How does energy travel in foodwebs?

Food webs represent the general framework of ecological communities. Our study unfold the structural complexity of weighted food webs, where energy flows are quantified, highlighting  the topologies that are mainly responsible for distributing the energy flow to the species. Energy travels in the ecological community through the whole food web, but it is unlikely that all trophic pathways share the same importance for energy delivery. Thermodynamic, as well as ecological, constraints such as efficiency, prey preference, resource availability, affect the way energy is distributed. Accordingly, preferential pathways may emerge as key players in this respect. Because any trophic passage from prey to predator (i.e. trophic levels) necessarily implies some energy loss, shortening the routes for distributing energy to the species would reduce the total energetic cost for the whole system to perform energy delivery. A seminal work of Lindeman (1942) anticipated that---because of the low efficiency of resource transfer between trophic levels---the main routes for energy delivery in food webs would be short. Following this idea other studies argued  that the trophic chains of maximum energy value would be the shortest.

We analyzed 30 well resolved weighted empirical food webs. Our major aim is to address whether the main energy flow in food webs travels along the shortest possible paths or whether other, longer structures can be responsible for distributing the main energy flow to the species. We discover that the trophic chains of major energy flow are not the shortest possible, but they are shorter than the average ensemble. The results show that in food webs, the bulk of energy travels along pathways that tend to be short. The short structure of the effective backbone for energy flows should be an optimized configuration that emerges from a trade-off between maximizing resource input with a minimum loss (minimum trophic level) and a suite of constraints that may include feeding preference, size effect in predator-prey interactions, dynamical features, and efficiency in energy transfer between trophic levels. This research would confirm Lindeman hypothesis that energy travels along short routes, confirming the thermodynamic view of earlier ecologists.

Photo: The picture represents the lake Scuro ecosystems (Parma, Italy), one of the food web analyzed in the paper, in the cold season (October 2003); from Prof. Gianpaolo Rossetti, Università di Parma, database. 

Michelle Bellingeri

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