PROTOZOAN DIVERSITY - CONVERGING ESTIMATES OF THE GLOBAL NUMBER OF FREE-LIVING CILIATE SPECIES

Protozoa are the most abundant phagotrophs in the biosphere, but no sc ientific strategy has emerged that might allow accurate definition of the dimensions of protozoan diversity on a global scale. We have begun this task by searching for the common ground between taxonomy and eco logy. We have used two methods - taxonomic analysis, and extrapolation from ecological datasets - to estimate the global species richness of free-living ciliated protozoa in the marine interstitial and freshwat er benthos. The methods provide estimates that agree within a factor o f two, and it is apparent that the species-area curves for ciliates mu st be almost flat (the slope z takes the very low value of 0.043 in th e equation: [number of species] = [constant][area](2)). Insofar as ind ependent ecological datasets can be extrapolated to show similiar, fla t, species-area relations, and that these converge with an independent estimate from taxonomic analysis, we conclude that the great majority of free-living ciliates are ubiquitous. This strengthens our recent c laim that the global species richness of free-living ciliated protozoa is relatively low (similar to 3000).