Interaction networks in coastal soft-sediments highlight the potential for change in ecological resilience

Recent studies emphasize the role of indirect relationships and feedback loops in maintaining ecosystem resilience. Environmental changes that impact on the organisms involved in these processes have the potential to initiate threshold responses and fundamentally shift the interactions within an ecosystem. However, empirical studies are hindered by the difficulty of designing appropriate manipulative experiments to capture this complexity. Here we employ structural equation modeling to define and test the architecture of ecosystem interaction networks. Using survey data from 19 estuaries we investigate the interactions between biological (abundance of large bioturbating macrofauna, microphytobenthos, and detrital matter) and physical (sediment grain size) processes. We assess the potential for abrupt changes in the architecture of the network and the strength of interactions to occur across environmental gradients. Our analysis identified a potential threshold in the relationship between sediment mud content and benthic chlorophyll a, at -12 microg/g, using quantile regression. Below this threshold, the interaction network involved different variables and fewer feedbacks than above. This approach has potential to improve our empirical understanding of thresholds in ecological systems and our ability to design manipulative experiments that test how and when a threshold will be passed. It can also be used to indicate to resource managers that a particular system has the potential to exhibit threshold responses to environmental change, emphasizing precautionary management and facilitating a better understanding of how persistent multiple stressors threaten the resilience and long-term use of natural ecosystems.