Biodiversity Assessment: moving towards an evidence-based index for biodiversity offsetting

Biodiversity offsetting is a mechanism for providing physical compensation to redress losses to species and habitats caused by development projects. As offsetting becomes more widespread, so has the evolution and development of frameworks, tools and methodological approaches for assessing biodiversity and implementing offsets. In this context and with a specific focus on assessment methodology this research takes a scientific and pragmatic approach to bridge the gap between empirical approaches to biodiversity assessment and the practical, often subjective, methods used by practitioners. Although commonalities among methodologies exist, systematically reviewing the state of the art, revealed a complicated situation which would benefit from methodological standardisation. The challenge of determining which components of biodiversity should be assessed by a standardised approach was informed with data gained through a survey that questioned biodiversity practitioners on which criteria and attributes they considered the most important indicators of biodiversity value. Results of an extensive field study of three habitat types are reported and the new data are employed; (a) to examine the sensitivity of a metric proposed for pilot offsets in England, and (b) to develop a novel multi-metric index with potential for wide use in biodiversity offsetting. From an array of forty five metrics a reduced index was produced which conveys information from measurements pertaining to four important biodiversity components. The new index is objective, relatively quick to produce, replicable and scientifically defensible. Compatible with existing frameworks the new index comprises information practitioners would expect to see i.e. biodiversity data (beta-diversity), temporal risk, (time to maturity) habitat rarity and structural connectivity. It can reliably provide a measure of value to biodiversity, inform spatial planning decisions, generate data for monitoring and aid the comparison of two or more sites of similar habitat. In concluding, the thesis discusses practical limitations of the index and, more generally, limitations for biodiversity offsetting.