Eelgrass & climate change: Storm disturbance erodes ecosystem engineering

Background/Question/Methods   Extreme storm events, such as hurricanes, are predicted to increase with climate change. On the coasts, such storms carry high winds that create damaging ocean waves. In this study, we investigate the ability of an established marine ecosystem engineer (eelgrass, Zostera marina) to ameliorate potentially damaging waves and allow for population persistence. Previously presented field data suggest that a canopy of adult eelgrass shoots creates a calm hydrodynamic environment, preventing eelgrass seedlings from washing away. Here, we use a stochastic matrix-modeling simulation to theoretically manipulate the strengths of disturbance via storm waves and positive density-dependence via ecosystem engineering. In the model, disturbance increased adult vegetative shoot mortality, and adult vegetative shoot density increased seedling survival. Model parameterization was inspired by over 3 years of experimental and observational field data.

Results/Conclusions   Our model shows that eelgrass is capable of persisting at low rates of disturbance, regardless of positive ecosystem engineering. At intermediate disturbance rates, a proportion of modeled populations went extinct over the period of the simulation, while others became stably vegetated. As disturbance rate increased, more of the populations went extinct, though populations with stronger positive density-dependence had lower extinction rates. Eventually, disturbance did erode positive density-dependent recruitment, as eelgrass did not persist at all at the highest rates of disturbance. These results suggest that eelgrass may be ably to persist as a patchy population at intermediate levels of disturbance. However, these populations are at risk of extinction if storm frequency increases with climate change. Extinctions of this habitat-forming plant are likely to have cascading effects on its animal inhabitants, including economically-relevant fish and shellfish.