ABSTRACT
The approach here is based on two programs of work with which I have been involved: The ¢rst is the study of the impacts of alien species and how to measure them2,3 and the second is the economics section of the Global Invasive Species Programme (GISP).4 It is important to note that economics is not accountancy, even though economic assessments will normally include a cost-bene¢t analysis. So, the GISP Economics Programme produced few cost estimates, and none that should be taken too seriously. The same applies to the cost ¢gures in this chapter. Although I give some numbers, the importance and effect of alien invasive plants in the British Isles are given more reliably by an understanding of how costs arise and the policy options to contain them rather than by concentrating on narrowly based ¢gures.
