Restoring and sustaining oyster reefs: drivers of spatial dynamics in oyster populations

Foundation species define ecosystems by building physical structure and sustaining ecosystem services, such as carbon sequestration, nutrient cycling, habitat provision, and erosion control. The decline of foundation species has prompted intensified restoration efforts, yet their restoration has yielded inconsistent levels of success. In the case of oyster reefs, whose 85% global loss is among the most staggering collapses of coastal marine foundation species, variations in oyster restoration outcomes has been linked to insufficient supplies of offspring, unfavorable abiotic conditions, and negative species interactions. Understanding the demographic processes and ecological interactions driving oyster population dynamics would aid in identifying the drivers of variability in oyster restoration success, enhancing the efficacy of restoration planning and project outcomes. For my dissertation, I explored how hydrodynamics (wind fetch and water residence time), geomorphology (topography and elevation), and biotic forces (competition and predation) influence oyster recruitment, growth, and survival. First, I examined the spatial patterns of recruitment in the eastern oyster Crassostrea virginica and the role of hydrodynamics, substrate elevation, and restoration age in driving these patterns using a standardized multi-year field study and a long-term oyster monitoring survey in coastal Virginia. To combine these data, I validated a method for measuring recruitment on standardized ceramic tiles by comparing recruitment densities with those on natural oyster shell on the reef substrate and found that tiles were a strong indicator of recruitment to oyster shells on the reef. Combining these datasets, recruitment across the coastal landscape (~670 km2) showed a unimodal relationship with wind fetch, indicating that sites with moderate wind-wave exposure yield greater recruitment than locations with excessively short or prolonged fetch. Recruitment showed no relationship to water residence time, an indicator of water flushing. Recruitment also increased with substratum elevation, possibly due to reduced sedimentation or predation on higher, shallower reefs (relative to reefs at lower or deeper elevations). Recruitment to natural reef substrate was higher on older reference reefs representing ideal restoration endpoints than restored reefs of any age and showed no difference between mature (7-16 years since reef construction) and developing (< 6 years) restored reefs. Second, to explore demographic trade-offs during the early life-stages of oysters, I identified the environmental conditions that maximize oyster recruitment, growth, and survival in Virginia using observational field studies and a predator-exclusion experiment. Recruitment was highest on oyster reefs with smoother topography (less variation in elevations), and in areas that were more sheltered and had lower water flushing. Oyster recruit size was largest on substrata with fewer barnacles competing for space. Oyster growth increased with water flushing (shorter water residence times), possibly due to enhanced food availability or feeding efficiency. Juvenile oyster survival was higher with fewer nearby barnacles and positively linked to oyster size, possibly due to reduced competition and predation. The 5-week field experiment revealed predation as a significant source of juvenile oyster mortality in the deeper subtidal zone compared to the upper and mid-intertidal zones. Overall, it appears that favorable recruitment environments might come at a cost to growth due to constraints in food resources. Conditions that promote survival may not necessarily be conducive to recruitment, as there are potential trade-offs associated with higher conspecific densities, such as competition for settling space and resources. Consequently, incorporating demographic trade-offs and predation into restoration planning would enhance the effectiveness of these projects. Third, I quantified the magnitude, uncertainty, and drivers of predator effects on oyster recruitment and mortality using a global meta-analysis synthesizing predator-exclusion experiments across four oyster species (C. virginica, C. gigas, Ostrea edulis, and O. lurida). Predators caused an average 46% decrease in oyster recruitment and an average 4.3× increase in oyster mortality. Predation increased with oyster size and varied with predator identity and richness (with gastropod predators showing the strongest negative effects on oysters). Unexpectedly, there were no effects of latitude, tidal zone, or tidal range on predation strength. Predator effects differed with experiment type (enclosures vs. exclosures), oyster tethering method (naturally settled vs. glued oysters), and experiment duration, indicating the importance of experimental design and the caution warranted in extrapolating results. These results suggest that consideration of the drivers of oyster predation in restoration and conservation planning may hasten recovery of oysters. Overall, my dissertation improves basic understanding of the causes of variation in oyster population dynamics by quantifying the drivers of oyster recruitment, growth, and survival. Furthermore, the knowledge gained from this research is useful for the spatial planning of future restoration projects, determining potential factors that impede or aid restoration success, and the management of oysters as a natural resource.