Prioritising surveillance for alien organisms transported as stowaways on ships travelling to South Africa

S1 Fig. The location of the five selected South African ports, and the benthic bioregions as per Sink et al. [70].S2 Fig. The number of ocean going vessels arriving at South African ports each year.S3 Fig. Examples of pairwise-comparisons of the climatic conditions of ports.S4 Fig. Examples of pairwise-comparisons of the marine environmental conditions of ports.S5 Fig. The biogeographical regions as classified by Mead et al. [69] and the position of the five selected South African ports.S6 Fig. The number of ship visits, number of days travelled, marine environmental distance and terrestrial environmental distance for the South African ports.S7 Fig. The seasonal, relative contribution of shipping routes from foreign ports to the marine and terrestrial establishment debt of South African ports.S8 Fig. Seasonal variation in the number of ship visits, number of days travelled, marine environmental distance and terrestrial environmental distance of the South African ports.S9 Fig. The number of marine and terrestrial watch list species that might be transported along the twenty shipping routes to each South African port with the highest relative contribution to marine and terrestrial establishment debt.S10 Fig. Temporal trends in the contribution of different regions to South African merchandise imports.S1 Table. Details of the foreign ports included in the analysis.S2 Table. Species on the watch list developed for South Africa by Faulkner et al. [71], the environment in which they occur (marine or terrestrial), whether they are found in climate zones or marine ecoregions that are associated with the source ports of high risk shipping routes, and if so whether they have a history of shipping-facilitated introduction.S3 Table. Marine and terrestrial watch list species that might be transported along the twenty shipping routes to each South African port with the highest relative contribution to marine and terrestrial establishment debt.S4 Table. The relative contribution of shipping routes from foreign ports to marine and terrestrial establishment debt.S5 Table. The seasonal, relative contribution of shipping routes from foreign ports to the marine and terrestrial establishment debt of South African ports.The global shipping network facilitates the transportation and introduction of marine and terrestrial organisms to regions where they are not native, and some of these organisms become invasive. South Africa was used as a case study to evaluate the potential for shipping to contribute to the introduction and establishment of marine and terrestrial alien species (i.e. establishment debt) and to assess how this varies across shipping routes and seasons. As a proxy for the number of species introduced (i.e. `colonisation pressure') shipping movement data were used to determine, for each season, the number of ships that visited South African ports from foreign ports and the number of days travelled between ports. Seasonal marine and terrestrial environmental similarity between South African and foreign ports was then used to estimate the likelihood that introduced species would establish. These data were used to determine the seasonal relative contribution of shipping routes to South Africa's marine and terrestrial establishment debt. Additionally, distribution data were used to identify marine and terrestrial species that are known to be invasive elsewhere and which might be introduced to each South African port through shipping routes that have a high relative contribution to establishment debt. Shipping routes from Asian ports, especially Singapore, have a particularly high relative contribution to South Africa's establishment debt, while among South African ports, Durban has the highest risk of being invaded. There was seasonal variation in the shipping routes that have a high relative contribution to the establishment debt of the South African ports. The presented method provides a simple way to prioritise surveillance effort and our results indicate that, for South Africa, port-specific prevention strategies should be developed, a large portion of the available resources should be allocated to Durban, and seasonal variations and their consequences for prevention strategies should be explored further.This work was supported by the South African National Department of Environment Affairs through its funding of the South African National Biodiversity Institute's Invasive Species Programme. Additional funding was provided by the DST-NRF Centre for Invasion Biology. MR acknowledges funding from the South African Research Chairs Initiative of the Department of Science and Technology and National Research Foundation of South Africa.http://www.plosone.orgam2017Zoology and Entomolog