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An ecological network is a representation of the biotic interactions in an ecosystem, in which species (nodes) are connected by pairwise interactions (links). These interactions can be trophic or symbiotic. Ecological networks are used to describe and compare the structures of real ecosystems, while network models are used to investigate the effects of network structure on properties such as ecosystem stability. == Properties of ecological networks == Historically, research into ecological networks developed from descriptions of trophic relationships in aquatic food webs; however, recent work has expanded to look at other food webs as well as webs of mutualists. Results of this work have identified several important properties of ecological networks. Complexity (linkage density): the average number of links per species. Explaining the observed high levels of complexity in ecosystems〔Williams R.J., Berlow, E.L., Dunne, J.A., Barabasi, A.L., and N.D. Martinez. (2002) Two degrees of separation in complex food webs. ''Proceedings of the National Academy of Sciences'', 99: 12913-12916.〕 has been one of the main challenges and motivations for ecological network analysis, since early theory predicted that complexity should lead to instability.〔Pimm, S.L. (1984) The complexity and stability of ecosystems. ''Nature'', 307: 321-326.〕 Connectance: the proportion of possible links between species that are realized (links/species2). In food webs, the level of connectance is related to the statistical distribution of the links per species. The distribution of links changes from (partial) power-law to exponential to uniform as the level of connectance increases.〔Dunne, J.A., Williams, R.J., and N.D. Martinez. (2002) Food-web structure and network theory: The role of connectance and size.''Proceedings of the National Academy of Sciences'', 99: 12917-12922.〕 The observed values of connectance in empirical food webs appear to be accountable for by constraints on an organisms diet breadth driven by optimal foraging behaviour.This links the structure of these ecological networks to the behaviour of individual organisms.〔Beckerman, A.P., Petchey, O.L. & Warren P.H. (2006) Foraging biology predicts food web complexity. ''Proceedings of the National Academy of Sciences'', 103: 13745-13749.〕 Degree distribution: the degree distribution of an ecological network is the cumulative distribution for the number of links each species has. The degree distributions of food webs have been found to display the same universal functional form. The degree distribution can be split into its two component parts, links to a species' prey (aka. in degree) and links to a species' predators (aka- out degree). Both the in degree and out degree distributions display their own universal functional forms. As there is a faster decay of the out-degree distribution than the in degree distribution we can expect that on average in a food web a species will have more in links than out links.〔Stouffer, D.B. (2010) Scaling from individuals to networks in food webs ''Functional Ecology'', 24: 44–51.〕 Clustering: the proportion of species that are directly linked to a focal species. A focal species in the middle of a cluster may be a keystone species, and its loss could have large effects on the network. Compartmentalization: the division of the network into relatively independent sub-networks. Some ecological networks have been observed to be compartmentalized by body size〔Schmid-Araya, J.M., Schmid, P.E., Robertson, A., Winterbottom, J., Gjerlov, C., and A.G. Hildrew. (2002) Connectance in stream food webs. ''Journal of Animal Ecology'', 71: 1056-1062.〕〔Reuman, D.C. and J.E. Cohen. (2004) Trophic links’ length and slope in the Tuesday Lake food web with species’ body mass and numerical abundance. ''Journal of Animal Ecology'', 73: 852–866.〕 and by spatial location.〔Krause, A.E., Frank, K.A., Mason, D.M., Ulanowicz, R.E., and W.W. Taylor. (2003) Compartments revealed in food-web structure. ''Nature'', 426: 282-285.〕 Evidence also exists which suggests that compartmentilization in food webs appears to result from patterns of species' diet contiguity 〔Guimera,R., Stouffer, D.B., Sales-Pardo, M., Leicht, E.A., Newman, M.E.J. and Ameral, L.A.N. (2010) Origin of compartmentalization in food webs. ''Ecology'', 91: 2941–2951.〕 Nestedness: the degree to which species with few links have a sub-set of the links of other species, rather than a different set of links. In highly nested networks, guilds of species that share an ecological niche contain both generalists (species with many links) and specialists (species with few links, all shared with the generalists). In mutualistic networks, nestedness is often asymmetrical, with specialists of one guild linked to the generalists of the partner guild.〔Bascompte, J., Jordano, P., Melian, C.J., and J.M. Olesen. (2003) The nested assembly of plant-animal mutualistic networks. ''Proceedings of the National Academy of Sciences'', 100: 9383-9387.〕 The level of nestedness is determined not by species features but overall network depictors (e.g. network size and connectance) and can be predicted by a dynamic adaptive model with species rewiring to maximize individual fitness〔Zhang, F., Hui, C., and Terblanche, J.S. (2011) An interaction switch predicts the nested architecture of mutualistic networks. ''Ecology Letters'', 14: 797-803.〕 or the fitness of the whole community.〔Suweis, S., Simini, F., Banavar, J and A. Maritan. (2013) Emergence of structural and dynamical properties of ecological mutualistic networks. ''Nature'', 500: 449-452.〕 Network motif: Motifs are unique sub-graphs composed of n-nodes found embedded in a network. For instance there exist thirteen unique motif structures containing three species, some of these correspond to familiar interaction modules studied by population ecologists such as food chains, apparent competition, or intraguild predation. Studies investigating motif structures of ecological networks, by examining patterns of under/over representation of certain motifs compared to a random graph, have found that food webs have particular motif structures 〔Stouffer, D.B., Sales-Pardo, Camacho, J.,Jiang, W., and Ameral, L.A.N. (2007) Evidence for the existence of a robust pattern of prey selection in food webs. ''Proc. R. Soc. B.'', 274: 1931–1940.〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「ecological network」の詳細全文を読む スポンサード リンク
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