Mussel beds are an important ecological component in the Wadden Sea. Mussels’ offspring settle massively in new suitable areas, forming seedbeds that may disappear again within months. The probability of a seedbed to survive the first winter is defined as seedbed stability; a definition that plays a very important role in the management of newly settled seedbeds. Many factors are important in the survival or extinction of seedbeds. Predation is thought to be particularly important during the first year after settlement and therefore key to survival. Many predators feed on mussel beds, but for most of them the potential to exterminate a seedbed is restricted by different factors such as prey selection or competition. Common starfish (Asterias rubens) are capable of concentrating/aggregating in high densities on mussel seedbeds making them an especially important factor limiting/affecting survival of mussel seedbeds. This study assesses the capacities of starfish as a mussel seed predator. It also provides tools and information to assess the risks of a seedbed being attacked and exterminated by starfish.
In Chapter 2 the role of temperature and shading on winter predation was studied. The results showed that temperature limits feeding rate and feeding activity of starfish during winter. However, starfish feeding rate exhibited very high sensitivity to temperature changes. Light intensity affected both feeding rate and feeding activity. We conclude that starfish may not be an important factor destabilizing seedbeds during the average winter, but its importance may grow along with the increasing mean winter temperature due to climate change.
In Chapter 3 the impact of salinity changes on predation performance and survival was assessed. Salinity is the main driver of species distributions in the Wadden Sea. Results show that salinity affected predation performance by reducing feeding activity and causing changes in prey selection. Moreover, as acclimation occurred, A. rubens predation performance improved in all treatments with survivors. We conclude that osmotic stress due to a salinity decreases determines A. rubens distribution, abundance and potential impact on the prey population. However this effect is influenced by the magnitude of the change in salinity and its timescale.
In Chapter 4 the effect of tidal currents on predation rate was assessed, however, the chapter also tackles the role of hydrodynamic stress amelioration by mussels on the A. rubens population. The results suggest that mussels interact with their own predator beyond the role of food source, by ameliorating environmental stress, creating an additional dependence link between the foundation species and the predator, which potentially has major implications for ecosystem structure and stability.
In Chapter 5, we assessed the role of mussel association with conspecifics at high densities on prey selection by A. rubens. We concluded that size selection does not always lead to an improvement in net profit. Size selection is a trade-off between energy yield and predation energy costs, which is affected by prey behaviour.
The results of the prior chapters were integrated in Chapter 6 with field observations and literature to develop a simulation model. This model was designed to simulate growth of mussels and starfish, predation by starfish and mussel mortality. It can also be used to predict the likely effect of future environmental change scenarios on the potential impact of A. rubens on this important resource.
In the general discussion, Chapter 7, previous literature, field data and the results from this thesis are summarised and reviewed to explain the spatial distribution of A. rubens in the Wadden Sea and the role of environmental conditions on A. rubens predation rate. Model simulations are used to answer the question: What is the role of A. rubens predation in mussel seedbed stability?