13 The global cargo ship network TABLE I: Characterization of different subnetworks. Number of ships, total gross tonnage [10 6 GT] and number of ports n in each subnetwork; together with network characteristics: mean degree 〈 k 〉 , clustering coefficient C , mean shortest path length 〈 l 〉 , mean journeys per link 〈 J 〉 , power-law exponents µ and η ; and trajectory properties: average number of distinct ports 〈 N 〉 , links 〈 L 〉 , port calls 〈 S 〉 per ship and regularity index 〈 p 〉 . Some notable values are highlighted in bold. other complex networks investigated in recent years, possesses the small world property as well as broad degree and weight distributions. Other features, like the importance of canals and the grouping of ports into regional clusters, are more specific to the shipping industry. An important characteristic of the network are differences in the movement patterns of different ship types. Bulk dry carriers and oil tankers tend to move in a less regular manner between ports than container ships. This is an important result regarding the spread of invasive species because bulk dry carriers and oil tankers often sail empty and therefore exchange large quantities of ballast water. The gravity model, which is the traditional approach to forecasting marine biological invasions, captures some broad trends of global cargo trade, but for many applications its results are too crude. Future strategies to curb invasions will have to take more details into account. The network structure presented in this article can be viewed as a first step in this direction. We thank B. Volk, K. H. Holocher, A. Wilkinson, J. M. Drake and H. Rosenthal for stimulating discussions and helpful suggestions. We also thank Lloyd’s Register Fairplay for providing their shipping data base. This work was supported by German VW-Stiftung and BMBF. Acknowledgments Ship class ships MGT n 〈 k 〉 C 〈 l 〉 〈 J 〉 µ η 〈 N 〉 〈 L 〉 〈 S 〉 〈 p 〉 Whole fleet 16363 664.7 951 76.4 0.49 2.5 13.57 1.71 1.02 10.4 15.6 31.8 0.63 Container ships 3100 116.8 378 32.4 0.52 2.76 24.25 1.42 1.05 11.2 21.2 48.9 1.84 Bulk dry carriers 5498 196.8 616 44.6 0.43 2.57 4.65 1.93 1.13 8.9 10.4 12.2 0.03 Oil tankers 2628 178.4 505 33.3 0.44 2.74 5.07 1.73 1.01 9.2 12.9 17.7 0.19