Protozoan diversity: Converging estimates of the global number of free-living ciliate species

Protozoa are the most abundant phagotrophs in the biosphere, but no scientific 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 ecology. 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 freshwater benthos. The methods provide estimates that agree within a factor of two, and it is apparent that the species-area curves for ciliates must be almost flat (the slope z takes the very low value of 0.043 in the equation: [number of species] = [constant][area](2)). Insofar as independent ecological datasets can be extrapolated to show similiar, flat, 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 claim that the global species richness of free-living ciliated protozoa is relatively low (similar to 3000).