Linking terrestrial and aquatic ecosystems : complexity, persistence and biodiversity in European food webs

In this work we have defined and analyzed the network structure, properties and composition of cohesive sub-webs in two food web networks at two selected European sites: Ferto lake in Hungary and Ria Formosa lagoon in Portugal. The main objective of this work was to test and to apply a coherent methodology for the definition and analysis of trophic networks in typical European ecosystems. Persistence and cohesion of the two networks have been determined and studied. Moreover the biodiversity in terms of number of species have been measured. Several preliminary conclusions can be drawn on a basis of results obtained from application of network analysis indices and dynamical simulation. In terms of ecosystem stability the lake Ferto network (persistence 0.42) seems more stable than Ria Formosa (persistence 0.26), but if we consider the stability of the main k-core, we obtain opposite results (0.48 and 0.50 respectively). Taking into account the distribution of species in the k-core partition, the general structure of Ria Formosa network appears more complex than Ferto lake. In both networks a main core containing species taxonomically different is present, but in the Ria Formosa network (37.7%) the proportion of species inside the core is lower than in Ferto lake (60%), this can be an indication that the dimension of the main core is not related to size of the network in real ecosystems. This hypothesis is confirmed by results obtained from k-core partition of simulated networks. In both studied networks the cohesion (measured in terms of mean density and degree) of k-cores is greater than k-cores extracted from simulated networks generated by different models, this may indicate a complex and not arbitrary structure of ecological networks, not easily reproducible by models. In general the approach used appears to be able of defining the main proprieties of the trophic networks examined. The extension to other European ecosystems is needed for having a clearer and complete picture and to use data collected to simulation of different scenarios, afterwards the effects on network structure and ecosystem biodiversity as a function of different European environmental policies could be assessed.