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In the last decades, many works investigated the trophic structure of communities stressing, in particular, the role played by species in food webs (e.g., their trophic level and, more recently, their centrality). There exist some encouraging applications, but few details are known about the relationships between centrality measurements and trophic levels. In addition, these studies almost refer to unweighted trophic networks, despite the acknowledged need of investigating weighted webs. Here we aim to contribute to the synthetic treatment of these complementary issues by analyzing several indices of centrality and trophic level. Studying 19 ecosystems, we ranked the nodes according to their positional importance values (based on various centrality indices) and we compared the rank order of coefficients with unweighted or weighted trophic levels. Our goal was revealing potential biases in finding high centrality nodes among basal, intermediate and top species. We found that key species occupy intermediate positions of the trophic hierarchy. In case of unweighted data, trophic levels of key nodes do not deviate from trends displayed by the whole dataset. Significant differences were observed when using weighted data. These results contradict the common belief of many ecologists that identified top-predators and charismatic megafauna as main targets of conservation policies. We discuss the potential consequences of the observed features on ecosystem dynamics.

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Community Ecology
Authors: U. Sommer, E. Charalampous, M. Scotti, and M. Moustaka-Gouni

Food chains in the pelagic zones of oceans and lakes are longer than in terrestrial ecosystems. The perception of the pelagic food web has become increasingly complex by progressing from a linear food chain (phytoplankton – crustacean zooplankton – planktivorous fish – predatory fish) to a food web because of an increasing appreciation of microbial trophic pathways, side-tracks by gelatinous zooplankton and a high prevalence of omnivory. The range of predator:prey size ratios by far exceeds the traditionally assumed range of 10:1 to 100:1, from almost equal length to 105:1. The size ratios between primary consumers and top predators are 3½ orders of magnitude bigger in pelagic than in terrestrial food webs. Comparisons between different pelagic ecosystems support ecosystem size as an important factor regulating the maximal trophic level, while energy limitation of the number of trophic levels is less well supported. An almost 1:1 relationship between ingestion by predators and prey mortality and a better chemical match between primary producer and herbivore biomass are further distinctive features of the pelagic food web whose role in explaining the higher number of trophic levels in pelagic systems needs further examination.

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Network models are traditional in community ecology. For example, they provide a rich analytical toolkit to put higher predators into a multispecies context. Better understanding their top-down effects and the potential bottom-up control on them would be of key importance for predictive ecosystem management. Food web architecture may be used to predict community dynamics, but it is an old question how reliable are the studies considering only static information. A general and intuitive assumption is that stronger links (with larger weights) mediate stronger effects. We study this statement and use an illustrative case study. We investigate the trophic structure of the Prince William Sound food web in terms of biomass flows, and study its simulated dynamics in a stochastic modelling framework. We aim to understand bottom-up effects of preys on consumers: we focus on the fluctuations of top predator populations, following disturbance on their prey. Several disturbance regimes are studied and compared. Food web structure and link weight generally predict well the average impacts of preys on top-predators, with larger flows mediating stronger effects. Most exceptions appear for weak links: these are less predictable, some of them can be surprisingly important.

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Community Ecology
Authors: R. Olmo Gilabert, A. F. Navia, G. De La Cruz-Agüero, J. C. Molinero, U. Sommer, and M. Scotti

Abstract

Anthropic activities impact ecosystems worldwide thus contributing to the rapid erosion of biodiversity. The failure of traditional strategies targeting single species highlighted ecosystems as the most suitable scale to plan biodiversity management. Network analysis represents an ideal tool to model interactions in ecosystems and centrality indices have been extensively applied to quantify the structural and functional importance of species in food webs. However, many network studies fail in deciphering the ecological mechanisms that lead some species to occupy the most central positions in food webs. To address this question, we built a high-resolution food web of the Gulf of California and quantified species position using 15 centrality indices and the trophic level. We then modelled the values of each index as a function of traits and other attributes (e.g., habitat). We found that body size and mobility are the best predictors of indices that characterize species importance at local, meso- and global scale, especially in presence of data accounting for energy direction. This result extends previous findings that illustrated how a restricted set of traitaxes can predict whether two species interact in food webs. In particular, we show that traits can also help understanding the way species are affected by and mediate indirect effects. The traits allow focusing on the processes that shape the food web, rather than providing case-specific indications as the taxonomy-based approach. We suggest that future network studies should consider the traits to explicitly identify the causal relationships that link anthropic impacts to role changes of species in food webs.

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