A simulation-based approach to assess the stability of marine food-webs and inform Good Environmental Status

The European Union Marine Strategy Framework Directive (MSFD) recognises that maintaining marine food-webs in Good Environmental Status (GES) is fundamental to ensure the long-term provision of essential ecosystem goods and services. However, operationalising food-web assessments is challenging due to difficulties in i) implementing simple but complete monitoring programmes, ii) identifying thresholds in monitoring indicators that inform when perturbations are diverting food-web state from GES and iii) in providing an integrative and complete picture of the (health) status of food-webs. In this context, stability assessments of marine food-webs could be useful to identifying the indicators that best track perturbation-induced changes in food-web state and the threshold boundaries that should not be exceeded to minimise the likelihood of losing stability. Yet, there is still a lack of systematic methods to perform such assessments. Here, we evaluate the potential of a simulation-based protocol to be used as a methodological standard for assessing the stability of marine food-webs. The protocol draws on the principles of ecological stability theory and provides a framework for assessing the trajectories of individual indicators during perturbation regimes and their robustness in detecting stability thresholds for marine food-webs. We tested the protocol on an open-ocean and deep-sea food-web modelled with the Ecopath with Ecosim suite. We concluded that indicators that quantify transfer efficiency through the food-web and measure the average trophic level of the community are optimal proxies for trophic functioning and structure to assess the stability of the system. Furthermore, we show how the approach can be applied to i) determine the impact of a loss of stability on the balance between trophic levels and ii) identify the biological components of the food-web that are most affected in scenarios of stability loss. Our findings could be useful for the ongoing debate on how trophic models and derived indicators can play a concrete and practical role in the food-web assessments in European seas.