Border control for stowaway alien species should be prioritised based on variations in establishment debt

Abstract

Highlights• We evaluated the number of stowaways that could be introduced and establish in South Africa.• The number of species that pose a risk varies across donor regions and seasons.• South Africa's risk is greatest in southern hemisphere spring and autumn.• In spring and autumn the risk posed by northern hemisphere countries increases.• Vectors from countries should be targeted during periods of the year when many species will be introduced and establish. AbstractBorder 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 1 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.

Indicators for monitoring biological invasions at a national level

“…In particular, although South Africa has started efforts at proactive management (Wilson, Ivey, Manyama, & Nänni, 2013), it is difficult to demonstrate the economic value of avoiding the predicted negative impacts of invasions that do not occur (Leung et al, 2002). We have started to estimate some aspects of invasion debt for South Africa (Faulkner, Robertson, Rouget, & Wilson, 2016;Rouget et al, 2016), but much more work is needed. Figure A1 (Indicator 1.3 van Wilgen, & McConnachie, 2012;van Wilgen, Fill, Govender, & Foxcroft, 2017).…”

Predicting the Risk of Biological Invasions Using Environmental Similarity and Transport Network Connectedness

“…(47) It should also be emphasized that many nations have land borders, providing additional transport pathways (both natural and through human transport) for the import of alien species, and this would need to be incorporated when applying this framework elsewhere. Understanding the links between transport pressure (as a reasonable proxy for the number of species that might be unintentionally transported between regions) and environmental similarity (and hence the capacity of those species to potentially survive) is critical for effective border control when biosecurity resources need to be prioritized, (48) and the approach we present here should provide valuable insight to this end.…”

The legacy of over a century of introductions: Spread debt of rainbow trout (Oncorhynchus mykiss) in Mpumalanga Province, South Africa

“…Spread debt can be used to identify additional areas that are likely to be invaded, and management implications include the identification of priority areas for control, the identification of areas where spread reduction methods are required, and spatial planning of the management of biological invasions (e.g., Khosa, Marr, Wasserman, Zengeya, & Weyl, 2019; Padayachee, Procheş, & Wilson, 2019). There are few studies that have investigated some aspects of invasion debt in South Africa, such as establishment debt (e.g., Faulkner, Robertson, Rouget, & Wilson, 2016; Novoa, Kaplan, Kumschick, Wilson, & Richardson, 2015; Walker, Gaertner, Robertson, & Richardson, 2017), spread of single species (e.g., Moore, Runge, Webber, & Wilson, 2011), and only one study has investigated all four components of invasion debt (Rouget et al, 2016).…”