Yurek S, Eaton MJ, Lavaud R, Laney RW, DeAngelis D, Pine III, WE, La Peyre MK, Martin J, Frederick P, Wang H, Lowe MR, Johnson F, Camp EV, Mordecai R. Modeling structural mechanics of oyster reef self-organization including environmental constraints and community interactions. Ecological Modelling 440:109389
Self-organization in reef-building systems is a process of establishing reef morphology on aquatic landscapes from substrate generated by the population, and reinforcing these structures through interactions between internal processes and external factors. In oyster reefs, internal dynamics include production of calcareous shell, which serves as settlement substrate for larval recruits. External factors include environmental conditions and predation, which regulate overall population size through growth and mortality, but also regulate settlement dynamics by exposing interior shell surfaces through mortality. Oyster reefs are also highly spatially constrained by aquatic conditions, thus their efficiency for producing settlement habitat under these constraints may be critical to self-organization and long term persistence. We developed an individual based model that simulates engineering of oyster reefs through individual contributions of shell, which slowly degrade and consolidate to form reef structure. Reef habitat has two aspects in this model, one for elevating the population above the benthos, and another for making exposed shell surfaces available for settlement. We applied the model to examine how these two aspects relate to the live population as a coupled system with complex feedbacks. In particular, we examined how temporal dynamics of the reef proceed through time as the size structure of the live population and relative composition of shell types change through time. To represent these dynamics, we simulated single restoration events and tracked ensuing dynamics over subsequent decades without additional enhancement, for an example study site in South Carolina (USA). To estimate uncertainty in restoration performance, we followed a biological ensemble modeling approach, varying selected model parameters over five scenarios of predator community composition. Our goal was to identify trends that were robust across simulations, which could serve as hypotheses and predictions for future field studies. The overall temporal pattern of simulations was three distinct phases: initial transient dynamics of the stocked population, followed by growth and saturation of the life population, and then saturation of settlement habitat several years later. All simulations incurred considerable loss of shell biomass during the transient phase when the live population was establishing and shell degradation exceeded production. Simulations with predators were able to recover from this decline through production of live oysters, while simulations without predators continued to decline throughout simulation runs. These results indicate that reefs can be productive with respect to the live population, but decline overall in reef substrate. We conclude with hypotheses relating the efficiency of generation of settlement habitat to measures of biomass and individual density, which suggest levels that may lead to reef self-organization.