Network theory is commonly applied to identify local and global properties of interconnected nodes, such as brain cells or websites on the internet (Newman, 2003a). Despite its potential to quantify disease spread (Newman, 2002) as well as the social interactions between individuals within and between groups, network theory has rarely been used to examine the social organisation of animal groups ([Lusseau, 2003], [Croft et al., 2004], [Croft et al., 2006] and [Godfrey et al., 2009]). The application of social network analysis to interactions in animal populations has great potential. For example, it may assist in the progress of research and analysis of co-operative behaviour, the transmission of information via social interactions and the mechanisms of disease and/or parasite transmission (Watts and Strogatz, 1998 D.J. Watts and S.H. Strogatz, Collective dynamics of small world networks, Nature 393 (1998), pp. 440–442. Full Text via CrossRef | View Record in Scopus | Cited By in Scopus (6835)[Watts and Strogatz, 1998], [Latora and Marchiori, 2001] and [Abramson and Kuperman, 2001]). It is also possible to predict a network's resistance to predation and the subsequent loss of individuals (Lusseau, 2003), as well as to examine social segregation (Newman, 2003b). Indeed, social network analysis may assist in the better understanding of the social integration of invasive species and the characteristics of a successful invasion. It may be particularly useful in uncovering new important social traits used in ecosystem colonisation.\ud \ud Many organisms form groups with benefits that have traditionally been explained by performance in predator–prey interactions and locomotion efficiency ([Magurran, 1990] and [Pitcher and Parrish, 1993]). But more recently, studies have suggested that abilities that enable social group-living may rather reveal direct information about individual performance that may have evolved as amplifiers of individual quality (Barber and Folstad, 2000). The drivers of social organisation include responses to predation pressure and inter-specific competition that can determine the success or failure of entire populations. For group-living organisms, such as shoaling fishes, social structure may be particularly important following the introduction of a new species, which may compete within the shoal for positions normally occupied by native fishes (Witte et al., 1992).\ud \ud In most organisms, early life is a crucial phase for growth and subsequent survival to recruitment. During their early life, fishes undergo a series of ontogenetic changes in morphology, behaviour, physiology, ecological interactions and thus in their performance-related capabilities ([Balon, 1975] and [Balon, 1990]). The aim of the present study was to apply the network approach to assess social network properties within a fish assemblage that has been invaded by non-native sunbleak Leucaspius delineatus (Heckel, 1843) during early life. A small cyprinid native to most of continental Europe, the sunbleak was introduced into the Stoneham Lakes (Southern England) in the mid-1980s via the aquaculture trade (Farr-Cox et al., 1996) and has since become well established in various canal systems, rivers and lakes of the South and South West of England. A few populations have also been discovered in the North West of the country ([Gozlan et al., 2003] and [Hickley and Chare, 2004]) and more recently in the Southeastern English counties of East Sussex and Kent (G. Zięba and G.H. Copp, unpubl. data, 2009).\ud \ud Sunbleak naturally aggregate in shoals throughout their life ([Rüppell and Gößwein, 1972], [Siegmund and Wolff, 1973a], [Siegmund and Wolff, 1973b], [Andoerfer, 1980], [Arnold and Längert, 1995] and [Pinder and Gozlan, 2004]), which suggests strong social connections between individual fish. As a means of identifying one or more mechanisms that may facilitate the invasion process of sunbleak, our objectives were to: (1) determine whether sunbleak integrate into the native species social network during their early life, and (2) assess the properties of the social ties between the invading sunbleak and native species during their early development. This is the first study that utilises network analysis to examine the social properties of a native fish assemblage containing a non-native fish species.\ud \u
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