Identifying hazards in complex ecological systems. Part 1: Fault-tree analysis for biological invasions

Ecological risk assessment is a common environmental management tool. It is routinely used for physical and chemical stressors, and has recently been used for biological stressors such as invasive species and genetically modified organisms. Identifying hazards correctly is critical to any risk assessment. In many ecological examples, however, this stage of the analysis is very poor. This paper applies a hazard-analysis tool that is commonly used in complex engineering systems - fault-tree analysis - to an ecological system - ballast-water introductions. The analysis, which is rigorous and systematic, highlights the complexity of the ballast-water introduction cycle. The top event in the fault-tree is the successful infection of a port. The fault-tree identifies the parallel and sequential events leading up to the top event, including multiple vessel-infection scenarios, determined by the origin (hard substrate, soft substrate, water column and another organism) and behaviour of the target organism. The analysis also identifies the role of ballast-tank populations, ballast-water carry-over, crevice-seeking species and third-party risks. These phenomena add considerably to the complexity of the problem. Fault-trees will have a limited application to ecological systems because of the difficulty of estimating the probability of the basic or undeveloped events in the tree. As this paper demonstrates, however, fault-tree analysis has considerable heuristic potential when applied to ecological systems.