Incorporating risk mapping at multiple spatial scales into eradication management plans

The success of pro-active management of invasive plants depends on the ability to rapidly detect invasive populations and individuals. However, the factors important for detection depend on the spatial scale examined. We propose a protocol for developing risk maps at national, landscape, and local scales to improve detection rates of invasive plant species. We test this approach in the context of developing an eradication plan for the invasive tree Acacia stricta in South Africa. At a national scale we used bioclimatic models coupled with the most likely sites of introduction (i.e. forestry nursery plantations) to identify areas where national-scale surveillance should be focussed. At the landscape and local scales we correlated the presence of A. stricta populations to various attributes. Regional populations were found in forestry plantations only, and mostly on highly used graded roads along which seeds are spread by road maintenance vehicles. Locally, previously recorded plant localities accurately predicted individuals in subsequent surveys. Using these variables, we produced a map of high-risk areas that facilitated targeted searches-which reduced the required search effort by ca. 83 %-and developed recommendations for site-specific surveying. With the high visibility of plants, and relatively small seed banks, long-term annual clearing should achieve eradication. We propose that such multi-scale risk mapping is valuable for prioritising management and surveillance efforts, though caution that the approach is correlative and so it does not represent all the sites that can be invaded.