Raphaël Trouvé1,2 and Andrew Robinson1
1School of Ecosystem and Forest Sciences, University of Melbourne, 500 Yarra Blvd, Richmond, Victoria 3121, Australia
2CEBRA & School of Mathematics and Statistics, University of Melbourne, Victoria 3010, Australia
The introduction of invasive plants with global trade is one of the most important socioecological challenge worldwide. One major option to reduce the introduction of invasive species is border inspection (i.e. reducing the risk at point of entry). Biosecurity inspection has most often been framed as an acceptance sampling problem at the consignment-level, i.e., inspecting each incoming consignment until we are confident that its infestation rate is below a chosen threshold. However, at the pathway-level, we currently lack a method to interpret the effect of inspection in terms of a risk assessment model (risk = probability of invasion times consequences).
Here, we provide an analytical framework and derive an explicit formula to predict propagule pressure reduction that arise from inspecting a pathway. The risk reduction from inspection arises from two processes (i) a reduction of the number of consignments allowed to enter the country due to non-compliance and (ii) a reduction in the mean infestation rate in the consignments allowed to enter the country compared to the mean infestation rate outside the country, as the inspection will preferentially filter highly infested consignments.
We illustrate our approach on several plant invasion pathways to Australia and New Zealand. We show that the risk reduction factor associated with an inspection can vary by more than one order of magnitude among pathways. Theoretical analysis reveals that we can predict the effectiveness of an inspection from the characteristics of the pathway (mean infestation rate and variability in infestation rate among different consignments of the pathway). Reframing biosecurity inspection within the framework of the risk assessment model not only allows optimizing the allocation of sampling effort for inspection, but it also allows comparing inspection effects with other potential risk reduction measures.