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The International Maritime Organization based in London estimates that 90 percent of the world’s trade is moved around the planet by sea. Given the fascination that complexity scientists have with rail, air and road networks, it seems strange that so little attention has been paid to the maritime network.

That’s a wrong that’s put right today by Pablo Kaluza and pals at Carl von Ossietzky University in Oldenburg, Germany. These guys have used the itineraries of 16,363 cargo ships during 2007 to construct a network of links between the world’s top 951 ports. The results, at least in part, are eyebrow raising.

First, the unsurprising news. Kaluza and co show that these links form a small world network in which it is easy to move from one port to another in a small number of jumps. That’s just what you’d expect given what we know about other transport networks.

However, the maritime network shows some surprising differences from the network that flight paths make between airports. For example, on average, it takes just 2.5 jumps to move from one port to another on the maritime network compared to an average of 4.4 to move between one airport and another. The maximum shortest path between ports is 8 jumps while between airports it is 15 jumps. It looks to be decidedly easier to travel the world by ship than by plane, at least in some respects.

One oddity, however, is that the maritime network is decidedly asymmetric: more than half of all ports are linked in only one direction, meaning that cargo ships do not routinely make round trip journeys between ports.

Perhaps most interesting of all is that different types of ship move in different patterns. Kaluza and co study three types of ship: container ships, bulk dry carriers and oil tankers. While container ships tend to follow regular schedules, the movement of bulk dry carriers and oil tankers is much less regular. That’s because their routes are determined by the price of the commodities they carry and which vary enormously. Bulk dry carriers and oil tankers are also more likely to sail empty.

That’s important because it gives a unique insight into the pattern of world trade. But there is another reason too.

One of the most significant methods of cross species invasion is from water sucked into ships’ ballast tanks in one part of the world and discharged in another; a particularly important factor when ships sail empty.

So the new network map should give marine biologists an insight into how bioinvasion occurs and what steps they can take to tackle it.




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