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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. 

## Acknowledgments 

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. 

