Patterns, pressures and processes : exploring biodiversity in eelgrass meadows and coral reefs

Biodiversity is the “variety of life” and refers to the collective variation at all levels of biological organization, including genetic, organismal, and ecological diversity. Higher levels of biodiversity are associated with healthy, productive ecosystems that provide crucial services to humans. However, climate change and increasing anthropogenic disturbances are driving global biodiversity loss. As a result, understanding how biodiversity is generated and maintained—and what this means for ecosystem function—is a central challenge in ecology. My thesis addresses this challenge by exploring mechanisms of community assembly and the ecological consequences of diversity in two productive coastal ecosystems: eelgrass (Zostera marina) meadows and coral reefs. Both systems are built upon foundation species, which provide habitat and alter the environment in ways that allow life to flourish. In Chapter 2, I draw from concepts in community ecology, testing for niche-based assembly processes by investigating spatial habitat partitioning among invertebrates within eelgrass shoots. I show that multiple mechanisms contribute to structuring these communities, with some species exploiting abundant resources found at the tips of shoots, while others avoid competition by spatially or temporally segregating. In Chapter 3, I investigate the impacts of recreational boating on eelgrass meadows and the biodiversity they support by assessing communities along a gradient of boat use intensity. I find that meadows within anchorages had shorter eelgrass shoots, lower biomass, and supported less abundant and diverse fish and invertebrate communities than meadows outside anchorages. Many conservation efforts are motivated by ensuring the presence of foundation species; however, I show that even if present the ability of foundation species to support life can be diminished by direct threats. In Chapter 4, I test whether increased herbivore diversity enhances algal consumption on Hawaiian coral reefs by assessing food preferences among different herbivorous fishes and conducting an ex-situ biodiversity–ecosystem functioning experiment. I show that increased herbivore diversity led to higher ecosystem function, with greater than expected consumption occurring at higher levels of diversity. Together, this thesis highlights the mechanisms driving biodiversity patterns and their role in supporting ecosystem function, while demonstrating the consequences of anthropogenic disturbance for coastal ecosystems.