ecological modelling 220 ( 2 0 0 9 ) 3070–3080 available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/ecolmodel Linking trophic positions and flow structure constraints in ecological networks: Energy transfer efficiency or topology effect? Marco Scotti ∗ , Cristina Bondavalli, Antonio Bodini Department of Environmental Sciences, University of Parma, Viale Usberti 11/A, 43100 Parma, Italy article info Article history: Available online 11 September 2008 Keywords: Food webs Energy flow Link distribution Interaction strength Trophic structure Flow diversity abstract In the present work we investigate whether the distribution of energy flows in ecosystems responds to criteria of trophic organization. We analyzed weighted and unweighted food webs estimating, for each node, trophic position (TP), Shannon’s index of inflow diversity (H) and individual contribution to the whole average mutual information (AMI). Finally, we performed the same analysis on simulated webs that were constructed using the following criteria: (a) preserving topology and varying link strength; (b) modifying position of links and their intensities. In real ecosystems, moving toward top species, higher pathway redundancy coupled to a strong specialistic trophic behavior was observed. This means that, beside the availabil- ity of multiple topological routes, top predators tend to establish “bipolar” predator–prey interactions feeding on preferred preys that, in turn, are mainly preyed by that special- ized consumer. Although the analysis shows that, in qualitative food webs, links should be more numerous at the top of the trophic hierarchy, with magnitude more evenly dis- tributed among the interactions, in weighted networks, the tendency of sharing the main flow between donor and receiving compartments (i.e. “bipolar” predator–prey interaction) increases from basal to top species. Patterns displayed by empirical data vanish when indices are calculated for simulated networks, pointing out how topology of energy delivery and flow intensities may be read as a function of the trophic hierarchy. © 2008 Elsevier B.V. All rights reserved. 1. Introduction Food webs are complex networks of trophic interactions among diverse species in ecosystems. Although extremely variable, their structure displays basic regularities (i.e. preda- tor/prey ratio, number of connections per species, food chain length) and simple models captured these patterns (Cohen et al., 1990; Williams and Martinez, 2004; Cattin et al., 2004; Stouffer et al., 2005). ∗ Corresponding author. Tel.: +39 0521 90 56 14; fax: +39 0521 90 54 02. E-mail address: scotti@dsa.unipr.it (M. Scotti). Ecologists have long been exploring the effects of food web structure on ecosystem properties as depending on the number of groups (May, 1972), the frequency of interactions (May, 1972; Yodzis, 1980), and the length of the food chains (Pimm and Lawton, 1977; Moore et al., 1993). Beside network topology, Paine (1969, 1974, 1980) demonstrated how inter- action strength can substantially affect food web features and that the combined knowledge of web structure and link strength is a key to understand how ecosystems function. In this framework, particularly intriguing is the statement that 0304-3800/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.ecolmodel.2008.07.034