Understanding consequences of the global erosion of species for the integrity of the biosphere is one of the grand challenges for biological sciences in the 21st century. Biodiversity determines the function of ecological communities, including how they respond to major threats to human welfare such as climate change and nutrient loading. The overall aim of the research in the group of Marine Ecology is therefore been to understand how biodiversity loss (in the broad sense) affects the function and resilience of coastal communities.

Current projects

The destruction of coastal fish communities through modification of our coastlines is one of the largest ecological catastrophes in northern Europe. Fish in the Wadden Sea is in dramatic decline. Especially threatened are those fish that migrate and use the Dutch coasts for part of their life-cycle and that are constrained by barriers such as seawalls and dams. Prioritized research goals are therefore to understand the importance of coastal habitats for fish, why coastal fish is in decline, and how fish in turn shape their own habitat and its wider ecosystem.

The swimway of fish

Currently we are exploring how fish use the Wadden Sea during different parts of their life-cycle. We have two main targets: 1) to document the function of foreshore salt-marshes for fish; and 2) to experimentally test the function of small-scale reef structures for fish.

Salt-marshes: The Netherlands have battled the sea for centuries and provide a template for the engineering solution to safe-guard highly populated coastal areas from rising sea levels. The Dutch solution is tempting because it provide instant safety by constructing a hard land-sea border that protects us from marine intrusion. At the same time it is an ecological catastrophe, transforming multi-functional wetlands that provide a portal between fresh and salt, into an impermeable barrier of land. Here we explore the function of the leftover salt-marches for fish. These man-made marshes outside the sea-wall are mainly managed for grazing livestock and birds, and are traditionally not acknowledged as valuable for marine organisms. We show that these foreshore marshes lack important aspects of habitat variability that occur in natural marshes, but that they still are valuable for fish and need to be accounted for when designing management plans for the failing coastal fish ecosystem in the Netherlands.

Reef restoration: Many fish use hard substrate habitat types during their life cycle for different functions; such as for spawning grounds, nurseries, hunting areas, and shelter. Historically, the Wadden Sea was connected to a large inland marsh landscape and was composed of a diversity of hard substrate types, including sublittoral shellfish and tube worm reefs, rocks of different sizes (from glacial deposits of boulders to gravel), driftwood, and hardened peat. The benthic habitats have been impacted by bottom-contact fisheries, including direct harvest of the hard substrates themselves, as well as coastal development and dredging practices homogenizing the bottom substrate. Consequently, the presence of sublittoral hard structures have  decreased dramatically in the Wadden Sea compared to historic records. In this project we test effects of different types of reef restoration on the fish community.

The soundscape of marine organisms

Another main interest of the group is to explore the sounds of mobile marine organisms. We deploy hydrophones to document the underwater soundscape with the purpose of documenting spatial and temporal distribution of fish and marine mammals. Our research questions include: Can we use sound to monitor fish communities and their behaviour in the Wadden Sea? What are the migration calls of fish? What do porpoise do along the Dutch coast?

The stickleback wave

Regime shifts are commonly deducted from simple time-series data that average across a larger system. The temporal-only perspective is limiting and represent spatially heterogeneous (natural) systems poorly, because theory predicts that in large ecosystems with environmental gradients, shifts should start locally and gradually spread through space. Thus, despite well-developed theory, many empirical examples, and implementation of critical transitions in European policy and management legislation; we are still far from understanding how to detect and manage regime shifts. By compiling a dataset with both a long temporal perspective and a high spatial resolution, we have been able to empirically document a spatially propagating shift in the trophic structure of a large aquatic ecosystem, from dominance of large predatory fish (perch, pike) to the small prey fish (the three-spined stickleback) (Eklöf et al. 2021). This shift has propagated like a slow moving wave through the system for the past 30 years. Mechanistic experiments and modelling also showed that the different “trophic states” were reinforced by strong biological feedbacks. Currently we explore options how to reverse the “stickleback wave”.

Biodiversity change

One of the large enigmas in biodiversity research is that the documented erosion of species globally, is not reflected by decreasing number of species in local monitoring data. Baffled by this phenomenon, we have together with internationally leading biodiversity researchers addressed the mechanics of biodiversity change (Elahi et al. 2017, Hillebrand et al. 2018, Eriksson and Hillebrand 2019). Thus, a major challenge for biodiversity research right now, is to predict the complex consequences of rapid compositional changes and turnover of species, which we are trying to do by analysing long-term datasets and developing species turnover metrics.