Variable Responses to Consumer and Nutrient Availability in Coastal Wetland Plants

Abstract

Top-down and bottom-up factors, such as grazing and nutrient availability, are powerful forces that control patterns in plant communities across diverse ecosystems. In most ecosystems, however, testing the effects of top-down and bottom-up forces on plant communities has been focused on a subset of plant traits or a small portion of the species in a plant community. Thus, the general effect of top-down and bottom-up controls may often be unknown. In this dissertation, to help address this intellectual void, I explore how grazers, a top-down force, and nutrient additions, a bottom-up force, influence multiple plant traits, plant diversity, and range limits in two coastal wetlands. In my first chapter, I analyzed data from a fully factorial field experiment in a salt marsh in Virginia, USA. I explored how grazing intensity and nutrient availability separately and interactively affect a suite of traits of the foundational salt marsh plant, Spartina alterniflora. This represents an analysis of unpublished data from experimental work first presented by Silliman and Zieman 2001. Three trait categories were measured for a total of nine responses for belowground characteristics, litter production, and reproduction. Fertilization led to greater cordgrass belowground biomass (p= 0.039). In comparison, the highest grazing intensity negatively impacted belowground biomass (p= 0.049). These grazing results contrast past studies suggesting grazer impacts are relegated to aboveground plant growth in this system. In addition, grazers and fertilization interacted to alter standing dead mass (p<0.0001), leaf litter (p=0.0001), and flowering (p=0.011). These results revealed both predicted and unexpected effects of grazing and nutrient availability on understudied traits in a foundational plant. These findings were not entirely predictable based on understanding the impacts on aboveground biomass alone. Next, I explored how nutrient addition and grazer presence impacted plant diversity and species-specific trait responses at the low-high marsh transition zone. The transition from one plant community to an adjacent plant community can be a zone of unique species diversity and interactions. These zones could be important indicators of environmental change due to climate change. From a multi-year factorial experiment, results show that species richness is negatively affected by grazer presence (p=0.0003) and nutrient additions (p=0.016). At the same time, nutrient additions alone negatively impacted Shannon’s diversity index (p=0.004). Additionally, my results show that grazers suppress while nutrients increase total aboveground biomass (gazers; p=0.001, nutrients; p=0.001). I also found that the top-down vs bottom-up effects varied by species. In plots with nutrient additions, the competitive inferior, S. alterniflora, increased in aboveground biomass, shoot length, and cover. This supports previous research in New England salt marshes. They found that nutrient additions invert the competitive hierarchy of marsh transition zones, favoring the stress-tolerant species. I predicted that grazers would act antagonistically to nutrient additions. The dominant grazers in U.S. southern salt marshes are thought to feed overwhelmingly on S. alterniflora. Therefore, I predicted grazers would help maintain diversity as S. alterniflora becomes the competitive dominant with nutrient enrichment. My results, however, do not support this hypothesis and suggest that grazers suppress diversity. This unexpected effect of grazers likely occurred because the marshes grazers in this study (insects and snails) acted as generalists, not specialist grazers in the high marsh. My results also contrast with the theory that states grazers generally help maintain plant species diversity in highly productive ecosystems. For the third chapter, I transitioned to seagrass habitats at their shoreward range limit. Here, physical forces, such as heat stress and desiccation, are thought to control the shoreward range limit. There has been limited consideration of testing the relative effects of biotic interactions in setting range margins under stressful conditions. Still, recent consumer-stress studies suggest these interactions could play a significant role. Using manipulative field experiments and observational surveys, I tested the effects of stingrays, a destructive forager in seagrass beds, and nutrient addition on the shoreward range limit of seagrasses. The stingray exclusion x nutrient addition experiment found that those exposed to stingrays had a greater loss in cover (p=0.037). In comparison, nutrient addition had no significant impact on seagrass cover at its upper range limit. In a follow-up experiment, I found that survivorship of seagrasses transplanted at higher intertidal elevations than observed increased when stingrays were excluded in 2022 (p<0.01) and again in 2023 (p<0.01). Finally, a multi-site survey found that stingray pit abundance strongly predicted the spatial extent of the intertidal range limit of seagrass. These findings suggest that consumers may be a more common contributing factor in setting range limits of plant communities on physically stressful boundaries than is currently thought. Throughout these experiments, I discovered that ecological theory, often used to inform and make decisions, does not always hold. In each chapter, I discuss how ecological theories should be more broadly tested across species ranges and traits, especially when used to make management decisions.