The Role of Environmental Gradients and Sea to Land Nutrient Transfer in Shaping the Ecology of a Large Herbivore

This dissertation examines the ecological dynamics of Sable Island, Nova Scotia, Canada, a remote and nutrient-limited island ecosystem where marine-to-terrestrial nutrient transfer plays a critical role in shaping primary productivity and trophic interactions. Island ecosystems, though relatively understudied, provide valuable insights into cross-boundary nutrient fluxes, energy pyramids, and biodiversity-ecosystem functioning. Understanding these systems contributes to broader ecological theory, particularly within the framework of meta-ecosystem theory, which emphasizes the importance of spatial resource exchange in structuring ecological communities. Chapter 1 introduces the theoretical foundations underpinning this research, situating Sable Island within the broader context of island ecology and meta-ecosystem theory. This chapter reviews key concepts related to energy flow, nutrient cycling, and resource regulation, synthesizing their relevance to the study of feral horses (Equus ferus caballus), a keystone species in this system. Additionally, it provides an overview of Sable Island’s ecological history, vegetation dynamics, and its unique role as a study system where terrestrial and marine influences intersect. Chapter 2 explores the fine-scale dietary ecology of Sable Island’s feral horses using a combination of fecal metabarcoding, individual horse metadata, and vegetation surveys. The analysis reveals significant spatiotemporal variation in diet composition, with horses on the east and west ends of the island exhibiting distinct seasonal dietary shifts. These findings underscore the role of localized resource availability and habitat heterogeneity in shaping foraging behavior. Chapter 3 extends this investigation to a broader temporal scale by utilizing stable isotope analysis to examine inter- and intra-annual dietary patterns. This chapter also introduces a novel method for anchoring segmented tail hair chronologies using environmental data, facilitating reconstruction of dietary histories in a system where direct observation is temporally constrained. Additionally, the study uncovers a link between δ¹⁵N variability in marram grass (Calamagrostis breviligulata) and grey seal (Halichoerus grypus) pup densities from the preceding winter, providing evidence of dynamic marine-to-terrestrial nutrient subsidies that shape the island’s ecological landscape. Chapter 4 addresses the decomposition dynamics of the two predominant sources of mammalian carrion on Sable Island—grey seals and horses—and examines their respective contributions to nutrient cycling. The findings indicate species-specific differences in the temporal and spatial extent of nitrogen enrichment in vegetation, further elucidating the complexity of nutrient transfer in this system. Chapter 5 revisits the movement ecology of Sable Island’s feral horses, building upon the dietary patterns identified in Chapter 2. This chapter investigates the drivers of seasonal migration on the island, highlighting the role of vegetation structure, landscape features, and water source availability in shaping movement patterns on the east and west sides of the island. Finally, Chapter 6 synthesizes the findings from preceding chapters, emphasizing the ecological significance of resource-mediated regulation in the system. My work contributes to a broader understanding of island ecosystems, demonstrating how marine subsidies, resource heterogeneity, and animal movement interact to shape ecological dynamics. By integrating fine scale dietary analyses, stable isotope analyses, and decomposition research, my work advances our understanding of the processes governing energy flow and nutrient cycling in island environments, with implications for meta-ecosystem theory and conservation biology.