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The Wadden Sea is one of the most important stop-over sites for breeding and migrating birds. About 10–12 million birds per year use the area for foraging and consume about 25%–45% of the standing stock of macrozoobenthos. However, little is known about the influence of birds on the entire ecosystem. We conducted ecological network analysis (ENA) on an important bird breeding and resting site, the Norderaue tidal basin in the northern German Wadden Sea, to determine the influence of birds on the entire food web. The model was based on the yearly average biomass of system compartments empirically sampled every season at the study site. The analysis revealed a complex and well-connected food web (high flow diversity and effective link-density) dominated by short pathways but with a high throughput of energy. The system's energy had a high degree of freedom for its development. However, there is a strong dependency on external imports of phytoplankton due to the dominance of suspension feeders suggested by a low recycling and the detritivory to herbivory ratio. A large variety of bird species uses the area for foraging and induces a negative impact on their prey items revealed by the mixed trophic impact analysis. The food sources and also the competitors of the birds' prey items were positively influenced by the birds via indirect pathways in the network. Furthermore, there is a negative impact among the bird compartments probably due to competition between the bird species. A sensitivity analysis, conducted by changing the bird biomass in the model, revealed only small changes in the activity, complexity and connectivity of the system. The linkage to other system components could amplify these changes in the system functioning. It is therefore recommended to include birds in coastal food web studies which has rarely been done before. The use of holistic approaches such as ENA would facilitate decisions on management measures by providing a simplified representation of the entire ecosystem including all direct and indirect connections between the system components.

About 10 to 12 million birds per year use the intertidal flats of the Wadden Sea in north-eastern Europe as a food source during their moulting, wintering or breeding season and as a stop-over site along the East Atlantic Flyway. The tidal flats are inhabited by a highly productive benthic fauna representing an important link between the marine microflora and -fauna and higher trophic levels such as birds. Due to differences in the food preference, bird species might distribute on different habitat types formed in the Wadden Sea. In the present study, the diversity of foraging birds was studied in four intertidal habitats (seagrass meadow, sand flat, mud flat, Elbe tidal flat) and their macrozoobenthic food sources were analysed. Samples of benthic macrofauna were taken seasonally while foraging birds were counted monthly at the studied sites. Results revealed different benthic communities in each of the studied sites with large variation in abundance and diversity of foraging birds. Peak numbers of birds during spring and autumn reflected migration through the region. While the majority of bird species shows an opportunistic habitat choice, some species (e.g. Bucephala clangula, Philomachus pugnax) were observed foraging at one site only. However, results revealed only a weak link between bird assemblages and benthic communities within the tidal flats. As indicated by clear relationships between bird species abundance and tidal stage, avian distribution patterns are probably also strongly dependent on abiotic factors such as distance to roosting sites, inundation time and anthropogenic influences. Therefore, it is not sufficient to study the distribution of favoured prey items in the tidal flats alone but additional information needs to be taken into account to understand the complex distribution patterns of birds in the intertidal environment.

Neobiotic species can have profound impacts on food webs and entire ecosystems. The American razor clam Ensis leei was introduced into the Wadden Sea by vessels in the late 1970s and has since spread widely. It has been suggested that Ensis does not interact strongly with other benthic species. The abundance and biomass of E. leei were recorded in 2393 samples in the north-eastern Wadden Sea and 800 samples in the south-eastern Wadden Sea over a total period of 9 years. Using an interdisciplinary approach, we developed a habitat prediction model using sedimentological and hydrodynamic predictors to help understand the shape of the ecological niche occupied by Ensis in the Wadden Sea. Our model showed that Ensis preferred areas with moderately high bed shear stress and prolonged or constant water coverage. Ensis preferred coarse sediments in the northern sub-area but coarse and muddy sediments in the southern sub-area and was negatively affected by the sand mason worm Lanice conchilega in the northern sub-area. Predictions of the spatial distribution of Ensis using the northern and southern datasets revealed major differences in predicted hot-spots throughout the entire study site. This study thus highlights the need to collect a sufficiently large dataset from different sub-areas of the Wadden Sea to allow valid conclusions to be drawn regarding the spatial distribution of Ensis. The negative effects of L. conchilega on Ensis abundance and biomass as well as the occurrence of Ensis in muddy sediments in the south suggest that the ecological niche of this neobiotic species is likely to overlap partly with the native fauna of the Wadden Sea.

There is substantial evidence that climate warming affects terrestrial and marine ecosystems. In addition to the geographic shifts of marine species and communities, comprehensive mesocosm experiments provide insight in the behavior of species and simplified ecosystems under climate warming conditions. Food web dynamics and stability has been topical in contemporary ecology, and while these aspects receive considerable attention, few studies have quantitatively examined the impact of climate warming on complex marine ecosystems and their food webs. Here we examine the response of a large marine ecosystem, the Sylt-Romo Bight in the northern German Wadden Sea, to warmer temperatures by means of ecological network analysis (ENA). Three quantitative network models (of 67 model compartments each) were constructed for each of 4 non-consecutive years (1995, 2007, 2011, 2013). A base-line model at the mean annual ambient temperature, and models at +3 °C and +5 °C above the annual mean were constructed for each year (a total of 12 models) and assessed by ENA protocols. Results showed i.a. an intensification of detrital production and consumption, substantial increase in the total system throughput (TSTP), decline in consumption of phytoplankton and macrophytes, an increase in the total overhead, an increase in community respiration, and an increase in the system's P/B and R/B ratios. The mean relative ascendency declined compared to the base models by 2.35% and 2.28% in the +3 °C and +5 °C networks models respectively. It is clear from the suite of system metrics and ratios that the ecosystem becomes less organized, more dissipative and shifts towards detritus based food webs at higher water temperatures.