrupt ecosystem services and functioning. Through the lens of plant defense theory, this dissertation examines abiotic and biotic control on traits and defense in the tidal marsh plant, Spartina alterniflora, and assesses their influence on further grazing and herbivore distribution. As a foundation species, Spartina regulates how marshes keep pace with sea-level rise, thus herbivore removal of Spartina directly affects marsh resilience. In mesocosms, I manipulated salinity and simulated herbivory on brackish and freshwater Spartina to evaluate the growth-rate hypothesis and the optimal defense theory (Chapter II). Simulated herbivory reduced tolerance traits and brackish Spartina was better defended than freshwater Spartina, supporting these hypotheses. Elevated salinity caused greater variation in freshwater Spartina traits, suggesting climate-driven saltwater intrusion may not affect brackish marshes, but could mediate freshwater Spartina response to herbivory. In mesocosms, I assessed how nutrient enrichment affected Spartina defense against grazing from the marsh periwinkle, Littoraria irrorata (‘resource-availability hypothesis’ or RAH) (Chapter III). Trait variation was assessed across plant age (original versus clonal new stems), which can influence traits in terrestrial plants. Nutrients promoted tolerance traits while decreasing constitutive resistance, supporting the RAH. Newer stems had higher tolerance and resistance traits, implying they are better defended than older stems. Neither nutrient availability nor plant age stimulated Littoraria consumption in feeding assays, suggesting nutrient loading will not intensify top-down control, and may increase vertical accretion through enhanced tolerance traits. Lastly, I examined if Spartina traits influenced consumer fronts created by the purple marsh crab, Sesarma reticulatum (Chapter IV). Despite causing marsh die-off in New England, in southern marshes, including Virginia, the Sesarma front is moving inland, allowing tall-form Spartina to revegetate and prevent marsh loss. Others hypothesized that sediment characteristics, abiotic conditions, and predation pressure drive this movement inland. Here, I tested if Spartina palatability, nutritional quality, and accessibility also act as a driver, as plant traits can determine herbivore distribution in terrestrial ecosystems. A caging study then evaluated if Sesarma grazing directly shapes Spartina traits. Intense predation pressure in the low marsh and enhanced Spartina forage quality in the high marsh were the only significant predictors of Sesarma front movement. Grazing from Sesarma affected short- and tall-form Spartina differently. Herbivory increased palatability and reduced short-form Spartina’s ability to mitigate damage, while having little effect on tall-form Spartina. Thus, higher constitutive defense in tall-form and increased palatability of short-form Spartina further propagate the Sesarma front inland. Overall, this dissertation demonstrates that plant traits can influence ecosystem resilience, directly through biomass production, and indirectly by shaping herbivore distribution, and should be considered when assessing how coastal vegetated ecosystems are affected by climate change and anthropogenic disturbance.
© The Author
Wittyngham, Serina Sebilian, "Spartina Alterniflora Defense Against Herbivory" (2022). Dissertations, Theses, and Masters Projects. William & Mary. Paper 1673281470.