AbstractThe internationally accepted definition of biodiversity creates difficulty in measuring difference and change. The authors suggest that well-sampled data can be used to generate a range of numerical indices reflecting species group characteristics/functionality (Species Richness, Simposons Index, Population Density, Biomass and Species Conservation Value) that can be viewed in combination to create a picture of biodiversity quality. This overall approach has considerable advantages over the currently accepted Convention on biological Diversity definition based on the "variability" of genes, species and ecosystems, since the numerical expression of the indices allows the probability of difference between biodiversity quality trends and values over time and between sites or taxonomic groups, to be assessed for statistical inference.