Abstract:
Border control is one of the major approaches used by countries to limit the number of
organisms introduced as stowaways. However, it is not feasible to inspect all passengers,
cargo and vehicles entering a country, and so efforts need to be prioritised. Here we use
South Africa as a case study to assess, based on tourism and trade data and climate
matching techniques, the number of stowaway species that might be introduced
(‘colonisation pressure’) and the likelihood that once introduced, these organisms will
establish (‘likelihood of establishment’). These results were used to explore how the number
of species that are likely to establish (‘establishment debt’) varies across donor regions and
seasons. A simple theoretical model was then used to compare four strategies for prioritising
border control inspections: no prioritisation; based on colonisation pressure; based on likelihood of establishment; and based on both colonisation pressure and likelihood of
establishment. Establishment debt was greatest in southern hemisphere spring and autumn
when South Africa is climatically similar to northern hemisphere countries with which there
are strong, consistent trade and tourism links (i.e. colonisation pressure varied little
seasonally, but likelihood of establishment did vary across the seasons). Prioritising
inspections based on both colonisation pressure and the likelihood of establishment was
clearly the most effective strategy, with this strategy detecting at least 6% more potential
invaders than the other strategies. While there are many practical limitations to the
implementation of such prioritised inspection strategies, the results highlight the importance
of national and regional studies of establishment debt.
