WP 5: Topology of the Barents Sea food web

Food webs are diagrams depicting who eats whom in ecosystems. An understanding of the structure and function of food webs is crucial for any study of how an ecosystem works and responds to change (Pimm 1991, Pimm 2002). For example, food web structure has provided the basis for models exploring how single species respond to perturbations in other parts of the web (e.g. Yodzis 2000), which has been important for evaluating marine management strategies (Yodzis 2001). An analysis of food web topology, the ensemble of species and their network of trophic interactions, allows addressing structural properties of communities with important dynamic implications. Structural characteristics known to influence the resilience and stability of communities can be quantified via appropriate metrics such as connectedness, compartmentalization and degree of omnivory (Dunne 2009). Spatio-temporal variation in these metrics can then be assessed and sources of variation investigated.

The topology of parts of the Barents Sea food web has been resolved in the BarEcoRe project (Proj.no. 200796). In WP5, we will expand this topology by including data from research cruises as they are produced and from literature sources, including Russian literature. We expect that considerable resolution will be added to the food web topology through this.

We will also fill in groups of species that are currently not represented in the food web topology, in particular infectious organisms (here defined broadly to include all types of organisms that exhibit a parasitic life style, including multicellular parasites, virus and bacteria). More than half of all species are infectious organisms, infectious organisms may profoundly affect the dynamics of their host populations (e.g. Tompkins et al 2002) and their inclusion in food web studies markedly alters the perceived structure and properties of food webs in terms of chain length, connectance and robustness. In addition, infectious organisms might affect food-web stability, interaction strength and energy flow (Lafferty et al 2008). Adding information on occurrence of infectious organisms that are food transmitted, can also give information on feeding interactions between free living species, thus helping to improve the resolution of the food web topology (e.g. Lafferty et al. 2006).

Diet data gives information on the food items eaten in a short period before sampling. In order to get data on feeding ecology that represents longer time spans, elements isotope analyses and fatty acid analysis will also be performed. In addition, to facilitate constructions of models based on the food web topology, data on life history traits, preferences in relation to abiotic factors and other factors that may influence interactions between species will be registered together with data on the trophic interactions.

The estimated food web topology will provide input for various multispecies models (e.g. Atlantis in WP 1, and energy flow models in WP 3). In addition, based on accumulated data from cruises and literature sources, WP4 will explore variation in time and space in the food web topology. Descriptive diet reports based on statistical analyses will also be performed, aiming to define important prey species and environmental conditions based on the revealed spatio-temporal distribution. These descriptive analyses will be summarized in scientific diet papers and may improve the understanding of feeding strategies. Together with functional response of some main components (WP6), this will strengthen our understanding of trophic interactions.

Finally, WP5 will address questions about dynamics and likely response to change in the ecosystem by exploring structures in the topology, such as connectedness, compartmentalization and degree of omnivory. To inform us on the need to include infectious organisms in food web studies in the Barents Sea, the influence on food web properties of adding these organisms to the topology will also be assessed.

Collaborating partners in WP5 will be Kevin Lafferty at the US Geological Survey / University of California, Santa Barbara, Ken MacKenzie at the University of Aberdeen and Willy Hemmingsen at the University of Tromsø. Lafferty is a world leading ecologist/parasitologist within the fields of food web topologies, their properties and how they are influenced by parasites. MacKenzie and Hemmingsen are marine parasitologists with long experience from work in the Barents Sea and will provide needed expertise on this in the project.

Specific tasks:

- Improve the current estimate of the Barents Sea food web topology by adding more data on free living species and the first data on infectious organisms with the aim of facilitating multispecies modelling.

- Explore variation in the food web topology in time and space.

- Perform diet analyses to improve our understanding of feeding strategies.

- Address questions about dynamics and likely response to change in the ecosystem by exploring structures in the food web topology.

- Assess the influence of adding infectious organisms on estimated properties of the Barents Sea food web.