The impact of birds on the Wadden Sea food web

The Wadden Sea at the western coasts of Germany, Denmark and the Netherlands is one of the globally most important foraging areas for breeding and migrating birds which act at the same time as indicators for the ecological condition of the ecosystem. However, little is known about how the intense predation pressure of birds influences the Wadden Sea food web. The aim of the interdisciplinary project STopP (From Sediment to Top Predator) is to determine the food web structure in different Wadden Sea habitat types in terms of the interaction between the basis of the food web and birds as top predators. Studied habitat types included the most important foraging areas of birds; mussel banks, cockle beds, sand flats, mud flats, seagrass meadows and beds of the immigrant razor clam Ensis directus a recently preferred prey item of several bird species. Data were analysed using the Ecological Network Analysis (ENA) that reflects trophic structures within the systems and reveals direct and indirect relations between the lower and the upper trophic levels. Preliminary results show that bird predation increases the complexity of the food web due to an increase in connections and a higher total system throughput. On the other hand the predation has also a destabilizing effect due to a high demand of system’s carbon stocks and increased exports out of the tidal system. In addition, analyses show considerable indirect dependencies of birds to lower trophic levels such as sediment POC and phytoplankton. Future scenarios modelled with ENA shall show how changes within the lower trophic levels would affect foraging birds due to anthropogenic or natural impacts. Further analysis will focus on the importance of special habitat types for different bird species and the influence of changes in the biomass of key species for the whole ecosystem food web

Food web characteristics of six intertidal habitat types of the Wadden Sea

Ecological network analysis (ENA) is a helpful tool to study complex ecosystem processes. The diverse species interactions are described as flows of energy providing a simplified representation of the natural system based on attributes and features. In the Wadden Sea, a unique ecosystem along the coastline of Denmark, Germany and the Netherlands, physical forces form the vast intertidal areas to diverse habitat types that differ in their species composition and abundance. Those habitats are important foraging areas for top predators such as birds. In the present study the food web of six habitats characteristic for the Wadden Sea (cockle field, razor clam field, mud flat, mussel bank, sand flat and seagrass meadow) were analyzed in a modelling approach. The functional and structural properties of the food webs were compared to assess differences and similarities in the system functioning. Although all systems revealed a good balance between their degree of organization and their robustness against external perturbations, they differed in their detailed features. The cockle field and the mussel bank both exhibited a strong dependence of their efficiency on external imports. The razor clam field, dominated by the introduced species Ensis directus, appeared to be a rather small and stressed system with low energy transfer. The mud flat system was characterized by a high use of microphytobenthos and appeared to be not fully developed yet. Bird predation was most pronounced in the sand flat and the seagrass meadow and led to an increase in energy transfer, parallel pathways and pathways lengths in these habitats. Each system shows characteristic features and plays a different role in the Wadden Sea ecosystem by contributing significantly to the whole system functioning

The influence of birds on the structure and functioning of coastal food webs

The Wadden Sea is one of the most important stop-over sites for migrating and breeding birds. About 10-12 million birds per year use the area for foraging and consume about 25 to 45% of the standing stock of macrozoobenthos. But little is known about the influence of birds on the entire ecosystem. We conducted Ecological Network Analysis (ENA) in an important breeding and resting site in the north-eastern German Wadden Sea to determine the influence of birds on the food web. The model was based on the yearly average of empirical data taken in the study site. In its current condition, the system appears to be in a well-balanced status, with a relative Ascendency of 32.3% and a robustness of 36.5%. The diversity of flows was high showed by a Flow Diversity of 5.1 and numerous parallel pathways represented by an Effective Link-Density of 3.3. A large variety of different bird species uses the area for foraging. As top predators, these bird species are included directly or indirectly in most of the pathways. Birds induce a huge negative impact on their prey items with a less pronounced positive feedback reaction to the competitors and food resources of those organisms. There is also a strong negative impact among the bird compartments probably due to competition between the bird species on the intertidal flats. Changes in the biomass of the birds revealed alterations in the food web structure. With a decline in the avian biomass the system showed a decrease in connectivity and diversity of flows but an increase in recycling. Birds therefore appear to be an important factor for the food web structure. Changes in the bird population could affect the complexity and functioning of the entire ecosystem. Thus, it is recommended to include birds in coastal food web studies which has rarely been done before. The use of such holistic approaches would facilitate undertaking management measures

Do birds influence the structure and functioning of coastal food webs?

The Wadden Sea is one of the most important stop-over sites for 10 to 12 Million migrating birds per year. But little is known about the influence of birds on the entire ecosystem. We conducted Ecological Network Analysis (ENA) in an important resting site in the Wadden Sea to determine the influence of birds on the food web. A large variety of different bird species uses the area for foraging and is included directly or indirectly in most of the pathways. Birds induce a negative impact on their prey items with a positive feedback reaction to the preys’ competitors and food sources. There is also a strong negative impact among the bird compartments probably due to competition between the birds. Changes in the biomass of the birds revealed alterations in the food web structure. With a decline in the avian biomass the system showed a decrease in connectivity and diversity of flows but an increase in recycling. Changes in the bird population could affect the complexity and functioning of the entire ecosystem. Thus, the use of such holistic approaches would facilitate undertaking management measures

Do drivers of biodiversity change differ in importance across marine and terrestrial systems — Or is it just different research communities' perspectives?

Cross-system studies on the response of different ecosystems to global change will support our understanding of ecological changes. Synoptic views on the planet's two main realms, the marine and terrestrial, however, are rare, owing to the development of rather disparate research communities.We combined questionnaires and a literature review to investigate howthe importance of anthropogenic drivers of biodiversity change differs amongmarine and terrestrial systems and whether differences perceived by marine vs. terrestrial researchers are reflected by the scientific literature. This included asking marine and terrestrial researchers to rate the relevance of different drivers of global change for either marine or terrestrial biodiversity. Land use and the associated loss of natural habitatswere rated as most important in the terrestrial realm,while the exploitation of the sea by fishing was rated as most important in the marine realm. The relevance of chemicals, climate change and the increasing atmospheric concentration of CO2 were rated differently for marine and terrestrial biodiversity respectively. Yet, our literature review provided less evidence for such differences leading to the conclusion that while the history of the use of land and sea differs, impacts of global change are likely to become increasingly similar

Ocean current connectivity propelling the secondary spread of a marine invasive comb jelly across western Eurasia

Publication history: Accepted - 15 February 2018; Published - 16 May 2018.Aim: Invasive species are of increasing global concern. Nevertheless, the mechanisms driving further distribution after the initial establishment of non-native species remain largely unresolved, especially in marine systems. Ocean currents can be a major driver governing range occupancy, but this has not been accounted for in most invasion ecology studies so far. We investigate how well initial establishment areas are interconnected to later occupancy regions to test for the potential role of ocean currents driving secondary spread dynamics in order to infer invasion corridors and the source–sink dynamics of a non-native holoplanktonic biological probe species on a continental scale. Location: Western Eurasia. Time period: 1980s–2016. Major taxa studied: ‘Comb jelly’ Mnemiopsis leidyi. Methods: Based on 12,400 geo-referenced occurrence data, we reconstruct the invasion history of M. leidyi in western Eurasia. We model ocean currents and calculate their stability to match the temporal and spatial spread dynamics with large-scale connectivity patterns via ocean currents. Additionally, genetic markers are used to test the predicted connectivity between subpopulations. Results: Ocean currents can explain secondary spread dynamics, matching observed range expansions and the timing of first occurrence of our holoplanktonic non-native biological probe species, leading to invasion corridors in western Eurasia. In northern Europe, regional extinctions after cold winters were followed by rapid recolonizations at a speed of up to 2,000 km per season. Source areas hosting year-round populations in highly interconnected regions can re-seed genotypes over large distances after local extinctions. Main conclusions: Although the release of ballast water from container ships may contribute to the dispersal of non-native species, our results highlight the importance of ocean currents driving secondary spread dynamics. Highly interconnected areas hosting invasive species are crucial for secondary spread dynamics on a continental scale. Invasion risk assessments should consider large-scale connectivity patterns and the potential source regions of non-native marine species.Danish Council for Independent Research; Grant/Award Number: DFF-1325-00102B; FP7 People: Marie-Curie Actions, Grant/Award Number: MOBILEX, DFF - 1325-00025; EU, BONUS, BMBF, Grant/ Award Number: 03F0682; Excellence Cluster “Future Ocean”, Grant/Award Number: CP153