AN ASSESSMENT OF SOME IMPROVED TECHNIQUES FOR ESTIMATING THE ABUNDANCE (FREQUENCY) OF SEDENTARY ORGANISMS

The traditional sampling method for estimating frequency (the number o f sub-quadrats containing a basal part of the organisms) is compared, using both computer simulations and direct comparison in the field, to two new methods that use a compound series of variable-sized concentr ic sub-quadrats. Both the new frequency-score and the new importance-s core methods are closer approximations of density than is the standard frequency method, and the estimates produced by both of the new metho ds are less affected by the choice of sub-quadrat size and the spatial distribution (dispersion) of the organisms (i.e. clumping and regular ity). Thus, the two nested-quadrat methods appear to ameliorate the us ual frequency limitations associated with sub-quadrat size and organis m dispersion, by the use of a range of different sub-quadrat sizes. Th is is important in community studies, where the component species may show a wide range of densities and dispersions. Both of the new method s are easily employed in the field. The importance-score method involv es no more sampling effort than does standard qualitative (presence-ab sence) sampling, and it can therefore be used to sample a larger quadr at area than would normally be used for frequency sampling. This makes the method much more cost-effective as a means of estimating abundanc e, and it allows a greater number of the rarer species to be included in the sampling. The frequency-score method is more time-consuming, bu t it is capable of detecting more subtle community patterns. This mean s that it is particularly useful for the study of species-pear communi ties or where small variations in composition need to be detected.