Protist, Vol. 149, 29-37, February 1998 © Gustav Fischer Verlag ORIGINAL PAPER Protist Protozoan Diversity: Converging Estimates of the Global Number of Free-Living Ciliate Species Bland J. Finlaya,1, Genoveva F. Esteban a , and Tom Fenchel b a Institute of Freshwater Ecology, Windermere laboratory, The Ferry House, Ambleside, Cumbria LA22 alP, UK b Marine Biological laboratory (University of Copenhagen), Strandpromenaden 5, DK - 3000, Helsingm, Denmark Submitted October 6, 1997; Accepted November 14, 1997 Monitoring Editor: Michael Melkonian 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 es- timate the global species richness of free-living ciliated protozoa in the marine interstitial and fresh- water 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 free- living ciliates are ubiquitous. This strengthens our recent claim that the global species richness of free-living ciliated protozoa is relatively low (-3000). Introduction It is rather difficult to provide accurate estimates for the number of species in any of the larger taxonomic groups (see May 1988, 1990). Furthermore, the scale of the problem seems to be inversely related to the size of the organisms concerned. Estimating the global species richness of birds is probably an achievable task (Zink 1996), but more difficult for the insects (Gaston 1992) and, apparently, extraordinar- ily difficult for micro-organisms (see UNEP 1995), where in many cases we do not even have a clear idea of what a species is, let alone a sound strategy for estimating global diversity. 1 Corresponding author; fax 44-15394-46914; e-mail b.finlay@ife.ac.uk Some of the methods used for estimating species richness of larger organisms are inade- quate when it comes to micro-organisms. For ex- ample, it may under certain circumstances be pos- sible to extrapolate species numbers in size cate- gories down to about 1 em, but this becomes a very dubious procedure for size classes smaller than this (Fenchel 1993). The main reason may be the marked tendency towards cosmopolitanism in the smallest organisms. For these, barriers to migration and dispersal appear to be ineffective: thus rates of extinction and speciation may be low, and the same species will tend to be distributed worldwide. So the global species richness of micro-organisms could be relatively low, and the task of accurately estimating the number of microbial species (includ- Protist, Vol. 149, 29-37, February 1998 © Gustav Fischer Verlag ORIGINAL PAPER Protist Protozoan Diversity: Converging Estimates of the Global Number of Free-Living Ciliate Species Bland J. Finlaya,1, Genoveva F. Esteban a , and Tom Fenchel b a Institute of Freshwater Ecology, Windermere laboratory, The Ferry House, Ambleside, Cumbria LA22 alP, UK b Marine Biological laboratory (University of Copenhagen), Strandpromenaden 5, DK - 3000, Helsingm, Denmark Submitted October 6, 1997; Accepted November 14, 1997 Monitoring Editor: Michael Melkonian 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 es- timate the global species richness of free-living ciliated protozoa in the marine interstitial and fresh- water 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 free- living ciliates are ubiquitous. This strengthens our recent claim that the global species richness of free-living ciliated protozoa is relatively low (-3000). Introduction It is rather difficult to provide accurate estimates for the number of species in any of the larger taxonomic groups (see May 1988, 1990). Furthermore, the scale of the problem seems to be inversely related to the size of the organisms concerned. Estimating the global species richness of birds is probably an achievable task (Zink 1996), but more difficult for the insects (Gaston 1992) and, apparently, extraordinar- ily difficult for micro-organisms (see UNEP 1995), where in many cases we do not even have a clear idea of what a species is, let alone a sound strategy for estimating global diversity. 1 Corresponding author; fax 44-15394-46914; e-mail b.finlay@ife.ac.uk Some of the methods used for estimating species richness of larger organisms are inade- quate when it comes to micro-organisms. For ex- ample, it may under certain circumstances be pos- sible to extrapolate species numbers in size cate- gories down to about 1 em, but this becomes a very dubious procedure for size classes smaller than this (Fenchel 1993). The main reason may be the marked tendency towards cosmopolitanism in the smallest organisms. For these, barriers to migration and dispersal appear to be ineffective: thus rates of extinction and speciation may be low, and the same species will tend to be distributed worldwide. So the global species richness of micro-organisms could be relatively low, and the task of accurately estimating the number of microbial species (includ-