Food web

FOOD WEB STRUCTURE, FONCTIONNING AND DYNAMICS

All the biotic compartments of an ecosystem is linked within the food web. An ecosystem approach to management of human activities must therefore consider the food web to be able to assess human impacts direct but also indirect impacts on the biotic compartments of the ecosystem. This area includes all research activities that aim to describe and understand the structure, functioning and dynamics of marine food web from the Channel. The studies will focus both on short food webs with two trophic levels (eg up to the agencies or suspension feeders filter of the epi-benthic) or long, that is to say, involving senior consumers (fish) or terminals (birds and marine mammals).

A. Food web structure: descriptive approach 

The description of the food web structure, first passes through the inventory of species that constitutes and trophic linkages between them to determine their position within the network in terms of trophic level. Two complementary approaches can be used for this purpose. Direct observation of the diet of organisms, including through the stomach contents and their isotopic composition. The first is limited to bodies having a size sufficient to easily lend itself to this kind of study and makes a point information, but the advantage of providing accurate information on species and quantity consumed. Conversely, the second can be applied to any type of organization, from the smallest to the largest and offers a view of time-integrated diet of organisms. However, it does not provide information on specific species and quantities consumed. In other words, direct observation provides an assessment of trophic links between species and material flow between them whereas the isotopic composition allows organizations to understand the trophic level of species and the network connectivity.

From information obtained by these two methods, a number of studies may then seek to characterize the food web structure. First, species diversity within trophic levels and in terms of trophic relationships can be studied in relation to the type of network, ie its size and connectivity, knowing that these various properties of food webs are often cited as having a role in the dynamics and resilience of ecosystems. Then a balance in the distribution and flow of biomass and energy between species, guilds or trophic level may be obtained by various methods (mass balance or energy, inverse method) in the event of a food web equilibrium. This balance allows a second step to address various issues relating to the identification of species, guilds or compartments keys, control the type of game (top-down, bottom-up "or" wasp-waist ") and performance and productivity of the network in terms of transfer of biomass and energy.

B. Food web functionning: Mechanistic approach 

Once the food web structure established, more targeted studies can be developed to elucidate the mechanisms that control the structure and determine its dynamics. Different processes can be approached from the bottom of the food web and up to the highest trophic levels.

Firstly, it is possible to study the influence of abiotic compartments on food web structure, such as weather systems, hydrodynamic and bio-geo-chemical that essentially affect the primary production and its spatial distribution. Second, the nature of primary production can be addressed in order to determine the relative importance of photosynthetic organisms (benthic and pelagic) and the microbial loop (bacteria, microflagellates, ciliates and viruses) in the entrance of carbon and the nitrogen and energy in the food web. This knowledge is essential to understand the effectiveness of transfer of matter and energy to higher trophic levels, and thus the productivity and performance of the food web. Indeed, the different types of primary production induced trophic paths of different lengths, usually involving a microbial loop road along the trophic transfer of production "novel" by photosynthetic organisms.

Moreover, the coupling between primary production and the rest of the food web through the primary consumers (zooplankton and epi-benthic fauna) also partly determines the efficiency of transfer of matter and energy to higher trophic levels. The study of assemblages of species or communities that constitute the dominant primary consumers is particularly important since they also influence the trophic path length according to their lifestyles (pelagic or benthic herbivores, suspension feeders, filtering, deposit feeders, scavengers).

Finally, predator-prey relationships between consumers and their implications for food web structure must be considered. These shape the emerging structure of the upper food web in the same way that individual characteristics determine the demographic structure of an emerging population. Identify the characteristics that determine the predator-prey relationships would be an asset for understanding the emergence of the structure of higher trophic networks. The morphology of species is an obvious trail. Studies have already shown how the diet of some fish was correlated with their morphological attributes, mainly in freshwater systems. The recent development of geometric morphometrics offers a tool to improve our understanding of the functional and trophic ecology of marine fish and identify morphological determinants of predator-prey relationships in particular through allometric relationships. This type of relationship would predict prey consumed by a species from the morphological attributes.

C. Food web dynamics: Theoretical approach 

Based on the knowledge accumulated on the food web structure and the mechanisms that underlie it, it is then possible to look at its dynamics in order to understand and predict. This kind of study is hardly feasible without the aid of modeling. Different possibilities so who depend mainly on the type of food web considered.

In the case of short food webs, that is to say that consumers terminals are suspension feeders, filter feeders, deposit feeders, or scavengers, models "explicit" can be developed. These models couple dynamic biogeochemical flows, population dynamics of algal species and population dynamics of a species of primary consumers (see sheet "Ecological Modelling Channel" by Alain Menesguen) and thus allow a relatively comprehensive description of system and its dynamics. One of the greatest limitations of such models is that it is now difficult to consider networks with more than 2 trophic levels with a limited number of species for each level (some species of primary producers , one to two species of primary consumers).

To understand the dynamics of food webs long, ie from top to consumers such as fisheries, birds or marine mammals, it is therefore necessary to simplify the system represented. Two approaches are possible which differ in their degree of simplification of the representation of the network. The mass balance models or inverse method is the most comprehensive approach, ie based on a simplification of the lower food web. These models can describe the distribution and flow of matter and energy between species or trophic guilds. However, they neglect any consideration of abiotic (hydrodynamics, physics and chemistry of water, nutrients), involve an arbitrary choice (or "expert opinion") the degree of detail with which the feeding guilds or functional groups are described and demanding large amounts of data for calibration, some difficult to obtain. The other approach, based on the spectra, is to oversimplify the description of the network by structuring the distribution of biomass in only one dimension: size (average) of individuals or trophic level. The dynamic models of spectra of size or trophic level have recently been used successfully for the interpretation of such data. However, they are an extreme simplification of the food web to which it is difficult to trust in terms of quantitative prediction. In addition, some simplifying assumptions, such as descriptions of predator-prey relationships as governed by the size ratio between individuals, not based on little or no empirical evidence. On the same principle, an alternative approach would be to reduce the size of the system by some structural determinants of morphological predator-prey relationships identified by geometric morphometry from empirical data (see previous section).

The models mentioned above can then be used to study a number of phenomena related to food web dynamics such as the relationship between diversity (specific, functional or trophic linkages) and biomass or productivity, the relationship between diversity and stability food web, and resilience of ecosystems to disturbance.

D. Food web interactions and antrophogenic activities: Applied approach 

Once the upstream knowledge acquired on the food web, a number of topics related to human activity can be addressed. Indeed, many human activities rely on marine food webs and affect. Among the most important issues, we note

  • The sustainability of fisheries depends on the productivity of the food web and directly affects the level of fishery resources and or indirectly including through the release;
  • The conservation of benthic communities are an important link between primary productivity and secondary consumer and a not insignificant prey fishery resources which are affected by trawling, gravel extraction and implementation of energy recovery systems ( wind, wave, current);
  • The impact of watersheds and river inputs on the food web mainly through the contributions of nutrients and organic matter and eutrophication problems;
  • Problems of chemical contamination (anthropogenic pollutants) and biological (toxic algae) food webs that threatens human health;
  • Marine protected areas and their effects on communities;
  • The impact of climate change on the food web.

Contacts: Ching VILLANUEVA and Bruno ERNANDE, Ifremer Boulogne-sur-Mer