21 The global cargo ship network FIG. 4: Motif distributions of the three main cargo fleets. A positive (negative) normalized Z score indicates that a motif is more (less) frequent in the real network than in random networks with the same degree sequence. For comparison, we overlay the Z scores of the World Wide Web and social networks. The agreement suggests that the ship networks fall in the same superfamily of networks (Milo et al. 2004). The motif distributions of the fleets are maintained even when 25%, 50% and 75% of the weakest connections are removed.