PRACTICAL APPROACHES TO THE ESTIMATION OF THE EXTENT OF BIODIVERSITY IN SPECIOSE GROUPS

The overall dimensions of global species richness remain very imprecis ely known and the manner in which this richness is distributed only sk etchily understood. This lamentable state of affairs is largely due to an inadequate appreciation of the contributions made by the most spec iose groups. The most reliable, practical and cost-effective means of documenting patterns and estimating species richness in these groups i s the use of a piecemeal, step by step, approach, eschewing the use of first principles, empirical relationships that are not directly amena ble to calibration, diversity indices and 'short cuts' that take no ac count of the effects of scale. Instead, simple ratios of species richn ess from taxon to taxon, focal group to more inclusive group, site to site, sample to inventory, and across spatial scales provide a basis f or extrapolation. Essential features of this approach are the calibrat ion of ratios, ensuring that like is compared with like, and the fulle st use of 'hands on' knowledge of the groups in question and the setti ngs in which they are found. The choice and use of focal groups for ex trapolation to larger groups and the choice and use of sampling method s to obtain reliable sample data from which to extrapolate to site inv entories are considered in some detail. The way that the interplay bet ween patchy distributions, method of sampling and sample 'dimensions' influences the reliability and precision of estimates is also discusse d. The importance of appropriate rigour in assembling the species data sets that form the basis of estimates, including care in the choice an d use of sampling regimes and accuracy in species recognition and sort ing, is stressed. Although species richness patterns in terrestrial ar thropods are used here as examples, the principle of employing simple ratios for extrapolation is also applicable to other speciose groups a nd other settings.