Ocean traffic is on the increase and it has been predicted that by 2030, shipping will have doubled. 'A single vessel releases as many chemical pollutants as 50 million cars.' It is also reported that pollution from vessels causes 60 000 premature deaths every year.

The harmful impacts on the environment and human health by the emission of nitrogen oxides, sulphur oxides and particulate matter are, however, not the only dangers shipping vessels pose. They are also carriers of alien species that threaten to invade foreign countries. Shipping can be a vector of both marine and terrestrial alien species – when ships leave a port, they pick up water for balance and so take up a lot of organisms. Organisms can also attach to the hulls of ships and get transported in this way. Terrestrial species can be found in cargo. If introduced, these species can cause major problems to the environment and human health, and can have an impact on agriculture and other economic activities.

Dr Katelyn Faulkner and Prof Mark Robertson, researchers from the Centre for Invasion Biology at the University of Pretoria's Department of Zoology and Entomology, looked at the risks posed by alien species transported as stowaways on ships traveling to South Africa. As part of their study, which was published in PLOS ONE with the title 'Prioritising surveillance for alien organisms transported as stowaways on ships travelling to South Africa', they created a model for identifying shipping routes that are likely to introduce alien species with the potential to become established. This information can then be used to direct action or resources to reduce the introduction of invasive species. The model can also be used in other countries. 

Faulkner and her team gathered data from across the globe to determine the likelihood that introduced species would become established in South Africa. Key aspects included the number of ships on shipping routes to South Africa, whether the ports where these ships come from are environmentally similar to South African ports, and the duration of the voyage. If ships took longer than three months to get to South Africa, they were presumed not to pose any threat as organisms would be unlikely to survive a voyage of that duration. Robertson noted, 'The likelihood of new species getting a foothold in South Africa is of great concern. We know of several species that are invasive in other parts of the world but there are probably many more that we do not know about which could become invasive if introduced.' The study involved the mammoth task of sifting through millions of ship records – ships are fitted with satellite tracking systems and Faulkner had access to information on all the movements ships make throughout the world. However, by focusing on the abovementioned aspects, the researchers were able to refine their study. Faulkner's skills in programme development also made it possible to filter and analyse the data and determine the threat posed to the country.

The study focused on South Africa's five busiest ports: Cape Town, Richards Bay, Saldanha Bay, Port Elizabeth and Durban. These ports are quite different from one another in terms of their climates, environments and the types of cargo that enter them. Durban port handles 60% of South Africa's imports and exports, and is said to be Africa's most active general cargo port. The study shows that more alien species are likely to be introduced and establish in Durban than at the other ports. The highest volume of traffic is from Asian ports, particularly Singapore. These routes were flagged as high risk in Faulkner's model.

Faulkner also created a watch list of alien species that are known to be invasive, but are not yet in South Africa. These species could pose a major threat to the health and safety of the country if introduced. A number of species on the watch list are linked to the busiest routes to South Africa, and results from the assessment suggest that they could establish here.

An example of such a species is the southern house mosquito (Culex quinquefasciatus), which is found in parts of Africa and Asia and is a vector of diseases affecting both humans and animals. A common mode of transport of mosquitos is on ships via transported tyres. Mosquito larvae are able to survive in the water that often lies inside tyres. The southern house mosquito is known to transmit diseases such as lymphatic filariasis, avian malaria, St Louis encephalitis, Western equine encephalitis, and West Nile fever. As a transmitter of arboviruses, they also pose the threat of outbreaks of diseases such as the Zika virus.

By combining the results of the model with data on the watch list species, they were also able to give direction on which routes should be monitored for these species. The model also provides information on which routes and ports should be the focus for surveillance, where introductions and invasions are most likely to occur.

Developing from this study, Faulkner is also looking at South Africa as a donor of invasive species to neighbouring countries, and vice versa. South Africa has a bio-security responsibility to its people and its neighbours, so the work of Faulkner and her colleagues is imperative.

*This was a collaborative research project with Mathieu Rouget from CIRAD, La Réunion, France and John Wilson from the Centre for Invasion Biology at the Department of Botany and Zoology, Stellenbosch University, South Africa.