players were always part of a well mixed population. [@now0; @now3; @sza2].

The evolutionary behaviour of the populations of surviving strategies of spatial games on networks can be affected by several features of the underlying topology as, for example, the degree distribution of the graph, the average distance between nodes, or the clustering coefficient [@kup1; @kup2; @dur; @moy; @roc].

The concept that cooperators can survive by grouping in clusters has been discussed and analyzed in many works [@oht; @now3; @doe]. Intuitively, the reasoning goes as follows. The effect of the cluster would be to screen the nodes at the interior from the presence of defectors. As defectors can only get an advantage from their interaction with cooperators, only those located next to the border of a cluster of cooperators should collect any benefits. In turn, although the cooperators at the border of the cluster should have lower payoffs because of their interaction with defectors, their cooperator neighbours at the interior of the cluster should perform better than the defectors at the border. Thus, imitating the internal cooperators should be always more convenient than imitating the bordering defectors, which should lead to the survival, and even expansion, of the cluster of 