Indications of marine bioinvasion from network theory

Abstract

The transport of huge amounts of small aquatic organisms in the ballasttanks and at the hull of large cargo ships leads to ever increasingrates of marine bioinvasion. In this study, we apply a network theoreticapproach to examine the introduction of invasive species into new portsby global shipping. This is the first stage of the invasion processwhere it is still possible to intervene with regulating measures. Wecompile a selection of widely used and newly developed networkproperties and apply these to analyse the structure and spreadcharacteristics of the directed and weighted global cargo ship network(GCSN). Our results reveal that the GCSN is highly efficient, showssmall world characteristics and is positive assortative, indicating thatquick spread of invasive organisms between ports is likely. The GCSNshows strong community structure and contains two large communities, theAtlantic and Pacific trading groups. Ports that appear as connector hubsand are of high centralities are the Suez and Panama Canal, Singaporeand Shanghai. Furthermore, from robustness analyses and the network'spercolation behaviour, we evaluate differences of onboard and in-portballast water treatment, set them into context with previous studies andadvise bioinvasion management strategies.