Monitoring of biological diversity in space and time

Monitoring programmes are being used increasingly to assess spatial and tem poral trends of biological diversity, with an emphasis on evaluating the ef ficiency of management policies. Recent reviews of the existing programmes, with a focus on their design in particular, have highlighted the main weak nesses: the lack of well-articulated objectives and the neglect of differen t sources of error in the estimation of biological diversity. We review rec ent developments in methods and designs that aim to integrate sources of er ror to provide unbiased estimates of change in biological diversity and to suggest the potential causes.