inventory of<1 mm species in two small water bodies, suggests that a similar
picture would have emerged irrespective of where the investigation had been
carried out. This interpretation is supported by the finding that protist species
richness is generally the same at all latitudes (Hillebrand & Azovsky,2001). This
is not to say that all protist species thrive at all latitudes. The conspicuous
pantropical ciliateNeobursaridium gigasgrows only within a narrow temperature
range of a few degrees on either side of 25 8 C (Dragesco, 1968 ) but it does so in
more that one geographical region (i.e. Africa and South-East Asia). The cold-
water planktonic foraminiferan species (Darlinget al., 2000), and the dinofla-
gellatePolarella glacialis(Montresoret al., 2003) have bi-polar distributions.
The vast majority of free-living protists probably tolerate fairly broad ranges
of environmental factors (e.g. salinity, temperature) and the few taxa that have
been investigated experimentally (e.g.Paraphysomonas,Cyclidium) indicate that
they also have great capacity for physiological adaptation within genetically
determined limits that are much broader than those of macrofauna. It is per-
haps not surprising then, that an intensive study of ciliate morphospecies in an
Australian crater-lake (Estebanet al., 2000) should reveal a species list consisting
entirely of species already known from northern Europe.
Neutral theory – local:global species ratios
Finlay and Fenchel ( 2004 ) showed that the size dependence of local:global
species ratios in small aquatic organisms appears to exist independently of the
taxonomic identity of the organisms concerned. The common factor underlying
these patterns is organism size, because size is inversely correlated with popu-
lation size, which largely determines the probability of dispersal (Finlay, 2002 ).
To some extent, a neutral model of community structure (Bell, 2001 ; Hubbell,
2001 ) is supported. While we do not necessarily endorse all of the assumptions
in current neutral community theories (e.g. that the aquatic species we have
catalogued are ecologically identical), our results support some of the predic-
tions of neutral theory – in particular that high absolute abundance of microbes
drives their large-scale random dispersal, resulting in high local species rich-
ness, correlated local and global abundances of individuals within species, and
the creation of metapopulations with global-scale distribution (Fenchel,1993;
Finlayet al., 2001).
Cosmopolitan distribution means that the global species richness of free-
living micro-organisms will be relatively small because a microbial species
carries out the same function in places that are geographically distant from
each other, which explains why there are apparently relatively few small spe-
cies, as originally highlighted by May (1988). These patterns support the domi-
nating role of dispersal and local extinction in moulding the composition of
biotic communities. We also note that in contrast to results obtained from
higher plant communities (Conditet al., 2002), the application of neutral models
BODY SIZE AND BIOGEOGRAPHY 181