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](z)). 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 freeliving ciliates are ubiquitous. This strengthens our recent claim that the global species richness of free-living ciliated protozoa is relatively low (∼3000).