Resilience and vulnerability of marine fish communities to change

Marine ecosystems are under pressure from a range of anthropogenic activities, such as overexploitation, habitat destruction and climate change. These pressures and their associated impacts have caused declines in many species across organism groups and ecosystems. This alteration of biodiversity is causing profound shifts in ecosystem structure and functioning, as well as the provisioning of goods and services vital for human wellbeing. Consequently, understanding ecosystem health and its underlying drivers have become a primary objective for science and management. One core element of ecosystem health is ecological resilience. Resilience in ecosystems refers to the ability to withstand disturbances and maintain its structure and functions. Resilient ecosystems have the capacity to absorb and recover from environmental changes, ensuring their long-term survival and the services they provide to society. Ultimately, studying ecosystem resilience in natural ecosystems plays a vital role in promoting the preservation and restoration of biodiversity, ensuring the long-term health and functioning of ecosystems.

Despite a solid theoretical understanding and a wide range of methodological approaches, estimating resilience in natural marine ecosystems is limited. This constrains the understanding and protection of resilience in marine ecosystems. Consequently, there is an urgent need to identify key indicators that can represent and serve to maintain ecological resilience. Ecological resilience depend on the diversity and distribution of species traits within and across scales. Therefore, the exploration of traits and functional diversity and their link to overall ecosystem resilience hold great potential to better understand resilience.

This thesis investigates how functional indicators can be estimated and quantified in natural marine ecosystems to assess ecological resilience. In Chapter 4, we applied an indicator-based approach to assess the potential resilience of ecosystems using the North Sea as an illustrative case study. More specifically, we use data on marine demersal (bottom-living) fish communities collected from scientific bottom trawl surveys combined with information on species traits. Furthermore, we investigate how key indicators of ecological resilience (i.e., functional richness, redundancy, response diversity and evenness) vary across demersal fish communities. In chapter 5, we provide insight into the relationship between the indicators of resilience, environment and overall stability using structural equation modelling (SEM). We find that environment is a strong driver of resilience and stability and that there is a tradeoff between the indicators of resilience in the European Sea. Finally in chapter 6., we investigate how biodiversity affects the overall measures of stability and resilience across scales, including both space and the level of biological organization. We demonstrate that the stability of a system is relative to the scale used. In this matter, we show how distance-based measures calculated from ordination scores can be used to quantitatively classify and evaluate the relative stability and resilience of ecological systems. This provide important insight into the driving processes of stability across scale and can help identify important drivers of stability across different scales.

This thesis can help guide future research and conservation efforts by providing an indicator-based assessment of the ability of functional indicators to represent resilience in natural ecosystems. We demonstrate the importance of using multiple indicators, an appropriate scale and applying system specific analysis when assessing resilience in natural marine ecosystems. Furthermore, we state that the inclusion of resilience indicators into management actions can help the preservation of biodiversity and resilience in natural ecosystems.