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      Compilation and Network Analyses of Cambrian Food Webs

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          A rich body of empirically grounded theory has developed about food webs—the networks of feeding relationships among species within habitats. However, detailed food-web data and analyses are lacking for ancient ecosystems, largely because of the low resolution of taxa coupled with uncertain and incomplete information about feeding interactions. These impediments appear insurmountable for most fossil assemblages; however, a few assemblages with excellent soft-body preservation across trophic levels are candidates for food-web data compilation and topological analysis. Here we present plausible, detailed food webs for the Chengjiang and Burgess Shale assemblages from the Cambrian Period. Analyses of degree distributions and other structural network properties, including sensitivity analyses of the effects of uncertainty associated with Cambrian diet designations, suggest that these early Paleozoic communities share remarkably similar topology with modern food webs. Observed regularities reflect a systematic dependence of structure on the numbers of taxa and links in a web. Most aspects of Cambrian food-web structure are well-characterized by a simple “niche model,” which was developed for modern food webs and takes into account this scale dependence. However, a few aspects of topology differ between the ancient and recent webs: longer path lengths between species and more species in feeding loops in the earlier Chengjiang web, and higher variability in the number of links per species for both Cambrian webs. Our results are relatively insensitive to the exclusion of low-certainty or random links. The many similarities between Cambrian and recent food webs point toward surprisingly strong and enduring constraints on the organization of complex feeding interactions among metazoan species. The few differences could reflect a transition to more strongly integrated and constrained trophic organization within ecosystems following the rapid diversification of species, body plans, and trophic roles during the Cambrian radiation. More research is needed to explore the generality of food-web structure through deep time and across habitats, especially to investigate potential mechanisms that could give rise to similar structure, as well as any differences.

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          Food webs, which depict the networks of feeding interactions among co-occurring species, display many regularities in their structure. For example, the distributions of links to prey and links from predators, the percentages of omnivores and herbivores, and the mean trophic level of species change systematically with the number of taxa and feeding links in a web. Such “scale-dependent” regularities are formalized by network models based on a few simple link distribution rules that successfully predict the network structure of complex food webs from a variety of habitats. To explore how long such regularities may have persisted, we compiled and analyzed detailed food-web data for two ancient fossil assemblages from the early Paleozoic, when rapid diversification of multicellular species, body plans, and trophic roles occurred. Our analyses show that for most aspects of network structure, the Early Cambrian Chengjiang Shale and Middle Cambrian Burgess Shale food webs are very similar to modern webs. This suggests that there are strong and enduring constraints on the organization of feeding interactions in ecosystems. However, a few differences, particularly in the Chengjiang Shale web, suggest that some aspects of network structure were still in flux during early phases of de novo ecosystem construction.


          Analyses of Chengjiang and Burgess Shale food-web data suggest that most, but not all, aspects of the trophic structure of modern ecosystems were in place over a half-billion years ago.

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          Most cited references 67

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          Network structure and biodiversity loss in food webs: robustness increases with connectance

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            Food-web structure and network theory: The role of connectance and size.

            Networks from a wide range of physical, biological, and social systems have been recently described as "small-world" and "scale-free." However, studies disagree whether ecological networks called food webs possess the characteristic path lengths, clustering coefficients, and degree distributions required for membership in these classes of networks. Our analysis suggests that the disagreements are based on selective use of relatively few food webs, as well as analytical decisions that obscure important variability in the data. We analyze a broad range of 16 high-quality food webs, with 25-172 nodes, from a variety of aquatic and terrestrial ecosystems. Food webs generally have much higher complexity, measured as connectance (the fraction of all possible links that are realized in a network), and much smaller size than other networks studied, which have important implications for network topology. Our results resolve prior conflicts by demonstrating that although some food webs have small-world and scale-free structure, most do not if they exceed a relatively low level of connectance. Although food-web degree distributions do not display a universal functional form, observed distributions are systematically related to network connectance and size. Also, although food webs often lack small-world structure because of low clustering, we identify a continuum of real-world networks including food webs whose ratios of observed to random clustering coefficients increase as a power-law function of network size over 7 orders of magnitude. Although food webs are generally not small-world, scale-free networks, food-web topology is consistent with patterns found within those classes of networks.
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              Complex Trophic Interactions in Deserts: An Empirical Critique of Food-Web Theory

               Gary A. Polis (1991)

                Author and article information

                Role: Academic Editor
                PLoS Biol
                PLoS Biology
                Public Library of Science (San Francisco, USA )
                April 2008
                29 April 2008
                : 6
                : 4
                [1 ] Santa Fe Institute, Santa Fe, New Mexico, United States of America
                [2 ] Microsoft Research Limited, Cambridge, United Kingdom
                [3 ] Pacific Ecoinformatics and Computational Ecology Lab, Berkeley, California, United States of America
                [4 ] National Center for Ecological Analysis and Synthesis, Santa Barbara, California, United States of America
                [5 ] Grant Institute, School of GeoSciences, University of Edinburgh, Edinburgh, United Kingdom
                [6 ] Edinburgh Collaborative of Subsurface Science and Engineering, University of Edinburgh, Edinburgh, United Kingdom
                [7 ] Department of Paleobiology, National Museum of Natural History, Washington, D.C., United States of America
                Princeton University, United States of America
                Author notes
                * To whom correspondence should be addressed. E-mail: jdunne@ 123456santafe.edu
                07-PLBI-RA-3144R3 plbi-06-04-24
                Copyright: © 2008 Dunne et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
                Page count
                Pages: 16
                Research Article
                Computational Biology
                Evolutionary Biology
                Custom metadata
                Dunne JA, Williams RJ, Martinez ND, Wood RA, Erwin DH (2008) Compilation and network analyses of Cambrian food webs. PLoS Biol 6(4): e102. doi: 10.1371/journal.pbio.0060102

                Life sciences


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