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Using trophic hierarchy to understand food web structure Marco Scotti, Cristina Bondavalli, Antonio Bodini and Stefano Allesina

Summary: Using trophic hierarchy to understand food web structure
Marco Scotti, Cristina Bondavalli, Antonio Bodini and Stefano Allesina
M. Scotti (marcoscot@gmail.com), Center for Network Science, Central European Univ., Zrinyi u. 14, HUÁ1051, Budapest, Hungary. Á MS,
C. Bondavalli, A. Bodini, Dept of Environmental Sciences, Univ. of Parma, Viale Usberti 11/A, ITÁ43100 Parma, Italy. Á S. Allesina,
National Center for Ecological Analysis and Synthesis, 735 State Street, Suite 300, Santa Barbara, CA 93101, USA.
Link arrangement in food webs is determined by the species' feeding habits. This work investigates whether food web
topology is organized in a gradient of trophic positions from producers to consumers. To this end, we analyzed 26 food
webs for which the consumption rate of each species was specified. We computed the trophic positions and the link
densities of all species in the food webs. Link density measures how much each species contributes to the distribution of
energy in the system. It is expressed as the number of links species establish with other nodes, weighted by their
magnitude. We computed these two metrics using various formulations developed in the ecological network analysis
framework. Results show a positive correlation between trophic position and link density across all the systems, regardless
the specific formulas used to measure the two quantities. We performed the same analysis on the corresponding binary
matrices (i.e. removing information about rates). In addition, we investigated the relation between trophic position and
link density in: a) simulated binary webs with same connectance as the original ones; b) weighted webs with constant
topology but randomized link strengths and c) weighted webs with constant connectance where both topology and link
strengths are randomized. The correlation between the two indices attenuates, vanishes or becomes negative in the case of
binary food webs and simulated data (weighted and unweighted).
According to our analysis, link density in food webs decreases with trophic position so that it is greatly reduced toward
the top of the trophic hierarchy. This outcome, that seems to challenge previous conclusions based on null models,


Source: Allesina, Stefano - Department of Ecology and Evolution, University of Chicago


Collections: Environmental Sciences and Ecology; Biology and Medicine